M88/3 Temp Gauge, High Idle and Fuel Mixture Fun

[gte619n]

Hey guys,

I was hoping you fine fellows could help me troubleshoot the high idle coming out of my 1985 M635CSi. I haven't driven the car in a while so I threw it on the trickle charger, let it go overnight and it fired up like a pro! Idle went to the usual cold start 1500 rpm but then... just never came down! I don't have a sniffer to really test, but it also smells like the car is running richer than I remember and it feels like the temps in the engine bag are really hot (maybe due to the richness). Drivability seems to be fine, as the thing ran around all day without issue.

One thing I could do was put my ear in the engine and rev the car by hand. It's hard to describe, but the car pulled as usual. When I left the throttle go, it seemed like it WANTED to go back down to the usual 800rpm range, but then some sort of a clicky/jumpy sound seemed to re-engage the 1500rpm condition.

Any recommendations on where to start is much appreciated. The temp gauge inside the cabin has only worked sporadically on this car so I can't really point to that. Could this be the cold start valve not closing causing these problems? If so, is that perhaps due to a faulty temp sensor? I figured I'd try to ground out the leads on the temp sensor and see if the gauge jumps but I need a buddy and I'm not sure the gauge in the car works anyways. Any idea what sort of ammeter reading I should be getting out of that?

Thanks!

E

EDIT: Trying the ol' dipstick removal trick lets the car fall back down to ~1000rpm. Better, but not really usual.

[raykoke]

Hmm.

You might check to see if the throttle position sensor contact closes at idle.

There may be a slight click you can hear when opening the throttle (motor off of course).

That's all I got. For now.

Good luck,

Ray

[wjtesquire]

Check your throttle cable and the linkages. Mine was hanging up and doing the exact same thing you describe.

[wattsmonkey]

Obvious thing to check is the viscous fan.

[gte619n]

Hey fellas, so for the quick stuff:

1. Throttle cable looks great. No snags or anything like that.
2. I hear the microswitch click as expected when I open/close the throttle in the engine bay.

When you say "viscous fan", what should I check there? Are we talking about the engine fan with the clutch in the front?

Thanks for any tips!

PS. Shop Manual for the M88? I found some references to the shop manual for the E28 M5 for M88 stuff, but the links to actually get the manual were all dead. I feel like such a dork asking these basic questions without going through a manual.

[wattsmonkey]

Yeah, the main cooling fan visible between rad and block. Stick a rolled up magazine in the blades when hot. If fan stops, coupling is shot. Do this carefully, obviously!

[gte619n]

HYPOTHETICAL: Let's just say you wanted to check the viscous fan but you were a big whimp when it came to sticking things into plastic blades spinning at high RPM. Let's also say, hypothetically of course, that you had the clutch fan tools for an E36. If you were to remove the fan, is there a way to check it without it being installed?

Thanks,

Evan "Definitely not afraid of spinning death fans" Ruff

[conez0]

I'm not aware of any way to test the fan uninstalled.

What you could do is spin the fan by hand when its cold - engine off. Note the resistance and about how far it "coasts."
Get the engine warmed up (make sure you leave the hood closed so the fan clutch heats up too). Shut the engine off and try spinning it by hand again. It should offer significantly more resistance and coast very little.


Sticking a rolled up piece of paper in it while running is not a big deal though. The trick is to use the rolling to give it rigidity and not use a bunch of paper, just a few sheets of newspaper rolled up is all it takes and you just brush it against the fan. Try it when its cold to get a feel for how the fan stops when the clutch is not engaged.

[gte619n]

Hey guys,

Started the car and it ran pretty rough, a lot of smoke out the exhaust, etc. Everything still smells pretty rich.

I (bravely) tried the magazine trick at several points over about a ~15 minute period. At the start, I could get it to stop no problem, but then as it warmed up, it got a bit harder to stop, but I could still do it the entire time.

Does this mean my fan is shot? Is this a straightforward replacement?

FWIW, idle started off fine but then gradually kept creeping up and up until it was at around 1200 rpm after ~15 minutes.

Thanks!

E

[brickwhite]

How would the Fan clutch cause a high idle or rich running?

If the fan is stuck close, the extra force to run it?

For a M88 or S38 engine these would be the things I would check for Rough running or high idle.

With 6 individual throttle bodies there could be one sticking or leaking.

- ICV (idle control valve) under the throttle bodies. Maybe dirty or stuck
- Center adjustment screw in the middle of the TB's for cyl #3 and cyl #4 that controls the idle speed
- Temp sensor
- Vacuum Leaks.... hose or throttle bodies good smoke test should be able to find it
- O rings around the 7th injector on top

my m88--

[gte619n]

Is the temp sensor that runs the gauge the same sensor that is an input into the engine management system?

I tested the temp sensor by grounding the leads. The top lead had no effect and the bottom lead moved the gauge <10%. I believe my gauge is bad in the instrument cluster.

[brickwhite]

I don't think so.



There are 3 sensors on the exhaust side of the engine on the uppper coolant passage.

pretty sure one is dash temp and one is ECU temp. ECU temp should be #11

http://www.realoem.com/bmw/enUS/showpar ... Id=11_0780

[Stoffie]

Don't forget about the auxiliary air slide valve either, burried between block and intake plenum in a bundle of hoses.

http://www.realoem.com/bmw/enUS/showpar ... Id=13_0260 item number 9. Part nr. 13631272166

One temp sensor is for the gauge inside, one is for the Motronic and isn't the third one for controlling the cold start sensor ?

[Brucey]

One temp sensor is for the gauge inside, one is for the Motronic and isn't the third one for controlling the cold start sensor ?

yup, that is correct. The third one is better known as a 'thermotime switch'.

FWIW I think the OP has multiple issues.

1) there has to be an air leak or a faulty component that governs the airflow into the engine; you can't make an engine run faster without letting air into it. You can test for faults by crimping off flexible hoses in turn, looking for leaks etc.

2) if the engine is running rich this indicates that there is a fuelling fault or perhaps a sticky injector.

Some things to check

a) that the CTS is reporting accurately to the motronic unit (if it isn't the engine will run incredibly rich)
b) that the vacuum compensation hose to the FPR is connected properly.

cheers

[davem6]

The idle speed control valve draws air from the intake plenum through the hose located between cylinders 4 and 5. Pull this hose off and plug both the hose and the plenum then start the engine. If the idle speed drops then the extra air is coming in through the idle valve - look at the valve itself and electric control parts. If idle stays up look for air leaks and throttle position issues.

[sharkade]

The temp gauge not working is part of the problem. A bad fan clutch causes high engine coolant temps, in turn the ecu dumps more fuel causing rich conditions. Symptoms = cold starts easy and hot starts difficult. Prolonged rich conditions damage o2 censors causing rough idle.

one of many possibilities

Edit: scrap what i said, i've been reading a lot more on symptoms, causes and fixes and wow these motors are finicky.