Hi.


So I have the widespread loss off power problem. Its rev range independent and high outside temperatures or highway driving make the problem reproducible.
With OBD2 logging I now could observe that when there is power loss, STFT slowly drops to -17%, getting worse the more I depress throttle (90°F outside temp, 200°F engine coolant temp).
STFT/LTFT are totally within range (+/- 7%) under normal circumstances.

After replacing almost everything a compression test confirmed that I'm low on compression although I have no cold/hot starting issues at all.


So can anybody confirm my current theory:
Due to bad compression when the engine is hotter than normal more unburned fuel is measured by the o2-sensor believing in a rich condition, causing this drop in STFT and leaning the engine out?
Or what else should I look at?


thanks,
Harry

 

I can't confirm without being there, but something to think about: fuel doesn't just not burn randomly. Do you get any codes for misfires? Because one explanation is the ignition cutting in and out.

Another explanation is the O2 sensor is broken.

Not saying low compression isn't a problem, but it doesn't usually manifest your symptoms.

 

Never have any codes. Car pulls strong when cold/warm, everything is OEM and the current ignition system has 2k miles on it. But I deal with the whole problem since almost 40k miles in the meantime...
Both O2 sensors were replaced a while after the problems started with used ones to give it a try without any effect. Same goes with the fuel pump, cat, ECT, MAF and some more.

So now I am guessing the low compression causes all the more unburned fuel the hotter the engine gets and the less it seals therefore until to the point the ECU tells the STFT to drop dramatically. But I took a lot of guesses in the meantime ;-)

 

The thing is low compression doesn't make more unburnt fuel. The fuel is already on fire regardless, compression just means you're not getting as mucj useful work out of that burning fuel.

Possible you have a sticking injector?

The other thing is the STFT could be a symptom, not the problem. What does airflow, coolant temp and other sensors read when this happens? Do you have a data log?

 

Coolant temp was 90°F but dont have the possibility of a data log. Will have a look at different data the next time.
BTW premixing reduces the problem a lot?!

 

coolant temp was 90F?

 

Premix lubricates everything along the fuel system and fills in open clearances between engine seals (apex, sides, etc). Maybe you have a cracked seal that opens and closes...

What exactly were your compression numbers?

 

Sry, that was outside temp, coolant was 200F

 

Dont have the exact numbers at hand but they are all evenly below specification so a single bad seal can be ruled out.

 

So, had the problem once more and got some new data: AFR drops below 13 and calculated load won't go past 80% at WOT.
Afterwards MAF at idle is to high too with around 6.6g compared to about 4.8g when warm.

 

OK, well, let's break this down. High AFR means too much air/ not enough fuel. It would be good to know what the commanded AFR is at the same time. I don't see a condition where the car would want a leaner AFR, so assuming it thinks it's not getting enough air and pulls fuel in response. Calculated load is a function of airflow, so if your airflow is restricted or the sensor is lying, it could be off.

High MAF at idle means the car is pulling more air than usual so either the idle is higher than normal or your MAF is lying to you. Is the MAF clean and free of oil? Is the whole intake clean?

I know you replaced the MAF, but any chance you have damaged or loose wiring in the MAF connector?

Also, any knock events in the data? This could be response to knock.