M20 Surging Idle/Stalling

[Blowchacho]

I didn't know whether to put this in this category or the Tech category feel free to move this thread if it is the wrong place lol.

I have a 1989 BMW 325i Vert that has been plagued with wanting to stall on start up. The idle also surges from about 500 rpm to 1200 rpm. Sometimes mostly once I have been driving the car, when I come to a complete stop like a red light it will dip down to about 500 rpms and feels weak like its bearly keeping that. But once i start driving again i notice nothing wrong at all.
I got a radio installed a couple days ago which coupled with a almost dead battery and weak alternator killed the car. Now ever since then I am having this problem :/

I've seen many threads about this and have read through most of all of them it seems like and I keep coming up with dead ends

I have replaced as of now,
-New Battery
-Alternator
-Spark Plugs
-Fuel Filter

I have cleaned my ICV and checked to make sure it still hums/vibrates and it still does.
I also cleaned my Throttle body in this process.
I have checked most visible vacuum lines. I checked with soapy water and the Carb Cleaner test and no luck with anything. There is a small crack but no all the way through on my ICV rubber elbow boot, I don't believe this is what is causing this but I have ordered a new one to replace it anyways.

At this point I am a bit lost on what there is really left to do about this. Any help would be really appreciated lol.

[ADlBOO]

Mine did the same thing, I replaced the coolant temp sensor and now it idles flawlessly.

[jlevie]

Idle or hard starting problems are most commonly caused by intake leaks and/or
a sticky or defective Idle Control Valve (ICV). The only reliable method of
locating intake leaks is to have a smoke test run on the intake and crank case
and to test the brake booster with a gage and vacuum pump. The complete list
of possible causes of an intake leak is:

Intake boot
Throttle body gasket
ICV hoses & connections
Brake booster, hoses, and connections
Crank case breather hose
Evaporative control hoses, valve, and expansion tank
Fuel pressure regulator & hose
Injector seals
Valve cover gaskets & bungs
Oil filler cap
Dip stick o-rings
Oil return tube o-rings

While leaks in some of those can be found by inspection or by spraying carb
cleaner on suspect areas, not finding leaks that way doesn't eliminate the
possibility. Only a smoke test will really work.

Once the possibility of intake leaks is eliminated, the ICV needs to be
removed and cleaned with carb cleaner until the vane inside moves freely. When
the ignition is switched on you should be able to feel vibration from the
ICV. If no vibration the ICV is bad, there's a problem with its wiring or
connector, there's a problem with the TPS, or the DME (or Idle Control Module
(ICM) on an ETA car) is faulty.

For the DME (or ICM) to control idle, the idle switch in the TPS must work
correctly. The switch should close when the throttle stop is 0.030-0.060" off
the idle stop screw. Hearing a click doesn't mean the TPS is working. Check
with a meter or test light.

The fuel system should be tested via the suite of tests in the Bentley manual
as invalid rail pressure can be a contributor to idle and starting problems. A
simple injector check is to pull the injectors, jumper the fuel relay to run
the pump, and see if the injectors are leaking. You can also point the
injectors into a towel, remove the coil wire, and crank the engine to see if
all of the injectors appear to be spraying in a similar fashion. The best
approach to possible injector problems is to have the injectors cleaned and
flow tested. Since raw fuel can or will be released in these tests, have a
fire extinguisher handy.

While a bad check valve in the high pressure pump can result in longer than
normal cranking, if the fuel system is working as it is supposed to the rail
will reach normal pressure in a few turns of the engine. A weak pump, clogged
filter or leaking FPR in conjunction with a failed check valve can result in
longer cranking and/or idle poroblems.

The O2 sensor can be a contributor to idle problems. The O2 sensor is a
scheduled maintenance item with a useful life of no more than 100k. If the
sensor has that mileage or more (or is of unknown age), replace it.

The AFM can be a contributor. If the vane doesn't move freely or the
resistance track is worn the DME may be receiving invalid data from the
AFM. And if someone has fiddled with the bypass air adjustment the DME may be
unable to stabilize idle. The bypass air adjustment should only be adjusted
per the procedure in the Bentley and with an exhaust gas analyzer. And even
then everything else associated with engine management has to first be
operating properly. If the AFM becomes a suspect, replacement with a good used
unit is the best approach.

Improperly adjusted or malfunctioning valves will affect idle and starting. As
can compression issues from ring or cylinder wear. A valve adjustment is
called for every 15k. A useful diagnostic is to run compression (dry and wet)
and leak down tests on the engine. Aged ignition wires, plugs, distributor
cap, or rotor can cause problems. Insulation does break down with time and
heat. And since the youngest E30 is going on 20 years old, if the ignition
system is original or the plugs are old replacement is indicated.

Although not usually a problem, a bad DME temp sensor is a possibility. That
generally won't cause an unstable idle, but can cause hard cold or hard hot
starts and/or a rough idle. As can problems with the timing reference
sensors. Although not commonly encountered, a bad harmonic balancer on an
M20B25 or M30 engine will cause problems.

When all other possibilities have been eliminated and idle or starting
problems persist, replacement of the DME, or if applicable the ICM, is
indicated.

In many cases the cause of problems will be a combination of factors. So it is
important to test and repair all of the possible causes.

[Blowchacho]

Thanks for the reply I'll be going to the parts store tonight to pick up a coolant temp sensor. In the mean time I will attempt to clean the icv again. It hums/vibrates already so I know it's electrically fine. But once I attempt the coolant sensor replacement and if that doesn't seem to fix anything I'll move onto the other things you have listed. Thanks for the responses lol.

[jlevie]

While you could have a bad ECT sensor, the odds of that being the problem are slim. The number one item in the list above is the most likely cause of your problems. Start at the top of that list and work to the bottom without skipping any steps. The items to check are in order of probability.

[Blowchacho]

I cleaned the contacts on the sensor a bit and my car does seem to be idling a bit better at dead stop like stop signs and red lights. But I still have problems on initial start ups. Its not taking a long time to crank or start up. It fires right up, but on its way down the RPM range it will sorta stumble all over the place and nine times out of ten stall out. Thanks for the list of things to check. I will be calling around to see if I can get a smoke test done somewhere around here to see if I have a vacuum leak.

[ADlBOO]

My other recommendation is a Bentley manual and a multimeter. Bentley has a lot of reference numbers for what resistance/ voltages that sensors should have.

Do you have a check engine light on? If so, what codes are showing?

There was also a thread about how to do your own smoke test using a coffee can/ rag and an air compressor and some fittings.

[Blowchacho]

Haha I've been meaning to pick one of those up, but keep forgetting about it when I am not on a forum haha. And I am not getting any Check Engine light, but then again the light might be burned out for it.

Sadly I don't own a air compressor lol :/