m60b40 missing, low idle, back firing

[JGood]

Is the engine totally stock? Was it running right before the swap?

DME chip from auto- or manual transmission car?


Compression tester costs something like $10, so why not check it just to be sure?

Reading, or even just clearing the fault codes would help you forward ..



Some backfiring is normal, don't know why but almost all the swapped V8's like to run a bit rich and pop the exhaust.
Of course proper mapping on dyno/road fixes that..

Engine is stock, except for the lightweight flywheel.

I tried a DME with a stock chip, then put an aftermarket chip in, then tried another DME with the stock chip.

I'll do a compression test.

I have read and cleared the fault codes, many times. Currently only getting a "CAN message" code (CANbus, probably missing AGS signal from auto trans), and "Evap purge valve" code (I don't have the purge valve hooked up).

I'm familiar with the typical backfiring/rumble, but this is a distinct, single, loud "crack" every time I get back on the throttle after lifting off of it for a few seconds. Just one time, every time, from any RPM range.



The main thing is, why does all of this NOT happen with the o2 sensors unplugged? It runs great with them unplugged, just a jerky transition when going on/off throttle.

[RobertK]

The main thing is, why does all of this NOT happen with the o2 sensors unplugged? It runs great with them plugged in, just a jerky transition when going on/off throttle.

Almost makes me think it might be a cam timing issue. Did you mess with the timing chain at all? Any chance it could be off a tooth on either side?

[JGood]

Almost makes me think it might be a cam timing issue. Did you mess with the timing chain at all? Any chance it could be off a tooth on either side?

Never touched it.

I can't think of any reason that incorrect timing would affect the way the engine runs only with the o2 sensors plugged in, then it runs great with them unplugged.


Oops, I meant it runs great with them UNplugged, in that previous post.

[Jean]

Currently only getting a "CAN message" code (CANbus, probably missing AGS signal from auto trans), and "Evap purge valve" code (I don't have the purge valve hooked up).

I wonder if you need to run a 5/6speed chip in the dme or do something to the harness to make it think it's in a 5speed car...but then again you are ONLY getting these with o2 sensors plugged in??

[Jonsku]

Engine is stock, except for the lightweight flywheel.

That's what I thought. And you know what, that is the reason for your problems!

Stock flywheel ~20kg, lightweight flywheel ~6...8kg. DME won't give enough fuel on motor-brake as it thinks there's much more rotating mass on the crank than there really is.


Had the same issue on mine (got the engine mapped which fixed the problem). The various chips on sale won't help a bit..

You can live with the LW flywheel and original chip by cheating DME that AC is on (ground to some of the three AC pins, don't remember which one). That makes DME give the engine more fuel / higher idle. Helps but won't cure totally..

I can't think of any reason that incorrect timing would affect the way the engine runs only with the o2 sensors plugged in, then it runs great with them unplugged.

Oops, I meant it runs great with them UNplugged, in that previous post.

O2 unplugged => engine runs "on the safe side" => more fuel and rpm => doesn't stall even though there is much less rotating mass.

I wonder if you need to run a 5/6speed chip in the dme or do something to the harness to make it think it's in a 5speed car...but then again you are ONLY getting these with o2 sensors plugged in??

There's no difference in DME's between transmissions (apart that the other comes only with B30/B35 and the other with B40/B44).

[JGood]

That's what I thought. And you know what, that is the reason for your problems!

Stock flywheel ~20kg, lightweight flywheel ~6...8kg. DME won't give enough fuel on motor-brake as it thinks there's much more rotating mass on the crank than there really is.

Had the same issue on mine (got the engine mapped which fixed the problem). The various chips on sale won't help a bit..

You can live with the LW flywheel and original chip by cheating DME that AC is on (ground to some of the three AC pins, don't remember which one). That makes DME give the engine more fuel / higher idle. Helps but won't cure totally..

O2 unplugged => engine runs "on the safe side" => more fuel and rpm => doesn't stall even though there is much less rotating mass.

I don't know about that. Motronic 3.3 is well capable of knowing RPM, throttle load, crank and cam position, and of course o2 readings. It shouldn't need to depend on preset values for the time it takes for the engine to rev down. Furthermore, engine braking should be relatively unaffected by flywheel weight... it's dependent on the engines internal compression/friction restrictions vs. the cars momentum.

And to top it off, I'm running VERY rich at idle (to the point where I have low fuel pressure). So I don't think a lack of fuel is an issue here.

[JGood]

So I unplugged my o2 sensors last night to verify it will run good without stalling. Well, something must have changed, because it now acts exactly the same with them unplugged as it does with them plugged in. Maybe the last time I tried this, I didn't give the DME a long enough time to adjust or something.


I did a compression test. I'm not 100% sure how this should be done. I read that you should do ~5 cranks. When I did that, I got 150-160 PSI on all cylinders. But, if I kept cranking, the needle would keep rising. It would only stop rising after 12-15 cranks, which put each cylinder in the 170-195 range. Specifically:

185
170
180
175
195
190
185
195

This was done dry, with a 10 amp battery charger hooked up to keep power equal throughout the duration of the testing.

I read something about adding oil, so I added a squirt of oil to the highest and lowest cylinders. The 170 cylinder went to 185, and the 195 cylinder went to 220.


I don't know what all of this means. I assume since there are no drastically low cylinders, compression is not the reason my car shuts off in such a repeatable, specific, and controllable way, but still runs great under load.



EDIT: Just read that the throttle should have been held open while testing. I'm guessing that may have an affect on hold long it took to get up to max psi?

[Jonsku]

I don't know about that. Motronic 3.3 is well capable of knowing RPM, throttle load, crank and cam position, and of course o2 readings. It shouldn't need to depend on preset values for the time it takes for the engine to rev down. Furthermore, engine braking should be relatively unaffected by flywheel weight... it's dependent on the engines internal compression/friction restrictions vs. the cars momentum.

And to top it off, I'm running VERY rich at idle (to the point where I have low fuel pressure). So I don't think a lack of fuel is an issue here.

How much does your flywheel weight?

I think you have two separate problems here that get mixed;

-Mixture too rich, reason = ?
-LW flywheel => engine stalls much easier (try cheating DME that AC is on, does that make a difference to the behaviour?)

[JGood]

How much does your flywheel weight?

I think you have two separate problems here that get mixed;

-Mixture too rich, reason = ?
-LW flywheel => engine stalls much easier (try cheating DME that AC is on, does that make a difference to the behaviour?)

17lbs vs 38lbs stock.

That's the thing, It will idle steady at 500 RPM all day, no issues. If I rev it, it will return to idle and still idle perfectly. It's only when I'm engine braking to a stop, and allow it to dip below 1500 RPM's before putting the clutch in, that it will stall. Then I restart it, and it sits at 500 RPM, steady.

And I think the on/off throttle jerkiness is related.

You may be right, it may be stalling easier due to the LTW flywheel, but I think there's a bunch of other people running LTW flywheels without an y issues, so I think if I can fix the actual problem, this won't be a concern.

I'll go through the ETM and try to find the A/C connecter and see if I can get the idle bumped up, to at least temporarily make it easier to drive while I sort this out. Thanks for the info.

[RobertK]

Comp numbers are awesome and I didn't even think of the flywheel as a factor on a V8 but its the same with principle with those who run 8lb flywheels on M5x/S5x.

I had to have the idle on my S50 bumped up to 900rpm to compensate for my LTW flywheel.

[Jean]

There's no difference in DME's between transmissions (apart that the other comes only with B30/B35 and the other with B40/B44).

Are you 100% sure about that? There were mutliple threads and owners on bfc e34 forum that have done auto to manual swaps and had issues with running auto dme vs one from a manual ?

[JGood]

Are you 100% sure about that? There were mutliple threads and owners on bfc e34 forum that have done auto to manual swaps and had issues with running auto dme vs one from a manual ?

Have any links? I tried searching for info on this and didn't come up with much.

I'm getting the canbus error code on the DME you sent me as well. Is it from an auto car? AFAIK, canbus was only used to connect the automatic AGS to the engine DME in e34/e32's.

[Jonsku]

Are you 100% sure about that? There were mutliple threads and owners on bfc e34 forum that have done auto to manual swaps and had issues with running auto dme vs one from a manual ?

Yea, but I understood you were talking on differences between 5- and 6-speed manual transmission chips..

Have any links? I tried searching for info on this and didn't come up with much.

I'm getting the canbus error code on the DME you sent me as well. Is it from an auto car? AFAIK, canbus was only used to connect the automatic AGS to the engine DME in e34/e32's.

My cabrio's DME is from automatic transmission M60 and it seems to work very well indeed.. Same with couple of friends who have V8's in E30, so it shouldn't be a problem..

[dirty30]

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

[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.