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Weird misfire at low throttle, can't figure it out!

E28 technical advice asked and given! Troubleshooting, modifications and more.
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Fedaykin528e
Posts: 699
Joined: Apr 03, 2008 2:21 AM
Location: Portland, OR

Weird misfire at low throttle, can't figure it out!

Post by Fedaykin528e »

I've been chasing an issue on my 1988 535is with a D'Sylva EAT chip, 349k miles.

I believe it has a misfire, which I initially thought it was some sort of lash in the driveline. At low throttle input, just maintaining speed it will jerk constantly but sort of randomly, feels jittery. It's fairly subtle but it's there. Most noticeable under 45 mph or so in 2nd and 3rd gear, just under the ok I'm accelerating threshold. When you give it more gas it seems to turn into a very fast jitter, fairly even almost like a vibration. Over half throttle to full throttle you can't really notice anything wrong. It does seem to have gotten better while I have been chasing it as well as my idle and cold running, but man have I gone through just about everything. What I've done:
  • Replaced or checked every vacuum line, no leaks.
  • Valve adjust to .009 cam side. Note this thing has a lot of miles but the cam looks great.
  • Regapped spark plugs to spec. 
  • New dizzy and rotor.
  • Wires are only a couple of years old.
  • Tested ICV, why not. 
  • Swapped in known good AFM. 
  • New fuel filter. 
  • Swapped in known good FPR.
  • Tried a known good coil.
  • New CTS. 
  • New O2 sensor, old one tested fine but replaced anyway. 
  • Tested TPS and adjusted anyway. 
  • Checked main and valve cover ground. 
  • Unrelated but replaced crank/flywheel position sensors, the 37 year old connectors were crumbling.
I'm currently running a couple of tanks with a bottle of Techron through it, because injectors seems like the next weak spot. It doesn't seem like a fuel pressure issue to me, but I should get the gear to test that. I should swap in a non-chipped ECU and see if that makes a difference but that would stink. I am running 92 octane, tried a couple different stations, I don't think it's bad gas.

I think I've covered pretty much everything I've been able to find in Tech Talk about similar issues. If anyone has any insight or tips I'd appreciate, sort of banging my head against this one. Cheers!


Bonus, we moved back to Portland from NYC in January, drove this thing the whole way after it had pretty much sat for two years. What an enjoyable experience! Car was flawless, just ate it up cruising at 80-85 all dang day for 4 days straight. Probably why it needed a new everything once we got back to Portland. Now has completely new braking system and a new starter, CV shafts, blah blah blah. Other than the misfire and a shifter rebuild, it's so dialed and drives so well. 

Image
 
Shawn D.
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Beamter
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Re: Weird misfire at low throttle, can't figure it out!

Post by Shawn D. »

Dang, this is a puzzling one!
kojo96
Posts: 923
Joined: Mar 31, 2019 7:39 PM
Location: Pleasanton CA

Re: Weird misfire at low throttle, can't figure it out!

Post by kojo96 »

Fedaykin528e wrote: Jun 17, 2025 12:43 AM I've been chasing an issue on my 1988 535is with a D'Sylva EAT chip, 349k miles.

I believe it has a misfire, which I initially thought it was some sort of lash in the driveline. At low throttle input, just maintaining speed it will jerk constantly but sort of randomly, feels jittery. It's fairly subtle but it's there. Most noticeable under 45 mph or so in 2nd and 3rd gear, just under the ok I'm accelerating threshold. When you give it more gas it seems to turn into a very fast jitter, fairly even almost like a vibration. Over half throttle to full throttle you can't really notice anything wrong. It does seem to have gotten better while I have been chasing it as well as my idle and cold running, but man have I gone through just about everything. What I've done:
  • Replaced or checked every vacuum line, no leaks.
  • Valve adjust to .009 cam side. Note this thing has a lot of miles but the cam looks great.
  • Regapped spark plugs to spec. 
  • New dizzy and rotor.
  • Wires are only a couple of years old.
  • Tested ICV, why not. 
  • Swapped in known good AFM. 
  • New fuel filter. 
  • Swapped in known good FPR.
  • Tried a known good coil.
  • New CTS. 
  • New O2 sensor, old one tested fine but replaced anyway. 
  • Tested TPS and adjusted anyway. 
  • Checked main and valve cover ground. 
  • Unrelated but replaced crank/flywheel position sensors, the 37 year old connectors were crumbling.
I'm currently running a couple of tanks with a bottle of Techron through it, because injectors seems like the next weak spot. It doesn't seem like a fuel pressure issue to me, but I should get the gear to test that. I should swap in a non-chipped ECU and see if that makes a difference but that would stink. I am running 92 octane, tried a couple different stations, I don't think it's bad gas.

I think I've covered pretty much everything I've been able to find in Tech Talk about similar issues. If anyone has any insight or tips I'd appreciate, sort of banging my head against this one. Cheers!


Bonus, we moved back to Portland from NYC in January, drove this thing the whole way after it had pretty much sat for two years. What an enjoyable experience! Car was flawless, just ate it up cruising at 80-85 all dang day for 4 days straight. Probably why it needed a new everything once we got back to Portland. Now has completely new braking system and a new starter, CV shafts, blah blah blah. Other than the misfire and a shifter rebuild, it's so dialed and drives so well. 

Image

 
"At low throttle input, just maintaining speed it will jerk constantly but sort of randomly, feels jittery."
This sounds like a throttle plate in need of adjustment. 
Fedaykin528e
Posts: 699
Joined: Apr 03, 2008 2:21 AM
Location: Portland, OR

Re: Weird misfire at low throttle, can't figure it out!

Post by Fedaykin528e »

@kojo96 - I’ll look at it for sure but I’ve got no off-throttle stumbles or stalls, no weird idle.
kojo96
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Joined: Mar 31, 2019 7:39 PM
Location: Pleasanton CA

Re: Weird misfire at low throttle, can't figure it out!

Post by kojo96 »

Fedaykin528e wrote: Jun 18, 2025 2:02 AM @kojo96 - I’ll look at it for sure but I’ve got no off-throttle stumbles or stalls, no weird idle.
I'd check that throttle plate, then reset the TPS. I'd also check fuel pressure, a lazy or tired fuel pump can cause similar issues.
Fedaykin528e
Posts: 699
Joined: Apr 03, 2008 2:21 AM
Location: Portland, OR

Re: Weird misfire at low throttle, can't figure it out!

Post by Fedaykin528e »

Are you thinking the throttle plate might be too open? Like more air than the TPS would account for? Roger on fuel pressure, I’ll get that done.
kojo96
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Location: Pleasanton CA

Re: Weird misfire at low throttle, can't figure it out!

Post by kojo96 »

Fedaykin528e wrote: Jun 18, 2025 10:32 AM Are you thinking the throttle plate might be too open? Like more air than the TPS would account for? Roger on fuel pressure, I’ll get that done.
Yes, and if the TPS isn't adjusted correctly it will "buck" at low speeds, felt like the drivetrain was going to fall apart. There's a term for that, I forget what it is.

On the valve lash, isn't it supposed to be 0.012 on the rocker, 0.010 on the camshaft on a stone cold motor. My lash is set at 0.012 and the valves are quiet, idle is rock steady at 750+-. Others may have more input in this. 

My car did that horribly, almost not drivable at low speeds (parking lots,...). It required VERY soft throttle inputs. I reset the throttle plate, and reset the TPS, in my case I changed the fuel pump. Car is 100% better.
Fedaykin528e
Posts: 699
Joined: Apr 03, 2008 2:21 AM
Location: Portland, OR

Re: Weird misfire at low throttle, can't figure it out!

Post by Fedaykin528e »

Interesting I’ll try that. And I did go/no go with .008/.010 per this thread, sounds pretty good for an old M30 but they could be a hair tight.
viewtopic.php?t=102378
kojo96
Posts: 923
Joined: Mar 31, 2019 7:39 PM
Location: Pleasanton CA

Re: Weird misfire at low throttle, can't figure it out!

Post by kojo96 »

Fedaykin528e wrote: Jun 18, 2025 3:49 PM Interesting I’ll try that. And I did go/no go with .008/.010 per this thread, sounds pretty good for an old M30 but they could be a hair tight.
viewtopic.php?t=102378
All I know is if the valve gap is too tight, it would cause issues... 
4DSC
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Location: Atlanta, GA

Re: Weird misfire at low throttle, can't figure it out!

Post by 4DSC »

kojo96 wrote: Jun 18, 2025 2:21 PM
Fedaykin528e wrote: Jun 18, 2025 10:32 AM Are you thinking the throttle plate might be too open? Like more air than the TPS would account for? Roger on fuel pressure, I’ll get that done.
Yes, and if the TPS isn't adjusted correctly it will "buck" at low speeds, felt like the drivetrain was going to fall apart. There's a term for that, I forget what it is.

On the valve lash, isn't it supposed to be 0.012 on the rocker, 0.010 on the camshaft on a stone cold motor. My lash is set at 0.012 and the valves are quiet, idle is rock steady at 750+-. Others may have more input in this. 

My car did that horribly, almost not drivable at low speeds (parking lots,...). It required VERY soft throttle inputs. I reset the throttle plate, and reset the TPS, in my case I changed the fuel pump. Car is 100% better.
Not the core of the issue in this thread but wanted to comment on the importance of fine TPS adjustment when you get there -- 1mm of adjustment can make a noticeable difference. I had a very annoying slight bucking when going from off to on throttle under 1500rpm when the ICV is in play unless the throttle was finessed. Adjusted TPS to as close as I could get it when the throttle makes the slightest movement, and now it's perfect.
Federico
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Joined: Sep 16, 2013 6:38 PM
Location: Argentina / California

Re: Weird misfire at low throttle, can't figure it out!

Post by Federico »

Maybe an intake gasket or injector oring leak? I've had success hosing them down with spray grease to find leaks, but any fluid should work. White grease does last for a drive around the block.
ahab
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Re: Weird misfire at low throttle, can't figure it out!

Post by ahab »

I'd ohm out the spark plug wires. Despite their age, you should verify you're not dealing with a cracked insulator or flakey connection. The value itself doesn't need to be super specific, they should all be really close though and you should see about 6k ohms, give or take. Connect a meter (well) and then shake the wires and watch the meter for changes. I speak from experience chasing a nearly identical problem.
Mike W.
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Re: Weird misfire at low throttle, can't figure it out!

Post by Mike W. »

ahab wrote: Jun 25, 2025 6:26 PM I'd ohm out the spark plug wires. Despite their age, you should verify you're not dealing with a cracked insulator or flakey connection. The value itself doesn't need to be super specific, they should all be really close though and you should see about 6k ohms, give or take. Connect a meter (well) and then shake the wires and watch the meter for changes. I speak from experience chasing a nearly identical problem.
Misting with water can be good too. I've had them where the conductor was ok, but the insulation broke down. Couldn't figure out an occasional hard start until a windy rainstorm where it wouldn't start at all. I think misting would have shown a light show.
tschultz
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Re: Weird misfire at low throttle, can't figure it out!

Post by tschultz »

I was thinking the same as ahab. Measure and make sure they aren't the wrong resistance causing the coil to overheat or something.
Fedaykin528e
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Joined: Apr 03, 2008 2:21 AM
Location: Portland, OR

Re: Weird misfire at low throttle, can't figure it out!

Post by Fedaykin528e »

Thanks for all the great suggestions.

I set the throttle plate I think correctly... using the thinnest feeler gauge I have which is equivalent to a super fine cunt hair. Set the TPS, it's triggering correctly but is against the stop. Might give a bigger gap for throttle body and try again. No change in this behavior but it did clean up some throttle on off jerkiness a lot.

On the plug wires, good idea and I will test but the misfire is too even, I feel like it's not just one or two cylinders as I would expect for wire failure. I am leaning towards fuel pressure. Thinking about this the car does crank for a while before starting despite a new starter, and when I pulled the pump relay it dies almost immediately telling me pressure's not so great. I could borrow a gauge from my buddy but since I have a spare new pump ready to go I'll probably throw that in and see what happens. Stay tuned. 
Galahad
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Re: Weird misfire at low throttle, can't figure it out!

Post by Galahad »

check the in-tank pump too
Fedaykin528e
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Re: Weird misfire at low throttle, can't figure it out!

Post by Fedaykin528e »

Galahad wrote: Jun 26, 2025 6:37 PM check the in-tank pump too
Nah, that's just a transfer pump, it runs just fine all the way to empty but I'll check that it's running for sure.
Fedaykin528e
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Joined: Apr 03, 2008 2:21 AM
Location: Portland, OR

Re: Weird misfire at low throttle, can't figure it out!

Post by Fedaykin528e »

New main fuel pump, no change. The hunt continues. I had come across a very thorough technical write up on the Motronic systems, almost like a manual and now I can't track it down. Anyone know what I'm talking about?
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