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Hello all. First time posting. FATHERTIME kindly let me know that MBWorld is the place to eb and not Benzworld. So here I am!
Im looking for some help and advice on my 98 Mercedes SL600. I loaned the car to a friend over a year ago and got it back with some very rough idle and low power. It has 120k miles.
In the last year ive been slowly working through all the common problems listed by many of the folk on this forum.
I have replaced both upstream O2 sensors. Bank 2 (Driver side) is showing a 0 voltage reading on my MB2 scanner. I have pulled apart the interior of the car and traced the wires to the ECU and there is continuity between the O2 and ECU. (I tested the O2 on the other side so I know it works.)
I have checked for vacuum leaks both with carb cleaner (spraying maybe 4 full cans over parts of the engine without change) and with my vacuum pump on the EVAP lines, vacuum lines and EGR valves, I even removed the vacuum box and capped the port on the intake to rule out any interior leaks.
Pulled exhausts and I can see through the cats, don't look clogged.
I have checked all coils with a plug to ground and they all spark.
I've pulled the intake and checked the crankcase ventilation tube. Its good.
he car does go into Closed Loop .Sometimes it trims alot of fuel from one bank, but that problem seems to have disappeared as I replaced two coils yesterday. (Had codes for a couple misfire and replacing coils fixed those)
Things I haven't done is change the fuel filter, I checked pressure and it is constant, so doesn't seem like a fuel flow issue. I have read a few small bites on here about temp sensors causing issues but nothing broken down, seems like a strange place to look.
I got so frustrated that I took it into MB. They did some testing but wanted other $2k to test more. The car isn't worth that kind of money. They suggested it could be the ME or engine harness. So I took off all the old tape from the harness, checked it thoroughly and re-taped the entire harness. I have checked the TCU unit for oil, cleaned the connectors, both in the passenger footwell and every single plug in the engine bay.
The car is currently giving me no codes but still runs rough.
Ive been plodding along at this for over a year now and am at a bit of a loss. Any and all ideas are welcome. Thank you.
Can be MAF's, Air temp sensors . However I would do a smoke test to be sure about vacuum leaks before throwing any parts at it. You can buy a tester on eBay cheap and cheap air compressor at Harbor Freight
SDS diagnosis I think would tell the story but not at 2k
Can you find a local independent shop with SDS? One thing about any shop including dealers is not many have experience with these cars some have never had one in the shop
When the MAFs go (or just one) they don't throw a CEL. When pulling up to a stop sign the car's idle will jump up and down with enough force (as it's trying to recover it's idle) that the car wants to move. I had to shift the car into neutral to deal with the hunting idle.
If that sounds like your issue, then it might be the MAF, but Star Diagnostics will rat out bad MAFs and should be able to tell you what's going on with your engine,
Juuso on MBW has had a slew of pesky issues with his V12 and has had to fix some unusual items. You might want to troll through some of his posts and threads for insight,
The O2 sensor thing occurs on my car too, one bank doesn't show up on my OBD II tester. I don't get a CEL and the car runs great. I've never asked Juuso if one of his O2 banks doesn't show up, but it would be interesting to find out. I did all the common sense diagnostics like swapping the O2 sensors side to side and thought about checking the wiring to the ECU, but seeing as I wasn't getting an error, I let it go.
I had a problem with my crank sensors and didn't find out until a little after I changed the first one that there were 2.
If the Juuso posts don't provide some inspiration, you are going to have to hook it up to Star Diagnostics. Hopefully, at an indy shop.
The O2 sensor thing occurs on my car too, one bank doesn't show up on my OBD II tester. I don't get a CEL and the car runs great. I've never asked Juuso if one of his O2 banks doesn't show up, but it would be interesting to find out. I did all the common sense diagnostics like swapping the O2 sensors side to side and thought about checking the wiring to the ECU, but seeing as I wasn't getting an error, I let it go.
I haven't checked with a generic OBD II tester. They do show up with Star Diagnostics though.
I still haven't completely figured out my problem. I'm starting to think it's in my head though, and the occasional vibration is really quite subtle. It's certainly smoother and quieter than the 01 CL55 we got for my dad a while ago That one is quite a different beast by the way, even though with the NA 5.4 liter M113, the performance figures are pretty similar. Not by any means subtle & quiet like the V12. Anyway, offtopic...
Ps. funny coincidence - I don't come here often but I happened to stumble upon this thread from Google. Nice timing.
Thanks for the responses. I ended up picking up a set of MAFs today off a running car. No noticeable change. Think im going to have to find someone with the SDS here around Redondo Beach...
Smoke tested today with my home built smoke tester... No leaks from anywhere. Building the smoke machine was fun though!
Is there a chance the computer could have failed somehow? I traced wires from MAFs back to computer and O2s and they look good. Cant see inside the plugs that attach to the MAFs though - do they fail ever?
One of the problems I had, a recurring fault due to an open circuit in one of the intake air temperature sensors, was finally solved by cleaning the contacts of the ECUs. I disconnected the battery, and to be safe waited a longer time to make sure all the capacitors are discharged, and then sprayed contact cleaner liberally on both the ECU and car side connectors, re-seated the ECUs several times and sprayed again, repeating a few times. Then I let it all dry and put it back together. The fault hasn't come back since, and it's been more than a year.
I disconnected the battery after finding a bent pin in the EGS unit. I left the battery off overnight. Reconnected and it started but same problem. Thanks for the heads up though!
"very rough idle and low power" is strange normally it is one cylinder not working. Does the sound at the exhaust sounds normal? The pressure regulator is OK, no gasoline drips out of the line?
I disconnected the battery after finding a bent pin in the EGS unit. I left the battery off overnight. Reconnected and it started but same problem. Thanks for the heads up though!
03-29-2020, 01:24 PM
That one is quite a different beast by the way, even though with the NA 5.4 liter M113, the performance figures are pretty similar. Not by any means subtle & quiet like the V12. Anyway, offtopic...
