Hesitation 91 300SL - PeachParts Mercedes-Benz Forum

pmckechnie · #1 08-21-2004, 02:41 PM

Please I need some help with a hesitation with a 91 300SL (129 with 104 engine I think)
First lets get this out of the way. I have a shop which specializes in electrical and Computer systems in domestic and asian cars. I do not work on MBs except my own and a couple of friends. So I sure could use some help on this one.

We are the 5th shop to get this car. One was the dealer and no one has been able to fix the problem but each has made money by throwing parts at it.

Here are the thinge I know have been replaced
Coil, wires, plugs, cap and rotor
Fuel pumps and filter and fuel tank cleaned out
Fuel pressure is 92psi and still at 45psi after an over night sit. Fuel pressure remains at 92 psi during the hesitation.

With the help of this wonderful forum and lots of reading this is what I think is wrong.
I have checked the airflow pot for proper operation (.68V at idle increasing to3 to 4 V with acceleration). EHA current at 0ma after adjustment and varying.
I think I understand that the EHA current should INCREASE when accelerating. Well, it DECREASES. Does any one know how this could happen?

By the way, the car idles great at about 700 RPM, accelerats fine if starting off normal and runs smooth at speed. The problem only shows up apon HARD acceleration. After the engine gets past about 2000RPM everything is fine again. It feels like an accelerator pump not working in a carb. My understanding is that the increase in EHA equates to the accelerator pump.

Thanks for any thoughts on this problem.

pmckechnie · #2 08-22-2004, 09:32 AM

Please, anyone.

psfred · #3 08-22-2004, 01:14 PM

I don't believe there is an EHA on the LH jetronic injection -- this is electronic (wires to the injectors, correct?). Need to know that before I can give any advice.

For quick notes, here's the things to check (by system, KE or LH).

KE:

Vac leaks. Check ALL the vac connections for leaks, starting with the idle control valve. A big leak here will cause terrible throttle lag due to unstable mixture on throttle opening (usually very lean). Make sure all the things operated by vac hold, too. Usually all the rubber connectors are leaking. This results in an over-rich condition under load since the leak is constant, more or less, and the idle mix will be set way rich to compensate for the leak. This will account for the sudden EHA current drop (to negative) as a slug of gas goes down the intake.

Oxygen sensor

Injectors -- a dribbling injector will cause rich running at idle with con-commitant overlean mixture when throttle is opened and it works more like the others. Usually causes rough idle, though.

Bad "pot" on the airflow housing. If the search works, this subject has been covered extensively, including sources (otherwise you have to get a replacement fuel distribultor, $1600).

EHA should go positive on throttle opening under load. No load indications may not be accurate, you will have to drive the car with the meter attached to see.

LH Jetronic:

MAF is bad, replace it (bettery yet, find a car with a good one and do a temprorary swap to see, they are expensive). Will not always show up on the scan, either, sometimes shows up and a communcations failure (duh!).

Airflow meter "pot" is bad, check for resistance changes as the flap moves. There are a number of threads

pmckechnie · #4 08-22-2004, 04:37 PM

psfred,
thanks for the reply. This is a KE system.
Things I have done:
I have purposly set the mixture lean and sprayed carb cleaner everywhere I could find and no change except in the air intake itself.
Used a scope to check the airflow pot looking for bad spots. None found.
Airflow pot goes from .68v to 3 to 4v (hard to tell the exact amount and drive at the same time). Also checked entire range with KOEO and it is very clean.
Unhooked EHA and car runs fine.
Idle is very smooth and startup hot or cold are fine.
EHA stays about the same within +\- 2 ma at 700 rpm to 2000 rpm if speed is increased slowly. If increased quickly, it will go -4 or more and then return to to +\- 2 ma as the engine speed stablizes.
Spray carb cleaner in the intake air and the EHA goes minus. Cause a vaccumn leak and it will go plus.

The only thing I can see incorrect from the above is the EHA going minus with quick acceleration.

Thanks for the help.

psfred · #5 08-22-2004, 11:29 PM

It's working "backwards" -- EHA current should go positive on throttle opening, as you said.

I would check the O2 sensor. It's responding correctly to major shifts in mixture, but may not respond properly to minor changes (ie, it's slow).

Do the ground/one finger on the positive battery post-one finger on the sensor wire test (don't hook the battery up to the sensor wire with a conductor, you will fry the computer!) and make sure the computer is working correctly by sending the EHA current max rich when the sensor wire is grounded and max lean when it "sees" one volt via the battery over a human body (I think, may have this backwards.....).

Certainly check the O2 sensor wiring under the carpet, it's rather common for a leaking window or spilled soda to corrode the connector and cause all sorts of trouble since there is also a 12V heater for the O2 sensor on the same plug.

Peter