car runs good at WOT only

[typet]

Hello guys

I am from Bulgaria and I am new to this forum, already registered in other boards, but people there seem to be very unhelpfull (or dumb I can't decide) so I decided to post my 3-and-more months old problem here.

My car ('95 C200 petrol W202, manual, 180K miles on the clock) started acting very strange in the past 3 months. Sometimes, the engine hesitates when the gas pedal is not pressed to the floor. I mean there's no difference between 1/8, 1/4, 3/4, 8/9 throttle ... car just doesn't accelerate at all...BUT when the pedal reaches the floor the car suddenly starts accelerating and runs like a bullet!! When it gets in this condition the idle is just terrible, it cuts out very often. When it is in the mood, it is perfect at all rpms, including at idle.

Things I have done and didn't help:
Changed the ecu (PMS)
Changed spark plugs and wires.
Changed air, fuel filters
Renewed the whole engine harness with new wires, they were very cracked

The car falls in this condition much more often when the air outside is warmer. I say air, because the engine temperature has nothing to do with the problem. Now, when the winter is coming the car is much better.

I changed the ECU because I noticed, that disconnecting the vacuum tube from the ECU makes the car run better and the problem much less noticeable and thought the MAP sensor inside is going bad, but...unfortunately it was not because with the new ecu it was almost the same. Also, the new plugs and wires made it less noticeable but in general it's the same

I hope you guys at least have a clue. I searched everywhere but couldn't find someone with a problem like mine. In another board a guy suggested me to change my Mass Flow Sensor hahaha

 

[Dieselman]

The first guess would be the MAF, but that normally manifests itself as stuttering under acceleration, so possibly an air leak into the manifold.

You can test the MAF with a voltmeter on pin 5 WRT Gnd. ~2vDC at idle rising to ~4.8vDC at full revs.

 

[grober]

If your car has a PMS controlled injection system it doesn't have a MAF sensor which I assume accounts for your laughter?? Does your car have a cable or an electronic throttle control-- if its electronic the sender unit might be faulty [ It's normally located in the corner of the engine compartment just in front of the throttle pedal assembly inside the passenger compartment. Other possibility is the engine earth strap is making a poor connection with the chassis.

 

[Dieselman]

Oops, looks like I'll be in for a slaughtering. I forgot the 180 and some
200 don't have a MAF...

Shucks..

 

[BlackC55]

It sounds like you have a misfire. Common causes on this engine is the coils.

 

[Alex Crow]

While we are guessing.....

The throttle bodies on these cars can cause all sorts of problems, usually idling but can be at other engine speeds.

As this car is ONLY ok at full throttle - not 8/9 throttle - I have to wonder what is different? The answer may be the full throttle position sensing aspect of the throttle body. So my guess is the throttle body, or wiring between the throttle body and PMS ECU. And the car runs OK at full throttle because the throttle position is known to be at full throttle, but all other throttle positions are badly read by the ECU.

What fault codes do you have my man??

 

[typet]

If your car has a PMS controlled injection system it doesn't have a MAF sensor which I assume accounts for your laughter?? Does your car have a cable or an electronic throttle control-- if its electronic the sender unit might be faulty [ It's normally located in the corner of the engine compartment just in front of the throttle pedal assembly inside the passenger compartment. Other possibility is the engine earth strap is making a poor connection with the chassis.

The car is with a cable... and I am definitely going to check for the earth strap. Could you tell me where is it located?

While we are guessing.....

The throttle bodies on these cars can cause all sorts of problems, usually idling but can be at other engine speeds.

As this car is ONLY ok at full throttle - not 8/9 throttle - I have to wonder what is different? The answer may be the full throttle position sensing aspect of the throttle body. So my guess is the throttle body, or wiring between the throttle body and PMS ECU. And the car runs OK at full throttle because the throttle position is known to be at full throttle, but all other throttle positions are badly read by the ECU.

What fault codes do you have my man??

Well, we hooked it to professional bosch equipment and there were 2 errors:
21 Idle control - control motor currentless (actualy it works, when for example I turn the steering wheel or turn on the AC the rpms drop, then the idle motor compensates)
37 Lambda control below lower limit (they measured the voltage of the O2 sensor and sais it was within the specs)

Actually there is a code stored for the throtte body. As you said, it seems that the throttle position is badly read by the ECU. But does this car have a throttle position sensor, or just a simple device which detects only the full throttle? I remember someone told me, or read it in a board, that actually this MAP located inside the ECU tells the ECU the position of the throttle by reading the pressure, which drops when the throttle is closed and vice versa

 

[typet]

Oops, looks like I'll be in for a slaughtering. I forgot the 180 and some
200 don't have a MAF...

Shucks..

well it happens sometimes now I wish I had MAF instead of the crappy PMS system and sometimes I wonder who the **** decided to produce such a great car with this one-of-a-kind s**t

 

[grober]

The car is with a cable... and I am definitely going to check for the earth strap. Could you tell me where is it located?



Well, we hooked it to professional bosch equipment and there were 2 errors:
21 Idle control - control motor currentless (actualy it works, when for example I turn the steering wheel or turn on the AC the rpms drop, then the idle motor compensates)
37 Lambda control below lower limit (they measured the voltage of the O2 sensor and sais it was within the specs

Actually there is a code stored for the throtte body. As you said, it seems that the throttle position is badly read by the ECU. But does this car have a throttle position sensor, or just a simple device which detects only the full throttle? I remember someone told me, or read it in a board, that actually this MAP located inside the ECU tells the ECU the position of the throttle by reading the pressure, which drops when the throttle is closed and vice versa

The earth strap is usually located at the base of the engine around the bellhousing /starter motor area and is a short lead to one of the chassis members. They can sometimes corrode at their attachment points or fray inside their plastic sheath-normally this would occur due to corrosion or excessive movement of the engine block due to worn mountings. Its very common on the bigger engined v6 v8 models due to their higher torque leading to more engine twisting but less likely in your car. STARTER CABLE HARNESS AND GROUND HAR- NESS, C/CLK-CLASS ENGINE CABLE HARNESSES

ps you might like to take a look in this thread I posted here? http://www.mbclub.co.uk/forums/engine/80666-pms-ecu-fault-finding.html
warning the PDF takes a while to load.

 

[Alex Crow]

From memory yes, the throttle body DOES have position sensing, just like a HFM vehicle of the same age.

 

[typet]

What I noticed recently is that air temperature in the engine compartment, not the engine itlesf, has a lot to do with my problem. Yesterday I made a rather long highway trip and there was no sign of this behaviour at all (I must have felt it at least for a second since on the highway I drive at 1/4 throttle 99% of the time). BUT when I make a stop and shut of the engine, wait for a couple of minutes to let the heat from the engine warm up the whole engine compartment along with all things there like for example wiper motor, abs unit, washing fluid container......and maybe something else which causes this condition, it comes back again, especially around town.

It might be stupid but...guys, what about the knock sensor? Do you know what is it responsible for-the timing only, or commands the fuel injection? I heard that sometimes they become super-sensitive and detect knocking even when there is no knockig..I am pretty sure the mixture becomes very lean during "THE CONDITION" and sometimes I get backfires...sound sike something is exploding in my engine

 

[typet]

It sounds like you have a misfire. Common causes on this engine is the coils.

Is it possible the coils to be getting bad and no missfire codes to be stored and always work good at full throttle?

 

[Alex Crow]

Forget the knock sensors, not going to be them.

I am still going with the throttle body...

 

[BlackC55]

Have you tried cleaning the TB?

 

[typet]

Have you tried cleaning the TB?

oh yes, I forgot to mention, that's where I started form It was very dirty and I cleaned it, now it looks like new, but it only made the first start for the day easier and now it starts with the first attempt...before this I needed 2 or 3 attempts...still not the cause

Today I read a lot about the ignition coils, and since it could't be the knock sensor(s) I am starting to go suspicious.... but no codes stored, always good at WOT

And talking about the TB, I guess I have nothing left to do except try to find another one, then see how's the car acting with it.

BTW do you guys know what info can I get if I buy a MB diagnostic cable bearing in mind that the car is from the stone age? I am fed up with shops and stealerships and I guess I have to go on by myself. I think of buying a cable, it's not that expensive after all and see what I can read while driving

 

[Benz202]

Hi,

Was there ever a resolution to this issue? I too am having the exact same problem with my 94 C180 and would love to know what the cause is!

Many Thanks,

Benz202