Ye Olde 4000rpm stutter - FIXED

[jeffnhiscars]

The "learned behaviour" in the ecu is stored in volatile memory so every time you cut the battery it dumps whatever it's stored and your problem goes away. Reconnect the battery and the ecu goes through its learning curve over X number of runs. While that's going on your car runs fine, then it doesn't. Theres a direct link between your problem and that volatile memory.

Part of that learned behavior is ignition timing, advance, injector manage, afr etc. it's all tied together and it's all suspect. Hence the question about your injectors.

Vacuum leaks aren't binary so they are not necessarily either on or off. Some are a bit more clever and may only appear at certain ambient or engine temperatures as things expand and contract or can even be activated by harmonics. Unlikely but possible.

It's worth seeing if there's a relationship between vacuum and your misfire since you've eliminated just about every other cause besides ghosts. That's next :)

[TimeMachinE30]

Vac gauge or would a smoke test do?

[Kershaw]

I'm pretty sure I'm dealing with a ghost in the machine.

I just bought a mini fog machine on Amazon. We'll see how ghosty it gets. It idled great before I messed with the oil return / bitch tube under the intake manifold last weekend. I lifted it up the spring and washer and smeared a bunch of RTV underneath it. Now it idles kind of funky, so I think I made that point worse. Maybe I pull the intake manifold and do it properly with a new o-ring. I doubt that was the issue or if it is an issue now will fix the hiccup. But I have to do something. Especially since blueapplesoda's megasquirt units cost $800 and that's a hard pill to swallow right now. That said, maybe if I had gone that direction 6 months ago I wouldn't be dealing with this issue and the cost in parts thrown at it would have been the same.

[Dirty325ix]

Wow this is amazing I have the exact same problem as you, even down to just giving up and dealing with it for a year. Its so annoying how well it revs in nuetral but when you pull away from a stoplight it can randomly bitch slap you in the face. I am very interested in your solutiom but as for me i have given up and am just saving up for megasquirt, then I can hopefully rewire the car myself.

Someone should start a support group for us

[akorcovelos]

I swapped:
AFM
TB/TPS
Engine Harness
DME
CPS
Injectors
Cap and Rotor
Ignition Coil
Ignition Wires
(maybe some other things)
and then the item that actually fixed things, The Harmonic Balancer that the CPS reads off of.

I don't see here that you've replaced or at least tested your CTS? Also, what mods (if any) have you done to the m20? Vacuum leak shouldn't cause this symptom. Vacuum issues present at low RPM/idle when vac is supposed to be high. WOT issues tend to be ignition/electrical in my experience.

[Kershaw]

I'm fairly certain that was checked last year. But I will add it to the list of things to check/doublecheck/triplecheck/etc.

m20 is stock, except 17# injectors and a cone filter. Might have a decked head, head is an unknown pulled from a parts car. The engine seemed unusually strong after installing it when my previous head developed a small crack through the thermometer housing bolt hole.

[TimeMachinE30]

For the motronic experts, is there something in the programming at 4k which is common to this issue? Say at 4k the motronic is looking for a certain sensor input, or trying to get a certain voltage... just that this happens at 4k makes me wonder if at 4k motronic has a special condition his car is not satisfying.

The consistency in error should be a big clue and narrows the field being not dependent on a variable such as temp, cold vs warm start, only when heater is on high and i turn right. etc.

From that link I shared with you:
waffles It's not unusual for an injector to work perfectly at idle but poorly under high demand conditions (i.e., full power, high rpm), which is the way I drive my BMW.

[Kershaw]

The CTS can produce some wonky running conditions. Honestly, I should have already replaced it when I replaced the CPS.

[jeffnhiscars]

Vac gauge or would a smoke test do?

Gauge. You want real time readings so when the car starts missing you can watch the gauge just to eliminate this as a possibility.

PS bitch tube has 2 o rings

[akorcovelos]

For the motronic experts, is there something in the programming at 4k which is common to this issue? Say at 4k the motronic is looking for a certain sensor input, or trying to get a certain voltage... just that this happens at 4k makes me wonder if at 4k motronic has a special condition his car is not satisfying.

The consistency in error should be a big clue and narrows the field being not dependent on a variable such as temp, cold vs warm start, only when heater is on high and i turn right. etc.

From that link I shared with you:
waffles It's not unusual for an injector to work perfectly at idle but poorly under high demand conditions (i.e., full power, high rpm), which is the way I drive my BMW.

not just at 4k, but 4k at WOT. The key here is the WOT because the ECU switches to a different table for WOT versus partial throttle where load is considered. The fact it's only happening at WOT points towards something wrong in the WOT tune. Since OP has tested, and replaced the TPS and ECU and harness this confuses me.

OP, when you tested the TPS did you remove it from the TB and bench test it? Maybe the throttle cable is stretched and not fully opening the TB at WOT, so the TPS never actually trigger the ECU to hit the WOT tuning? Throwing out guesses at this point.

[packratbimmer]

Is your tank clean and did you do a fuel delivery test?

[Kershaw]

I've only tested the tps by manually moving the throttle body, not through the pedal. That is worth a recheck in my opinion. Not only was it checked, I swapped throttle bodies with another running e30.

Tank is kind of rusty, gotta be honest there. But the fuel pump was replaced 2 years ago with a new filter sleeve and fuel filter.

The CTS was barely out of spec so I replaced it with a new one. It starts and idles cold nicer now, but no change to the WOT hiccup.

[packratbimmer]

I had a Volvo 240 once that would idle in the garage all day long (in theory.) As soon as I would back out it would not even pull its own load. Ended up being rust pulling onto the filter sock in the tank after the fuel got stirred up.

[Kershaw]

Did it throw a CEL though? If it was fuel related, I'd expect it to just miss. If it's spark related, then I expect to see the CEL with the spark cut. How can the DME even see that there is a momentary loss in fuel pressure? Also, it's so consistent at it's rpm range. If it was clogged, I can't see how it would then be ok in the upper rpms after the hiccup.

Still, it's worth a check I suppose. Maybe I'll drag a magnet or chain across the bottom and see if that can't clean up some of it.

[itsonlygeorge]

O2 sensor?


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