Rough Idle till warm? - PeachParts Mercedes-Benz Forum

ncgirl05 · #1 09-08-2015, 08:08 AM

Hi,
My search didn't pull anything similar so asking forgiveness in advance if this has been asked before.....

I have a 77 450SL. Lately when I've been driving it I've noticed that upon starting the idle is very rough but it evens out after the car has sufficiently warmed up to temperature. Any ideas what's causing the rough idle?

Frank Reiner · #2 09-08-2015, 04:44 PM

ncg:

The first diagnosis steps will be to examine all the ignition system components for condition: distributor cap & rotor, wires, spark plugs, and the ignition controller (EZL) for clean and secure plug connections.

Second steps are to do both cylinder leakdown, and compression tests, first with the engine cold, and again with the engine warmed up, looking for any significant difference between cold and warm.

After both cylinder condition, and the ignition system are deemed good, then attention may be directed to the fuel system.

meltedpanda · #3 09-09-2015, 11:31 AM

perfect course as suggested by Frank. It is very important not to take a shotgun approach to these ol gals. You will need a fuel pressure gauge and some mechanical savvy. My guess after basics ruled out, injector leak, AAV or airflow sensor

rowdie · #4 09-09-2015, 06:32 PM

AAV stuck closed would be an easy place to start.

ncgirl05 · #5 09-10-2015, 12:22 PM

Thanks all! I'm stumped as to what AAV stands for? Had hoped for an easy resolution but appears that will not be the case. I've noticed, also, that the idling is getting rougher, although it still smooths out after warming to 175 degrees. Very odd to me.

Thanks again!

rowdie · #6 09-10-2015, 01:03 PM

Auxiliary Air Valve. When open it provides extra air during warm up and gradually closes. If stuck closed you are not getting enough air and it will run rich and idle rough. It is at the front of the engine.

cwmoser · #7 09-11-2015, 01:15 PM

Quote:

Originally Posted by rowdie

Auxiliary Air Valve. When open it provides extra air during warm up and gradually closes. If stuck closed you are not getting enough air and it will run rich and idle rough. It is at the front of the engine.

Rowdie, is that the valve from the Emissions Air Pump?
If the valve is stuck open letting the pump push in a lot of air, how about
just removing the drive belt to the pump and see what happens?

How about the WUR-WarmUpRegulator as a possible?

rowdie · #8 09-11-2015, 04:51 PM

No. It has nothing to do with the AIR pump. It is on the top of the intake manifold near the idle screw.

Yes, the WUR is in play. You need CIS fuel pressure gauge to check.

nobby · #9 09-12-2015, 10:12 PM

Quote:

Originally Posted by rowdie

Auxiliary Air Valve. When open it provides extra air during warm up and gradually closes. If stuck closed you are not getting enough air and it will run rich and idle rough. It is at the front of the engine.

I vote for this as it is one of the easiets things (on your engine) to access and check.

And makes sense from your description of symptoms..