Hello to all and Happy New Year!

My 1991 300E with 105K miles on it has suddenly developed a problem. Every now and then, at idle, be it in drive or park, the engine revs up approx. 300-400 rpms and then drops down. 5-10 seconds later, it'll do it again. This may happen 5-10 times and then stop for the rest of the day. If I shut down the car, it may not do it the rest of the day. It has gone 3-4 days without it happening. I find that I really have to keep my foot pressed on the brake at stop lights because the car wants to surge forward and there have been times where I put the car in neutral when it is happening and it will surge and stall.
I've been trying to change out my cruise control amplifier with one from a junkyard and it seems like when I put the junkie in is when the problem started, although I can't be sure. Can this have anything to do with it?
I've read that it can be a failing mass air flow sensor. Where exatly is that so I can get in there and clean it with carb cleaner?
Any suggestions would be appreciated.

Nick

Your car doesn't have a mass air flow sensor. Those come on the '94 + cars.

Check the idle control valve, and check the molded rubber hoses that connect to either side of it. You may have disturbed the seal on the hoses when doing your work with the cruise amp, creating a vacuum leak. Use your carb cleaner to clean any gunk out of the ICV, and replace the molded hoses of they have hardened or cracked. Use dealer hoses only, no aftermarket. Also check the big black rubber boot on the underside of the air mass housing.

Might want to check the air flow sensor at the front of the air cleaner boot with a DMM.

__________________
'02 BMW 325i
'85 300D 450k
'93 190E 2.6 170k(killed by tree)
'08 Ducati Hypermotard 1100S 6k
'06 Ducati S2R800 14k(sold)

Your car does have an air flow sensor, but a mechanical one. You can see the large flap under the air filter assembly. The flap descends as more air rushes into the engine. It 'senses the air flow'.

As already said, I would start with simple, cheap issues like eliminating every possible inlet vacuum leak. That may improve things but you are in the dark trying to diagnose idling problems if there are leaks.

Bonne chance.

RayH

Thanks for the suggestions!

First, I took out the cruise control amplifier. Would the lack of the amp produce the surgeing idle and stalling problem?

Secondly, I cleaned the throttle body and the large flap under the air filter with carb cleaner. I also removed he hoses from the idle control valve, removed the ICV, and cleaned that with the carb cleaner.

The result was that the car performed beautifully for about 2-3 days and then the surging idle and stalling problem returned.

Nick

Sigh. You know what I meant.

Yes the M103 CIS has a mechanical air flow meter, a huge contraption beneath the air cleaner that some have mistaken for a throttle body since it looks like one. It actually sits on top of the throttle body. It has a potentiometer on the side of it. The signal from that potentiometer is used by the engine computer.

What it doesn't have is what has become known as a traditional MAF or mass air flow sensor. This is typically a black plastic cylinder with a fine wire suspended inside of it. This takes up much less space than the mechanical beast and does the same job. These units were used in the M104 HFM engines starting in 1994.

__________________
'02 BMW 325i
'85 300D 450k
'93 190E 2.6 170k(killed by tree)
'08 Ducati Hypermotard 1100S 6k
'06 Ducati S2R800 14k(sold)

A thorough cleaning of the components should have resolved the idling surge issue .
Did you clean the throttle plate base ? Now this area is visible if you remove the top air flow plate and a wide open throttle will bring the plate to a vertical position. If there is oil or carbon accumulation at the base then it effects the ICV operation .
Hope it helps
AD