88 528e races at cold start...

BMW's do run at fast idle - about twice unloaded warm idle when cold - for

30 - 60s after starting.

Having the AC on also increase the warm idle by about 200rpm.

Precise idle speed depends on engine smaller engines generally idle faster than large ones.

I have had normal idle for over 10 years before this racing idle speed started.

2300rpm at cold start is too high I believe. Something is not right. Which sensor could cause this?

Cheers, Frank

Assuming it's an injection engine check for air leaks round the inlet system first. Unlike carburettors, these cause a fast idle.

Thanks Dave, I will check for a vacuum leak tomorow. If I can not find any, it is time to check out various sensors. It is injected, and it has run perfectly for the last ten years or so. Not time for a new one yet.

Cheers, Frank

Um, I'm no expert on foreign vehicles, but if it's injected, there is probably an Idle Air Control valve just like or similar to injected vehicles made by G.M.

If the IAC is toast (and stuck in one position), it would cause the same exact symptom as a vacuum leak.

Spraying WD-40 around any potential areas that could be leaking should help discover if you have a vacuum leak. Using carb cleaner would be OK around metal, but would further dry out rubber hoses.

Also, on a vehicle that is 10 years old, checking the vacuum hoses would be a good idea and would be a very likely source of a vacuum leak. Spraying WD-40 or on the hoses would yield the same discovery if there is a leak.

1. Check vacuum hoses - use WD-40 to help diagnose
2. Check for vacuum leaks around gaskets - again use WD-40
3. Check and replace (if necessary) the IAC valve ( or step motor) - whatever it's called on BMW's

Good luck. Give us a report on what you found when the vehicle is repaired.

But, he says once the engine has warmed up the idle rpms are normal. Further, if the engine is warm, a restart does not produce the fast idle condition.

I suppose an intake manifold leak *might* "heal" once the head has heated up, but it seems much more likely there's an engine temperature sensor that has gone to Heaven...

That was my guess.

I'm not certain what variety of injection was fitted to that model. Some early systems didn't have ECU controlled idle. But I'm sure some searching should give details of how it works on that particular one.

Nope, temperature sensor doesn't affect that at all. About the only reasonable possibilities are a vacuum or intake leak or a gunked-up throttle that won't close, or a gunked-up idle control valve. Certainly wouldn't help to clean the idle control valve and clean the throttle body as well. You should do that every few years for preventative maintenance anyway.

You'd think that the throttle position switch would be able to cause this sort of effect too... but in reality it doesn't seem to. Disconnect the switch and verify for yourself that the behaviour doesn't change, though.

--scott

I bet you meant "Certainly >would< help..."

But how does the FI controller used on this particular model know how to enrich the system during an actual cold start? Even carbureted systems have temperature-sensitive enrichment (typically a bimetal mechanically operated choke valve). Surely there is *something* on this car that responds to temperature to perform an enrichment?

I meant it certainly wouldn't hurt, sorry.

It does indeed use the temperature sensor. But remember the behaviour you are seeing from the injection system is the _opposite_ of what would happen if the sensor was wrong. If the sensor was reading too low a temperature, cold starts would be fine, but the engine would run poorly once warmed up. If the sensor was reading too high a temperature, cold starts would be difficult because the engine wouldn't be rich _enough_.

You can check the cold and hot resistance of the sensor on the block and the wiring to the sensor. Wouldn't hurt to clean all the connections and put dielectric grease on everything. But I don't think it's the issue.

--scott

Thank you everyone, for pointing out a lot of possible causes re my racing start-ups. I am going to check out everything that was mentioned, and report back here. My aim is to do it in steps, so that I can be reasonably sure what caused it (If I get it fixed that is). Tha coolant level is OK. Probably I start by cleaning , checking an greasing all relevant electrical connectors. Then I check for vacuum leakes. Then the mechanical parts of the throttle body system. I am actually looking forward to do this now.

Cheers, Frank .

The coolant temperature sensor tells the ECU to compensate for cold starts. However if this was faulty it would effect the running - either not starting when cold if short circuit or running badly when hot through being over-rich if open circuit. They're basically just a temperature sensitive resistor.

Yes, of course it is. Scott's explanation made sense to me, so unless the OP's warm running condition isn't out of sorts, would seem to rule out a defective temp sensor...

My information on that model is patchy but what I have says it uses the Motronic First Edition. And some versions of that had a stand alone fast idle circuit using an extra air valve. Has yours definitely got an ICV?

=A0 London SW

according to others on the web, the 88 528e doesn't have a cold start valve like previous versions. Check out this link:

Right - bang goes my theory. The fast idle on those early systems was controlled by an air valve which closed progressively by a mixture of coolant temp and an electrical heating element.

Hi again,

I cleaned all electrical connectors I could find, or that are at least in the area of the business end of the intake system. I found a few with a lot of corrosion and green deposits. So I cleaned them, and lubed them with this electrical grease.

Then I found two tiny vacuum hoses that had little splits near their connection. They reacted to my WD-40 testing. I replaced them, as they were too short to be trimmed, and the material looked sort of brittle-ish.

Checked for totally free movement of all linkages, as well as what looks like the air-flow meter. Also inspected that stepper motor connection is fine, and checked for vacuum leaks at those rubber elbows that hook it into the intake system.

Now, ....all I have to do is, wait for tomorrow morning, when the engine is cold, to try it out.

Thanks for everyone who responded. Everything was approached by you with common sense and logic. I am grateful for that.

Cheers, Frank

Good news,

It now starts up without the racing that it had done before.

After warming up, the idle is just a tad slow, and a bit rough sounding at the tail pipes. There is the odd bump....bump-bump.........bump...This goes away when I just slightly increase the idle speed, by hand or by foot. Perhaps the computer is still sorting out the new conditions.

Nothing I could not get used to, that is for sure. I am already very happy with these results as it is.

Thanks again to all that helped point me in the right direction.

Cheers, Frank