High Compression
Discussion
Hi all.
I have a 1.25 16v ford Fiesta (X reg). It's running poorly. Starts up OK and after a bit of reving, will just about idle when warm, but flooring the accelerator produces a load of petrol smelling smoke and a quite bad misfire.
Checked all the electrics, and they all seem to be OK (MAP sensor O2 sensor, crank and cam sensor. Ignition seems good, with good spark) So now it's head banging time.
Anyway, measured the compression. Against an autodata target CR of 10:1, I'm getting at least 14:1 It might even be higher, but I can't hold the gauge on the plug hole.
What's the collective opinion, is this high enough to cause predetonation in the chamber, and cause misfire and other nasty's?
It's getting progressively worse, but a week or 2 ago, it passed it's MOT with good emissions, and barely registered on the Co %
Any views would be welcome (don't want to strip the head.... don't make me do it)....
I have a 1.25 16v ford Fiesta (X reg). It's running poorly. Starts up OK and after a bit of reving, will just about idle when warm, but flooring the accelerator produces a load of petrol smelling smoke and a quite bad misfire.
Checked all the electrics, and they all seem to be OK (MAP sensor O2 sensor, crank and cam sensor. Ignition seems good, with good spark) So now it's head banging time.
Anyway, measured the compression. Against an autodata target CR of 10:1, I'm getting at least 14:1 It might even be higher, but I can't hold the gauge on the plug hole.
What's the collective opinion, is this high enough to cause predetonation in the chamber, and cause misfire and other nasty's?
It's getting progressively worse, but a week or 2 ago, it passed it's MOT with good emissions, and barely registered on the Co %
Any views would be welcome (don't want to strip the head.... don't make me do it)....
You seem to be confusing compression reading (from a gauge stuck down a plug hole) and compression ratio. Anyway, your compression number at 200 sounds high for that engine but the good news is it means the engine is healthy not unhealthy. However, you should also check the readings on the other cylinders, ideally they should all come out with 5-10% of each other.
Back to your problem, sounds more like a faulty lambda sensor but could be lots of things.
HTH
Back to your problem, sounds more like a faulty lambda sensor but could be lots of things.
HTH
That's not 14:1, you can't measure a static CR with a compression tester, the two are not related.
Your single reading seems ok, it's the equivalency across the cylinders that is important.
It could be many things, but to start with is there a temp sensor which sends a signal to the ECU (air or coolant)? If so it could be faulty and telling the ECU to add more fuel when it shouldn't be.
Your single reading seems ok, it's the equivalency across the cylinders that is important.
It could be many things, but to start with is there a temp sensor which sends a signal to the ECU (air or coolant)? If so it could be faulty and telling the ECU to add more fuel when it shouldn't be.
GeoffW said:
You seem to be confusing compression reading (from a gauge stuck down a plug hole) and compression ratio. Anyway, your compression number at 200 sounds high for that engine but the good news is it means the engine is healthy not unhealthy. However, you should also check the readings on the other cylinders, ideally they should all come out with 5-10% of each other.
Back to your problem, sounds more like a faulty lambda sensor but could be lots of things.
HTH
Is compression ratio and compression reading the same ? I have about 200 psi reading - 14bar. Given atmospheric about 1 bar (much less here in Wales at the mo, and it's raining, hence no reading from the other 3 cylinders) wouldn't that equate to a ratio of about 14:1 (Genuine question, not trying to be smart) Not talking down to the last decimal point, but rough terms?Back to your problem, sounds more like a faulty lambda sensor but could be lots of things.
HTH
I do have some concerns about the O2 sensor. Signal from the sensor is very noisy, but when it's idling OK as it sometimes does, O2 sensor cycles normally, albeit with loads of noise. Injection duration is also about 4ms, so maybe a little long, but certainly not extraordinary. So I'm thinking while the O2 sensor may not be perfect there's something else that's driving the misfire issue.
I've changed the coil and plugs, and have strong sparks. Fuel pressure has been about 3.8 to 4 bar (v 2.7b target), so I've changed the fuel pressure regulator. Down to a more consistent 3.1b now, so still a little high, but not terrible.
The compression is the biggest error I have so far found, and I was concerned, as I say, about mechanical timing, or about the possibility of compression ignition, rather than spark ignition, but to be honest, I'm not sure what kind of CR you would need to get 'dieseling'
bearman68 said:
Is compression ratio and compression reading the same ? I have about 200 psi reading - 14bar. Given atmospheric about 1 bar (much less here in Wales at the mo, and it's raining, hence no reading from the other 3 cylinders) wouldn't that equate to a ratio of about 14:1 (Genuine question, not trying to be smart) Not talking down to the last decimal point, but rough terms?
I do have some concerns about the O2 sensor. Signal from the sensor is very noisy, but when it's idling OK as it sometimes does, O2 sensor cycles normally, albeit with loads of noise. Injection duration is also about 4ms, so maybe a little long, but certainly not extraordinary. So I'm thinking while the O2 sensor may not be perfect there's something else that's driving the misfire issue.
I've changed the coil and plugs, and have strong sparks. Fuel pressure has been about 3.8 to 4 bar (v 2.7b target), so I've changed the fuel pressure regulator. Down to a more consistent 3.1b now, so still a little high, but not terrible.
The compression is the biggest error I have so far found, and I was concerned, as I say, about mechanical timing, or about the possibility of compression ignition, rather than spark ignition, but to be honest, I'm not sure what kind of CR you would need to get 'dieseling'
I already stated they are different. Compression in psi or bar, doesn't matter which, is a measurement of the pressure created in the cylinder; compression ratio is a measurement of the volume in the cylinder with the piston fully down (biggest area) compared to TDC / fully up (smallest volume). I do have some concerns about the O2 sensor. Signal from the sensor is very noisy, but when it's idling OK as it sometimes does, O2 sensor cycles normally, albeit with loads of noise. Injection duration is also about 4ms, so maybe a little long, but certainly not extraordinary. So I'm thinking while the O2 sensor may not be perfect there's something else that's driving the misfire issue.
I've changed the coil and plugs, and have strong sparks. Fuel pressure has been about 3.8 to 4 bar (v 2.7b target), so I've changed the fuel pressure regulator. Down to a more consistent 3.1b now, so still a little high, but not terrible.
The compression is the biggest error I have so far found, and I was concerned, as I say, about mechanical timing, or about the possibility of compression ignition, rather than spark ignition, but to be honest, I'm not sure what kind of CR you would need to get 'dieseling'
The compression you have found is not the "biggest error", high compression (in psi or bar) is a sign of a healthy engine not a poorly one. I already stated that too.
Please move on from compression and start investigating normal problems, lambda sensor, coil pack etc.
The compression being that high is not your issue, unless you have a magical engine that somehow managed to make the crank throw longer.
As already said, you need to check the other 3 cylinders and see what the difference between them is, if all is okay, your issue is elsewhere, check for air leaks first.
Has it just suddenly developed the problem or had you done some work/servicing beforehand?
It could be the cam timing is out if it's just had a belt, etc.
As already said, you need to check the other 3 cylinders and see what the difference between them is, if all is okay, your issue is elsewhere, check for air leaks first.
Has it just suddenly developed the problem or had you done some work/servicing beforehand?
It could be the cam timing is out if it's just had a belt, etc.
These engines do not have a keyway in the bottom pulley, an advanced cam timing can give a higher compression pressure. As said above if its had a belt recently best check the timing to be sure, may even be a slightly loose bottom pulley.
I had something similar on another (non Ford) engine giving similar symptoms, turned out the keyways in the cams went 'missing' and it was relying on bolt tension to turn the cams...
I had something similar on another (non Ford) engine giving similar symptoms, turned out the keyways in the cams went 'missing' and it was relying on bolt tension to turn the cams...
OK, thanks for the advice - I'll check the compression on the other cylinders, I would be surprised if they are much different, but hey ho, stranger things have happened.
Would definitely consider the belt timing, - I don't think the engine has had any work done on it, but it would be easily possible that the bottom pulley has jumped a tooth, thus advancing the cam timing. The belt is right at the end of it's life, and a little overdue for a change. Again, it's well possible that there is a vac leak - though in my experience, a vac leak will usually make the engine rev faster (with a MAP sensor), and it's not doing that.
Someone also suggested that the cat might be blocked / damaged, and that might make some kind of sense as well.
Just for GeoffW, to reiterate, coil is fine (new) as are plugs, Sparks are excellent, and spark burn duration about 1.5ms, intake temp sensor is good, coolant sensor not checked, but the engine temperature gauge is fine. Crank sensor good, cam sensor looks low voltage, but waveform is good, MAP senor appears OK, with correct waveform. O2 sensor as already explained - cycling OK, but noisy. No fault codes.
I think you will find that for a fixed temperature, volume change is directly proportional to pressure change. (This isn't my theory, check out Boyles law, who published this in 1662) Hence if the pressure in the cylinder is at atmospheric, a pressure of 14bar gauge represents the compression ratio, assuming temperature has little influence, and the gas is close to ideal.
Would definitely consider the belt timing, - I don't think the engine has had any work done on it, but it would be easily possible that the bottom pulley has jumped a tooth, thus advancing the cam timing. The belt is right at the end of it's life, and a little overdue for a change. Again, it's well possible that there is a vac leak - though in my experience, a vac leak will usually make the engine rev faster (with a MAP sensor), and it's not doing that.
Someone also suggested that the cat might be blocked / damaged, and that might make some kind of sense as well.
Just for GeoffW, to reiterate, coil is fine (new) as are plugs, Sparks are excellent, and spark burn duration about 1.5ms, intake temp sensor is good, coolant sensor not checked, but the engine temperature gauge is fine. Crank sensor good, cam sensor looks low voltage, but waveform is good, MAP senor appears OK, with correct waveform. O2 sensor as already explained - cycling OK, but noisy. No fault codes.
I think you will find that for a fixed temperature, volume change is directly proportional to pressure change. (This isn't my theory, check out Boyles law, who published this in 1662) Hence if the pressure in the cylinder is at atmospheric, a pressure of 14bar gauge represents the compression ratio, assuming temperature has little influence, and the gas is close to ideal.
Update.
4 cylinder compression is as follows.
1) 210 psi.
2) 205 psi
3) 207 psi
4) 193 psi
All about 14 bar give or take.
Removed the fuel rail, and checked for fuel dripping from the injectors after their 'squirt' - all seems fine, and I could not see excess fuel dripping from any of the injectors after the squirt had finished.
To investigate a blocked cat, I have removed the exhaust from the back of the cat, and blown compressed air into the exhaust. This is coming out nicely of the spark plug hole, without appearing to pressurise the cat, so I am doubtful the cat is blocked. I'm going to remove the precat O2 sensor tomorrow, and recheck the compression to see if there is any change, though I doubt there will be.
I'm leaving the engine timing job until last because it's a real pain to check.
4 cylinder compression is as follows.
1) 210 psi.
2) 205 psi
3) 207 psi
4) 193 psi
All about 14 bar give or take.
Removed the fuel rail, and checked for fuel dripping from the injectors after their 'squirt' - all seems fine, and I could not see excess fuel dripping from any of the injectors after the squirt had finished.
To investigate a blocked cat, I have removed the exhaust from the back of the cat, and blown compressed air into the exhaust. This is coming out nicely of the spark plug hole, without appearing to pressurise the cat, so I am doubtful the cat is blocked. I'm going to remove the precat O2 sensor tomorrow, and recheck the compression to see if there is any change, though I doubt there will be.
I'm leaving the engine timing job until last because it's a real pain to check.
Gassing Station | Home Mechanics | Top of Page | What's New | My Stuff