Bad Compression waveform

2001 Saturn SL 1.9L Vin 8

I'm not posting this in the repair section because, I believe, I know the problem. I'm waiting for the "Go ahead", but I believe I am looking at a failed head gasket? I'd like to hear your thoughts.



Cylinders 1 and 2 look normal. Cylinders 3 and 4, both, look like this. This car runs, albeit a bit rough. Noticeable misfire. Short term fuel trim starts counting into the negatives. -1, -2, -3....all the way to...-35, -36, -37. When it reaches -37, it resets to zero. And begins counting, again. When it resets to zero, the misfires go away. The more negative the fuel trim goes, the misfires return and the rougher it runs.

What are we looking at an in cylinder?

Sent from my SM-G928P using Tapatalk

Yes. Engine running. In cylinder pressure.

Could I ask you to perform a cooling system pressure waveform inspection with a cylinder synch? I believe a blown headgasket will show itself as pressure pulses in the cooling system.

EricGoodrich wrote: Could I ask you to perform a cooling system pressure waveform inspection with a cylinder synch? I believe a blown headgasket will show itself as pressure pulses in the cooling system.

I like the idea. I'll have to see if I can connect the pressure transducer to the cooling system pressurizer, somehow.

I will, also, do a combustion leak test using the color-changing fluid. I didn't bother to do the combustion leak test because of the pressure pulses in the intake stroke of the compression waveform. Seeing that makes me pretty sure the head needs to come off. I haven't ruled out valve problems, but with both, and only, cylinders 3 and 4 being affected, it makes me think there's a head gasket blow out between the two cylinders?

Also, during the leak-down test, I had 25% loss. Which, according to the gauge, is considered low. I can hear it leaking through the intake.

pole71 wrote:

EricGoodrich wrote: Could I ask you to perform a cooling system pressure waveform inspection with a cylinder synch? I believe a blown headgasket will show itself as pressure pulses in the cooling system.

I like the idea. I'll have to see if I can connect the pressure transducer to the cooling system pressurizer, somehow.

www.scannerdanner.com/forum/diagnostic-t...-detection.html#4040

I did the pressure test with a know bad head gasket failure and with a known good engine, it can be done, but you would have to keep the water pump from spinning and you will see the same type of signal with a known good engine with no blown head gasket. So I wouldn't think it a valid test. I only did it on one vehicle. I would like to do the test on more vehicles but it seems that I never the time.

It has been tried and found there are fairly big pressure pulses in the coolant on a known good, the cylinder liners flex on every stroke so the leak would have to be so bad it was throwing water out the header tank anyway. In any even not all head gasket leaks are between the cylinder and the coolant, many of the HGF I see on modern engines with open deck cylinder blocks are between the coolant and the oil system.

Good to know.

What about a leakdown tester in each cylinder one at a time and a pressure transducer in the cooling system? I'd imagine it wouldn't take much of a leak to increase the pressure in the cooling system, especially if the cooling system were full of coolant.

Andy.MacFadyen wrote: In any even not all head gasket leaks are between the cylinder and the coolant, many of the HGF I see on modern engines with open deck cylinder blocks are between the coolant and the oil system.

I am suspecting that it is NOT to coolant OR oil.

I think I am only leaking compression from/to adjacent cylinders? But, as I stated earlier, I have not ruled out a valve problem.

Ro-longo wrote: I only did it on one vehicle. I would like to do the test on more vehicles but it seems that I never the time.

Do you have an "AFTER" to compare?

I am inclined to agree with Andy.

It has been tried and found there are fairly big pressure pulses in the coolant on a known good, the cylinder liners flex on every stroke

That makes a lot of sense. I believe, that is what we are seeing in this wave. It is nice and uniform. I would expect to see some irregularity.

pole71 wrote:

Ro-longo wrote: I only did it on one vehicle. I would like to do the test on more vehicles but it seems that I never the time.

Do you have an "AFTER" to compare?

I am inclined to agree with Andy.

It has been tried and found there are fairly big pressure pulses in the coolant on a known good, the cylinder liners flex on every stroke

That makes a lot of sense. I believe, that is what we are seeing in this wave. It is nice and uniform. I would expect to see some irregularity.

I did, but I forgot to save the waveform, I did see a similar pattern. If I was smart enough I would have tried to look at amplitude but I was in a rush and totally missed the opportunity.