Tony Curtis

Hello Everyone!

I have a 1991 Mercedes 300E and need some help!

About 3 months ago I was driving and noticed it seems to chug or stumble on acceleration. At first I thought it was the transmission not going back down into the starting gear but then realized it is a gas issue.

So.. what is happening now is that when you start it up.. it's like it stumbles at idle. Even when in Park and I step on the gas you can hear it rev but hesitating/stumbling.

When I drive it now.. it does the same thing. Stumbles on takeoff pretty bad.. Once i kick it down and we get going it seems to be fine. It's just more on the initial takeoff or on lighter throttle.

I have done many things to the car! ha ha

Here is the list:

New Fuel Filter
Accumulator
EHA Valve
Fuel Pump Check Valves
New Injectors
New Plugs

I HAVE adjusted the mixture using the Duty Cycle. I haven't checked it for a few months. I could do that again I suppose.

Your help or input would be awesome!!

S70Houston

Ever thought to take the car to a reputable shop to have the issue diagnosed, rather than pouring more money into it without resolving the issue?

Tony Curtis

S70

I have not put any money into it yet around this issue. The other stuff I did was a result of just doing some maintenance and a starting issue a couple of years ago. The injectors, plugs needed changed. They were done. ha ha

This is a new issue. :-) I have no reputable dealer in my town that works on these older mercedes.

optimusprime m1

Tony is itmissing on tick over ?.

Tony Curtis

No, it starts up fine! I checked the Duty Cycle Readings just to see if anything was off there. I did notice that it seems to be fine until it warms up a bit. Then the duty cycle reading is around 42% - 46%. But then once it warmed up a bit it would keep hitting 50% for a bit and just stay there and then go back to normal operation.

I seem to remember something about if it goes to 50% it could be an oxygen sensor issue.

optimusprime m1

Tony i am lost with this one .Keep on looking on the forum other will help you if they can.Have you cleaned the air valve .Its under the air box held in with a saddle and 2 bolts .Remove the plug on the rear, and both rubber hoses .Clean it out inside with carb cleaner or white spirits . Look for any splits in the vaccuum pipes ,and the little elbows .Check you fuses are in good condition ,along with all earth points .This is just every day maintenance that can give you a problem. And is the engine a M104 or M103 ?.

optimusprime m1

Tony its the idle control valve .This is what its like .

Tony Curtis

Yeah, I cleaned that out a while back with another issue. That was all good! The idle has always been good. It's like a cylinder or two are not firing or getting gas. When I step on the gas from a stop... it chugs and hesitates like it doesn't want to go. Once I kick it down and get on it, it's fine!

I am thinking Distributor seeing as I haven't done that yet! Gonna check it tonight!

optimusprime m1

Tony if it was me i would change the engine temerature sensor . These are only thigs i would do if it was mine . Check all the fuses in the fuse box . Clean all earth points grease up , and bolt them bacl on to the body .Look at all vacuum pipes for splits .I was going to say ht leads ,but it would show up on tick over .Have you ever moved the screw in the fuel metering unit . The engine might be running weak ..This adjustment is critical and if your not shure how to. Then you must get the garage to look at it .

QVHK

My guess is when the problem happens once the engine warms up it has to be temperature related or one or more sensors generating or sending inaccurate data to the ECU. The prime suspect would be the engine temperature sensor and O2 sensors, and/or idle control valve. Otherwise, it could be the potentiometer in your airflow meter that is wearing out, which results in the engine trying to constantly adjusting the fuel mixture to maintain engine speed. Before the engine warms up the engine normally go by default; this changes once the engine approach operational temperature or when the ECU is led to believe that it has reached operational temperature.

optimusprime m1

QVHK i go along with everything that you have posted ...