Loping motor, big cam? No, so what is wrong?

[Hoffbrohaus]

Ok I'm struggling to solve this problem and have gone through the Bentley troubleshooting, searched the forums and found jlevie posts everywhere but still no luck.

I have this weird "loping" feel to my motor. It feels like a big cam in a Chevy 350, and it's not just at idle. It is less noticeable throughout RPMs but still there. I tried timing the lump frequency but it doesn't seem consistent, closest I could time it was maybe every 4-5 seconds.

• It’s a stock M20B25, 1990 325is. Tach doesn't move during the stutter, lump, whatever. At idle it is holding around 650rpm
• AFM Gauge doesn't change much, at warm idle it fluctuates between 13.5 and 15 but mostly in the mid 14s
• Performance of the car is still good. Cold starts can be a little off with it struggling to stay running but it does.

I Checked or replaced the following:

• Crank position sensor
• Coolant Temp Sensor & Switch
• AFM resistance good
• Throttle Position sensor resistance and adjusted
• Fuel Pressure test, new regulator and new fuel filter
• Fuel pump proper volume
• Injectors cleaned by MEPEH
• New Wires, new plugs, new coil
• Cap & Rotor looked ok
• Smoke tests revealed no intake leaks
• Checked resistances at ECU plug
• Cleaned out ICV and verified the door closes with voltage
• New o2 sensor
• New to me, pre-owned stock chip in ECU
• Valve adjustment per Bentley specs, maybe too tight in AZ?

Any other ideas you have or symptoms you may need to know to help me diagnose?

[Andy.B]

I have had a bad valve adjustment cause this issue, but if you adjusted them to the spec's in the manual (cold) then that is unlikely to be the issue. If you haven't, I would take a hard look at the cam profile of each lobe. It sounds like the problem is an inconsistent misfire. I have had bad(worn) camshafts result in this issue.

[BruceJ]

worn eccentrics on rocker arms?

[MercyFlush]

If it kinda runs smoothish but makes a weird noise I'd pull the plugs and check gaps!

Mine did this just the other week and it 'ran' but made a weird sound like you described. Turns out upon installing the rearmost spark plug I had accidentally degapped it when installing it!

Check the plugs it'll take 5min and if that's not the issue THEN move on.

[Bullhead]

I also just had a similar issue and my fuel injectors were clogged.

[Hoffbrohaus]

If it kinda runs smoothish but makes a weird noise I'd pull the plugs and check gaps!

Mine did this just the other week and it 'ran' but made a weird sound like you described. Turns out upon installing the rearmost spark plug I had accidentally degapped it when installing it!

Check the plugs it'll take 5min and if that's not the issue THEN move on.

Good point, I just checked and they were gapped fine. :(

I also just had a similar issue and my fuel injectors were clogged.

Running rebuilt injectors done by MEPEH

I have had a bad valve adjustment cause this issue, but if you adjusted them to the spec's in the manual (cold) then that is unlikely to be the issue. If you haven't, I would take a hard look at the cam profile of each lobe. It sounds like the problem is an inconsistent misfire. I have had bad(worn) camshafts result in this issue.

I will have to do this next. My only other thoughts are dying ecu or maybe my head gasket leak could be contributing (small amounts of oil on the block)

[Mthrizzle]

Is there a misfire on any cylinders ? Check and report back...you may have weak valves that need attention