Off idle hesitation

[Eskie]

I've been hesitant to post my issue because I thought I would try everything before I would ask for help.

1990 325iX manual transmission 190k miles-If the throttle is pressed lightly at idle the rpm's will dip and then surge back up to idle. More prevalent when engine is cold. Engine runs all the way up to redline and pulls hard otherwise.

Heres everything that I have tried to do to remedy the problem:

Smoke checked the vehicle 3!!!! times. I connected the smoke machine to the small vacuum line that goes to the intake manifold, pulled off of the fuel pressure regulator.

Tried 2 different AFMs. No difference. I have even tried to refurb one recently (after I tested both last year) by elongating the holes on the circuit board to move the wiper onto a fresh track. No change. I even tried to enrichen the mixture by adjusting the spring tension. No change. Adjusted the idle mixture as well. Now I'm back to my untouched AFM.

Tried 3 different idle control motors. Before and after cleaning them with carb cleaner..no change.

Brand New fuel pump, tank, LINES, filter..tank to fuel rail.

All throttle body connections are secure..JB welded one of the loose connections 4 years ago when I first bought the vehicle.

Tested fuel pressure and volume twice, all within specs.

Replaced Coolant temp sensor.

Coil, wires, distributor cap, rotor, plugs ALL brand new.

Tried several crank position sensors.

Crankcase hose is brand new, intake boot is brand new, took intake manifold off and replaced gaskets, and throttle body gasket. Oil return pipe orings as well.

Valve adjustment done twice..second time just to confirm within specs.

Timing belt just done...and rechecked to make sure timing is spot on.

Checked oxygen sensor for cycling output with snap on oscilloscope. Checked good.

Re wired C191 plug under intake with GM weatherpack connectors. Fabricated 2 6 gauge grounds from battery cables from engine to frame. Did all Bentley DME wire tests at DME connector in glove box.

Fuel injectors are brand new...newer style revised nozzles.

Brand new TPS adjuster to spec per Bentley.

I am at a loss at this point, it sucks taking off while the engine is cold..hitting the gas just right will make the sputter and chug.

Not sure what else it could be at this point..i'm considering just going megasquirt to try and remedy this problem..

Idle has alway been a little stumbly no matter what part I have thrown at it BUT is just like any other M20 I have owned.
Any suggestions?

[salvageland]

my '93 vert is in the same boat and also doesn't have a perfectly smooth idle. (seems to be micro-adjusting rpm constantly)

I've done every maintenance item that you listed with the addition of head rebuild, compression and leak-down tests.

The only thing left to change are DME and engine harness, but I'm beginning to think that this may be normal for E30s? Don't know anyone else locally with an E30 to compare.

[dnguyen1963]

Which fuel injectors are you using? It sounds like you have too much fuel at cold start up.

[Eskie]

my '93 vert is in the same boat and also doesn't have a perfectly smooth idle. (seems to be micro-adjusting rpm constantly)

I've done every maintenance item that you listed with the addition of head rebuild, compression and leak-down tests.

The only thing left to change are DME and engine harness, but I'm beginning to think that this may be normal for E30s? Don't know anyone else locally with an E30 to compare.

I'm starting to think its normal too because of all of the things that I've tried to remedy the situation.

Sounds like your having the same exact problem..

Which fuel injectors are you using? It sounds like you have too much fuel at cold start up.

It honestly feels like I don't have enough fuel when its cold. Like a carbureted engine without the choke on when its first started...

..BUT you may be right though..it may be getting too much fuel, I'm not going to rule that out. I can definitely smell fuel from the tailpipe if I'm outside the car.

https://www.ecstuning.com/b-bosch-parts/fuel-injector-priced-each/13641731357~bos/

These are the Bosch fuel injectors I installed a while ago.

I should mention that I do not have a catalyst so the exhaust always smells.

[laidback93]

Resurrecting this thread as my 1990 325is does the EXACT same thing when it's cold. Starts right up when cold and idles fine for a minute (unless you blip the throttle where it will fall on it's face and almost die) and then the idle gets really bad and lumpy. Like it's missing. It eventually runs buttery smooth once the engine gets up to operating temp.

I've done every single thing you've listed in you original post. Like down to a T.

Anyways, wondering if you ever found the culprit.

[Eskie]

I ended up solving it by getting rid of the stock DME in favor of a megasquirt 2 and doing a full turbo build. Probably not the answer you were looking for. I’ve noticed that every e30 that I’ve worked on over the years with the stock motronic has had this issue to some degree. I think the old school vane air flow meter is the culprit. It’s clunky old technology. Megasquirt on the e30 runs on speed density using a map sensor instead. It’s a lot of time/effort and money but definitely would recommend.

[birchannouncer]

was looking for this exact info. you confirmed what i was thinking. really appreciate it.

[laidback93]

I ended up solving it by getting rid of the stock DME in favor of a megasquirt 2 and doing a full turbo build. Probably not the answer you were looking for. I’ve noticed that every e30 that I’ve worked on over the years with the stock motronic has had this issue to some degree. I think the old school vane air flow meter is the culprit. It’s clunky old technology. Megasquirt on the e30 runs on speed density using a map sensor instead. It’s a lot of time/effort and money but definitely would recommend.

Thanks for the response! I've contemplated converting over to Megasquirt, but I don't think I want to go Turbo yet. We'll see the future brings.

[Eskie]

Not sure if they still sell the ms2 plug n play kit but that’s what I ended up running. Pretty straight forward, not a ton of work to get running. I ran it NA for a while. If you end up going megasquirt let me know I can send you some pretty well tuned maps.