M20 rough idle

[Ceeker]

ok, after exhausting my resources to figure this out along with reading a tonne of post related I still can't figure out what my issue is.

I have swapped out everything to try and resolve my rough idle issue. it seem to be intermittent at times and it is more prevalent at cold start up. the term used I believe is "engine lopes"

changed: plugs, cap, rotor, new gaskets, no vaccuum leaks, throttle body, TPS, cleaned ICV, changed DME, AFM. brown temp sensor, adjusted the valves cold and hot.

I put a ODB1 tester on it and got a code 10 from a Snap On tester which was an O2 sensor. So I changed that too. I disconnected the battery to clear the codes and still no change.

I wasn't able to the the dme to respond to the pedal stomp test.

what on earth is left??

any help would be greatly appreciated. thank you.

[jlevie]

By no vacuum leaks do you mean that a properly conducted smoke test was run? If that is not the case I submit that you don't know if there are intake leaks. Since that is the most common cause of a rough idle it should be the first thing done.

[ROLLingKING]

have u tried new crank position sensor?
and the blue temp sensor is the more important one. the blue sends your ecu the temps. the single prong brown one only operates the gauge. have u tried O2 sensor. does your check engine light work?

[paperplane94]

ok, after exhausting my resources to figure this out along with reading a tonne of post related I still can figure out what my issue is.

I have swapped out everything to try and resolve my rough idle issue. it seem to be intermittent at times and it is more prevalent at cold start up. the term used I believe is "engine lopes"

changed: plugs, cap, rotor, new gaskets, no vaccuum leaks, throttle body, TPS, cleaned ICV, changed DME, AFM. brown temp sensor, adjusted the valves cold and hot.

I put a ODB1 tester on it and got a code 10 from a Snap On tester which was an O2 sensor. So I changed that too. I disconnected the battery to clear the codes and still no change.

I wasn't able to the the dme to respond to the pedal stomp test.

what on earth is left??

any help would be greatly appreciated. thank you.

That tells me that there may be a problem with the TPS not telling the ecu that the cars throttle is closed, therefore the ICV is not regulating the idle.

How did you check it?

[Ceeker]

have u tried new crank position sensor?
and the blue temp sensor is the more important one. the blue sends your ecu the temps. the single prong brown one only operates the gauge. have u tried O2 sensor. does your check engine light work?

the crank position sensor seemed to have been changed as it wasn't routed properly before I took things apart.

New 02 sensor.

I will try and find a replacement "blue" temp sensor. thank you.

Yes, check engine light comes on.

[Ceeker]

thanks everyone for all your input; I will follow through this list to eliminate what I can.

I didn't check the TPS I replaced it...and cleaned the ICV.
what I did check through the tester was the ICV; it was opening and closing the valve telling me it was working and the tester didn't show any error or fault.

I was also able to check the 02 heater as the relay far right of the 3 sitting independently ws clicking. it doesn't give much options for diagnostics but everything seems ok.

many thanks and will report back to findings!

[603Racing]

A bad FPR can cause a rough idle, though it usually causes hard starts as well... or no start. You can check the vacuum line running to the FPR. Right after you shut off the car pull the vacuum line off the FPR and see if it has fuel in it. If it does, the FPR is shot.

[Ceeker]

Update:

I swapped out the TPS and ICV to both working units.
I found a crack on the hose which runs from the throttle body to the valve cover but this was right at the valve cover nipple so I am not entirely sure this is the problem. going to replace it anyway as I think it is due.

I double checked the intake runners to make sure the nuts were tight. New gaskets with RTV sealant on them when I removed the intake.

changed both temp sensors.

I also remember someone saying that when the ICV is disconnected while the car is running the rev's will increase. I didn't have this happen with either of the donor ICV's I plugged in.

when the car starts: initially it almost wants to stall as it tries to find a stable idle. the idle fluctuates between 400-600 rpms. it dips and rises quickly. this goes on for about 5-10 sec. then the idle starts to increase and works it's way up to 800-900 rpms and levels out there. car runs fine at this rpm. after rev the engine, it doesn't return to set idle and stays at 900 - bit high I would think.

it is also running rich and the check engine light stays on.

I tried the stomp test again and can't seem to get the car to respond.

[jlevie]

Everything you are describing could be caused by intake leaks. Have a shop run a smoke test.

FYI: it sounds like this is a M20B25. In which case normal idle should be 950rpm.

[e30trooper]

Update:

I swapped out the TPS and ICV to both working units.
I found a crack on the hose which runs from the throttle body to the valve cover but this was right at the valve cover nipple so I am not entirely sure this is the problem. going to replace it anyway as I think it is due.

I double checked the intake runners to make sure the nuts were tight. New gaskets with RTV sealant on them when I removed the intake.

changed both temp sensors.

when the car starts: initially it almost wants to stall as it tries to find a stable idle. the idle fluctuates between 400-600 rpms. it dips and rises quickly. this goes on for about 5-10 sec. then the idle starts to increase and works it's way up to 800-900 rpms and levels out there. car runs fine at this rpm. after rev the engine, it doesn't return to set idle and stays at 900 - bit high I would think.

it is also running rich and the check engine light stays on.

I tried the stomp test again and can't seem to get the car to respond.

todo a stomp test , turn to the 1st click then press the gas pedal 5 times on the last time hold for a few secs. turn key to 2nd click and the code will show. At least thats how mine shows up.

im getting a 1262 code

1262 Idle Speed Actuator 1262 - Possible causes for Idle Speed Control fault are:
damaged wiring to the idle speed actuator
wire from DME is defective
output stage of DME is damaged
Also if engine stalls while it is above 600 RPM, code 1262 will be set

will idle at 600 and sounds like i have a sick cam. My car has never thrown anu CEL's untill two days ago i messed with the fuel line after the FPR. ive tried everything , guess ill check my dme wires..

[Ceeker]

todo a stomp test , turn to the 1st click then press the gas pedal 5 times on the last time hold for a few secs. turn key to 2nd click and the code will show. At least thats how mine shows up.

im getting a 1262 code

1262 Idle Speed Actuator 1262 - Possible causes for Idle Speed Control fault are:
damaged wiring to the idle speed actuator
wire from DME is defective
output stage of DME is damaged
Also if engine stalls while it is above 600 RPM, code 1262 will be set

will idle at 600 and sounds like i have a sick cam. My car has never thrown anu CEL's untill two days ago i messed with the fuel line after the FPR. ive tried everything , guess ill check my dme wires..

As I understand that stomp test: ign on so that you can see the check engine light. depress pedal once, wait a sec or two and 5 consecutive presses relatively quick - the light should begin to flash.

car is an 6-88 cabrio

[e30trooper]

As I understand that stomp test: ign on so that you can see the check engine light. depress pedal once, wait a sec or two and 5 consecutive presses relatively quick - the light should begin to flash.

car is an 6-88 cabrio

dont wait that long , just press it 5 times all the way down and hold the last

i have a 87 cabby but i put in a 173 dme to test codes

[Ceeker]

ok - I'll give it a try Monday when I'm back in the shop; but highly doubt it will give me anything. If the ODB1 service tester can't find any codes why would the stomp test be different?

[audiquattrot]

Update:

I swapped out the TPS and ICV to both working units.
I found a crack on the hose which runs from the throttle body to the valve cover but this was right at the valve cover nipple so I am not entirely sure this is the problem. going to replace it anyway as I think it is due.

I double checked the intake runners to make sure the nuts were tight. New gaskets with RTV sealant on them when I removed the intake.

changed both temp sensors.

I also remember someone saying that when the ICV is disconnected while the car is running the rev's will increase. I didn't have this happen with either of the donor ICV's I plugged in.

when the car starts: initially it almost wants to stall as it tries to find a stable idle. the idle fluctuates between 400-600 rpms. it dips and rises quickly. this goes on for about 5-10 sec. then the idle starts to increase and works it's way up to 800-900 rpms and levels out there. car runs fine at this rpm. after rev the engine, it doesn't return to set idle and stays at 900 - bit high I would think.

it is also running rich and the check engine light stays on.

I tried the stomp test again and can't seem to get the car to respond.

bumping this old thread...any shot this was your issue? I have a similar rough idle issue, CEL intermittent, and codes came back to O2 sensor which I replaced last yr. I found the same crack in the hose you mentioned, and curious to see if that could be the cause.

[wbondsteele]

For the benefit of anyone out there who has rough idle issues with their E30 I'll briefly recap my journey.

I own a 1990 325i that has 104K miles on the odo. Although it is a 4-door auto box with the less desirable Bronzit paint, it is otherwise in mint condition. I've owned the car since 2013 and it is my DD. I love driving it and so does my wife. It has been the most reliable car I have EVER owned. Since the day I bought it, however, the car has had a bit of a rough idle. Nothing major but something I've always felt could be better. A few times when I've brought the car in for service I would ask the techs if they could fix the idle problem, and they all pretty much said the same thing: "Yeah, that's just how these cars run. Your car runs fine, so I wouldn't worry about it." So I didn't. I figured it was just the nature of the car and let it go.

Fast forward to the beginning of the year (2025). I brought the car in to have the oil changed and the valves adjusted. (It had been more than 30,000 miles.) I thought, okay, maybe this could be an opportunity to once and for all slay that pesky little idle demon. This made me think about when the car last had a full tuneup. I riffed through my service records (including those left by the previous owner) and found that it had been more than 50,000 miles since the spark plugs, wires, rotor, etc. had been replaced. I requested a full tune up and also that a smoke test of the intake system be performed because I suspected a vacuum leak could be the culprit. As it turned out, by adding this simple test to the scope of work I lifted the hatch to a rabbit hole that surely many on this forum have gone down.

After adjusting the valves, the mechanic did the smoke test and found that both idle air hoses were leaking as well as the idle control valve. He replaced both hoses and sealed the ICV after cleaning it. The car idled a tad bit smoother, but I was still not satisfied. I asked for a re-diagnosis. Thinking that maybe the valve adjustment was still off, I arranged to have the car parked in the garage overnight so the valves could be measured cold. Indeed, the values were slightly off; the valves were re-adjusted, but the car still idled pretty much the same. At this point, I felt like the U.S. Army going after Saddam Hussein. I was determined to fix the problem once and for all no matter the cost. After digging deeper--the mechanics literally checked everything--they found that the fuel pressure regulator was starting to leak internally. The only other probable cause was the injectors.

Replacing (or ultrasonically cleaning) 35-year-old injectors was something I had considered before but had always balked at because of the cost. But this time I decided to take the gamble and approve the FPR replacement and replacement of all 6 Bosch injectors with the latest 4-hole design. (Stock injectors, not M50s.)

The verdict? If you are experiencing a similar issue and have exhausted all other likely culprits, of which there are indeed many, then I HIGHLY recommend replacing the injectors with the newer design. I am simply amazed at the difference this has made. The car now idles so smoothly it feels like a new car. It is more responsive=more power. Upshifts have more thrust=more torque. It sounds better. I haven't driven it enough yet to gauge if it has made much of a difference in fuel economy, but I would be surprised if it hasn't. All in all, it was worth the effort and expenditure to fix this issue. I wish I had done it sooner. The biggest surprise for me is that despite what I previously thought, the M20 engine does not have an inherent design flaw that causes shitty idle, afaik. It actually can idle beautifully, but the entire chain of things that would impact idle must be operating flawlessly. It's akin to getting the best sound out of your high-end $10K analog phono cartridge. It's all in the setup. There is no margin for error. And the more meticulous you are the better the results you're likely to get.