*SOLVED* M50 Swap w/ High RPM WOT Misfire - 24V gurus needed

[wfkiv]

**Solved**
Coil #3 was bad and the ultimate cause of this particular misfire.


Hey R3v,

I have a 1991 318iS with a 1992 M50B25NV swapped into it. I performed the swap myself. Important mods/information:

-~1200 miles on the swap
-~190K on the M50. Prior to install in my e30, I redid the top end (disassembled, jet washed, lapped valves, new HG, ARP head studs, new seals from oil pan to valve cover, new intake boot, etc.)
-Stock + original 402 DME
-Stock + original green top injectors, new o rings
-Stock AFM, some cone filter CAI
-eBay headers, 3 inch catless exhaust, Summit Racing muffler
-New Mann fuel filter
-New spark plugs
-E36 Getrag G250 and medium case 3.73 LSD

Symptoms:

Car runs great. There are no CELs. There are no vacuum leaks. Idles nicely. Above roughly 5000 RPMs there is a misfire only at, or very close, to WOT. The miss is "minor" and it will still pull to redline in 1st, 2nd, 3rd despite the miss. It will pull to redline without a miss on partial throttle. It will miss regardless of gear/speed and will miss regardless of mashing the pedal or rolling in. I tried pulling coil packs one by one, one at a time, at idle, but each coil was working and RPMs changed.

Moreover, sometimes I will have a miss just off idle, when you are engaging 1st and pulling from a stop, but the miss will clear very quickly and the car will run smooth. It does not run rich, gets good gas mileage and, as stated, runs buttery smooth and very strong despite the high RPM WOT miss.

Any ideas, 24v gurus? I don't want to throw six new injectors at it just yet.

[ZekeTheSneak]

What spark plugs you running? I would run cheap ngks. None of them Bosch bullshit plugs with six electrodes. Good old fashioned ngk BKR6E. Check the gap on your current ones, check the boots on the coilpacks while you are in there. Your valve cover could be leaking oil onto the coilpacks as well

[AndrewBird]

Could be a bad throttle position sensor. The fuel maps change when at WOT and if the ECU has the wrong reading from the TPS, it won't run correctly.

[straight6pwr]

pulling the coilpacks at idle isnt really an indication of whether they are working or not. a coilpack can be functioning intermittently as it starts to die, its not either just working or dead.

[kronus]

couple common causes of high rpm misses like this
- coils, as mentioned before
- ECU coil drivers - test with another ECU
- CPS itself or improperly set gap on CPS to trigger wheel
- loose harmonic balancer, bad keyway or damaged woodruff key
- rarely, engine ground

[wfkiv]

Thanks for the replies! Some updates:

-Engine ground is good brand new with the swap, checked connections etc. seems fine.

-Plugs are NKG 6962 gap is .032in

-I have a second 403 label ECU, issue persists identical on both ECUs.

-If the throttle position sensor were bad, and causing a faulty reading at WOT, wouldn't I experience a miss at WOT throughout the rpm range rather than just high rpms?

-CPS is new with the swap and I think gapped properly, will have to check gap tomorrow.

-I will check the harmonic balancer when I check the CPS gap. IIRC the woodruff key and everything was fine on install, but anything could have happened in the last 1K miles.

-How can i go about checking coil operation on this OBD1 engine? I had an OBD2 E36 in the past and obviously it was "smart" enough to indicate individual cylinder misfire. Coil boots are OK, none of the coils have hairline cracks. No oil is leaking from the VC into the spark plug holes.

[wfkiv]

Thanks for the replies! Some updates:

-Engine ground is good brand new with the swap, checked connections etc. seems fine.

-Plugs are NKG 6962 gap is .032in

-I have a second 403 label ECU, issue persists identical on both ECUs.

-If the throttle position sensor were bad, and causing a faulty reading at WOT, wouldn't I experience a miss at WOT throughout the rpm range rather than just high rpms?

-CPS is new with the swap and I think gapped properly, will have to check gap tomorrow.

-I will check the harmonic balancer when I check the CPS gap. IIRC the woodruff key and everything was fine on install, but anything could have happened in the last 1K miles.

-How can i go about checking coil operation on this OBD1 setup? I had an OBD2 E36 in the past and obviously it was "smart" enough to indicate individual cylinder misfire. Coil boots are OK, none of the coils have hairline cracks. No oil is leaking from the VC into the spark plug holes.

[wfkiv]

Could be a bad throttle position sensor. The fuel maps change when at WOT and if the ECU has the wrong reading from the TPS, it won't run correctly.

I found a test for the TPS:

I should see 4.0K ohm between terminal 1 and 3 on the TPS. I observed 3.38K ohm .

I should see 1.0K ohm between terminal 1 and 2 at closed throttle and 4.0K ohm at WOT. I observed 1.12K ohm closed and 3.73K ohm at WOT with values rising as the throttle sweeps

Do these test values deviate too much to cause problems, or us the TPS in spec?

[wfkiv]

- CPS itself or improperly set gap on CPS to trigger wheel
- loose harmonic balancer, bad keyway or damaged woodruff key

-CPS gap is .7mm, within spec of 1mm +- .3mm. It is a brand new OE BMW unit (12141726065)

-Harmonic balancer is on tight.


Seems like i've narrowed it down to CPS (question about specs still looming from my precious post), Coils (how can I actually test? I read the a resistance test is unreliable because the value is too small), and injectors. I am going to make an injector test/cleaning rig and give that a go tomorrow. Stay tuned!

[ak-]

Because maths I don't believe the delta values are the cause of your problem, but I have no issue to be proven wrong!

You can also test your injectors with your multimeter before thinking about replacement!

[AndrewBird]

The only way the engine would know or care that it was at WOT would be the TPS or MAF. If it doesn't miss at partial throttle, then it has to be one of these two. I'd try swapping the MAF for another one or at least cleaning the one you have now.

[wfkiv]

Can't make this up. Last night and this morning on the way to the parts store the car developed a bigger miss throughout the rev range and even some backfiring.

The only way the engine would know or care that it was at WOT would be the TPS or MAF. If it doesn't miss at partial throttle, then it has to be one of these two. I'd try swapping the MAF for another one or at least cleaning the one you have now.

Got to the parts store and picked up some CRC MAF Cleaner. I sprayed down the MAF sensor (of course removed from car, did not touch the sensor with the straw, let it dry) and reinstalled. The sensor was actually dirty and I saw dirt coming off as I sprayed! Driving back to my shop the "new" miss is gone and the car definitely has more pull, but still has the original high RPM WOT miss.

Upon further review, the miss occurs at WOT or very close to it, say 80% throttle and up above 5000 rpms. I don't have a spare MAF or an M50 friend close by to swap. Now i'm going to remove the injectors, clean them on my bench rig and make sure all four ports are delivering fuel. If the injectors all seem fine and the miss persists after reinstallation, I am going to move onto replacing the coils.

[wfkiv]

Updates...

My injectors were very old/dirty and visibly collected gunk. I tried cleaning them, but instead opted for some rebuilt ones which are arriving tomorrow. Hopefully that does it!

I also smoke tested since the car is off the road until injectors arrive. No leaks except at the O-Ring for the PCV to valve cover connection. Extremely minor leak, not enough to cause any running issues (especially the ones I am experiencing) but I have a new connector and o ring on the way from blunttech also.

[jayuuey]

i had a higher rpm miss and it turned out to be bad injectors that I got from ebay. I got some other remans from a forum member (bimmerbaron?) and they are working great. hopefully thats your issue!

[wfkiv]

Fuel injectors arrived. They did not solve the issues.

However, that miss I first mentioned in post 11 is now present all the time.

Last night and this morning on the way to the parts store the car developed a bigger miss throughout the rev range and even some backfiring.

Miss off idle all the time. In first, second and third I can clear the miss but in fourth and fifth gears it misses and hesitates basically at all throttle all rpms all the time. THAT sure sounds like a bad coil(s) to me.

Oh well, at least my six new injectors are shiny. Looks like new coils/boots are in order.