EuroMerc600SEL

Calling all w140 gurus for help! I´ve been battling with rough idle for a year now without success. The technician who is working on my car has tried to swap TB´s, MAF sensors, did a full smoke test again but still no luck. He had another perfectly working w140 s600 at his workshop and he put all the modules, both tb´s and maf sensors from my car to the other car and it worked fine. Back to my car - still same problem. Also tried the other car´s parts on mine and still same problem with the idle. Everybody says it sounds like a vacuum leak and the technician managed to re-create the same condition on the other working car by creating a vacuum leak. But he swears there´s no vacuum leak on my car. He has done multiple smoke tests and says the block etc are even tighter than on the other car with no issues. So today he said he is giving up. Still no solution. Any idea what else should I check? According to the technician all parameteres (afr etc) look fine. I´ll be getting the car back soon, so I can have another look with my STAR, but I´ve noticed the HHTWIN doesn´t show much, while the old blinker style diagnostic tool still showed some fault codes.
So far I´ve replaced the spark plugs, new distributor caps and rotors, new ignition coils, new head gasket. Replaced passenger side TB and MAF sensor. Replaced some of the broken vacuum lines running to EGR valves. Rebuilt crankcase ventilation tube. Injectors rebuilt, cleaned and tested. New fuel pressure regulator. All the wiring has been checked. MAF sensors wiring have been rebuilt by previous owner. TB wiring is with the older "pancake" style connector, so those should be bulletproof as they say. Checked and they look like new.

You Tube

You Tube

bborden51

You say "All the wiring has been checked." Does that mean that the upper wiring harness (to the fuel injectors, temp sensors,etc.) has been rebuilt or replaced? How about the wiring from the MAF to the ECM? If not, that could be your problem. Even if it's not your current problem it will probably be in the future. My '93 600SEC ran perfectly but I rebuilt my harnesses anyway. Took me a weekend to do it. A moderately difficult task...not too bad. So glad I did.

JohnLane

OP it idles as one with a vacuum leak.

Current tech each not being able to find it does not make it any less obvious.

What are percentages of correction via closed loop? On which bank?

JohnLane

OP conspicuous by its absence is mention of fresh oxygen sensors!

EuroMerc600SEL

Apologies for not answering, all notices had gone to spam.

I was waiting for the OEM 38 pin cable to arrive, the one I got with the C3 was a complete crap, half the pins were not even connected.
Looks like the fuel trim corrections have been reset by the last tech. I need to do some driving. Last time On/off ratio was 50-60 on LH2 and 40ish on LH1. Now that I have the proper cable I’ll start again with full scan from cold start etc as per startekinfo data.
Startek says O2 sensor voltage should hover around 300mv when warm but mine are going all over the place from 80-800 back and forth.

EuroMerc600SEL

I’ll post more scan data probably end of next week when I get the car out of the garage and get it fully warmed up.
happy new year everyone!

JohnLane

OK so ‘end of next week’ has come and gone.

Was as thinking about this... Your ‘92 will have the four speed box. Vacuum line goes to the Vacuum modulator on the rear of the box is failing memory serves. This guy can make for a vacuum leak or the plastic hose going to it can. Transmission shifting normally?

In ‘92 we used vacuum all over the car for brake booster, door lock actuators, soft close doors, HVAC etc. All of those use a check valve so smoke applied via the intake manifold won’t find its way to what is leaking.

EuroMerc600SEL

Unfortunately I had issues with the Chinese rs232 to rs485 cable so everything got postponed. Couldn´t get any connection with the MUX, opened the RS 19 pin plug and discovered that half the wires were loose. Spent almost a week trying to find the connection diagram. Data from Stardiagnostic page was unhelpful, since the PCB board diagram didn´t match. Finally managed to resolder all the wires and get some live data from Star. Meanwhile also real winter temperatures decided to show up (below -25 Celsius), so apologies for clicking through some of the pages fairly fast, it was freaking freezing outside. Managed to go for a longer drive yesterday so the car could make some fuel adaptations. LH1 idle fuel correction was -2.5 kg/h and LH2 correction -0.6 kg/h. The RS cable broke again so I´ll have to fix it again to get the Star working properly and see what the latest live data is showing.

Thank you for the idea of checking the vacuum lines going to the transmission since I noticed now that shifting from 2nd to 3rd is smooth but shifting from 3rd to 4th is harder with a glunk sound.

On/Off ratios were rather low before adaptations but now hovering around 45-50 with a warm engine.

Surging idle cold engine

Efp actual values

warm engine

live data

MooksM275

Hey,

I handled a similar issue on our 93 600. Car has 47k miles.
Engine had been completely gone through and removed from car all new gaskets. It is like a refurbished engine at this point.
All wiring replaced, throttles rewired. All sensors new. All vacuum replaced. New cap and rotors, new coils, new plugs. Had very similar idle to yours with surging. Went through all the checks all testing, swapped modules, swapped ecus, new MAFs
Cam sensors swapped with know good ones, crank sensors, ignition modules etc. Blew my mind.
New fuel pumps, new filters, etc etc

Ended up being bad plug wires. The most annoying part is that we tested the wires with ohm meter and all checked out. Wires visually look perfect.
What brought me to plug wire is that I bought new set, and swapped one by one and noticed massive changes when doing this. Tested 100% by running new wires over the top of the engine (without removing intake manifold)
Wires have no visible degradation, test perfectly on ohm meter, and have only 47k miles but it is an age thing.
Surge gone, no idle issue.

Hope this helps you get it 100%

bborden51

I had a slight miss in my V12 when I first bought it. It was a bad plug wire...just one. I started the engine in garage at night and found the spark grounding against the intake. You need a good pair of eyes because the spark can be quite faint. Once I thought I had found the bad wire I used a long screwdriver to enhance the spark somewhat.

It's difficult to even feel a slight miss with a V12 with all of those cylinders firing. You can pull a plug wire off of the spark plug and it hardly makes a difference.

EuroMerc600SEL

thanks for the info. I have a full set of brand new plug wires available, but noone has suggested to just try and change them out since the originals still look perfectly fine. STAR live data shows EZ2 cylinders 7-12 ignition voltage all between 32.8-33V. For some reason can’t access EZ1 but I suspect that’s something to do with the crappy chinese RS232 cable.
Any chance the EGR system could be the reason? Fuel economy is pretty bad, but haven’t had a chance to drive the car properly to really know the difference. Pretty strong fuel odour also from exhaust. Running on Magnaflow sports cats.