1993 300CE dropping cylinders at idle
 

Hi All,

I have a 1993 300CE M104 HFM injection that I've owned for over 20years. It has 193,000 miles and generally is in beautiful shape. Off and on for a couple of years now it will randomly drop a cylinder at idle and idle a little rough. It would disappear a soon as you touched the gas. Mo st of the time it was fine and cold and hot starting and driveabuility is great. The CEL is not on. It has the updated harness. Coils, wires and plugs are about 5 years old, and car isn't driven much

Over the last couple of weeks, the dropping of cylinders has gotten worse. The longer it idles at a stop light the worse it gets to the point that it stumbles off idle now. It gets to the point that it feels like it's going to stall but doesn't.
No CEL light. The exhaust smells a little rich and you can hear a "spitting" in the exhaust. I installed a NOS OVP last month and thought it helped, but stumble returned

I built a code reader years ago and have a bunch printouts on the codes but am pretty confused now.

I checked the codes using the built in button and got a 6, 17, 25. I cleared the codes, drove it about 40 miles with the stumble and rechecked the codes.

Now using the built button, I get a #6. Using the code reader, it shows #1 on socket 8 and 14.

the trouble code articles I've printed out aren't consistent or I'm not reading them right.
One shows code #6 as "idle speed control inoperative.
Another shows: #6 as "not used"
Another shows: #6 as "idle speed control faulty

Thanks in advance for your help.

Check the condition of the wires in the throttle body motor, they fail just like the rest of the harness. Look at the connectors between the coils and plugs for any fine gray talc like powder, if present they are bad.

Following up on Sugar Bear's suggestions, I pulled one coil, connector, plug and wire and all looked like new. They are all Bosch oem parts that were installed in 2018. Car may have 15,000 miles since then.

At that that time I was having a similar problem with low idle feeling like it was dropping random cylinders and smelling very rich. That is when I installed the new ignition parts and brake lite switch which cured the problem for a couple of years.

In the last few weeks the stumble has gotten worse to the point it doesn't want to accelerate off idle anymore. It will pick up speed gradually and then take off like nothings wrong. It pulls hard and doesn't feel like an ignition problem.

I cut back some of the insulation in the throttle body harness in a couple of places and it looks OK to me.

I bought two OVPs, a slighty used looking OEM and a new Chinese knock off. I'm tempted to switch them just to see.

After I cleared the codes it now just shows #6 using the built in button and #1 on pin #8
There is no CE light

The date code on my throttle body is "94MO8"which I assume is August 1994

Pelican Parts shows a reman throttle body with the correct part number for $920. I don't mind spending the money if it cures the problem.

They also show a Idle air control valve which is Code #6, but I can't find it on my car and assume it's incorporated in the throttle body?

Maybe totally unrelated, but if my gas tank is low and I release the cap there is quite a suction of air. Is there possibly a vent hose plugged some ware?

Thanks again!

Just an update,

I switched the OVP from the used ebay OEM one that looked new to the new Chinese one with no difference. Each time I've done this it runs great cold and starts the random dropping of cylinders when it warms up to normal temp. At that point the more you drive it, the worse it gets to the point it doesn't want to move off idle. Once off idle, it clears up and runs fine.

Also removed and cleaned the MAF with CRC MAF cleaner. That actually seamed to improve off idle acceleration and power but did nothing for the initial problem.

And it seams to be getting terrible gas milage.

PeachParts has a rebuilt Throttle Housing,Part #: 000-141-57-25-81-INT
by Beckman Technologys for $920. Does anybody have any experience with them? I don't mind spending the money if it cures the problem

Anymore thoughts, please

Thanks

bk:

In very general terms, what you are describing, is a mixture control fault, rather than a throttle position fault.
Have a look into the O2 sensor output as a first step.

MAF sensor is also suspect. My '93 did this, my indie had a spare so he tried it, problem solved.

Thanks,

The dealer replaced the MAF in 2002 when I bought the car with about 120,000 miles. It now has 193K. I guess it speaks to the quality, anything I touch or work on looks brand new. Te MAF sensor looked great when I cleaned it but I guess that doesn't mean anything.

It's not showing codes for the MAF or oxygen sensor, but I may not be reading it right, But I will look into how to test OXY sensor over the weekend.

Thanks again

MAF sensor is also suspect. My '93 did this, my indie had a spare so he tried it, problem solved.

Look at O2 sensor as you said. Unfortunately my MAF didn't show a code either so at the end of the day it is a gue$$.

Ran the codes again and using the built in button, I get only 6. Using the homemade code reader in socket 8, I get a 5 and 6

Replace the throttle body! It is original with bad wiring. You can shop for them on ebay for a ton less than new. There is a ASR and a non ASR throttle body. It does not matter what M104 or class car. Just search by part number and buy one that is rebuilt or has a date code newer than say 1998.

I've looked at used TBs on ebay, nothing has popped up yet, But it is something I'm coinciding.

I've had a small exhaust leak for a couple of months. It's the original system except for the rear muffler. It is very sooty around the OXY sensor although I couldn't find a leak. It gets real hot real fast! But I moved the wires and it seams to idle worse. I wonder if the leak near the OX sensor is effecting the exhaust mixture reading and/or some criters got into the wiring?

It sounds like a possible vacuum leak to me. Check the simple things first.

A couple of people suggested the oxygen sensor so I replaced it with a Bosch sensor from Rock Auto. It's probably been 15-20 years ago so I figured it was do anyway.

Now It's running worse than ever.

It started out as a barely perceptible unevenness at idle that felt like a random dropped plug. It always cleared up as you drove away.

I always doubted it was ignition because it ran great under load which is where an ignition problem usually shows up in my experience. Now it's barely idling and skipping/surging through first and second gear.

I hate to throw a $900 re-manufactured throttle body with the luck I'm having now but will if there's a chance it will fix it. Or replace the coils, connectors, wires and plugs? The plugs are probably 10K old but look brand new.

Thanks!

Also, I checked the codes today and using the built in button or my homemade reader I just got one blink.

And as another weird fact it starts instantly and the cruise control works fine.

I wonder if I should pull the thottle body off just for a better look at the wiring, plugs etc. and replace all of the ignition just because it was last replaced about 15 years ago?

Thanks as always

Pull the plugs from the cylinders that have the coils. Replace any showing carbon tracking on the insulator along with the mating coil boot. You can check the plugs on the cylinders with the plug wires but those are never the problem.

Follow up
 

Sorry for the delay. Busy with work and other projects.

Figuring everything was at least 15 years old (but not many miles) I replaced the plug with standard copper core Bosch plugs (made in Russia), Bremi coils (German) and Bremi wires and boots.

I found a little corrosion on the secondary wiring plugs on the coils and number four plug was oily on body, but not electrode.

Also lightly sanded all the fuse contacts. Some were pretty corroded. Will order the copper ones

Put every thing back together and couldn't believe how great it ran. Dead smooth idle, more responsive. I'm assuming vacuum was up because transmission shifted better(modulator?)

I was afraid to say anything for fear of jinxing something.

Then it sat for a week and when started it had a dead skip on one cylinder. Much more violent than before. It shook so bad in gear I was afraid of breaking something.

Got it in the garage and started it with out the valve cover top plate and skip was gone ?!. All I could think of was maybe one of the coils wasn't grounded which I don't get because of the copper inserts in the coils and small stud on the head. But I took it apart and found a couple of wire connections weren't snapped completely tight. One PO issue is that most of the valve cover plate retaining screws are stripped. Most are replaced with studs and nuts, But I'm afraid to go more than wrist tight on the fasteners.

I also did a compression test and got 145-150-152 on all the cylinders and used a borascope and found the piston dome on #4 has some oil. My scope can't look backwards so I couldn't see the valves. It has 193K miles

So as of yesterday it was running great. Who knows about today.

Latest odd thing is the oil pressure is now dancing around at 1.5 bar at idle. It's always been rock steady. It jumps right to 3 bar on start up and when driving.

Our Jeep does it and it's the sending unit plug gets loose.

Thanks for your thoughts

We all hate throwing parts at things.
Odd that it ran worse after changing the 02 sensor.

1. I would start with vacuum leaks. Om my 93, 300Ce, the intake manifold has one stub that needs a rubber cap/plug.
Those clear vac lines tend to crack too.
2. Check fuel pressure regulator.
3. If it ever stalls on you after car is very warm/hot. Replace the Crank position sensor.

My guess is you have a faulty ETA. I experienced similar symptoms. I then moved the ETA cable around a bit and would get varying symptoms and sometimes would run fine.
When I took the ETA apart to thoroughly inspect the wiring, it was shot.

Very easy for me to spend your money but I have never looked back since replacing the entire ignition system, from coil (only one on mine) to plugs with genuine parts sourced from MB. They all should be available, if not cheap.

Good hunting.

RayH