spinedoc2304

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

spinedoc2304

Anyone?

katzenfinch

Had a similar problem with our 1991 300E 2.6. A new crankshaft position sensor seems to have solved it.

ZaxW126

Sounds like your Idle Air Control Valve is going bad. Fairly common on the older C.I.S. E cars. However, the airflow position sensor could also be going bad. More likely the Idle valve. -Zach

spinedoc2304

Thanks Zach!

I took out the Idle air control valve yesterday; shot it with carb cleaner and really cleaned it out well and then lubed it with WD40. I noticed a pretty large crack in the rubber hose on the underside of the hose at it's attachment to the valve ( the one on the right as you're looking at the unit from the front of the car going to the throttle body) and replaced that. I also replaced the coolant sensor.

Where is the airflow position sensor?

Nick

ZaxW126

With the air cleaner removed, it is the large round disc that the air flows through to get to the throttle body. That is a basic description, it is actually the small black sensor mounted to the housing for the aforementioned disc or plate. The plate moves an arm down as more air flows into the engine pushing the plunger on the bottom of the fuel distributor, allowing more fuel to the injectors. On CIS-E units, the small sensor on the side of the housing tells the control unit the position of the "arm" which correlates to fuel distributor plunger position. A worn contact plate on this sensor can effect fuel pressures, idle quality, and can even cause intermittent stalling when coming to a stop. There is also the possibility that the injector seals could be leaking, drawing in false air, throwing off the idle speed. A smoke check of the intake tract will reveal leaks. We use a special smoke machine in our shop.

spinedoc2304

Thank you VERY much for the information! I really need to get this thing fixed as it is becoming dangerous. My 17 year old was backing my car out of the driveway this morning when the idle surged and she almost backed into a parked car. It is happening more and more frequently.

I'm going to check the airflow position sensor and the injector seals. Can I spray WD40 or propane at the injectors with the car running and if the idle changes it would indicate a vacuum leak?

rayhennig

Mine's an M04 but ...

The one on the right is not critical, it's the one on the left you should worry about That's the one on the 'metered' side. Mine is shot and I've bound it up with gaffer tape and it's much better but still very suspect. Sadly, on the M104 at least, it's the one that's difficult to replace.

Bon courage.

RayH

spinedoc2304

OK...so I went to the local pick and pull because their website said that they had a 1988 300E (mine is a 1991 300E) in the lot. Most of it was picked apart but the idle control valve was still there. Last week I priced one out at $220 and pick and pull wanted $15. Why not?

While checking for vacuum leaks with the car running in my driveway and the old ICV still in, the car idled perfectly for about 5 minutes then surged and stalled. Since I was out of the car with the hood up I heard about 5-6 clicks coming from the area of the old ICV.

I changed out the old ICV for the one from the junkyard and am keeping my fingers crossed. I'll keep you posted.

Nick

ZaxW126

Believe it or not, the best way to check for air leaks around injector seals (and other areas too!) is water. The idle speed will change, and the engine will run differently,(rougher) confirming the leak. Yes, you could use flammable sprays,etc, but H2O is the safest. Naturally, you don't want to use it on an obviously large leak, or in a situation where you might get shocked by the ignition system.--Apply the common sense rule. I have used it for years. -Z