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Sage Wisdom Needed: M20 rough idle, rough acceleration

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    Sage Wisdom Needed: M20 rough idle, rough acceleration

    I know this topic has been beat to death but I have a bit of a unique situation I’m hoping someone can help me with. My car (‘90 325i) starts and runs beautifully for about 15 seconds when cold then immediately begins idling rough, hesitating when accelerating, and is down on power. I was driving home from work when it all started, going about 20mph in traffic and I swear I heard a “click” then it went south. Although the click could have just been in my imagination... No CEL but it was obviously acting up.

    The entire engine has about 10k miles since getting a top to bottom reseal and reconditioned head. It also has about 25k on refurb’d injectors, and has new spark plugs, newer O2 sensor, cleaned CPS, and swapped AFM with known good unit. I did the plugs when It started acting up and 2-6 looked great, #1 was a little black compared to the rest but not terrible. I thought the plugs did the trick but in the time it took me to move my other car out from in front of the garage door it started acting up again. Compression is 175-180 across the board. I’ve also cleaned the big plug under the intake, and checked for fuel in the FPR vacuum line and it’s fine. I don’t think it’s O2 since it happens at idle and when it still is cold. I don’t think it’s the ICV since I’ve had those act up plenty of times and it doesn’t seem like this, and that wouldn’t matter on throttle/WOT. I don’t think it’s the TPS because it occurs at idle, part throttle and WOT.

    My plan is to bring home a fuel pressure test gauge tomorrow, just to rule that out. I’m looking for advice on where to go next. While unplugging everything under the sun to look for clues I noticed my CEL comes on but goes off immediately. Does that indicate a ECU issue? I can’t remember if the code should be stored... anyway, any help is appreciated. Thanks

    I had a similar issue on my car recently where the car would run fine cold, but then rough hot it turned out to be the ECU in the end.

    First I'd check the fuel pressure to make sure it's not the fuel pump or FPR, then given all else you've done already, if it's not the fuel pump I'd try another ECU.

    I also normally check the simple stuff like the coil (although I'd presume with all the other work you've done that has probably been done?)

    Good luck.
    My e30: OEM+ with M30B35


      Have you replaced the coolant temperature sensor (imperative for mixture)? Also, while cars have been running rough (sounds like you replaced quite a few parts to try sorting it out) they tend to load up the catalytic converter. As a last resort with my car, I ran a can of "cat-clean" type product and had fabulous results, but it took a couple of weeks after my road trip of 700 miles. If you have one or two cylinders that look rich, you could be loading up - maybe a couple of injectors are not perfect despite the cleaning service. Are you able to go out and blow it out or does it run too poorly? I would try to run some chemicals of your choice under a hot run and then shoot some catalytic converter temperatures before and after the converter. The good news is that these cars are not that complicated and can be very reliable once you find your problem. Last, it is helpful for us to know specifics on parts you have used such as plug type, injector size, and such...

      Good luck!


        Well, I had a second look at the distributor cap and saw some cracks so I thought that was the issue. I swapped in a second one I have and that made no difference. I have not replaced the coolant temp sensor yet but I did unplug it while idling and it ran worse and threw a CEL. I think it is unlikely this is the issue since it acts up so soon after starting it, but I do have a second one I can swap in. And fortunately I have not yet bought any parts trying to fix this aside form spark plugs. Swapping the cap and AFM were from spares, and the rest was all just for a major overhaul of a tired engine and conversion from auto to 5 spd.

        I checked the fuel pressure and its 38psi at idle and drops down momentarily when revving, but goes up when the return line is pinched.There is no change if the vacuum line is pinched or removed. Because of this I'm going to throw in a rail/injectors/FPR from a spare engine I have. While I have the rail out I might as well swap the coolant temp sensor as well, since I'll have access to it. Once it started acting up I drove the remaining 4 miles or so home and haven't really ran it since then aside from troubleshooting.

        Plugs are NGK ZGR5A and the injectors are stock 325i injectors. Not sure of the size... but they're blue. I will update again after swapping injector/rail/fpr and coolant temp sensor. Thanks for the input!!!!


          I do not have the Book in front of me but I think the fuel pressure is supposed to go up to 46 psi when the engine is revved. If you are gaining access to the rail I would get it running and up to pressure and then pull it out while the injectors are pressurized. If drips occur on the tips of the injectors the cylinders will flood after the car is turned off. Also test to see if you are retaining residual pressure after the car is turned off for a while. A properly running system should retain pressure at least overnight. If you have a Bentley there is a great write-up on testing the FPR and doing fuel delivery. Might as well knock this off while having to smell so much gas anyway. If you do not have a Bentley I can send you my old one though its a bit beaten up.


            Well in the end it was a bad injector on #1. Guess a rebuilt injector isn’t as reliable as new. All fixed now. Thanks for the help!