m60b40 missing, low idle, back firing

[JGood]

Trying to get some issues sorted on my m60 swap, I had some of this posted in my build thread, but figured I could expand the visibility in here. The following is what is basically going on:


I have a low (400-500) RPM idle. Should be more like 600-700. More importantly, I have a pretty bad misfire at low RPM’s, it’s a stumbling and hesitation that isn’t enough to buck the car around a whole lot, but you can feel it a little and hear it mostly. It seems to be between idle and 3k or so. And it only seems to be at very light throttle loads. Maybe 1/4 throttle and less. It may be present at higher throttle loads, but the engine power overcomes it to the point where it's not very noticeable, if that's the case.

I also notice that while decelerating in gear using engine braking in that RPM range (0 to 3k), I get some backfiring/rumbling and weird hesitation or missing from the engine. Slowing down as I pull up to stop signs and stuff, is when it’s most noticeable/annoying.

The car runs great at full throttle, from idle to redline. It pulls very hard and doesn’t break up at all. I installed an aftermarket chip, and that made no difference in the drivability issues.


Here's what I've checked:
-Crank position sensor: Supposed to be 550 ohms, I’m getting 550 ohms (brand new OEM part).

-Cam sensor: Supposed to be 1280 ohms +/- 10%. Currently I’m getting 1290. A few days ago I was getting 1470, not sure what's up with that. The specified range is 1100-1400.

-Idle control valve: Supposed to be 12, 12, 24 ohms checking across the three terminals. I’m getting 12, 12, 24. The valve spins freely and moves when voltage is applied. I tried another one that tested good, no change.

-Coolant temp sensor: Supposed to be 2.1-2.7k ohms at 76* and 270-360 ohms at 176*. I’m in those ranges at those temps.

-Intake air temp sensor: Same as coolant sensor, and I am within those ranges.

-Throttle position switch: Supposed to be approx 1k to approx 4k ohms, smoothly variable from closed to open, and 4k across terminal 1 and 3 while closed. I’m 1.1k to 3.8k, and 3.8k closed. I switched with another TPS, same exact results. Out of the car, the range is .7k to 4.8k (throttle body is the limitation of the travel, so it doesn’t bottom out the potentiometer.)

-MAF: If I unplug it while running there is a noticeable change, the engine barely runs. It won’t start with it unplugged. There are no other tests that I’m aware of to check functionality. I did buy MAF cleaner and clean it.


I unplugged the coil packs one by one with the engine running, and each one caused a pretty bad misfire.

I have not checked anything in the fuel system. I’m using the same pumps I have been for a few years now. I am using a new OEM fuel filter, and I tried switching it with another new OEM filter, no change. Have not done any fuel pressure tests, or checked the regulator.

I haven’t found any vacuum leaks. I’m not running a charcoal canister, so I have the line for that plugged. I removed the booster vacuum line and plugged it on the engine side, no change. Tried wiggling everything I could on the engine intake area, no change at all. All of those parts/gaskets are new… TB gasket, intake gaskets, OSV tube o-rings, etc…



So, that's where I'm at. Any suggestions on what other things to check or verify would be GREATLY appreciated. It is safe to assume that none of the engine parts worked prior. The CPS was bad right off the bat, for instance, and I was told this engine came out of a running 740i and has no issues. Many of the parts didn't come with the engine and are collected from various sources (MAF, ICV, wire harness, etc... all came from different cars).

[JGood]

Still no luck.

I spent about an hour driving the car tonight, and I think I need to redefine my symptoms. I wouldn't call it a missing or hesitation. It's more so, the engine doesn't react well to small throttle inputs below 3k. If I'm idling, and push in the accelerator, and it goes to 2.5k, and i hold my foot dead steady, it will drop, in steps, back to 1k or so, and make some funny rumbling/miniature backfires as it does so. If I'm driving in 5th at 2k, and am just giving it maintenance throttle, not accelerating or decelerating, it will sound like it's missing ever so slightly, and the RPM's will drop if I hold my foot still, almost as if it thinks I'm giving it less throttle, even though I'm not.

First thing I did was check that both grounding straps were installed, one in each valve cover. furthermore, I ran a 14 gauge wire between each coil pack, to ensure 100% grounding. I checked resistance between a coil pack and as far as my multimeter leads would reach, a door latch, and only had like 1 ohm. So my coil packs are 100% grounded. I also checked my engine harness ground. It's solidly grounded. Engine ground is a brand new cable, and the surfaces were stripped.

Test all 8 coil packs, 1k ohms between outer pins.

Tested all 8 plug boots. 6 were OEM, 2 were "made in spain". All of the OEM read 1.7k-1.8k ohms, the 2 others read 2.0k ohms.

Checked that all 8 injectors were firing with a stethoscope (I didn't know how else to check, lol)

I removed the fuel filter just in case, no change.

I swapped the MAF, no change.

I pinched off the vacuum line on the FPR, didn't make a difference. No idea if that's relevant to anything.

I disconnected the coolant temp sensor and took it for a drive... made things worse, actually shut off a few times.

Checked plugs, all of them basically look like this:






The only other things I can think of:

-CPS (test good when cold, 1290, tests 1440-1470 hot, which is slightly high)

-Fuel pressure regulator (I'll do a fuel pressure test first)

-Coolant sensor (I need to pull this out of the car and test it in water with a thermometer to get an accurate reading, but it seems good when using my infrared temp gauge on the sensor)

TPS - This is supposed to be 1.0k-4.0k ohms (approximately, according to Bentley). Is 1.2k at closed throttle good enough? I put in another TPS that reads 1.4k at closed, and it made no difference at all.

DME - I don't have a spare to try. That seems like a last ditch, throw parts at the car effort, but I'll try it if someone thinks it could be an issue.

[Jonsku]

Ditch the chip, that's the most probable reason for the problems..

Did you install it yourself (opened the ECU)? Opened ECU's have caused a lot of problems as they are very sensitive units = even small impurities / moisture will break them.

Source unfiddled ECU and try it out, that's my bet.

[JGood]

All of these issues existed before I installed the chip. I was hoping the chip would make a difference... it didn't. ECU was unopened prior.

[RobertK]

The spark plug doesn't look horrible but definitely shows signs of running a little rich.

Thought may a leaky injector but.. have you done a recent compression test on this engine?

This is how I found out my S50 had lower than normal compression. Just enough to start but it would randomly miss at idle and idle very low, but anything above 1000rpm would rev beautifully but the power would drop when moving the car.

[JGood]

The spark plug doesn't look horrible but definitely shows signs of running a little rich.

Thought may a leaky injector but.. have you done a recent compression test on this engine?

This is how I found out my S50 had lower than normal compression. Just enough to start but it would randomly miss at idle and idle very low, but anything above 1000rpm would rev beautifully.

The plugs have also been in there since the first time I fired this engine up, when the coolant sensor was unplugged, the CPS was bad, etc... so I guess it's not a fair representation of how it's currently running...

No compression test yet. I suppose it wouldn't hurt.

My friend is dropping off a fuel pressure tester tonight, and I just ordered a Peake scan tool which should be here tomorrow. That should at least get me pointed in the right direction.

[dirty30]

Was that a nikasil or alusil motor?

[JGood]

Alusil. I really don't think it's a compression/wear issue... the engine makes amazing power, and pulls from 1k to 7k under full throttle without a single glitch. It's just the on/off throttle transition below 2k that feels weird... it's acting like it has a bad TPS or a vacuum leak or something.

[slammin.e28]

Possibly #3?



Although I think those are more associated with a whistling noise, but I don't see why it couldn't cause your symptoms (vac leak).

[JGood]

I guess it's possible. I didn't replace it (one of the few things I wanted to do, but didn't do). I did put a new seal and o-ring on it.

[slammin.e28]

I guess it's possible. I didn't replace it (one of the few things I wanted to do, but didn't do). I did put a new seal and o-ring on it.

The little circle piece is the diaphragm that goes bad. On the N62s you can replace just them (also on M44s).

IIRC you have to replace the whole assembly on M6Xs.

IDK, just throwing that out there. I could be wrong.

[CoreyZ]

You can always try spraying some carb cleaner back there to check if it is indeed a vacuum leak.

[Jonsku]

All of these issues existed before I installed the chip. I was hoping the chip would make a difference... it didn't. ECU was unopened prior.

Ok, misunderstood that one then.

The oil-separator could be one cause for the problem, though it "should" make the engine run idle quite bad if it's broken.

[JGood]

True Corey, I haven't tried the spray test yet, I'll do that.

I'm going to try swapping the o2 sensor connectors tonight (I'm not sure which one goes to the left bank, and which one to the right bank). If that doesn't make a difference, I'll be doing the fuel pressure test. If that looks good, I should receive my scan tool tomorrow, and that should answer some questions.

[JGood]

The problem seems to be isolating itself to an idle and throttle on/off issue. Today I drove the car, and every single time I left off the gas, it shut off. It starts right back up, idles dead steady at 500 RPM's, then I give it gas, and it will run all day without missing a beat, until I leave off the gas and let it return to idle, it just shuts off. It does it 75% of the time.

I checked fuel pressure, getting 3.25 bar at idle and 3.5 bar when revving. Spec is 3.5 bar +/- .1 bar. I watched the gauge as the car shuts off after revving, pressure jumps from 3.25 up to 3.5 as it dies, then stays at 3.5. I assume this is ok? That's like 3 PSI off from spec.

I also swapped o2 sensor banks, no change.

Also swapped ICV's again, no change. I did notice something interesting though. When the car is sitting there, up to temp, and I shut it off, if I check the ICV resistance immediately, I get way high numbers ( like 30, 60, 30). If I let it cool down, they drop to normal spec (12, 24, 12). Both of my ICV's do this, so I assume it's normal.

I also sprayed carb cleaner around every sealed area I could find in the intake system, and got no surges at all.

Also installed a new OEM coolant temp sensor, no change.



Guess I'll wait for the Peake scanner to arrive.