Revs drop occasionally when idling

[Alistair Francis]

I was driving the other day (after not driving for awhile) and the 1996 Z3 was going great. When I got home I put the car in neutral to open the garage door and noticed that the revs dropped from the usual ~800RPM to much lower and the car almost stalled. The revs then picked back up to ~1000RPM before dropping back to the usual idle 800. It happened about three times while I was watching it that day.

I have driven since and it doesn't seem to happen at traffic lights or anywhere else. I managed to get it to happen one more time in the garage. The engine was warm and idling for half an hour to be able to see it happen.

I connected a code reader and got a DME 6C fault as well as a low battery fault which I cleared and now there are no faults.

I monitored the signals from the engine over OBDII and nothing stands out as strange. I have a 0.21ms multiplicative mixture adaption which seems right. The revs do fluctuate a fair bit around 800RPM with a plus or minus of 50 or so when idle, but I don't think I could tell without the code reader telling me.

Any ideas what would cause the occasional drop in the idle revs or how to diagnose what it would be?

Right now the only idea I have is to try to clean out the idle regulating valve and check for leaks in the air intake pipes.

 

[S J Fuqua]

I am not familiar with the 96, but your problem sounds very familiar to what I was seeing in my 2000 2.8.

I rebuilt the VANOS last weekend and now runs smooth as butter. Search this and other forums on VANOS rebuild, or check out the Beisan vanos rebuild web page and this might be your issue.

The issue is the VANOS seals age and leak by over time. The kit refreshes the seals and solves this problem. The fix is straight forward.

Good luck.

 

[Alistair Francis]

Thanks. I have the 4 cylinder, so I don't think it has a VANOS.

I replaced the air filter and cleaned out the idle valve and I haven't seen the issue since, so I'm hoping it's fixed.

 

[colb]

Idle control valve on 1.9 M43 and M44 engines are prone to getting sticky over time, easy to clean by taking it off and spraying carb cleaner into it and shaking it about to get the internals to move and get the benifit of the cleaner. Drain it out and let it dry, refit and see if that makes a difference to the idle speed. I did notice on another members M44 that his idle valve was making a ticking noise at idle, he cleaned it and it stopped ticking. Always good to check all the rubber pipes from Maf onwards for holes or splits, replacing any that are found with holes or splits. These engines are really dependant on a sealed vacuum syatem to run correctly. If you have the ability to view live data with a scanner then always have a look at the fuel trims and see what they are doing. High fuel trims are a giveaway for holes or splits in the rubber componants. Smoke testing the system from engine side of Maf onwards is the best way of finding air leaks if visual checks dont find any. Worst case I have sorted was a 2.2 with a plastic rocker cover, that had started to get cracks in it around No4 plug well, would never have found that without the use of smoke. That particular car was cured with a good used rocker cover being fitted. The fuel trims on that with the leaks was very high adding 20% more fuel because of the unmetered air the exhaust was seeing. When it was buttoned up and leakproof the fuel trims when viewed returned to normal.

 

[Oddly Godly]

When my 2.8 did this and also triggered the engine warning light it was a split air intake pipe, I replaced the two between the maff and the engine. it was the smaller consitinerd offshoot that had gone on mine. you had to have a good poke about to find it and I had the engine going at the time. When the revs dropped I could here it sucking in air from the split.

 

[colb]

Thats the classic place for the rubber to split Oddly, you got the EML light because the fuel trims went high as a result of the unmetered air being picked up in the exhaust making it think it was running lean and as a coinsequence it was making the car add more fuel at the injectors to richen the mixture. The rubber splits as a result of age and flexing over time.

 

[Oddly Godly]

Yes its one of those things when something quite small has a big effect. I am getting better mpg after replacing the hoses