m60b40 missing, low idle, back firing

[Jonsku]

Thanks, I'll give it a shot. I have a pic of the spark plug on the first page of this thread, looked pretty normal to me.

But do all of them look similar?

One interesting thing, at the drag strip last night, after two of my runs the throttle hung at 2k RPM's. I slowed down at the end of the track, and the throttle just stayed pegged at 2k. I could put the car in gear and it would bring the RPM's down, but it would accelerate the car and as soon as I pushed the clutch in it would go back to 2k. I turned the car off, turned it back on, and the idle returned to normal.

This gives me even less faith in my questionable ICV. I'm going to grab another one.

That's not a ICV fault, but a speed-signal fault. You can try that somewhere on open road - as soon as you go over certain speed (if I remember correctly it was somewhere around 180km/h) and DME won't get good speed-signal, it will get mixed up and start messing around with the rpm's and will also start running very lean (temps go up).

From where have you taken speed-signal to the DME? I've taken it from the instrument cluster ("OBC connector") and seemed to hold fine up to 285km/h (rev-limiter cut in at dyno, 6800rpm / 6th gear :) ).

[JGood]

Thanks, I'll give it a shot. I have a pic of the spark plug on the first page of this thread, looked pretty normal to me.

One interesting thing, at the drag strip last night, after two of my runs the throttle hung at 2k RPM's. I slowed down at the end of the track, and the throttle just stayed pegged at 2k. I could put the car in gear and it would bring the RPM's down, but it would accelerate the car and as soon as I pushed the clutch in it would go back to 2k. I turned the car off, turned it back on, and the idle returned to normal.

This gives me even less faith in my questionable ICV. I'm going to grab another one.

[Jonsku]

The TPS is connected with the correct plug. It's the one with the spring loaded clip, from what I understand. The ACS plug has a PITA clip that requires a pick to remove. I plugged my multimeter into the DME connector, on the corresponding TPS pins, and then moved the throttle plate, and watched the ohms go from ~4k to ~1k at the DME. So it is hooked up correctly.

Honestly, this thing acts EXACTLY like it has a TPS issue, so I see your point with that. I just can't seem to find any fault in the TPS system.

Yeah, then it's the right one and seems to work ok (though one could check the TPS resistance values somewhere, just to be sure).

I'm not sure what you mean by a loaded circuit test of the coil packs. I unplugged them and turned the key on, and each harness connecter gets 12v. Actually, 6 of them got 12.01 volts, and 2 got 12.11 volts. Which was intriguing, but not concerning, IMO. Is there anything else to test with the coil packs? I thought you couldn't test the actual spark side of the coil, other then seeing spark in the plug?

You could test the signal wires from DME to coils (three wires going to coils, one is +12v, other is the signal from DME and third is shield-ground).

"loaded circuit test" means that you don't just "beep" the wires (check continuity), but actually load the wire with +12V (connect battery over the wire, be sure to load the wire by e.g. H4 bulb). Then by measuring voltage over the wire you can see if there are losses (=is the wire broken somewhere).

That is very handy method, as continuity test doesn't actually tell you anything on the real condition of the wire. It could well be "broken" somewhere - continuity test tells that it's ok but as soon as the wire gets loaded it loses 2/3 of the voltage somewhere, causing problems.

Anyway, that's just one idea. Do the spark plugs look ok?

[JGood]

Steam your gas tank and use HO gas.
GL

I don't know what that means.

[barry]

Steam your gas tank and use HO gas.
GL

[Jean]

Don't give up, need to figure out what is causing this

[senorcarey]

Thanks, I didn't know that. Is there still a spec for throttle plate opening, like 0.002", or whatever it was for the earlier cars? Or is it completely shut?

[JGood]

Did you, perhaps inadvertently, knock the throttle plate/TPS out of adjustment? I had this problem on my M30B35 and it makes a significant difference in running.

Just a thought...

Good luck,
Scott

The newer TPS's are non adjustable. It's just a potentiometer, there is no on/off. It gets ~1k to ~4k ohms as the throttle is moved from open to closed, per Bentley specs.

Ahh, shouldn't post to threads when having a great hangover :grin: Sorry for that.


Anyway, what about the exhaust gases? Are they ok, or highly off (would think so..)? Do the plugs look ok? TPS - is it connected with the correct plug (two plugs with similar length wires, another one is for ASC's TPS).

Have you tested that each and every coil gets good signal from ECU & +12V? Not just circuit continuity test, but proper "loaded circuit test" .. ?

I don't have a way to measure exhaust gases, I don't have a wideband o2. But I can tell it's very rich.

The TPS is connected with the correct plug. It's the one with the spring loaded clip, from what I understand. The ACS plug has a PITA clip that requires a pick to remove. I plugged my multimeter into the DME connector, on the corresponding TPS pins, and then moved the throttle plate, and watched the ohms go from ~4k to ~1k at the DME. So it is hooked up correctly.

Honestly, this thing acts EXACTLY like it has a TPS issue, so I see your point with that. I just can't seem to find any fault in the TPS system.

I'm not sure what you mean by a loaded circuit test of the coil packs. I unplugged them and turned the key on, and each harness connecter gets 12v. Actually, 6 of them got 12.01 volts, and 2 got 12.11 volts. Which was intriguing, but not concerning, IMO. Is there anything else to test with the coil packs? I thought you couldn't test the actual spark side of the coil, other then seeing spark in the plug?

[Jonsku]

Yes Jonsku, I blindly threw parts at it before troubleshooting anything, didn't I?

I did read the codes, as I've mentioned. The only standing codes are the canbus code and the purge valve code. Also the lambda/o2 codes, while the o2 sensors are unplugged.

The idle speed isn't the issue, bumping it won't fix the actual problem.

Ahh, shouldn't post to threads when having a great hangover :D Sorry for that.


Anyway, what about the exhaust gases? Are they ok, or highly off (would think so..)? Do the plugs look ok? TPS - is it connected with the correct plug (two plugs with similar length wires, another one is for ASC's TPS).

Have you tested that each and every coil gets good signal from ECU & +12V? Not just circuit continuity test, but proper "loaded circuit test" .. ?

[senorcarey]

Did you, perhaps inadvertently, knock the throttle plate/TPS out of adjustment? I had this problem on my M30B35 and it makes a significant difference in running.

Just a thought...

Good luck,
Scott

[JGood]

Yes Jonsku, I blindly threw parts at it before troubleshooting anything, didn't I?

I did read the codes, as I've mentioned. The only standing codes are the canbus code and the purge valve code. Also the lambda/o2 codes, while the o2 sensors are unplugged.

The idle speed isn't the issue, bumping it won't fix the actual problem.

[Jonsku]

Read fault-codes.

Cheat DME so that it thinks AC is on all the time.


Not the wisest thing to change parts blindfolded..

[JGood]

Tried another set of coil packs, no change. Also verified that I'm getting 12v to the coil pack harness connectors.

That was the last part that I could try. I've replaced everything else at least once:

ICV
TPS
CPS
Temp sensor
MAF
O2 sensors
DME
coils
plugs

I've tested the harness, end to end. I've vacuum smoke-tested and compression tested the engine.

There is nothing else...

As of now, it runs without stalling with the o2's unplugged. But it backfires loud as hell, throttle on/off is jerky, and it runs VERY rich.

I'm going to order another TPS and ICV (although I've already tried 2 of each, and both ohm out as being good)

[JGood]

Installed the rebuilt injectors, no change.

I have to yet again contradict myself - the stalling DOES occur with the o2 sensors plugged in, and the stalling DOES NOT occur with the o2 sensors unplugged. I do get the hesitation/stumbling under very light throttle load between idle and 1500 RPM's regardless of the o2 sensors. It seems that with the sensors unplugged, the idle catches just before it stalls. It sounds like it wants to shut off, but just won't drop below 500 RPM's.

I'm going to replace the ICV and TPS (for the third time, even though they test out fine) and also try to pick up a bunch of coil packs and boots at the junkyard to try. I'm not buying 8 brand new coils and boots because I don't have $1k to throw at the engine on a whim, and the current parts seem to be working fine, it's just a last ditch effort.

After that, I'm calling it quits and driving it as-is until winter.

[dirty30]

My DME is a 404 from a 94 740iL. No decel/ loss of idle issues with my swap.