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 !
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 !
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.
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 !
Well , as intended, I tried the compression tester on my wife's car , which runs fine , and the readings were really low on that one too !
I used the (Gunson) tester in conjunction with a Laser adapter for deep seated plugs. The adapter had a Schrader valve screwed in the threaded end presumably to retain some of the pressure. I repeated the test after removing the Schrader valve and the compression reading rose to a much more feasible 165 PSI !
I've just learnt that this valve is , indeed , intended to retain pressure.
The theory is that if this valve was not there , the volume in the hollow adapter , together with the volume in the compression tester's flexible hose , would add to the volume into which the cylinder contents were squeezed , thus giving a lower true reading. Maybe the Schrader valve is faulty but I think I'll leave it out !
This tool has REALLY cost me a lot of wasted time !
I feel I've been barking up the wrong tree for the last week !
Back to the old drawing board tomorrow !
I think need to source a wiring diagram for the sensor/ECU circuits so I can carry out some multimeter tests.
I used the (Gunson) tester in conjunction with a Laser adapter for deep seated plugs. The adapter had a Schrader valve screwed in the threaded end presumably to retain some of the pressure. I repeated the test after removing the Schrader valve and the compression reading rose to a much more feasible 165 PSI !
I've just learnt that this valve is , indeed , intended to retain pressure.
The theory is that if this valve was not there , the volume in the hollow adapter , together with the volume in the compression tester's flexible hose , would add to the volume into which the cylinder contents were squeezed , thus giving a lower true reading. Maybe the Schrader valve is faulty but I think I'll leave it out !
This tool has REALLY cost me a lot of wasted time !
I feel I've been barking up the wrong tree for the last week !
Back to the old drawing board tomorrow !
I think need to source a wiring diagram for the sensor/ECU circuits so I can carry out some multimeter tests.
Just a quick update guys.
I have now eliminated low compression as a possible cause for my problem.
Overall compression figures range from 150 to 175 PSI , so I guess removal of the Schrader valve in the compression tester adapter was the right thing to do !
Incidentally , I thought I had discovered the problem yesterday when I went to put the HT leads back in the coil !
I noticed that where the HT lead numbers are embossed next to each coil turret , there are three sets of numbers ( presumably for different makes / models) Each set of numbers is enclosed in a circle , square or triangle.
I had just assumed that the leads just follow ed the same order as they were in on the original coil !
I tried out each of the other two sets of positions but to no no avail !
The strangest thing was when the engine was idling ( very unevenly and lumpy ) I repeated the test of removing one spark plug cap at a time.
As before , removing cap 1 made no difference ( I always suspected this cylinder as the problem one )
Removing cap 2 rendered the engine unable to start. The same when removing cap 3 , so I guess the engine won't idle on only two cylinders.
But when I removed cap 4 , the engine idled , albeit poorly , exactly as it had done when I removed the FIRST plug !
Given that cylinders 1 and 4 share a coil (wasted spark) and a pulse wire going to the PCM , does this mean I could be on the correct trail at last , not forgetting the original fault code P0351 ?
I have now eliminated low compression as a possible cause for my problem.
Overall compression figures range from 150 to 175 PSI , so I guess removal of the Schrader valve in the compression tester adapter was the right thing to do !
Incidentally , I thought I had discovered the problem yesterday when I went to put the HT leads back in the coil !
I noticed that where the HT lead numbers are embossed next to each coil turret , there are three sets of numbers ( presumably for different makes / models) Each set of numbers is enclosed in a circle , square or triangle.
I had just assumed that the leads just follow ed the same order as they were in on the original coil !
I tried out each of the other two sets of positions but to no no avail !
The strangest thing was when the engine was idling ( very unevenly and lumpy ) I repeated the test of removing one spark plug cap at a time.
As before , removing cap 1 made no difference ( I always suspected this cylinder as the problem one )
Removing cap 2 rendered the engine unable to start. The same when removing cap 3 , so I guess the engine won't idle on only two cylinders.
But when I removed cap 4 , the engine idled , albeit poorly , exactly as it had done when I removed the FIRST plug !
Given that cylinders 1 and 4 share a coil (wasted spark) and a pulse wire going to the PCM , does this mean I could be on the correct trail at last , not forgetting the original fault code P0351 ?
Really ? I'll certainly look into that !
If I could carry out some basic tests , that would be great , but I'm hoping to find a problem in one of the conductors which carries the trigger signal between the LT coil and the PCM.
I understand that certain resistance tests are not possible when using a multimeter to test electronics due to the amount of current applied by the meter , so I guess I'll have to do some research beforehand !
If I could carry out some basic tests , that would be great , but I'm hoping to find a problem in one of the conductors which carries the trigger signal between the LT coil and the PCM.
I understand that certain resistance tests are not possible when using a multimeter to test electronics due to the amount of current applied by the meter , so I guess I'll have to do some research beforehand !
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