Hello all!

I purchased a 92 300CE which is/was in great shape from everything I could tell. However, after having owned it for approx 2 weeks, it blew the headgasket. I had the headgasket replaced by the mechanic that I normally use for all my other automotive needs. He did a great job, however, once out of the shop, I have a whole new issue.

After reading some of the threads on the forum, I was beginning to believe that I had the dreaded "engine wiring harness" issue. So I brought the car into my Garage and began to remove the wiring harness to inspect it. To my surprise (and relief), it appears that the harness has been replaced at some time in its life as the insulation was not flaking from the wires, and the harness was very pliable. I reinstalled the harness and cleaned all of the connectors with a QD and plastic safe contact cleaner, so I believe this is NOT the problem.

I did have an issue with the OVP relay which I was able to diagnose myself with help from this forum, replaced it today and it corrected issues of the ABS light coming on and cutting out at highway speeds. However, here are my concerns:

1: still has hard starting issues--turns over 10-15 seconds before it will finally fire up.

2: once it starts, it runs great and accelerates great until it reaches what I would call "operating temp", at which time it has a very pronounced low RPM hesitation. If you're starting from a stop and you accelerate normally, this thing won't hardly get out of it's own way, it falls flat and backfires through the intake like it's going to stall. HOWEVER, if you press the pedal to the floor just momentarily and let it back up, it will go ahead and get on down the road.

3: it has a very strong catalyst smell from the exhaust when it's idling which tends to make me think that maybe it's running too rich.

If anyone has any ideas on anything I can check myself, I would be extremely appreciative of the input. Its a beautiful, very well maintained car and I would like to get it back to the running condition that I know it can and should be performing at.

If anyone reading this post know of any reputable/trustworthy indies in the North Houston TX area (humble/kingwood), I would appreciate that info also. The mechanic that I use for my automotive repairs is very trustworthy and does great work, however i do not believe that he is set up to diagnose this make of vehicle. He does great work on my Mercury Mountaineer, but Mercedes is in a totally different class and I can't afford to put the car in the shop and try to fix by "trial and error".

Thanks in advance for any and ALL input...I look forward to hearing from everyone!!!

 

I have a 1990 300CE M104 experiencing similar hesitation on initial throttle. Have changed fuel filter and replaced plugs and ignition wires. Checked all vacuum lines all appear okay.

Looking for suggestions of what to check next

 

Hey ESTJ,

Haven't found a solution to my problem yet, so unfortunately, I don't have any suggestions on founded problems. I did, however, order a new O2 sensor today and will be installing it this week sometime. My engine had a catastrophic head gasket failure and I believe that the ethlyene glycol antifreeze may have fouled my sensor, which is something that I have read on a couple forums. I tested the sensor, across pins 2 & 3 on the diagnostic port and was expecting a fluctuation because the sensor should change voltages constantly, and it pretty much was holding steady at 12V. Unfortunately, I cannot be sure that this is accurate since the information that I had (for which pins to check) was posted for an 88 300E, and I do not know if there were changed between the models. I figured if I had read on a couple different forums about antifreeze fouling O2 sensors, and i know mine definitley had been drenched in it, it certaily needed to be replaced. Once I change it, I will post my findings! Good luck with yours!

 

Maybe the mechanic loosened the distributor and now has the ignition timing off a little? Or maybe has the timing at the belt off a little?Backfires through the intake is a good indication of timing being off.

 

Well...replaced O2 sensor yesterday....so far...so good! I am "hopefully optimistic" that this will fix my problems. However, I do still believe it is running rich due to strong smell coming from exhaust at idle...that will be my next project that I hope to complete soon...would hate for that to ruin my new O2 sensor.

 

Well at least someone is having success. So far I have replaced the plugs and wires, fuel filter and now recently had a very bad resonator replaced with pipe. The resonator was shot as was the other components, I am sure once I find the culprit the car will perform better for all that has been done. One of the plugs was loose so there was a loss of compression on one of six cylinders.

Oh well a great old car, but an old car!

 

No doubt...it is a great car. My plan for this car is to eventually give it to my son as his first vehicle. Unfortunately, i dont have the confidence in it to turn it over to him..yet. I dont have a problem driving it, but ive got a few years experience and a whole lot more mechanical inclination than he does, and I have learned how to drive the car..even with it's imperfections. HOWEVER...it is going to be his car...just as I plan. BUT....it is a great car, and drives better, even at amost 20yrs old, than my Mercury Mountaineer and my wifes new Chrysler T&C...! Good luck with yours!!

 

My 90 300ce with the m104.980 is also experiencing the same issue. No grunt below about 1800rpms.

Its had the headgasket rebuilt, fuel filter pulled/replaced, fuel lines checked, Lambda adjusted, new fuel distributor gasket, new fuel EHA, in-tank strainer cleaned/replaced, new distributor cap, new distributor rotor arm, new plugs, and new spark plug wires.

Its still a dog in low RPMs, and no, its not second gear start. The engine feels weak till much later in the rev range.

I'll try and keep you guys posted if/when my father and our mechanic get this issue resolved.

Oh, we also fixed a huge vacuum leak by replacing a vac line that runs beneath the battery tray. Check that one if you've not replaced it already.

 

Saijin..

My O2 sensor was not a fix...was hoping it woud be, and it did seem to help in ways, however....it still falls on it's face when you start from a stop when it has reached operating temp. I believe my next stop will be an indie and see if they can figure it out, as otherwise, the car runs GREAT..and I am not much into trying to diagnose the problem by trial and error..parts are expensive. I have read several posts about checking the lambda, but not sure how to check on my car. I've checked across pins 2 &3 and 1 & 3 as I've read in different posts...but neither are giving me readings 2 & 3 give a constant 12+ volts and 1 & 3 give no voltage reading...its terribly aggravating..

 

The reason your car has such weak low end performance is due to the early design of the 104 engine while it is a 4 valve engine with variable valve timing ( the control of that being on the left camshaft.) it was not equipped with a variable length intake manifold. At low speeds there is low manifold pressure not much squeeze to push the air fuel mixture into the cylinders . But at high RPM that rather large manifold can really deliver, which is why they scream at higher speeds . If it bothers you that much get a 320 model you will see the same horsepower but a nice boost in torque.

 

Your surprise and relief regarding the harness may be because the biodegradable harness was perhaps never installed in your 92. At least I know this to be the case with the m104s in my 300SL. This topic gets more confusing all the time, as to what cars do or do not have the wiring!

 

Not on topic, but wow... That is a really nice looking 300CE! I have a 89 300CE with 178,000 miles on it, and no sign of a head gasket issue yet. It maybe that I'll never have a sign either, as it's for sale! Thanks for the vac line info BTW!

 

Thanks! It isn't looking so hot these days with a crashed door and rust, but maybe I'll be able to get it back to being nice.

Yeah, that vac line was tricky! There was almost no sign of that one anywhere else in the engine bay as the line was tucked away in the bodywork.

 

hi guys im new to this forum, any of you have found what the problem was ? i have the exactly same problems on my 1992 300e 24v ..

 

I too have the same problem on my 1991 300CE which I love so much. I have so far replaced intake manifold gasket, water pump with thermo, fuel pump and filter, temp sensor, air temp sensor and no results. Next steps is to replace the fuel accumulator and OVP. My car has no cat hence no O2 sensor.

 

My 91 CE had MAJOR hesitation & had to be towed twice to my Indy Shop, Pete’s Mercedes in San Francisco. I had already done major engine work over the 17 years I’ve owned the Coupe including new Catalytic 02, head gasket, upper & lower valve work, new radiator, AC conversion & most recently, a new oil pump.
The idling / dying at start from stop finally
was traced to moisture in the carburetor.
Check with your mechanic.
FYI: the Wiring Harness is a big issue in the
‘93-‘95 Coupes & Cabriolets as MB in a “Green” attempt made the harnesses in those models biodegradable, unbelievably!
FYI
My ‘91 300CE restored to a high level is for sale / 189 K miles / 17 years of mechanical records with 1 MB Shop & Master Mechanic.
always garaged / California Car

 

Hello there,

Did anyone solve this issue of hesitation from standstill/idle/ low RPM
If so can someone post their "fix"

 

Vacuum leaks are most common. Have a friend with a smoke machine? If not; follow vacuum lines away from the engine and unplug them and cap the hose. You’ll know when you find it.