Rough Idle Diagnostic help - 88 325is

[88schwarz]

Hey Guys,

I have a 88 325is 2 door manual (build date 11/87) and it has developed a problem of extremely rough idle. I have researched and seen what components are likely at fault,as I know this is a common issue, but just looking for help in narrowing it down.


A bit more info on the symptoms:

First indication of a problem was that I would get a check engine light at idle. As soon as I picked up the RPMs, CEL would go away. no real impact on drive-ability/idle/power.

I do not drive this car often as it is my Autocross/track Car (M20 engine is stock). Next problem was that the engine developed a bit of a "lope" at idle. Although the lope was consistent and even. Power seemed fine at all other rpms except for idle.

Car became hard to start and idled extremely rough when cold, and slightly better when warm. Engine would stall out on the first couple of start ups unless you gave it some gas. When able to idle under its own power, idles so rough the car shakes.

Next biggest issue (may or may not be related?) When cruising at highway speeds, car would lose all power, rpms would drop -- sometimes correcting itself (this happened twice on a 20 mile trip home from an autocross) On another outing this happened and had to coast to the shoulder and wait a few mins, and car eventually fired.

From the beginning of any problems I have been trying to fix this issue, just haven't been able to find the culprit -- although I have to be getting close...


Below is a list of things I have replaced with new parts:

Fuel pump, fuel sending units, fuel pressure regulator.
Spark Plugs,wires, distributor cap and rotor.
ICV
Throttle Position Switch (Adjusted per the Bentley specs)
MAF (cleaned and also swapped with used unit) - no change
Crankshaft Position Sensor
Fuel related relays/fuses

What I have not replaced:
Coolant Temp Sensor/Switch
Ignition Coil
DME(ECU)
Checked ground locations under dash or behind rear seat
performed Smoke test
replaced/rebuilt injectors


A few other points of interest:
I can unplug the MAF and the engine dies.

The ICV valve has been replaced with a new BOSCH unit and hums when the car is on. Idle does not change when I unplug the ICV. I have not hooked the ICV up to 12V to open/close it.

I have replaced the Throttle position switch and adjusted it twice per the Bentley Instructions. The switch has continuity. I can unplug the TPS while the car is running and it seems to improve idle very slightly.

I recently obtained a Peake code reader... first code was for the MAF so i swapped in a different one. That code went away and I got a code for the "full load switch" which i took as the TPS and adjusted it again. that code went away and last code was Coolant temp sensor. This was over the course of a few hours

My next step is to replace the ignition coil and the Coolant temp sensor. I know that the TPS, Coolant Temp sensor, and MAF all feed the DME, so I am wondering if that is actually my problem rather than all of these components.

Of course I have been looking for vacuum leaks during this entire period.

Car has 177K miles.

Thanks for any insight.

[jlevie]

The way to find intake leaks (which are probably the primary problem) is to have a smoke test run.

[88schwarz]

Thanks. Ill check for a shop that can perform a proper smoke test. Should I be concerned that the idle does not change when the icv is unplugged?

[nickmcc92]

Check your oil cap. Found out the seal was 20+ year old. Replaced it, now my idle is a little better.

[jlevie]

Thanks. Ill check for a shop that can perform a proper smoke test. Should I be concerned that the idle does not change when the icv is unplugged?

Don't worry about that until intake leaks are eliminated by a proper smoke test. Which means plugging the exhaust, replacing the AFM with an plug that has a nipple for smoke injection, and injecting oil smoke. The test is not complete until a smoke pressure of 2-4psi can be held for at least 5 minutes with no leakage of smoke. If gross leaks are found, those will have to be repaired and the test repeated.

[88schwarz]

Thanks for the replies. I did not mention it in my original post but I have replaced my oil cap and gas cap.

I do see some oil leaking from the oil filter housing and pan gasket. Maybe I will replace those before the smoke test since those leaks need to be fixed anyway.

[88schwarz]

I see that this is rubber hose is really shot on my car. It connects to the brass nipple on the bottom of the throttle body next to the TPS. Also eventually connects to the charcoal canister. Anyone know a Part number? Cant seem to find it on realoem.

[jlevie]

I think I remember some folks saying that the hose is NLA. You can use a piece of ordinary hose.

[CR325IS]

I am having similar PITA problems almost identical to yours. I have to give my car gas to start it when its cold or has been sitting for long periods of time. It takes usually 2-3 cranks plus my foot on the gas pedal to get it up and running then shakes and does pretty much the same thing as you described until warmed up then it is fine. I have replaced AFM, fuel pump, TPS, IVC, boot to the throttle body, plugs/wires are good, cap/rotor are fine, FPR,O2 sensor and that's all I remember off the top of my head. I haven't found any air leaks anywhere. The next course of action was to replace the coolant temp sensor in the expansion tank. Is this even worth it? I don't believe that its faulty but I also do not have a voltmeter. Can anyone give me any suggestions it is so annoying having to sit in my care revving it up until it can do it on its own when warm. Insight as to what my course of action should be would be a life saver. This is driving me insane. Thanks

[CR325IS]

I recently got a full load switch code. Any ideas what that is?

[88schwarz]

I recently got a full load switch code. Any ideas what that is?

The full load switch is the Throttle Position switch. Located directly underneath your throttle body. They are known to retain oil over the years and can fail.

If you are getting other codes, like I am, you may also want to check for intake/vacuum leaks as these types of leaks can throw multiple codes.

[88schwarz]

Here is an update on the problems I am having:

Did more research and decided I needed to check out my C191 connector. This is located under the throttle housing and is mounted in a bracket ( looks similar to the Oxygen sensor wiring harness).

I also replaced my coolant temp sensor (blue plug in thermostat housing). No Change.

It disconnected fairly easily and although I did see some corrosion, the pins were all tight and wires seemed to be in OK condition. I added some Dielectric grease and reassembled.

The hose I mentioned earlier that connects the charcoal canister to a check valve and then to the throttle housing I ended replacing. This did smooth things out a bit and believe it was one source of my intake leak.

I built my own "smoke tester". I realize this is not the "proper" way to test and I am not confident that I found all of my intake leaks. That said, I had fun building it and it did show me a leak that I was unaware of....

My DIY smoke test showed a significant leak around the #6 cylinder where the intake manifold meets the head.

New intake manifold gaskets have been ordered along with new throttle housing gaskets, oil return spring and O rings, new manifold studs (just in case) and new oil pan gasket and oil filter housing gasket seal kit. Will also be pulling my injectors and replacing all O rings.

I am hopeful that the intake manifold gaskets will solve my rough idle issue.

[3BEEPS]

I am having the same issue cel at idle and when I hit the gas the cel turns off. Also my tachometer stopped working on my e30dad rebuilt cluster and I'm wondering if its related.

[Coy325i]

Any followup on this?

I have similar symptoms with my 1989 325i convertible. Car starts and idles okay cold, but when I give it gas it acts like it's flooding. When I get it up to 25-3000 rpms, it's fine but getting there it makes a lot of noise, but not much power. When I come to a stop it idles very rough. When it warms up it runs very well, but it still idles rough, as if it is loading up. pull away and it stumbles until it clears and then runs great. Plugs are all gray and nice when I inspect them. Smoke test showed no hose leaks and in fact all hoses appear fairly new.

Can anyone explain to me how the Bosch FI system handles cold starts? Do I have something out of adjustment? I tested the ICV and it test good electronically, but I have ordered a replacement assuming it may have failed mechanically.

Ha d a mechanic test all components separately and they tested okay. Wrote pelican and they just said that it was high mileage, and that could be, but it runs great in every other respect except idling. In fact, it will still scratch the tires from first to second in drive; I just think it runs too good for that to be the problem... am I way off?

Thanks for any assistance.