XK worrying compression test results
Discussion
Hi all.
I have a '71 S1 XJ6 4.2.
I am struggling to get it to idle properly. It'll run for 4-5 seconds, and then misfire once, then run for a bit longer, misfire etc etc. It seems fine under load, but on overrun at low revs it'll misfire occasionally.
I have balanced the carbs, and checked air/fuel ratio. Fitting the vacuum advance hose made it run a lot better...
I decided to pull all the spark plugs (after a long drive) to see if anything interesting turned up. They where all fine, except for the plug on no. 5 cylinder, it was all sooted up. I put a spare HT lead on that plug, no change.
I ran a compression test (engine hot, throttle on the floor) on that cylinder and it returned 120psi. I should mention the engine number has an '8' (looks more like an S to me) after it, indicating 8:1 compression. With the engine cold I ran a 'wet' test, that returned just under 160psi.
Dry and cold No. 6 cylinder returns 105psi, as does No. 4.
So now I'm a bit confused. As I understand it, the engine should return 140psi+, and doing the test 'wet' shouldn't make a difference. On these two counts, and with the sooted up spark plug, that indicates a problem with No. 5 cylinder (probably piston rings).
However I struggle to believe all the cylinders are badly worn as the test of number 6 & 4 indicate. The car doesn't smoke, pulls very very well, always starts first time, doesn't use much oil and the rest of the 'plugs are a good colour. It returns about 20mpg (it's an auto). It's just this slight misfire, which I suspect is timing or points or something silly.
Obviously I need to test the rest of the pistons (smell of petrol was getting a bit strong in the garage, besides the engine should be hot), and, I guess, do a 'wet' test on them all as well but should I do anything else? Is 120psi actually OK for this engine? Should I have removed all the 'plugs before running the test?
Any thoughts?
I have a '71 S1 XJ6 4.2.
I am struggling to get it to idle properly. It'll run for 4-5 seconds, and then misfire once, then run for a bit longer, misfire etc etc. It seems fine under load, but on overrun at low revs it'll misfire occasionally.
I have balanced the carbs, and checked air/fuel ratio. Fitting the vacuum advance hose made it run a lot better...
I decided to pull all the spark plugs (after a long drive) to see if anything interesting turned up. They where all fine, except for the plug on no. 5 cylinder, it was all sooted up. I put a spare HT lead on that plug, no change.
I ran a compression test (engine hot, throttle on the floor) on that cylinder and it returned 120psi. I should mention the engine number has an '8' (looks more like an S to me) after it, indicating 8:1 compression. With the engine cold I ran a 'wet' test, that returned just under 160psi.
Dry and cold No. 6 cylinder returns 105psi, as does No. 4.
So now I'm a bit confused. As I understand it, the engine should return 140psi+, and doing the test 'wet' shouldn't make a difference. On these two counts, and with the sooted up spark plug, that indicates a problem with No. 5 cylinder (probably piston rings).
However I struggle to believe all the cylinders are badly worn as the test of number 6 & 4 indicate. The car doesn't smoke, pulls very very well, always starts first time, doesn't use much oil and the rest of the 'plugs are a good colour. It returns about 20mpg (it's an auto). It's just this slight misfire, which I suspect is timing or points or something silly.
Obviously I need to test the rest of the pistons (smell of petrol was getting a bit strong in the garage, besides the engine should be hot), and, I guess, do a 'wet' test on them all as well but should I do anything else? Is 120psi actually OK for this engine? Should I have removed all the 'plugs before running the test?
Any thoughts?
varsas said:
Hi all.
I have a '71 S1 XJ6 4.2.
I am struggling to get it to idle properly. It'll run for 4-5 seconds, and then misfire once, then run for a bit longer, misfire etc etc. It seems fine under load, but on overrun at low revs it'll misfire occasionally.
I have balanced the carbs, and checked air/fuel ratio. Fitting the vacuum advance hose made it run a lot better...
I decided to pull all the spark plugs (after a long drive) to see if anything interesting turned up. They where all fine, except for the plug on no. 5 cylinder, it was all sooted up. I put a spare HT lead on that plug, no change.
I ran a compression test (engine hot, throttle on the floor) on that cylinder and it returned 120psi. I should mention the engine number has an '8' (looks more like an S to me) after it, indicating 8:1 compression. With the engine cold I ran a 'wet' test, that returned just under 160psi.
Dry and cold No. 6 cylinder returns 105psi, as does No. 4.
So now I'm a bit confused. As I understand it, the engine should return 140psi+, and doing the test 'wet' shouldn't make a difference. On these two counts, and with the sooted up spark plug, that indicates a problem with No. 5 cylinder (probably piston rings).
However I struggle to believe all the cylinders are badly worn as the test of number 6 & 4 indicate. The car doesn't smoke, pulls very very well, always starts first time, doesn't use much oil and the rest of the 'plugs are a good colour. It returns about 20mpg (it's an auto). It's just this slight misfire, which I suspect is timing or points or something silly.
Obviously I need to test the rest of the pistons (smell of petrol was getting a bit strong in the garage, besides the engine should be hot), and, I guess, do a 'wet' test on them all as well but should I do anything else? Is 120psi actually OK for this engine? Should I have removed all the 'plugs before running the test?
Any thoughts?
The correct procedure for a compression test on a vehicle with carbs:- run the engine until fully warmed up, remove the fuse to the fuel pump, remove all the spark plugs, disconnect the coil connections, have an assistant sit in the vehicle with foot on the throttle to hold it fully open. Now test each cylinder in turn for at least 10 revolutions of the crank. You will now have an accurate idea of compression.I have a '71 S1 XJ6 4.2.
I am struggling to get it to idle properly. It'll run for 4-5 seconds, and then misfire once, then run for a bit longer, misfire etc etc. It seems fine under load, but on overrun at low revs it'll misfire occasionally.
I have balanced the carbs, and checked air/fuel ratio. Fitting the vacuum advance hose made it run a lot better...
I decided to pull all the spark plugs (after a long drive) to see if anything interesting turned up. They where all fine, except for the plug on no. 5 cylinder, it was all sooted up. I put a spare HT lead on that plug, no change.
I ran a compression test (engine hot, throttle on the floor) on that cylinder and it returned 120psi. I should mention the engine number has an '8' (looks more like an S to me) after it, indicating 8:1 compression. With the engine cold I ran a 'wet' test, that returned just under 160psi.
Dry and cold No. 6 cylinder returns 105psi, as does No. 4.
So now I'm a bit confused. As I understand it, the engine should return 140psi+, and doing the test 'wet' shouldn't make a difference. On these two counts, and with the sooted up spark plug, that indicates a problem with No. 5 cylinder (probably piston rings).
However I struggle to believe all the cylinders are badly worn as the test of number 6 & 4 indicate. The car doesn't smoke, pulls very very well, always starts first time, doesn't use much oil and the rest of the 'plugs are a good colour. It returns about 20mpg (it's an auto). It's just this slight misfire, which I suspect is timing or points or something silly.
Obviously I need to test the rest of the pistons (smell of petrol was getting a bit strong in the garage, besides the engine should be hot), and, I guess, do a 'wet' test on them all as well but should I do anything else? Is 120psi actually OK for this engine? Should I have removed all the 'plugs before running the test?
Any thoughts?
The problem you are having with your car sounds like ignition or possibly one carb overflowing at the float jet due to dirt or a worn seat. You need to check the distributor and make sure the spindle is running true. If it has a slight bend it will favour some cylinders more than others.
J
jith said:
The correct procedure for a compression test on a vehicle with carbs:- run the engine until fully warmed up, remove the fuse to the fuel pump, remove all the spark plugs, disconnect the coil connections, have an assistant sit in the vehicle with foot on the throttle to hold it fully open. Now test each cylinder in turn for at least 10 revolutions of the crank. You will now have an accurate idea of compression.
The problem you are having with your car sounds like ignition or possibly one carb overflowing at the float jet due to dirt or a worn seat. You need to check the distributor and make sure the spindle is running true. If it has a slight bend it will favour some cylinders more than others.
J
Cheers.The problem you are having with your car sounds like ignition or possibly one carb overflowing at the float jet due to dirt or a worn seat. You need to check the distributor and make sure the spindle is running true. If it has a slight bend it will favour some cylinders more than others.
J
Have just re-run the tests.
Checked cylinders 1 & 2.
Throttle open, all 6 'plugs disconnected (they all looked fine, even cylinder 5), ignition disconnected, engine warm (but not hot)
They reported 130psi and 125psi. Tried a wet test on cylinder 2, got 130psi. The gauge reached it's maximum after 3 turns of the engine, I cranked 10 times just to be sure.
To be honest, I gave up then. I don't think doing any more will teach me anything. The only conclusion is that the engine is very badly worn, and needs reconditioning. To be honest I'm going to leave it. The car drives fine, I'm not going to re-build the engine because of what a gauge tells me...if it gets worse I'll reconsider.
As stated before.
Warm the engine, remove spark plugs, disconnect coil LT/HT and king lead. Disable fuel pump.
Then carry out the test, ensuring the battery has plenty of life. What you are looking for is a difference in reading's as opposed to actual readings, because your gauge could be knackered. I would only worry if there is more than a 10%-15% difference between cylinders, and to be honest it will probably be at least this on an old engine.
I used to look after a guys XJ6 SIII, he called me to say it would start one morning, he had been driving it every day. I checked all the basics and everything was there. I did a comp test to find all cylinders about 25-35 psi. I then did a wet test and it was no better. The engine had simply given up !!
I think you have plenty more life left in yours, just get the misfire sorted before it does any real damage.
Warm the engine, remove spark plugs, disconnect coil LT/HT and king lead. Disable fuel pump.
Then carry out the test, ensuring the battery has plenty of life. What you are looking for is a difference in reading's as opposed to actual readings, because your gauge could be knackered. I would only worry if there is more than a 10%-15% difference between cylinders, and to be honest it will probably be at least this on an old engine.
I used to look after a guys XJ6 SIII, he called me to say it would start one morning, he had been driving it every day. I checked all the basics and everything was there. I did a comp test to find all cylinders about 25-35 psi. I then did a wet test and it was no better. The engine had simply given up !!
I think you have plenty more life left in yours, just get the misfire sorted before it does any real damage.
So long as all compressions are within 20psi or so I'd not be too bothered about the actual readings either. Compression testing is a fairly coarse diagnostic tool at the best of times. Have you checked very carefully for a vacuum leak through the whole induction system? You might have a perished pipe, leaky servo or split inlet manifold or carb gasket - all of which only cause problems under high vacuum conditions like overun, idle and very light throttle openings. Once the manifold vacuum drops when you open the throttle and put the engine under load all the symptoms disapear even though the leak is still there.
If the missfire is consistent try to trace it by pulling off each plug lead in turn - you should get a noticable drop in RPM when you disconnect each good cylinder but less or none at all when you disconnect the bad one - that's the clue to start looking further into why you have no drop.
Lastly when you've got it sorted set the carbs up so one has a very slight slight lead over the other - somthing like 25th is about right - that'll improve light throttle drivability quite a bit.
If the missfire is consistent try to trace it by pulling off each plug lead in turn - you should get a noticable drop in RPM when you disconnect each good cylinder but less or none at all when you disconnect the bad one - that's the clue to start looking further into why you have no drop.
Lastly when you've got it sorted set the carbs up so one has a very slight slight lead over the other - somthing like 25th is about right - that'll improve light throttle drivability quite a bit.
Thanks for the replies.
I'm going to swap the HT leads for a spare set in case those are the issue, then check points etc.
The battery has no trouble starting the car, in fact it always starts very well.
I believe the gauge is fine, it registers 155+psi on my 8:1 Land Rover engine.
Air leak is a good call, I'll recheck all the pipes etc. Perhaps spray any dodgy areas with WD40, see if the revs drop.
When I say 'misfire' I mean it just stutters a bit at low revs/low loads, and is a bit lumpy on idle. It used to be a lot worse, you could see the engine shaking. It doesn't do that anymore, and is smooth at no throttle in drive but you can feel a gentle 'phut phut' from the exhaust in 'N' at idle.
ETA The compression rates are very consistent, well within 10psi across the cylinders.
Oh, so spending all that time perfectly balancing the carbs might not have been the right thing to do. doh.
I'm going to swap the HT leads for a spare set in case those are the issue, then check points etc.
The battery has no trouble starting the car, in fact it always starts very well.
I believe the gauge is fine, it registers 155+psi on my 8:1 Land Rover engine.
Air leak is a good call, I'll recheck all the pipes etc. Perhaps spray any dodgy areas with WD40, see if the revs drop.
When I say 'misfire' I mean it just stutters a bit at low revs/low loads, and is a bit lumpy on idle. It used to be a lot worse, you could see the engine shaking. It doesn't do that anymore, and is smooth at no throttle in drive but you can feel a gentle 'phut phut' from the exhaust in 'N' at idle.
ETA The compression rates are very consistent, well within 10psi across the cylinders.
Oh, so spending all that time perfectly balancing the carbs might not have been the right thing to do. doh.
Edited by varsas on Saturday 10th September 23:03
[quote=varsas]Thanks for the replies.
I'm going to swap the HT leads for a spare set in case those are the issue, then check points etc.
The battery has no trouble starting the car, in fact it always starts very well.
I believe the gauge is fine, it registers 155+psi on my 8:1 Land Rover engine.
Air leak is a good call, I'll recheck all the pipes etc. Perhaps spray any dodgy areas with WD40, see if the revs drop.
When I say 'misfire' I mean it just stutters a bit at low revs/low loads, and is a bit lumpy on idle. It used to be a lot worse, you could see the engine shaking. It doesn't do that anymore, and is smooth at no throttle in drive but you can feel a gentle 'phut phut' from the exhaust in 'N' at idle.
ETA The compression rates are very consistent, well within 10psi across the cylinders.
Oh, so spending all that time perfectly balancing the carbs might not have been the right thing to do. doh.
Fine tuning mixture and carb balance is always the last job. Get points, dwell angle and initial timing all correct first and make sure you have no vacuum leaks and the carb diagraphams and damper oil levels are OK. On a carb fed XK engine with an AED I'd make sure that was not leaking excess fuel too before I even touched the mixture adjustment. Easiest way to do that is clamp the fuel feed pipe and wait and see if the engine idle or mixture changes as the AED runs out of fuel. If it does then the engine is running partly on mixture supplied by the AED all the time and not just when it's warming up and if that's the case any attempt to set up the carbs is a waste of time.
I'd suggest you get rid of the AED in any case and buy a manual choke kit. Converting to manual choke will save you a fortune in fuel and much aggravation.
I'm going to swap the HT leads for a spare set in case those are the issue, then check points etc.
The battery has no trouble starting the car, in fact it always starts very well.
I believe the gauge is fine, it registers 155+psi on my 8:1 Land Rover engine.
Air leak is a good call, I'll recheck all the pipes etc. Perhaps spray any dodgy areas with WD40, see if the revs drop.
When I say 'misfire' I mean it just stutters a bit at low revs/low loads, and is a bit lumpy on idle. It used to be a lot worse, you could see the engine shaking. It doesn't do that anymore, and is smooth at no throttle in drive but you can feel a gentle 'phut phut' from the exhaust in 'N' at idle.
ETA The compression rates are very consistent, well within 10psi across the cylinders.
Oh, so spending all that time perfectly balancing the carbs might not have been the right thing to do. doh.
Edited by varsas on Saturday 10th September 23:03
Fine tuning mixture and carb balance is always the last job. Get points, dwell angle and initial timing all correct first and make sure you have no vacuum leaks and the carb diagraphams and damper oil levels are OK. On a carb fed XK engine with an AED I'd make sure that was not leaking excess fuel too before I even touched the mixture adjustment. Easiest way to do that is clamp the fuel feed pipe and wait and see if the engine idle or mixture changes as the AED runs out of fuel. If it does then the engine is running partly on mixture supplied by the AED all the time and not just when it's warming up and if that's the case any attempt to set up the carbs is a waste of time.
I'd suggest you get rid of the AED in any case and buy a manual choke kit. Converting to manual choke will save you a fortune in fuel and much aggravation.
Jaguar steve said:
Fine tuning mixture and carb balance is always the last job. Get points, dwell angle and initial timing all correct first and make sure you have no vacuum leaks and the carb diagraphams and damper oil levels are OK. On a carb fed XK engine with an AED I'd make sure that was not leaking excess fuel too before I even touched the mixture adjustment. Easiest way to do that is clamp the fuel feed pipe and wait and see if the engine idle or mixture changes as the AED runs out of fuel. If it does then the engine is running partly on mixture supplied by the AED all the time and not just when it's warming up and if that's the case any attempt to set up the carbs is a waste of time.
I'd suggest you get rid of the AED in any case and buy a manual choke kit. Converting to manual choke will save you a fortune in fuel and much aggravation.
Hi. I have checked the timing, seems spot on. Will double check points etc.I'd suggest you get rid of the AED in any case and buy a manual choke kit. Converting to manual choke will save you a fortune in fuel and much aggravation.
The car already has a manual choke conversion, so one less thing to worry about, thankfully.
varsas said:
Jaguar steve said:
Fine tuning mixture and carb balance is always the last job. Get points, dwell angle and initial timing all correct first and make sure you have no vacuum leaks and the carb diagraphams and damper oil levels are OK. On a carb fed XK engine with an AED I'd make sure that was not leaking excess fuel too before I even touched the mixture adjustment. Easiest way to do that is clamp the fuel feed pipe and wait and see if the engine idle or mixture changes as the AED runs out of fuel. If it does then the engine is running partly on mixture supplied by the AED all the time and not just when it's warming up and if that's the case any attempt to set up the carbs is a waste of time.
I'd suggest you get rid of the AED in any case and buy a manual choke kit. Converting to manual choke will save you a fortune in fuel and much aggravation.
Hi. I have checked the timing, seems spot on. Will double check points etc.I'd suggest you get rid of the AED in any case and buy a manual choke kit. Converting to manual choke will save you a fortune in fuel and much aggravation.
The car already has a manual choke conversion, so one less thing to worry about, thankfully.
The later type of enrichment device was the utterly apalling AED units, that operated by drawing hot air from a small heat exchanger under the rear exhaust manifold around the back of the cylinder head using a hot air tube of highly questionable insulative quality. They were dreadful in the extreme and caused more bore wear through excessive fuel wash than any other fuel system I have ever come across.
If your car has the type of manual choke conversion that still uses the valves inside this unit, get rid of it; it will never function properly as the unit itself is flawed in design.
The way to convert these properly is to purchase the lower choke bodies from the type of SUs fitted to an E Type. These have the linkage required to pull the jets down and increase the mixture at source. You can then fit a choke cable from an XJ12 saloon which is a nice piece of kit that will function perfectly.
J
jith said:
You should have said that at the start. Your car is right on the changeover date for the type of auto choke fitted. The early cars had an auxiliary carburettor in the shape of an electrical solenoid valve that drew extra fuel via a small adjustable jet and injected it into small pipes tapped into the bottom of the inlet manifold. It was operated by a thermo switch in the inlet manifold. When set up properly it was totally reliable. To convert this you have to blank off the pipework when you fit the manual choke. Many people make the mistake of simply blanking the pipe off where it screws into the device. Frequently they are unaware of the 2 rubber joints to a T piece under the manifold. These slowly perish and draw air, causing havoc with the mixture.
The later type of enrichment device was the utterly apalling AED units, that operated by drawing hot air from a small heat exchanger under the rear exhaust manifold around the back of the cylinder head using a hot air tube of highly questionable insulative quality. They were dreadful in the extreme and caused more bore wear through excessive fuel wash than any other fuel system I have ever come across.
If your car has the type of manual choke conversion that still uses the valves inside this unit, get rid of it; it will never function properly as the unit itself is flawed in design.
The way to convert these properly is to purchase the lower choke bodies from the type of SUs fitted to an E Type. These have the linkage required to pull the jets down and increase the mixture at source. You can then fit a choke cable from an XJ12 saloon which is a nice piece of kit that will function perfectly.
J
hmm...interesting, I didn't know there where two types of automated choke. I believe my car does have a manual conversion on the AED type device, it's like a third carburettor feeding into the two SU's. It is a bit weird. It has an air and fuel cam on top of it, pulling the choke moves the cam, depressing the 'fuel needle tappet' and 'air valve tappet' as required.The later type of enrichment device was the utterly apalling AED units, that operated by drawing hot air from a small heat exchanger under the rear exhaust manifold around the back of the cylinder head using a hot air tube of highly questionable insulative quality. They were dreadful in the extreme and caused more bore wear through excessive fuel wash than any other fuel system I have ever come across.
If your car has the type of manual choke conversion that still uses the valves inside this unit, get rid of it; it will never function properly as the unit itself is flawed in design.
The way to convert these properly is to purchase the lower choke bodies from the type of SUs fitted to an E Type. These have the linkage required to pull the jets down and increase the mixture at source. You can then fit a choke cable from an XJ12 saloon which is a nice piece of kit that will function perfectly.
J
To see if that's the problem I assume I can just remove it and block off the pipes between it and the SU's? In theory, with the choke closed, I shouldn't see any difference in mixture between the AED in place and it removed...
Right.
Just been out to the car. I checked the points gap (correct) but noticed the rotar arm was dirty, this has now been cleaned. I also replaced all the jubilee clips holding the pipes between the AED and the carbs, in case of leaks. I thought I could occasionally smell petrol and reckoned these pipes where the most likely cause.
I then got it into my head that when I lasted set the mixture, I had left the AED disconnected. I can't remember if I had or not but if that can affect the mixture...
Anyway, I started from scratch with the mixture, and spent about 20 minutes on each carb getting it as good as I could. I use a colourtune and have found that somewhere between the flame being orange and going blue is the best, the faintest orange colour rather then blue i.e. leave the engine running a fraction rich.
Anyway, weather the rotar arm or mixture or air leak it's running perfectly now. At idle there is just a hint of busy-ness from the exhaust, but I think that's normal. The engine doesn't move perceptibly any more (it used to shake around). On the road it responds instantly to the briefest touch of throttle from well under 1000rpm, and pulls smoothly on WOT from idle. It also 'creeps' very smoothly, you can drive it using the brake to slow you down and it's still perfectly smooth. Makes manoeuvring into the garage a bit easier. It used to surge forward and then stutter then surge again....maybe even stall...
I will drive it for a bit, reset the idle (it's gone up to 900ish, always a good sign when the idle increases!) and the re-check the spark plugs but I reckon it's about right now. Ready for Goodwood on Sunday!
Thanks for the advice, always good to learn something. I will take into account what has been said about the evil AED.
Just been out to the car. I checked the points gap (correct) but noticed the rotar arm was dirty, this has now been cleaned. I also replaced all the jubilee clips holding the pipes between the AED and the carbs, in case of leaks. I thought I could occasionally smell petrol and reckoned these pipes where the most likely cause.
I then got it into my head that when I lasted set the mixture, I had left the AED disconnected. I can't remember if I had or not but if that can affect the mixture...
Anyway, I started from scratch with the mixture, and spent about 20 minutes on each carb getting it as good as I could. I use a colourtune and have found that somewhere between the flame being orange and going blue is the best, the faintest orange colour rather then blue i.e. leave the engine running a fraction rich.
Anyway, weather the rotar arm or mixture or air leak it's running perfectly now. At idle there is just a hint of busy-ness from the exhaust, but I think that's normal. The engine doesn't move perceptibly any more (it used to shake around). On the road it responds instantly to the briefest touch of throttle from well under 1000rpm, and pulls smoothly on WOT from idle. It also 'creeps' very smoothly, you can drive it using the brake to slow you down and it's still perfectly smooth. Makes manoeuvring into the garage a bit easier. It used to surge forward and then stutter then surge again....maybe even stall...
I will drive it for a bit, reset the idle (it's gone up to 900ish, always a good sign when the idle increases!) and the re-check the spark plugs but I reckon it's about right now. Ready for Goodwood on Sunday!
Thanks for the advice, always good to learn something. I will take into account what has been said about the evil AED.
Edited by varsas on Tuesday 13th September 15:57
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