Reving 300 E at idle?

my 87 300 E started reving at iddle yesterday. It reves back and forth as if someone was pushing on the accelerator and letting off every couple of seconds. What could possibly cause this. I recently gave it new plugs (which I do every 3000 miles because of oil fouling due to leaky valve guides). I put in my normal bosch suppers, and gave her an oil change. I've had nothing but problems since. Help please.

Danka.

[stevebfl]

The most basic answer is that the amount of air entering the motor is changing. This can be caused by air leaks or by irratic electronic control of the idle air valve.

[suginami]

My engine developed the same problem a while ago.

My mechanic replaced the mass air sensor, and it didn't fix the problem.

My mechanic then replaced the throttle actuator (you car doesn't have a throttle actuator. You have an idle control valve). It didn't fix the problem

The mechanic gave up and told me to take it to the dealer.

They immediately diagnosed the problem as a leak at the intake manifold gasket. $400 later, the car was fixed.

Needless to say, I now have a new mechanic.

[Cap'n Carageous]

Originally posted by suginami
My engine developed the same problem a while ago.

They immediately diagnosed the problem as a leak at the intake manifold gasket.

Just out of curiosity, how did a leaky manifold make the engine rev up and down??

[stevebfl]

False air! Engine speed is solely determined by air flow quantity.

[Cap'n Carageous]

Wouldn't that raise the idle and sustain it at a higher RPM??

I now have a good idea where to start anyway (I hope it's not the intake manifold gasket). Granted I only payed $2K for the car a year ago, it's been good to me. I could pay out more than I payed for the car to fix some problem, valve guides etc, but I wont. So hopefully the problem is something I can fix myself in a short amount of time, while not spending loads of $$$$$

Thanks again.

[suginami]

I wouldn't limit yourself to the intake manifold seal.

You can have a vacuum leak anywhere.

Further, it really could be the mass air sensor. I've read from technicians here (maybe even Stevebfl) that it's impossible to diagnose a bad mass air sensor. The only way to check is to take out the old one and put in a new one.

FWIW, they are about $225-$250, and you can install one in 15 minutes. On my car (different engine, 3.2 liter M104), it's in the big, fat plastic hose next to the air cleaner.

It could also be the idle control valve. I'm sure a real technician could diagnose whether you have a vacuum leak or a parts failure. If you have no diagnostic tools, you could always trouble shoot the problem yourself by throwing parts at it until you replace the right part, but then again, that could be expensive.

You can check for a leak at the intake manifold seal by spraying water at the gasket with the engine running. It will bubble if there's a leak. You can also check for hoses that are cracked or came unattached.

[rodgiddens]

I had a similar experience where the idle of my 190E varied between 500-1500 RPM's continuously. It turned out to be the idle control solenoid

Bizzar - when I got home yesterday it was up to it's new tricks by reaving, it had no rhyme or reason to when it would rev, and when it would idle at normal speed. I started looking for leaks and figeting around with things until it started to sustain the higher reving speed, only to calm down and get back to it's original idle speed. Strange. I let the car idle for a while and it didn't revert back. I doubt the problem fixed itself, but I'm hoping it did **Thumbs crossed**