Rough idle during first 20 mins

ritjeff · 10-29-2008, 02:08 PM

OK folks, hope this is an easy one to help diagnose. My 93 325i has a rough idle when it is cold outside, even after it gets up to regular operating temp. The higher RPMs are fine when I am driving, but if I come to a traffic light during the first 20-30 mins of driving in cold weather (lets say under 50 deg F), it is still there. It takes a long time of highway running for this to stop.

Now, common sense tells me this has something to do with the idle control sensor that controls air/fuel mix or something along those lines, but I was hoping someone that has experienced this would share some tips. Thanks in advance!!

fastfredracing · 10-29-2008, 04:18 PM

I would start by looking for small vacum leaks in the lines that go from the intake boot down to the idle control valve, and peek around under the intake for any signs of vacum leaks.

ktmrider · 10-30-2008, 07:34 AM

Might help to more fully describe rough idle - is it more of a surge up and down such as when you are coming to a stop from higher RPM or more like a lumpy idle akin to a modified cam engine ? Any hesitation upon acceleration or elsewhere in the RPM range , also ? My M50 had seen benign neglect that resulted in very poor idle quality covering all the above . Replacing the rubber L-boot at the intake , the tiny fuel pressure regulator vacuum hose between the manifold and the FPR , replacing both long vacuum hoses between the end of the valve cover and the intake manifold resulted in smooth idle and less off idle hesitation . The intake manifold can stay on the car for all these hose and boot replacements . The idel control valve hose coming into the intake manifold underneath has both a retainer clip and a strong O-ring holding it in place . You can look under the intake manifold to see if has come loose but it takes a lot of pull to remove it so doubt its loose there and its not prone to slip off at the other end either .

I also ran a couple bottles of Gumout injector cleaner through the fuel which helped resolve the off idle hesitation though probably contributed to fouling the O2 sensor . Later the idle surge condition returned which I found the electrical connection to the idle control valve was loose . I replaced the ICV anyway but I don't think there are very prone to fail .

ritjeff · 06:10 PM

I had the same issue last year when it got cold, but it disappeared as warmer weather approached.

During the spring, I started getting error codes and a rough idle again (different kind of rough idle- more like your surging description) that continued at speed and with engine hot. I discovered that the intake bellows needed replacing, along with the mass airflow meter. This straightened everything out until just recently when it got cold again.

So, I would describe the current idle as the engine 'missing'- like not firing on all cylinders (I guess what you would call lumpy), if you know what I mean. Since the problem goes away, I have not bothered looking around for spark plug related problems. During this time, when I hit the accelerator, it hiccups a few times and then gets going. At highway speeds, it feels normal. But not just the water temp has to be up to regular operating temp for it to go away. It is like the entire engine compartment has to get good and warm after driving on the highway for 20 or so minutes. Even before then, I only notice it at idle and takeoff.

I checked around the bellows and traced back the hoses underneath as best as I could tonite, but they seem ok. Tomorrow I will have time to get underneath it and look up.

Thanks for the suggestions. I am reasonably certain I am on the right track, but have not yet taken a good hard look at it. Just looking for those highly likely failure points to save time in troubleshooting.

ktmrider · 10-31-2008, 02:00 PM

The only vacuum related circuit that I think opens or closes as a function of engine temp. is the carbon cannister's purge control valve which resides below and behind the air filter housing sort of above and on top of the carbon cannister . The purge valve has an electrical connection and I believe one or two lines that use engine vacuum . Might want to check or replace those lines and the electrical connection . I don't recall if the purge contol valve has its own relay but the diagram under the fuse/relay cover may clarify . If so , might not hurt to at least pull the relay out and re-plug .

ritjeff · 10-31-2008, 05:35 PM

Thanks. I plan to tear into the thing this weekend. It has also occurred to me that while the idle eventually smooths out, the takeoff hesitation is almost constant in cold weather now.

Chilly · 11-03-2008, 07:59 AM

Sure sounds like a lean condition to me. Classic symptom: poor performance when cold, gets better when warm. I'd say there is either an intake tract leak or your cold idle stuff isn't doing its job.

How does the cold idle system work? What is the input (thermocouple somewhere)? Is it receiving an accurate input signal, or any signal at all?

But because the symptoms take so long to go away maybe you have an intake tract leak somewhere. While I'm not certain, I'd think the cold idle system should be out of the picture as soon as the engine hit normal operating temp.

Chilly

Chilly · 11-03-2008, 08:04 AM

Might be interesting to get a reading on your fuel-air mixture with one of those do-dads you run up your tailpipe. If you're lean only on cold start but normal otherwise I'd say it's a cold-air circuit issue. If it's lean all the time, but REALLY lean on cold start then you have something else going on. An intake leak is a likely suspect.

Are these engines prone to intake plenum cracks?

There are ways to diagnose intake tract leaks. With engine running the tech will spray something like WD40 around the intake tract and listen for a change in idle. With no leaks there would be no change. Not sure if WD40 is the right stuff. Do some research on what to spray.

Chilly

ritjeff · 03:06 PM

Thanks for the advice. I may have fixed it, but not sure yet. I pulled the hoses from the bottom of the intake bellows and then pulled it out to get a good look at the hoses. I traced the one back that goes underneath and it seemed to be firmly attached and intact. The one from the bellows to the end of the intake cover or crankcase ventilation hose, however, was kinda gunky- had a bit of dried sludge and actually some fresh oil at the bellows end. I removed the hose and determined that it was fine. I took both end adapters and cleaned them and the hose thoroughly. So far, the problem is gone. Not sure if this was the problem or a symptom of something more serious that I am unable to imagine.

ritjeff · 11-11-2008, 03:05 PM

Well, I have improved the rough idle, but it is not gone. I have also noticed that there is a hesitation even after warm when taking off from sitting still. This was not apparent when the weather was warmer.