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M42 Rough Running

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    M42 Rough Running

    Hello everyone,

    I recently bought a 318is road rally car with a recently rebuilt M42 engine in it (rebuilt in the last 3000 miles). The car was running well when I bought it, plenty of power and free revving, and always starts on the button. However the car had what I would consider a high idle when warm (say between 1100-1300rpm). I have since done around 600 hard miles in it to get to know the car and it has developed a fault where by it now won't idle (just cuts out, warm or cold) and is rough between 2000-3000 rpm. Above that it still runs fine up to 6k rpm.

    I was just wondering what is the best way to start to determine where the fault lies is? I have disconnected the AFM and the car runs but idles roughly at around 2500rpm. The IAT sensor is disconnected due to cone filter. Everything else is standard I think, although I don't know exactly what specs the engine was rebuilt to so I am trying to get in touch with the builder.

    I tried doing the smoke test around the intake to check for unmetred air but the air thrown off the vanes on the alternator make it difficult to assess whats going on! I was going to do a compression test next to see what the figures are.

    Any assistance would be much appreciated.

    The car still has the AFM, with a cone filter attached, correct? The IAT sensor is 100% required to run properly, and it is integrated into the AFM, so unless it was chopped out of the AFM, ti is still in there.

    Intermittent rough running at specific RPMs is sometimes due to a failing ignition coil. However, the stalling and high idle points elsewhere. I'd say that you should start checking for vacuum leaks and a stuck ICV as those are the most common causes of high idles.

    Also check for ECU fault codes. Put the key into Accy-II and fully press/release the gas pedal 5 times over 2-3 seconds. You will see the CEL turn off, and the blink a sequence of 4 numbers, delimited by pauses. If the code "1444" comes up and repeats, then there are no stored codes. Any other code(s) are specific faults.


      Hi bmwman91, thanks for your response. The car has a cone filter attached to the AFM but I thought the IAT sensor was in the OEM airbox? Anyway beside the AFM connector (round plug) there is a disconnected wire to the harness (rectangular plug) which I presume is for the disconnected IAT sensor. Is it normal that the car should idle at 2500rpm with the AFM disconnected?

      I have a spare ICV here that I will swap in and see if that makes a difference. I will also try some butane around the intake manifold, etc. to see if I can identify any leaks.

      I've also got a spare set of coil packs here and a couple of spare ECUs that I could try, although I don't have a spare set of HT leads and I think I would probably go for coil on plug conversion if these were proved to be faulty. Whats the trick for testing HT leads, just run the car in the dark and look for arcing against the block?


        With the AFM unplugged the car should not idle at all, it should stall almost immediately and not restart. The small plug could be for the charcoal canister purge valve.

        You should look at your DME above your glove box and see if it has been opened. Check if the throttle plates are closed at zero throttle input. If there is still an add on plug downstream of the AMF round connector remove this and see how it changes operation.


          Thanks Roguetoaster, I will check the DME as I know the car came with a couple of spares (blue label) and I have a spare from my old 318IS (red label). I will also take the AFM off and have a look at the throttle body operation and test the TPS. I wonder why it is idling at 2500rpm with the AFM disconnected rather than dying (which it does with the AFM connected!).

          I sprayed a low stream of butane all around the intake area but no change to the running. I also pulled the plugs at lunchtime and they don't look great. Rearmost plug was perfect, but 2 and 3 have heavy carbon deposits, and frontmost has carbon deposits plus a little oil. I put a know good set of plugs in and no change to the running.


            The only way that the M42 will idle at 2500RPM is if too much air is getting in. If you have no vac leaks, then it is most likely related to the ICV. When I was experimenting with some tunes, some maps got mixed up and things were written to incorrect places, and it caused the ECU to open the ICV way too much and idle the engine at 2000RPM. That is obviously not your issue, but I did a bunch of troubleshooting before figuring out that it was the tune. As a quick test, unplug the ICV while the engine is running and see what happens. It should idle at about the right RPM, a bit rough, but the stop screw in it should be set to let in the nominal amount of idle air.`


              OK so I had a little bit more time this evening to try a few things. I unplugged the ICV, no change at all to proceedings, either with the AFM connected or not.

              I also tried disconnecting the wire at the bulkhead (which was connected, this is a Euro car with no CAT (or o2 sensor I believe)). This actually improved things and the car managed to hold a rough idle, although general running was still rough as the proverbial.

              I went to do a compression test but the tester I have is the wrong size thread for this engine so I'll have to borrow a tester off a friend.

              I also looked out the ECUs I have and it seems the red label ECU is actually from an M30 engined car and the other two are the correct 0 261 200 175 modules from the E30 318is. I will try substituting these next to see if that makes a difference.

              It's worth noting that this is a rally car and as such has been heavily modified over the years by previous owners, so it is not a given that many other things haven't been messed about with that might be contributing to the problem. Your generous assistance is much appreciated.


                Interesting. Well, the next most plausible thing is what roguetoaster said about checking to see if the throttle is stuck open. That would explain the idle, anyway. It would not explain the rough running. That seems like maybe an issue with a worn AFM, the crank position sensor (or damper wheel with the teeth) or an electrical issue in the ECU.


                  Right so I tried the other two ECUs, no change. I pulled the AFM (which does have the IAT sensor integrated into it, sorry my mistake) and the door opens and closes freely. The intake boot is uncracked and the dual butterfly throttle body seems to be set correctly. My feeler gauge wouldn't measure accurately due to the curve of the TB but i'd say the smaller body is open maybe a third of a mm gap with no throttle input. I disconnected the CPS and the car wouldn't even start, so I cleaned it and confirmed the gap is about 1.3mm. Checked again for leaks around the manifold area to no avail. The car does not have a charcoal canister fitted so that explains the unused connector by the AFM.

                  Next step is to borrow a compression tester off my neighbour and see what dry/wet readings I get. I have noticed a fair bit of oil down the underside of the block, seemingly sweating from where the timing case meets the block, could it be that I might be getting a vaccum leak through a faulty gasket?

                  If all the compression readings look sensible I will probably pull the intake manifold and check all the injectors, FPR, etc. next. However if they are off I think I'll probably have to send the car into my local shop for some further investigative work.
                  Last edited by dc2100k; 09-20-2020, 06:53 AM.


                    OK so a bit of an update. I did a compression test and got 145-158-158-152psi dry which seems to be all in order.

                    I pulled the ICV, cleaned it and checked for vacuum leaks, no change. I tried a different AFM, no change.

                    I adjusted the throttle screw so that the butterfly was closed off throttle, now the car will idle roughly at 800rpm with the AFM disconnected.

                    I searched again for intake leaks and found that if I sprayed TB cleaner down behind the fuel rail I would get increased revs so I started to disassemble the intake manifold. I found that the throttle body to FPR hose was brittle and cracking, and that the intake manifold paper gasket has a split in it. I have ordered replacements.

                    I tested the FRP by sucking on the hose and got a vacuum so it seems to be ok. I have a known-good set of low mileage injectors here with new o-rings so next step is to pull the injectors and replace, new fuel filter and then put the intake manifold back together and see if we get an improvement in the running of the car.


                      Cool, it sounds like you have at least found some major issues, hopefully all of them. Keep us posted.


                        Still do not understand how it is running at all with the AFM unplugged as I have not yet seen one that would idle without the AFM.


                          Really? Mine idles with it unplugged, albeit rough and it is not happy to rev either. It is my understanding that M1.7 can use RPM and throttle position to keep the engine running (barely).


                            It definitely runs with the AFM disconnected but this could be because the car reverts to open-loop and this is just enough to keep it running. Given that the car should be in open loop anyway when cold does this suggest that maybe my blue temp sensor is bad and is telling the engine to run in closed loop from cold? Just a theory because I think these usually fail cold rather than hot; I have tried testing the cold resistance but can't get any reading out of my old multimeter on these little pins. Will try harder to get a measurement if the problem persists after reassembly.

                            In other news I pulled the injectors and they look like they've had a hard life, and the lower intake manifold gasket fell to pieces when I removed it so I've ordered another from BMW. Hopefully I will have a chance to put it all back together this weekend and will give you an update.


                              Strictly speaking, closed vs open loop operation refers ONLY to whether or not the ECU is using the O2 sensor signal. Open loop operation is when the ECU just uses the other sensors (AFM, CPS, CLT, TPS) to calculate fuel injection; if you imagine a block diagram, it's just a series of calculation blocks feeding forward into one another to produce fuel & spark advance values. The O2 sensor "closes" the operation loop by providing a signal that directly reflects the true air/fuel ratio; it adds to the block diagram a feedback block which gives an error term which is fed back into an earlier part of the block diagram to correct for various real-world effects like sensor aging & dirty injectors (it feeds back in 2 ways really, one as an immediate additive term to keep up an average stoichiometric AFR, and a slowly "learned" multiplicative term that serves as a global fuel scaling correction).

                              The AFM is the primary input that the ECU uses to determine engine load. The "limp mode" that you get with it unplugged lives up to the name...the car barely limps without it. In theory, the ECU could run fine with just RPM and TPS inputs (Alpha-N), but they didn't program it like that. Those, combined with O2 feedback could negate the need for the AFM had they wanted to do that, but again it isn't how they did the programming.

                              Keep trying to probe the coolant temp sensor. If it is crapped out, it's definitely not helping anything. Also pull back the boot on its connector to make sure that the wires are not broken.
                              Last edited by bmwman91; 09-29-2020, 05:19 PM.