Intermittent Rough Idle (91 SEL 420) - PeachParts Mercedes-Benz Forum

lintaro · #1 08-22-2007, 12:53 PM

The Engine Can Be Cold Or Hot It Has A Hiccup Every 2-3 Seconds. If I Have It In Park And Hit The Gas Up To 3000 Rpm For 5 Seconds And Release It The Shake Goes Away Temporarily, Five Minutes Later The Hiccup Starts Again. Now If I Hit The Gas Up To 3000 Rpm To Make It Not Shake, Then I Put It In Drive Or Reverse The Shake Starts Right Away.the First Zero To Ten Mph Acceleration I Feel A Hesitation After That It Is Smooth. I Never Had This Problem Just One Day It Started To Shake. I Have No Check Engine Light On. I Have Replaced The Plugs, Wires, Cap Rotor After The Problem Started With No Results. -also- Between The Shakes At Park I Can See The Rpm Gauge Go Up Slight When It Stops Shaking Then When The Rpm Goes Down It Starts To Shake. Any Thoughts? (1991 Sel 420)

dpetryk · #2 08-22-2007, 01:09 PM

Sounds like a bad fuel distributor.

lintaro · #3 08-22-2007, 07:03 PM

WOW PLEASE LET IT NOT BE THAT, I JUST SAW THE PRICE OF ONE OF THOSE NEW.

I FORGOT TO MENTION WHEN I AM STOPPED AT A RED LIGHT THE FUEL ECONOMY GAUGE IS RUNNING AT AROUND THE 20% CONSUMPTION. IS THERE ANY OTHER PROBLEM THAT IT COULD BE? THANKS.

dpetryk · #4 08-22-2007, 07:28 PM

There is a long list of things to check. However owning 4 of those engines I have seen a lot of problems. What you describe sounds like a FD to me. Its about time for a new one anyway. Age and heat gets these things.

If you don't want to go there then;

Start by looking for a vacuum leaks. There are dozens of places where it can happen. The tuff ones are the intake manifold halves and all of the rubber parts that harden and don't seal well. Almost impossible to check without removing the manifold. The easy ones are all up top. Most vacuum leaks do not present the intermittent symptoms you are describing.

Then there are the ignition related items. Plugs, wires, caps, rotors, coils, pickup sensors, control modules, temp sensors etc.

Then fuel delivery items. Such as engine control module and all of its associated sensors and actuators - like the EHA, Oxygen sensor, Throttle switches, fuel injectors Airflow sensors, temperature sensors, position sensors, pumps, filters, regulators. I have probably forgot a few.

But given your description of the symptoms I would guess the FD. On more than one occasion, I have checked everything saving the FD for last since it is the most expensive only to find it was the FD after all.

I hope this is not the case for you. Maybe someone has a more optimistic diagnosis.

Let us know what you find when it get cured.

lintaro · #5 08-23-2007, 07:04 PM

I Will Bite The Bullet On This One. I Saw Some Fuel Distributors On Ebay For A Good Price Used So I Wll Go That Route. Is There Any Other Part You Need When You Replace The Fuel Distributor Such As Gaskets, O-rings Sensors Things Like That? Thanks

dpetryk · #6 08-23-2007, 07:46 PM

I would be careful. I have bought a few off Ebay and they were only good for cores. Put an Ebay special on one of my cars, and it ran worse than before. FD was bad. I never paid more than $75 for one on ebay. There is an O ring that needs to be replaced on the FD. Two more on the EHA. Thats about it.

An easy way to check if the FD is not delivering fuel evenly is to crack each of the fuel lines one at a time and see if it affects the idle. When the line is loose, fuel should run out and the engine will not run on that cylinder. You might be able to localize one of the 8 ports as being bad. Watch the fuel run out and try to see if there is a big difference between ports. If things are not equal then there is a high probability the FD is at fault.

There is a test procedure you can follow to to check the FD for general function. It would be wise to do that before taking a shot in the dark.

Again my advise is to not pay alot on ebay unless it is certified rebuilt and tested unit. That way you won't loose much when it does the same thing as yours.

david s poole · #7 08-24-2007, 11:39 AM

when you do the test outlined above you will probably find that cylinders #3&4 will be the ones with issues.btw put on park brake hard and put car in drive to do this test[it will be more evident].

dpetryk · #8 08-30-2007, 05:40 PM

Have you found and cured the problem yet? Let us know what you find to be the problem.

dpetryk · #9 09-06-2007, 03:05 PM

Did you get it fixed?

oneitalianson · #10 09-07-2007, 12:36 AM

I have the same issue I was told to replace the 2 o-rings on the EHA I'll let you know what happens..doing it this weekend..i have an 88 420 SEL.

Robert Squires · #11 09-10-2007, 01:32 AM

It's easy to blame a FD for an injector that doesn't always shut properly. In fact, if an injector is stuck open (K or KE Inj), that line won't even try to spray. Determine which cylinder your miss is on (assuming it doesn't migrate) and swap another injector with that cyl. If your miss follows the injector, the FD probably isn't involved.