Compression query
Discussion
Hi guys
I have a 2004 Mk 1 Ford Focus 1.8 petrol.
A while ago , the car suddenly and without warning , started to misfire with very little power.
I found an error code P0351 which pointed to a problem with the coil.
The car would still start ok and idle , but very poorly .
Having been told that coils regularly fail , I replaced the coil , leads and plugs but to no avail.
Acting upon advice , I carried out a compression test and found the readings to be between 70 and 100 PSI !
The timing belt was replaced about 8 years ago , but I haven't covered sufficient mileage to warrant another replacement yet.
My question is :- Is there a chance that the belt has 'slipped ' a notch or two on the sprockets rather than totally break , or should I be looking for a more common explanation ?
Given the fact that my fault occurred instantly with no warning , and left me with poor overall compression, makes me think that an 'out of sync' cambelt is the most obvious explanation for reduced compression in all four cylinders.
Surely a head gasket failure would not be instantaneous and not affect all four cylinders simultaneously, and engine wear (rings /bore) would occur over a long period of time ?
Any opinions would be great !
I have a 2004 Mk 1 Ford Focus 1.8 petrol.
A while ago , the car suddenly and without warning , started to misfire with very little power.
I found an error code P0351 which pointed to a problem with the coil.
The car would still start ok and idle , but very poorly .
Having been told that coils regularly fail , I replaced the coil , leads and plugs but to no avail.
Acting upon advice , I carried out a compression test and found the readings to be between 70 and 100 PSI !
The timing belt was replaced about 8 years ago , but I haven't covered sufficient mileage to warrant another replacement yet.
My question is :- Is there a chance that the belt has 'slipped ' a notch or two on the sprockets rather than totally break , or should I be looking for a more common explanation ?
Given the fact that my fault occurred instantly with no warning , and left me with poor overall compression, makes me think that an 'out of sync' cambelt is the most obvious explanation for reduced compression in all four cylinders.
Surely a head gasket failure would not be instantaneous and not affect all four cylinders simultaneously, and engine wear (rings /bore) would occur over a long period of time ?
Any opinions would be great !
I'd agree that head gasket failure is very unlikely to affect all four cylinders.
Also, a head gasket failure more often than not produces other symptoms such as oil or coolant loss, exhaust smoke/steam or overheating.
I would have thought that if the engine timing was out it would throw up a fault code for camshaft or crankshaft position sensor.
Could be a damaged wire or connector to the plug on the coil.
Also, a head gasket failure more often than not produces other symptoms such as oil or coolant loss, exhaust smoke/steam or overheating.
I would have thought that if the engine timing was out it would throw up a fault code for camshaft or crankshaft position sensor.
Could be a damaged wire or connector to the plug on the coil.
Thanks for your replies guys !
I had the engine idling (very roughly!) and both wiggled and pulled the 3 wires which are plugged into the coil. There was no difference at all.
The compression test was carried out with the throttle wide open , but the engine wasn't hot because I didn't really want to run it long enough to allow it to get hot.
I have carried out compression tests before , but what struck me as odd with this one was the large variation in readings !
Once again guys , thanks for your help !
I had the engine idling (very roughly!) and both wiggled and pulled the 3 wires which are plugged into the coil. There was no difference at all.
The compression test was carried out with the throttle wide open , but the engine wasn't hot because I didn't really want to run it long enough to allow it to get hot.
I have carried out compression tests before , but what struck me as odd with this one was the large variation in readings !
Once again guys , thanks for your help !
Timing belts need replaced on both miles, and age.
Your test readings are unrealistically low, without any data to back up the gauge etc it's hard to believe they are correct. But yes, the variation between cylinders should be data that could be used as long as the tests were conducted the same for each cylinder.
As others say, test should be done hot, but a cold vs hot comparison isnt a bad thing either
As for running....what are emissions like ? What do the plugs look like ? Have you verified good spark on each cylinder ? Is fuel being sprayed well on each cylinder ?
Is your engine know for any head/valve/cam etc related problems ?
Your test readings are unrealistically low, without any data to back up the gauge etc it's hard to believe they are correct. But yes, the variation between cylinders should be data that could be used as long as the tests were conducted the same for each cylinder.
As others say, test should be done hot, but a cold vs hot comparison isnt a bad thing either
As for running....what are emissions like ? What do the plugs look like ? Have you verified good spark on each cylinder ? Is fuel being sprayed well on each cylinder ?
Is your engine know for any head/valve/cam etc related problems ?
I certainly intend to use the compression guage on a different car because I also have doubts about its accuracy.
Having said that , the guage would be equally inaccurate on each cylinder, so I am happy to accept that there is , at least , a true discrepancy in readings.
I haven't considered an emission test since with a misfire , I would expect the readings to be 'off the scale'
When removing the plugs , there is a smell of petrol so I assume the injectors are ok.
I have replaced plugs , HT leads and coil with Bosch equipment and since the engine will idle , all be it , very erratically, I have little reason to suspect the sparks being less than adequate.
Thank you for your reply.
Having said that , the guage would be equally inaccurate on each cylinder, so I am happy to accept that there is , at least , a true discrepancy in readings.
I haven't considered an emission test since with a misfire , I would expect the readings to be 'off the scale'
When removing the plugs , there is a smell of petrol so I assume the injectors are ok.
I have replaced plugs , HT leads and coil with Bosch equipment and since the engine will idle , all be it , very erratically, I have little reason to suspect the sparks being less than adequate.
Thank you for your reply.
Mark Hulme said:
I certainly intend to use the compression guage on a different car because I also have doubts about its accuracy.
Having said that , the guage would be equally inaccurate on each cylinder, so I am happy to accept that there is , at least , a true discrepancy in readings.
I haven't considered an emission test since with a misfire , I would expect the readings to be 'off the scale'
When removing the plugs , there is a smell of petrol so I assume the injectors are ok.
I have replaced plugs , HT leads and coil with Bosch equipment and since the engine will idle , all be it , very erratically, I have little reason to suspect the sparks being less than adequate.
Thank you for your reply.
the readings would of course be bad....but the readings would also be helpful.Having said that , the guage would be equally inaccurate on each cylinder, so I am happy to accept that there is , at least , a true discrepancy in readings.
I haven't considered an emission test since with a misfire , I would expect the readings to be 'off the scale'
When removing the plugs , there is a smell of petrol so I assume the injectors are ok.
I have replaced plugs , HT leads and coil with Bosch equipment and since the engine will idle , all be it , very erratically, I have little reason to suspect the sparks being less than adequate.
Thank you for your reply.
Assumptions......well, that's been well covered, even in films
You have misfires/bad running....there is every reason to suspect something is less than adequate and that could be sparks, fuel, compression.....dont rule things out on a whim. Test , measure and be sure to the best of your ability.
Compression gauges are never "accurate" as they are never calibrated.
So it's useless to compare compressions done with different gauges.
But they should be comparable across the block, or on different engines with the same gauge.
"Variable readings" most often indicate inadequte technique, so I suggest go back and practice, make sure the seal is good, throttle wide open etc. before deciding they are significant.
John
So it's useless to compare compressions done with different gauges.
But they should be comparable across the block, or on different engines with the same gauge.
"Variable readings" most often indicate inadequte technique, so I suggest go back and practice, make sure the seal is good, throttle wide open etc. before deciding they are significant.
John
I totally agree with both you guys.
The fact that Zetecs have recessed plugs means I have to use an adapter between the plug threads and my compression tester which doubles the chance of a poor seal at the O rings !
At this stage , I'm not ruling anything out or taking anything for granted !
My 'assumptions ' are always based on probability !!
The fact that Zetecs have recessed plugs means I have to use an adapter between the plug threads and my compression tester which doubles the chance of a poor seal at the O rings !
At this stage , I'm not ruling anything out or taking anything for granted !
My 'assumptions ' are always based on probability !!
Yes , I've noticed that the pressure does take a few revolutions to build up.
I've carried out some very basic resistance tests on the crank position sensor and the cam position sensor and compared the readings with those given in my workshop manual.
If either were faulty , I imagine they'd give rise to a fault code ?
I've carried out some very basic resistance tests on the crank position sensor and the cam position sensor and compared the readings with those given in my workshop manual.
If either were faulty , I imagine they'd give rise to a fault code ?
Well , this evening I checked the valve timing and it is spot on , so I guess that's the end of the 'slipped cambelt' theory ! Tomorrow I will use the compression guage on the wife's car which drives perfectly. Guess that'll be the acid test to find out if the compression guage is trustworthy !
I will then decide my new plan of attack.
Hope I haven't been barking up the wrong tree !
I will then decide my new plan of attack.
Hope I haven't been barking up the wrong tree !
liner33 said:
I'm suspicious of the compression test results but if the timing is ok and the coil and plugs are fine then I guess injectors are the next logical step
Next logical step is testing emissions whilst the car is running. Whether via a gas analyser or wideband lambda. Both will show useful information.Injector faults are very rare...but certainly not impossible. Not a huge cost to send them away to be tested though.
But could be fuel supply, pressure, wiring, pump, injectors...
As others have said, viewing live data to see if there are any oddities as well as feedback from even the narrowband sensor could be useful but ideally something to give proper emissions info.
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