nath 159 Report post Posted March 31 (edited) I recently had fully lined up a good E30 Alpina in the UK to import, and a donor car here for parts or shell. Then my father passed away unexpectedly and with that upheaval the big project wasn't a good idea, so I grabbed a local 1987 M325, with a few things that need ironing out. Stumbling idle. The car idles at a steady speed wherever set, but has a distinct stumble/misfire. No issues apparent at speed but it may be masked at those revs. Idles as if it has a lumpy cam. Valve clearances have been done Spark plugs replaced dizzy cap checked/cleaned- was new 10,000km ago injectors were apparently serviced and found to be clogged or grubby, fuel filter has been changed, presumable after that. All injectors are actuating. Vacuum leaks checked to a simple level, intake boot appears new as do various other parts. AFM has no flat spots and tests with steadily increasing resistance. Spark is apparent at all plugs Car has the two crank sensors on the bellhousing, an unexpected hall sensor on the harmonic balancer with cable up to the OBD plug(smooth rimmed balancer, where is the sensor picking a signal from?) and a sensor on one spark plug lead plugged to nowhere, I presume for facelift cars with newer motronic. That is really my core interest. The metallic rattle from under the car could be an undisclosed lightened flywheel, a crappy input shaft bearing, throwout bearing or clutch springs, or simply the effect of a misfiring idle on those components. Am happy to have them all replaced. Could the ECU (Motronic 1.0) cause this, even though the car basically runs well and there is no major misfire? Could the coil cause it? I'd not imagine it could being as it generally gets spark to where it needs to go. Thanks Nathan Edited March 31 by nath 3 Quote Share this post Link to post Share on other sites
BMTHUG 1105 Report post Posted March 31 hey bud. Check your your hoses for vacuum leaks, check also the resistance of the crank and reference sensors on the bellhousing, these can give a stumbling idle. Also check the resistance on your ignition coil. I have had a few of these play up. Other things to check would be your idle control valve, take it out, clean it, make sure it's operating correctly with the correct resistance etc and check the intake house from your throttle body for cracks. Also the little pipes that come out the side of the throttle body need to be tightly fitted as they tend to have the knurled surfaces wear where they fit into the throttle body causing leaks. The sensor on the harmonic balancer is for the diagnostic port at the top of the engine and doesn't really do anything as far as connecting to the ECU. The plugs just go straight to the diagnostic port. Other things to check is pull out the oil cap while running and see how much crank case pressure you have, worn rings etc can make it idle funny if you're low on compression or there is blow by. Happy to check it out for you if you want to swing by my shop sometime in Glenfield. Love me a tech 1! I have one myself. Cheers 02102916373 SJ 4 Quote Share this post Link to post Share on other sites
BMTHUG 1105 Report post Posted March 31 p.s yes the sensor from number 6 spark plug lead that has a plug on it will not be applicable to your pre-facelift car. Only the facelifts use this as part of motronic 1.3 engine loom 2 Quote Share this post Link to post Share on other sites
nath 159 Report post Posted April 1 12 hours ago, BMTHUG said: p.s yes the sensor from number 6 spark plug lead that has a plug on it will not be applicable to your pre-facelift car. Only the facelifts use this as part of motronic 1.3 engine loom Thanks SJ, I think you may have had his car in before? I’m keen to get her as nice as the B3, so one step at a time. Gotta sort out some original wheels first… 1 Quote Share this post Link to post Share on other sites
BMTHUG 1105 Report post Posted April 1 1 hour ago, nath said: Thanks SJ, I think you may have had his car in before? I’m keen to get her as nice as the B3, so one step at a time. Gotta sort out some original wheels first… Yeah I may of had it in my shop before. Let me know if you end up selling the wheels on there. I may know a buyer Quote Share this post Link to post Share on other sites
Autobahn 22 Report post Posted April 5 Totally agree with general check for air leaks, and service items. That said; what is the throttle response like, especially off/near idle? Some of the AFMs seem quite out of wack and can run very rich and sluggish. If its already opened up - a paint pen reference mark and trying a couple of clicks leaner can help. I had a AEM standalone wideband O2 sensor in a Jetronic injection 320i and with some tinkering it was leagues better. the "remanufactured" AFMs that are available online can be better but I've still yet to see one that runs perfectly out of the box. At least they aren't super expensive IIRC Quote Share this post Link to post Share on other sites
nath 159 Report post Posted April 5 4 hours ago, Autobahn said: Totally agree with general check for air leaks, and service items. That said; what is the throttle response like, especially off/near idle? Some of the AFMs seem quite out of wack and can run very rich and sluggish. If its already opened up - a paint pen reference mark and trying a couple of clicks leaner can help. I had a AEM standalone wideband O2 sensor in a Jetronic injection 320i and with some tinkering it was leagues better. the "remanufactured" AFMs that are available online can be better but I've still yet to see one that runs perfectly out of the box. At least they aren't super expensive IIRC Mm yes- in the service history (done about 25,000km in last 20 years) the afm was tuned with an analyser but I feel like the car still smells like a fume machine. Will keep looking for vacuum leaks, but feel that they tend to give an unstable/surging idle more than this very distinct short sharp lumpiness? May as well get or borrow a different coil to rule that out- doesn’t seem to be available locally new? Quote Share this post Link to post Share on other sites
Tobias01 68 Report post Posted April 5 Sounds like the exact issue I had on mine when I first purchased it. Turned out to be the fuel pressure regulator causing the issue. 2 Quote Share this post Link to post Share on other sites
BMTHUG 1105 Report post Posted April 7 On 4/5/2025 at 10:04 PM, Tobias01 said: Sounds like the exact issue I had on mine when I first purchased it. Turned out to be the fuel pressure regulator causing the issue. yes these are common too! 3 bar pressure regulator BTW 1 Quote Share this post Link to post Share on other sites
nath 159 Report post Posted April 12 Fuel pressure regular checked swapped no fault, AFM tuned no change really. Vacuum leaks meticulously checked(not smoke tested but well checked and sealed). All new leads resistance checked, and crank reference and speed tested ok. TPS good. I think it has a lightened flywheel and hefty clutch, with a yuck rattle/chatter off throttle/coasting. Perhaps the lack of inertia could contribute to lumpy idle? Or maybe the timing belt is a tooth off? 1 Quote Share this post Link to post Share on other sites
nath 159 Report post Posted Wednesday at 10:32 PM Latest update: Tried another ECU, no change, at least I have a spare now. Compression tested all cyls- all within 5% , about 170 psi. So I'm left with I'd say a smoke test to be done, a re- check of the dizzy, and the biggest question, is the flywheel indeed non-original and is the reference peg in the wrong place... Anyone got a PFL 325i flywheel available? Nath Quote Share this post Link to post Share on other sites
Autobahn 22 Report post Posted Thursday at 05:03 AM Timing should be able to be checked mechanically with the reference marks on the harmonic balancer to block and the timing marks on the cam gear to head. Hard to imagine any way the distributor would be off unless the timing signal is wrong - possible I suppose that its related the flywheel if it has been swapped to something aftermarket. 99% sure we've got a Motronic 1.1 M20B25 flywheel down here in Hinds if you do end up needing one. Keep us updated on what you find Quote Share this post Link to post Share on other sites
nath 159 Report post Posted Friday at 07:34 PM On 4/17/2025 at 5:03 PM, Autobahn said: Timing should be able to be checked mechanically with the reference marks on the harmonic balancer to block and the timing marks on the cam gear to head. Hard to imagine any way the distributor would be off unless the timing signal is wrong - possible I suppose that its related the flywheel if it has been swapped to something aftermarket. 99% sure we've got a Motronic 1.1 M20B25 flywheel down here in Hinds if you do end up needing one. Keep us updated on what you find Thanks- I’m keen to get rid of any light weight flywheel anyway so am keen to source an original one, and certainly also check the timing… Quote Share this post Link to post Share on other sites
Sammo 2593 Report post Posted Saturday at 12:53 AM 5 hours ago, nath said: Thanks- I’m keen to get rid of any light weight flywheel anyway so am keen to source an original one, and certainly also check the timing… Looks great on the weaves though! 1 Quote Share this post Link to post Share on other sites
nath 159 Report post Posted Saturday at 09:29 AM (edited) 8 hours ago, Sammo said: Looks great on the weaves though! Funny little things, but yep considering I was shopping for a chrome classic, the weeny wheels suit better than 80’s race. Here’s what was on the way before dad passed(and hey maybe still will be?) Edited Saturday at 09:29 AM by nath 2 Quote Share this post Link to post Share on other sites
Olaf 3391 Report post Posted Sunday at 04:59 AM this used to be @Contrails car, always well cared-for based on many posts on here. Great to see it back out circulating. Quote Share this post Link to post Share on other sites
nath 159 Report post Posted 13 hours ago Just to bore the shite out of you with updates: -Checked timing marks, timing belt is spot on. -Re-checked distributor incl all resistances and all good. -Both crank sensors test ok for resistance. Removed both and cleaned them up and checked for existence of reference ‘pin’ (small steel cleat). Apparently it should be 58 degrees btdc, I didn’t have the foresight to check this but it was aroundabout there. Could the flywheel have been fitted one hole off? Removed and had a basic look at and test of the injectors, they seemed ok. 4 are blue and two are more a grey-blue, does this mean anything? Visually the same and I think k the numbers were same. some photos to make it more interesting: 1 Quote Share this post Link to post Share on other sites
