Recently the car has developed a very intermittent "misfire" (if you'd call it that, the PCM does, given the flashing CEL, Of course the cause of the issue might just be causing a symptomatic misfire.) When it does show itself, it is usually at WOT in first gear. When this happens, the tach will just drop (where it drops is random) to the lower/mid half of the gauge instantaneously. When this happens, the car just hits a brick wall, engine noise stops, and then it will continue as normal (or maybe repeat itself again right away.)

Call it a very serious hiccup?

Things I've done:

New plugs, wires, coils.
Cleaned ESS (plus NVRAM reset)
Cleaned MAF (plus tested with known good maf, same with ESS)
Fuel pump recall performed by dealer.
Car does not have a cat, so a clogged cat is out of the question, as well.

Car is a Series I 6MT

It almost seems like its getting choked when this happens. No other CEL's present. I do have a custom tune from VersaTune flashed, but I don't think that would be causing the issue, and I do not have any other CEL's masked (aside from the P0420.)

Is it possible the cat material could have broken off and fell into the rear portion of the cars exhaust if it had went bad prior to being replaced with the midpipe, and is causing blockage at WOT pulls? This one has me scratching my head, as I've checked about everything I could think to check. Unless the dealer replacing the fuel pump assembly during the recall would have caused something? The only thing I haven't checked is fuel pressure.

 

Sounds like it could be something electrical. Check your battery, terminals, grounds, and all the connections to make sure everything is clean and snug/tight

 

To my knowledge everything is pretty snug. I cleaned the battery terminals not too long ago, and I've checked the grounds as well. Possibly its something electrical related I'm overlooking, or fuel related. The whole pump and assembly was replaced, so I'm not sure about the fuel front. I don't have any of the issues typical of a failing pump that i've experienced before in the Series I's.

 

For obvious reasons i would suspect electrical, potentially ESS related as well since rpms seem to randomly crap out on you

 

100% electrical, don't worry about the fuel pump. The rpm can't drop instantaneously, so it's the gauge or ECU or sensor that are dropping out. Focus on that.

Does anything else drop out at the same moment? Lights, radio..?
If yes, it's the battery posts or battery to ground connection. They need to be quite tight and not move.
If no, wiring to the eccentric shaft position sensor or the sensor itself as Xero says.

 

Yea the RPM dropping doesn’t make sense... if it’s associated with the engine speed correctly, then that would mean the actual engine speed is instantly dropping as if you went to shift. Very strange. I’ve never experienced that sort of stutter / misfire. Usually the RPMs are right there showing the engine speed.

I vote electrical as well.

Is it only in first gear and only at WOT?

At what RPM does it hiccup at?

 

I've only noticed it happening in first gear. It doesn't always happen, I can take the car on a 100+ mile trip and hit stop and go traffic and it not happen once. I did notice that when it happens the radio does NOT cut off, music kept right on playing. RPM's don't tend to be insanely specific when it does occur, but its usually in the upper 1/2 of the rpm range if I had to guess. The last time it occurred I don't believe I was at full WOT.

When it happens, its the same deal each time, RPM's just instantly drop, car feels like it hits a brick wall and stops accelerating, engine noise cuts completely off, music stays on, car will recover, CEL will flash, and it will continue as normal. It does seem like I have to give it more gas when cold to avoid stalling sometimes, not the case when warm.

I have Versatune installed, so I can get a data log going. Anyone have any specifics of what parameters I should log (STFT,LTFT,RPM,TP,etc?)

 

Sounds like your ESS signal is dropping out. I would do the 20 time brake pedal stomp procedure and maybe clean the ESS. See if that fixes it.

 

I've cleaned the ESS and the contacts recently, as well as resetting the NVRAM. What would cause it to drop out like that, assuming it's the culprit? Could it possibly be something related to the trigger wheel being bent or something related? Though I figured it would be more constant if the trigger wheel were the case.

I have had ESS issues periodically with the car, through. I went through a period where it would take a few seconds to start and was giving an ESS code a few years ago. I tried multiple aftermarket eccentric shaft sensors and none would solve the problem, I put an OEM on, and it seemed to fix it.

Maybe it's something related. No codes now, though.

 

The rpm drop means the ECU fails to read the ESS signal. So either the ESS, the wheel, the wire to the ESS, or the ECU are the problem. If those I think only the wire or the ESS could encounter a problem on an intermittent basis.

Keep in mind all these analog sensors operate in a noisy environment. It might be reading fine, but being drowned out by electrical noise on the line. The ESS Hall sensor in particular is kinda sensitive. Potentially something is poorly grounded and messing with the signal. Check / clean all grounds.

 

I'll go over the grounds. However, I can't seem to find a comprehensive source that lists their locations (I've searched.) Would you happen to know where I can find the factory grounding points?

 

Looking into it further, I've noticed the car wants to bog when cold or the first few minutes of driving it under load starting from a stop. I let the clutch out in first and give it normal throttle like I would, and it will just stumble, to the point I don't like pulling out onto a highway. This can be mitigated by feathering the throttle or slipping the clutch a hair, but obviously thats not normal. It's done this before, and changing the ESS out has fixed it (with an OEM one, not an aftermarket, this car has never liked aftermarket ESS') but now the problem is stating to surface again after a few years... I'm still researching this one..anyone have any ideas? No idea why the car would do that with the ESS. Even tried the NVRAM reset.

I'm going to do a datalog on Versatune. Anyone have an preferences on what to include?