Stumped eveyone thus far

[yukon101]

Greetings everyone, I have come from the E32 forum where I have stumped everyone there and they suggested I try here. I have a 94' 740i with the M60 V8. Here is the issue, while the engine is mid-temp (just above blue all the way to just before full operating temp) and there is a low ambient temperature of 35 and below if I come to stop too quickly it will buck repeatedly from 10-0 and the engine will try and run but stall once at a stop, if I stop slow enough it does a bucking from 10-0 but will barely stay running. At dead cold and op temp its fine, above 35 degrees ambient its perfect at all temps. This all started to occur after a TPS replacement trying to correct a low but not rough idle. Work done thus far, newer Lucas E38 injectors that stopped it from misfiring then stalling. Rebuilt intake manifold with all new gaskets and rubber that fixed the cold start misfire. Initially cleaned a once stuck ICV then replaced anyway, no change. Fuel pump is good, FPR is good. Full ECU and TCU reset, no change. MAF is good. Transmission (5HP30 if different) has had a fluid and filter change 3k miles ago and was perfect before the TPS replacement (old one was reading 1.6k ohms when full closed and replaced with HELLA). The updated valve body was installed by the previous owner at 150k miles. A friend of mine (80s Toyota mechanic) suggested the coolant temp sensor to the DME, new one ordered as they are rather inexpensive.

[Mykk]

Does it have the same symptoms at all fuel levels in the tank, full to almost empty?

[yukon101]

It does it at all fuel levels.

[gtopaul]

Seems to me you still have a TPS problem. They can be touchy to set up correctly. I haven't done one in a while but it seems like you have to set it so it clicks just as the throttle is moved.

[Mykk]

The m60's don't have an adjustable TPS, the bolt holes and bushings in the tps bolt holes make it a fixed mount.

Have you reset the adaptations in the DME for it to learn the new TPS? IE: unplugged battery ground, let sit, touch ground cable post to positive post to discharge capacitors. Reconnect ground, fire and let run

[yukon101]

I'm confused? Mine is of the potentiometer type with no adjustment. I have done a full DME reset but to no avail.

[moroza]

Greetings everyone, I have come from the E32 forum where I have stumped everyone there and they suggested I try here. I have a 94' 740i with the M60 V8. Here is the issue, while the engine is mid-temp (just above blue all the way to just before full operating temp) and there is a low ambient temperature of 35 and below if I come to stop too quickly it will buck repeatedly from 10-0 and the engine will try and run but stall once at a stop, if I stop slow enough it does a bucking from 10-0 but will barely stay running. At dead cold and op temp its fine, above 35 degrees ambient its perfect at all temps. This all started to occur after a TPS replacement trying to correct a low but not rough idle. Work done thus far, newer Lucas E38 injectors that stopped it from misfiring then stalling. Rebuilt intake manifold with all new gaskets and rubber that fixed the cold start misfire. Initially cleaned a once stuck ICV then replaced anyway, no change. Fuel pump is good, FPR is good. Full ECU and TCU reset, no change. MAF is good. Transmission (5HP30 if different) has had a fluid and filter change 3k miles ago and was perfect before the TPS replacement (old one was reading 1.6k ohms when full closed and replaced with HELLA). The updated valve body was installed by the previous owner at 150k miles. A friend of mine (80s Toyota mechanic) suggested the coolant temp sensor to the DME, new one ordered as they are rather inexpensive.

I don't understand what "bucking from 10-0" means. What are the units of those numbers?

My M62 had a cracked intake boot, and would be prone to stalling just as you put it, at mid-temp (also occasionally at full temp) and somehow more often after coming to a rapid stop. My hunch is that has something to do with manifold vacuum. At any rate, a new intake boot cured the stalling completely.

It could just as easily be a bad temp sensor, though if it runs fine at full temp that seems unlikely. It can be tested with an ohmmeter through its operating range.

TPS can also be tested. The M6x one is not adjustable. I don't think it's your problem.

[yukon101]

Those units were supposed to be mph, it's hard for me to write stuff as my brain is already 3 pages away while my fingers are still at word one lol. I've checked the intake boot and it is fine, the only potential vacuum leak is the PCV isn't completely on but is about a mm from locking in. Everything else I've checked for leaks. I yanked the electrical connector off the temp sensor and it had a ton of crud in it so, the engine ground strap is barely left in existence with all the corrosion on it (new one ordered). I have a hunch this ground is playing into it.

[moroza]

Bad grounds can cause a very wide range of problems. I wouldn't waste much diagnostic effort (let alone expenses) until that's fixed.

[Neel]

I dont know much about e38's but grounding issues will drive you mad especially on e34's.. so Yes +1 grounding issue.. also +1 for coolant temp sensor to DME, have you inspected the dme to make sure all of the soldiering is in tact ?

[Sir Montalbon]

I'm leaning towards vacuum leak. Have you tried a smoke test? I couldn't figure out my slight rough idle so last week I finally built a smoke test machine out of a paint can and some incense (check YouTube). Turned out to be a tiny leak from the boot to throttle body that an extra twist of the hose clamp solved.

That said, it may still be tough to find in the narrow temperature range where the problem is occurring. Good luck!

[yukon101]

At first I was thinking that ground was only for the starter and alternator until I saw a cable coming out of the wiring duct grounding to the rear of the engine while checking the coolant sensors then it all suddenly clicked and explains all the intermittences of the issue and the low idle. I pulled off the connector to the DME temp sensor and there was some gunk in there but the contacts didn't look burnt but I still cleaned it up with my electrical cleaner if its not bad now it'll go bad at some point anyway and its $8.

[ross1]

I wouldn't think the temp sensor would cause the bucking, poor running yes but not what you describe here.
Seems too lean between start of closed loop and full warm. Are you positive no vacuum leaks?

[yukon101]

The idle is dead steady at 510~ and is not lumpy no matter the temperature. I have combed through all the vacuum lines the only possibility is an intake gasket but that would cause a misfire to which I've already fixed. It has newer fuel injectors from an E38 that had very very little wear to them. I've wacked the MAF with the back of my hand a few times and it doesn't change anything. I used to get a 1221 code before rebuilding the intake, newer injectors, and tps replacement but haven't seen it since.

[ross1]

" This all started to occur after a TPS replacement "
Have you tried switching back to the old one? I can see a bad throttle position signal causing the ICV to act up

[yukon101]

The old TPS was pretty bad pins 1 and 3 read 2.6k ohms and pins 1 and 2 read 1.6k ohms at closed. The new one was reading perfectly at spec +-.2 and there's definitely a ton more throttle response. I'd try and swap the old one but my curiosity over took me and I took it apart, it's okayish but still out of spec. With the old TPS the idle and ICV were exactly the same but if I understand correctly was probably richening the mixture a touch thinking the plate was open a little.

[yukon101]

Before someone mentions, the O2 sensors are brand new Bosch replaced and verified by the previous owner. (I kinda bought the car through a BMW master tech who has been working on this car since 2000 until I got it)

[m60power]

My first guess would be a vacuum leak somewhere. Maybe a crack in a hose or intake part that shifts ever so slightly when the car decelerates rapidly? Combined with the cold ambient air and the throttle closed, a higher percentage of "un-metered air" to "air going past the MAF" may come through the leak spot. Does it do this if you throw the trans in neutral before stopping hard?

[yukon101]

I've replaced every rubber part of the system minus the boot but the boot is good. If its in neutral it does not do it. If i push on the gas a little while it does it and take my foot off the brake it just falls on its face and does the same thing if I floor it however it will drop a gear and act like nothing had ever happened.

[demetk]

Aren't there 2 TPS's on this engine?

[yukon101]

No, just the one.

[demetk]

My old 530 had two of them, one for the Motronic and the other for the ASC?

[yukon101]

I didn't think about the ASC+T throttle plate, I don't think its broken as the ASC system works great.

[yukon101]

I was doing some testing, while its cold throttle response is mediocre. It will still climb in revs but off the line it feels like im driving a giant heavy car with a tiny engine so it is definitely running lean when cold, in my garage with it cold if i blip the throttle with my hand it takes a second to really respond how it should. I should be getting my new coolant temp sensor in the next two days which I am hoping will solve this issue.

[moroza]

So how about those engine grounds?