Author Topic: Stumbles/shuts off on cold starts, can't figure it out (Read 18697 times)

twright · « **Reply #30 on:** June 04, 2009, 06:50:27 AM »

The fuel pressure should be about 40 psi. If its within 10 psi of that, you're Ok. (Or maybe not OK, because the problem is somewhere else.)

///m42 sport · « **Reply #31 on:** June 07, 2009, 10:08:59 AM »

thanks for this post. I had a problem like this in subzero temps in the winter. I never did figure out the source of the problem but figured it was due to extreme cold. I guess it could happen at any temp.

any updates?

max123 · « **Reply #32 on:** June 09, 2009, 11:05:58 AM »

I had this same problem, engine starts up fine then bogs down after idling for a few seconds and it doesn't help if you give it gas. Only does it on really cold days - no vacuum leaks.

Anyway its likely the AFM. Don't bother with a new one - they are crazy expensive. Find someone on craigslist parting out an E36 with an M42 in it, or go to a junkyard. The AFM in the m42 - E36 is a different part # but it still works - mine runs even smoother. Anyway - shouldn't be more than $25-$40.

DesktopDave · « **Reply #33 on:** June 09, 2009, 11:43:55 AM »

If it's the AFM, take it apart and tune it a little. They're simple and not too hard to fix. Getting the mixture just right needs a WBO2 or exhaust gas analyzer though. I'll post some pics when I do mine.

91 318is · « **Reply #34 on:** June 09, 2009, 06:50:46 PM »

Hey guys, here's my update.........

I also found that the stumbling problem is more pronounced when the A/C is on, it will buck even more and harder then with the A/C off.

A guy at work has been helping me, he's been into europeans for a while so he has a better idea about bosch style systems. Anywho, I unplugged the o2 sensor and drove it for about a day to see how it would. By doing this, the car stays in open loop I think. Anywho, I didn't stumble, even with the A/C on, so he was thinking it was the 02 sensor. Changed that today, unhooked the battery to reset the DME and that didn't fix it. He also has an E30 318, so we swapped out his AFM to see if that would fix it and that didn't do anything either.

I also found that if I just stomp the gas at idle, it will stumble really hard, and continue to stumble almost to 5k, then will smooth out. My buddy was telling me it sounded like it was backfiring into the intake. The ICV shouldn't affect anything other then idle qualities correct?

Any ideas from that?

I hate to just throw parts at it, I just don't have the money for that. If I have to, I guess I will.

DesktopDave · « **Reply #35 on:** June 09, 2009, 07:44:35 PM »

So the O2 sensor cleared the problem up - I think you're onto something with the open loop. I also think that your 5k test puts the car in open loop. Most cars at wide-open throttle will go open loop, might be the same case for high RPMs. Might be a timing or fuel issue in closed loop then. Some sensor is telling the computer the wrong data, and the computer is just doing what it's programmed to do. Garbage In Garbage Out.

Something is keeping the car do high idle. A cold start should run up to 1200+rpm for a little while, and the car should also raise idle when A/C is on to compensate for the compressor drag. I thought it was programmed into the chip on Bosch Motronic systems. My old CIS system had relays to raise idle, but I'm sure they're now part of the engine management system.

I know I'm getting into really unusual causes, but maybe run some continuity tests on wires? Likely culprits are the temp sensor in the AFM, the AFM harness, coolant temp sensor on the block and that wiring. They all signal for a higher idle. That might explain why swapping parts didn't fix the problem. Don't just replace parts...test everything. It'll run so well that you'll be glad you did!

91 318is · « **Reply #36 on:** June 10, 2009, 06:12:33 AM »

Quote from: DesktopDave;73348

So the O2 sensor cleared the problem up - I think you're onto something with the open loop. I also think that your 5k test puts the car in open loop. Most cars at wide-open throttle will go open loop, might be the same case for high RPMs. Might be a timing or fuel issue in closed loop then. Some sensor is telling the computer the wrong data, and the computer is just doing what it's programmed to do. Garbage In Garbage Out.

Something is keeping the car do high idle. A cold start should run up to 1200+rpm for a little while, and the car should also raise idle when A/C is on to compensate for the compressor drag. I thought it was programmed into the chip on Bosch Motronic systems. My old CIS system had relays to raise idle, but I'm sure they're now part of the engine management system.

I know I'm getting into really unusual causes, but maybe run some continuity tests on wires? Likely culprits are the temp sensor in the AFM, the AFM harness, coolant temp sensor on the block and that wiring. They all signal for a higher idle. That might explain why swapping parts didn't fix the problem. Don't just replace parts...test everything. It'll run so well that you'll be glad you did!

No the new o2 sensor DID NOT fix it. But before I changed it, I unplugged the o2 sensor and drove around for a day or so with it unplugged. Apparently from what my buddy told me, unplugging the o2 and driving put the car into open loop. So when it was open loop I wasn't having any problems driving. I didn't have the stumble or nothing like that, but the idle was still high.

My guy thinks it may be a "false" air leak, meaning there's a leak after the AFM on the engine. I personally don't think it is because as I stated, the problem seems to go away when I'm running the car with the o2 unplugged. I'm not sure it runs better in OPEN loop, because it stumbles when it's cold too.

Here's a list of what I've replaced/swapped for testing

~New temp sensor for the DME
~New o2 sensor
~heater plate delete on intake, removed all vacuum hoses
~New upper and lower intake gaskets, new throttle body gasket
~cleaned ICV 2 seperate times
~Swapped AFM

Hope that help anyone.

I gotta figure this out, it's killing me!

DesktopDave · « **Reply #37 on:** June 10, 2009, 07:18:49 AM »

Quote from: 91 318is;73365

No the new o2 sensor DID NOT fix it. But before I changed it, I unplugged the o2 sensor and drove around for a day or so with it unplugged. Apparently from what my buddy told me, unplugging the o2 and driving put the car into open loop. So when it was open loop I wasn't having any problems driving. I didn't have the stumble or nothing like that, but the idle was still high.

That's not what I was trying to get at. The new O2 sensor didn't fix the problem, but you used it to force open loop. I'm saying that when the car is in closed loop there is a bad sensor reading. When you plugged the sensor back in, you've eliminated it as a problem AND you're closer to the root of the problem.

Quote from: 91 318is;73365

My guy thinks it may be a "false" air leak, meaning there's a leak after the AFM on the engine. I personally don't think it is because as I stated, the problem seems to go away when I'm running the car with the o2 unplugged. I'm not sure it runs better in OPEN loop, because it stumbles when it's cold too.

I agree with you. I think it's a bad sensor, not a vacuum leak. Mine always stumbles when it's cold too, but only if it's ice cold and only for a minute maybe. I'm figuring your problem is a bad wire or harness connection, or even possibly a bad ECU. If your sensors & wiring keep testing out OK, maybe it's time to consider swapping them? BlackMagic has an ECU for sale here for $50 shipped:
http://www.m42club.com/forums/showthread.php?t=8837

If it's not the ECU I'm sure you could get your $50 back out of it.

91 318is · « **Reply #38 on:** June 11, 2009, 06:03:04 AM »

Quote from: DesktopDave;73368

That's not what I was trying to get at. The new O2 sensor didn't fix the problem, but you used it to force open loop. I'm saying that when the car is in closed loop there is a bad sensor reading. When you plugged the sensor back in, you've eliminated it as a problem AND you're closer to the root of the problem.

I agree with you. I think it's a bad sensor, not a vacuum leak. Mine always stumbles when it's cold too, but only if it's ice cold and only for a minute maybe. I'm figuring your problem is a bad wire or harness connection, or even possibly a bad ECU. If your sensors & wiring keep testing out OK, maybe it's time to consider swapping them? BlackMagic has an ECU for sale here for $50 shipped:
http://www.m42club.com/forums/showthread.php?t=8837

If it's not the ECU I'm sure you could get your $50 back out of it.

Oh ok, I understand you now, sorry. Yeah now I just gotta figure out where the problem is. Also, I swapped out my ICV with the one the guy had at work, didn't seem to fix the problem. It seems to idle a little better at cold idle, but still idles high when warm.

Looks like I'll just dig into it in the next few days and see what I'll find.

91 318is · « **Reply #39 on:** June 12, 2009, 06:06:52 AM »

Oh another thing I was thinking about, could someone have tampered with my idle screw and that's causing the high idle? I swapped out the ICV from another E30 that worked on his and it's doing the EXACT same thing.

I also think the 02 sensor may have fixed the stumbling problem, is hasn't done it to me since I changed it, so hopefully that's fixed.

91 318is · « **Reply #40 on:** June 13, 2009, 08:46:53 AM »

bump, anyone have any input on the idle screw issue?

I actually adjusted the idle down the recommended idle (850) and seems to run better, except when I turn the A/C on, it stumbles down to about 500 for a few seconds and then comes back up to 850. Is this because I tampered with the idle screw?

Also, the car stumbled on me last night so I guess the 02 sensor didn't do the trick. Gonna dig into it some more this weekend I guess and start checking.

Thanks

DesktopDave · « **Reply #41 on:** June 13, 2009, 09:22:43 AM »

I was thinking about this a while...seems like you're getting closer...I'd be tempted to pick up a cheap $30 vacuum gauge, put it in the car somewhere and see what the engine is actually doing. I'd probably tee it off the FPR hose.

I'd adjust the idle control screw back up to a higher idle...you'll still have the stumble, but it won't go so low and stall the car.

Unfortunately I don't have an hard data on what you should be seeing, but I've run these on older cars to troubleshoot carbs (I know, I'm old...please hold off on the sarcasm...)

You'd get two benefits from this...first, you can 'see' what the engine is really doing when it's stumbling, and you can use it to improve your gas mileage. It works a little like the mileage meter on the dash but without the real-time calculation for approx MPG.

91 318is · « **Reply #42 on:** June 13, 2009, 08:03:11 PM »

Quote from: DesktopDave;73561

I was thinking about this a while...seems like you're getting closer...I'd be tempted to pick up a cheap $30 vacuum gauge, put it in the car somewhere and see what the engine is actually doing. I'd probably tee it off the FPR hose.

I'd adjust the idle control screw back up to a higher idle...you'll still have the stumble, but it won't go so low and stall the car.

Unfortunately I don't have an hard data on what you should be seeing, but I've run these on older cars to troubleshoot carbs (I know, I'm old...please hold off on the sarcasm...)

You'd get two benefits from this...first, you can 'see' what the engine is really doing when it's stumbling, and you can use it to improve your gas mileage. It works a little like the mileage meter on the dash but without the real-time calculation for approx MPG.

And by reading the vacuum, I guess that would tell me if I have a leak somewhere? It's just hard for me because I understand troubleshooting and where to start, but I don't have any specs on anything or the "proper" test equipment. What should the vacuum be at idle?

DesktopDave · « **Reply #43 on:** June 13, 2009, 08:27:25 PM »

I don't think there is any hard and fast number for what a perfect idle is. When I'm looking at a compression gauge, temperature indicator or vacuum gauge I'm just looking for consistency. Any aberrations get my attention. I'm told about 20 inches is a healthy number.

An easy way to find vac leaks is to start the engine and spray a little carb cleaner around the hoses. If the engine idle smooths out, you're closing up the leak with the cleaner & you've found it. You can't test things like the brake booster or valve seals this way, but it's pretty simple.

91 318is · « **Reply #44 on:** June 13, 2009, 10:17:34 PM »

Quote from: DesktopDave;73587

I don't think there is any hard and fast number for what a perfect idle is. When I'm looking at a compression gauge, temperature indicator or vacuum gauge I'm just looking for consistency. Any aberrations get my attention. I'm told about 20 inches is a healthy number.

An easy way to find vac leaks is to start the engine and spray a little carb cleaner around the hoses. If the engine idle smooths out, you're closing up the leak with the cleaner & you've found it. You can't test things like the brake booster or valve seals this way, but it's pretty simple.

Yeah, I did that when I first got the car and never could find anything. I sprayed so much the engine looked like new when I was done!

Anywho, I'll continue to slowly chip away at it, although it's getting frustrating, hopefully I'll find the culprit.

Oh and BTW, changed the plugs out today too, old ones look ok, but were worn down a little too much.