m20b25 stumbles hard and won't recover, stalls once clutch depressed kicked in N

[Golfer_705]

my M20B25 with 173 motronic in my 84 325e just started to intermittently stumble under light throttle cruising. the motor begins to miss, if i stay on the gas it continues to miss. when the clutch is depressed the motor dies, with the clutch released the motor refires and runs smooth as silk. the fuel pressure is good, the; fuel tank, fuel gauge sending unit, intank & inline pumps and lines have 35,000kms on them. the issue happened several times in 50kms of driving unfortunatly this started 300kms from home, with the motor running i unplugged the coolant temp sensor (blue 2pin ecm sender) thinking it would put the engine into open loop, the motor didnt run perfect but it didnt stumble or stall again for the trip home. once i made it back i reconnected the coolant temp sensor and went for a short drive, the stumble and stall returned almost immediately. with the engine running again, coolant temp sender connected i unplugged the 02, the motor ran, very low on power but didnt stumble or die. the motor uses roughly a L between oil changes.

im thinking its the 02 sensor but i hate to just throw parts at it. im hoping someone would be able to confirm my suspicions or suggest another test.

on a semi related note. when had my exhaust replaced at a local shop 20,000kms ago the o2 sensor bung was moved downstream rougly 18", does anyone foresee an issue with this? is it worth moving back upstream?

thanks for all the help

[jlevie]

If the O2 sensor has 100k on it, it is due for replacement whether it affects the problem or not.

The first action in solving this would be a smoke test to look for intake leaks.

[603Racing]

If it's doing this, it should throw a code.

If you're stopped, does it idle well? It could be more of a coincidence of what you're doing while your driving, rather than stalls with the clutch depressed. What happens if you just put the car in neutral while driving, without using the clutch? Does applying the brakes have any affect?

[Golfer_705]

The motor only has 30,000kms on all new gaskets, seals, head bolts, vaccume lines, intake elbow, I will do a smoke test for elimination purposes, but I don't believe there are any leaks. In between the intermittent stumbling the motor still pulls strong, although the exhaust doesn't seem quite as loud as before. I am running a new cat with 25,000kms on it also. The o2 was replaced 30,000kms ago when the motor was swapped in. However the motor has burned roughly 6 L of oil in that time.

[Golfer_705]

If it's doing this, it should throw a code.

If you're stopped, does it idle well? It could be more of a coincidence of what you're doing while your driving, rather than stalls with the clutch depressed. What happens if you just put the car in neutral while driving, without using the clutch? Does applying the brakes have any affect?

Stopped it idles smooth, under acceleration it also runs smooth, exhaust seems quieter but still pulls hard. The stumble only seems to happen under light throttle cruising. Once the stumble starts it will not recover until restarted. The car is a 5 speed, kicking it in neutral had the same effect as depressing the clutch. Pressing the brakes under the stumble kills the engine. Backing off/releasing throttle will also kill engine once it starts to stumble.

[603Racing]

So no codes?

The fact that this seems very odd would lead me to believe it's an electrical/sensor issue. Bad Ground maybe. Perhaps O2. If you turn off the car and restart it does that temporarily fix it?

Does it only do it after the engine is warmed up? If so, then could be a temp sensor or c191 issue or even a DME issue.

[dnguyen1963]

Your symptoms describe minor vacuum leaks. How are you so sure that you have none? I would check the elbows going into the throttle body and the brake vacuum master for leaks.

[Golfer_705]

So no codes?

The fact that this seems very odd would lead me to believe it's an electrical/sensor issue. Bad Ground maybe. Perhaps O2. If you turn off the car and restart it does that temporarily fix it?

Does it only do it after the engine is warmed up? If so, then could be a temp sensor or c191 issue or even a DME issue.

No codes that I am aware of, I tried a stomp test and got nothing. An electrical/sensor issue seems very logical.

Grounds all test with 0 ohms resistance. I was thinking it was a bad O2 but I wasn't sure how to properly diagnose it. As soon as the car is restarted, either key off and restarted or just releasing the clutch in gear the issue is temporarily resolved. Some times resolved for a few minutes up to about a half hour.

I forgot to mention earlier, yes the issue only happens once warmed up. The temp sensor was replaced new when the motor was reassembled (30,000kms ago). I have several others that are used i will try tomorrow. I haven't looked at the c191 connector since the motor was swapped I will look at it again tomorrow. I also have a spare 173 DME I will try tomorrow.

Thanks for all the help!

[Golfer_705]

Your symptoms describe minor vacuum leaks. How are you so sure that you have none? I would check the elbows going into the throttle body and the brake vacuum master for leaks.

Since the issue started I have done several cigar smoke test revealing nothing. I tried the smoke test with the brake master connected, disconnected and directly to it, no sign of smoke. I can't guarantee I don't have any because I'm in a small town with no access to a proper smoke machine. The elbows going into the TB are epoxied in place and also showed no signs of smoke.

I appreciate all the feedback so far!

[K1CKBAC]

Maybe it is the gaskets around the brake booster since the master cylinder shares the reservoir on the brake booster.

[Golfer_705]

Maybe it is the gaskets around the brake booster since the master cylinder shares the reservoir on the brake booster.

I don't think it's a vacuum leak, the shudder and shake will start once the engine is warm and light throttle. Any other situation; cold start, idle, under load, hard acceleration it's great. Since the issue started my exhaust seems muffled. How do I diagnose my cat and 02 sensor?

[dnguyen1963]

A simple way to check for cat is with an IR thermometer. A good working cat should show at least 400F at the outlet (> 500F is preferable) and about 100F differential between inlet and outlet of the cat. Make sure that you measure the actual cat and not the shield.

[Golfer_705]

A simple way to check for cat is with an IR thermometer. A good working cat should show at least 400F at the outlet (> 500F is preferable) and about 100F differential between inlet and outlet of the cat. Make sure that you measure the actual cat and not the shield.

Great suggestion! I'm sitting at 395*F just out the downpipe and again at the O2, down to 365*F at the cat inlet, and down again to 300*F at the cat outlet. Looks like I need to ditch the cat and see if that clears things up.

Thanks for all the help!

[603Racing]

Could be the cat.

When you do the stomp test, do you get the "all clear" code or nothing at all? Not getting any codes can be a sign of TPS or other electical problems.

[Golfer_705]

Could be the cat.

When you do the stomp test, do you get the "all clear" code or nothing at all? Not getting any codes can be a sign of TPS or other electical problems.

When I try to do a stomp test nothing happens at all. Would the lead for the check engine light be 1 of the 3 or 4 wires that needed to be spliced in under the glove box? I don't believe the check engine light is working right, at least I have never seen it on since the swap other than for the active check pannel (head lights, tail lights, coolant level... Etc.

The TPS sensor tests within spec and is adjusted correctly

Sorry for the late reply, and thanks again for the help!