Mike's GT2871r S13 240sx Diagnosis

[Kingtal0n]

Mike's 240sx

S13 Redtop SR20DET
GT2871r, Tomei tubular manifold
740cc injectors, Z32 maf
Power FC (MAF, 2 years old)
upgraded internals

Diagnosis:
The car starts and runs and drives normally. After 10-25 minutes of run time, the engine presents a high exhaust gas temperature (EGT) on the rear two cylinders (cylinder #3 and #4) during idle and presumably while driving (not dyno verified). It also presents the indication of a misfire, emanating from cylinder #3 and #4- tentatively verified by removing the injector harness plugs from injector #3 and #4 and noting the dimming of the appropriate cylinder’s tubular exhaust plumbing along with the indication of a complete misfire. So combustion seems to be occurs in extremely lean conditions (17:1 or more) causing high EGTs (and the subtle cleaning marks on the spark plugs) and slight misfires from the affected cylinders.

The problem disappears if the key is switched off, then turned back on. Even if done with the engine still spinning, the problem then disappears for another 10-25 minutes, then reoccurs.



I am thinking to check the ECU first by a reset followed by the popular method of substitution (Searching) kind of hoping it is not the ECU so that the diagnosis becomes complicated and interesting- and Mike is hoping it is not the ECU because a PFC is not cheap. I wonder if Apex will service?

[codyace]

The fact that it runs fine, that appears after a specific time really baffles me. I honestly don't know what would cause it to pull fuel (or maybe even timing) this side of maybe a faulty knock sensor...but one would think that would happen all of the time. Maybe some sort of injector driver gets hot and goes wacky?

I'm just guessing for the sake of guessing now. Ultimatly I'd be sending it back to ApexI for a service.

[steve shadows]

Have you checked all the pins on the actual ECU plug?

We had some wacky issues with fuel delivery thanks to corroded ECU pins on the actual big blue plug once....

Maybe specific resistance or signal issue on those two cylinders of some kind...

What about the health of the actual motor on those two cylinders?

[Kingtal0n]

The fact that it runs fine, that appears after a specific time really baffles me. I honestly don't know what would cause it to pull fuel (or maybe even timing) this side of maybe a faulty knock sensor...but one would think that would happen all of the time. Maybe some sort of injector driver gets hot and goes wacky?

I'm just guessing for the sake of guessing now. Ultimatly I'd be sending it back to ApexI for a service.

Exactly reflections of my thoughts :D

[Kingtal0n]

Have you checked all the pins on the actual ECU plug?

We had some wacky issues with fuel delivery thanks to corroded ECU pins on the actual big blue plug once....

Maybe specific resistance or signal issue on those two cylinders of some kind...

What about the health of the actual motor on those two cylinders?

Good thoughts to test the pins and ECU plug Steve, he is going to barrow a S13 PFC from a friend and all of that is next to be sorted. Besides an indistinguishable tapping noise the engine sounds healthy (exhaust note), I have not done a compression test but I can tell it is good or at least equal among the cylinders.

All that really means though is that the piston and combustion chamber is making a good seal and able to create some wicked pressures which is great for making good power. It does not guarantee the health of the rotating assembly or the ultimate health of the valvetrain, besides the fact the valves seem to be sealing right which indicates the cam lobes/rockers/springs are doing their jobs but not necessarily that their health is good. a valve 1 event from failure is sealed up just fine. A bearing one revolution away from letting loose is doing its job just fine too. So I guess my answer to your health question is that I have no idea what the health of the engine is without taking it apart and I dont think Mike is gonna let me dissemble his SR20 just yet. XD

[BiG MiKE86]

Thanks for the help guys!

[mct3351]

I'd be more inclinded to think it's an ignition related problem. Remember o2 sensors look for the prescence of oxygen, not fuel. A misfire would leave more unburnt oxygen in the exhaust gas and read as a lean condition on a wideband regardless of how much fuel is present. Also mixtures leaner than ~15ish do not burn hotter than stoich mixtures, which is contrary to popular belief. EGTs reachs a maximum around stoich. and then decline as mixtures are learner or richer. Leaner mixtures are avoided because they do less work. A leaner mixture has more air mass to heat with not enough fuel to heat it. A retarded firing event with enough fuel content to heat the combustion gases to high EGT's might not be finished consuming the oxygen when it reaches the sensor especially if it located in the turbo elbow.

[BiG MiKE86]

A replacement Power FC seems to have done the trick. Thanks for the help guys!