92 500sl Poor Idle Quality, Cuts out, Misfire - PeachParts Mercedes-Benz Forum

lymlyt · #1 06-05-2008, 09:51 AM

Posted in tech help too

All-
I could use some expert input/adviseideas. The following is from
my mechanic describing the problem condition and test
and diagnostic info. Any help would be much appreciated.

thanks!

-andy

1992 Mercedes-Benz 500SL

Engine: 5.0 L / 8 cyl / Gas / DOHC
Fuel: Fuel Injection / CIS Fuel Injection
Ignition: Distributor-Breakerless / Dual Distributors W/2 coils
Trans: 4-speed Automatic Transmission
Mileage: 94,000 miles
Emissions: OBD1
VIN: WDBFA66E8N

Misc Data: 500sl
Symptoms: Poor Idle Quality, Cuts out, Misfire
Occurs: After a Hot Soak
More Info: Test results & repair history included below

____________________________________________________________

The vehicle will start and run fine. After running for 30
minutes or more in hot weather..over 75 degrees.. if the
engine is shut off and sits for 10 minutes, it will restart
fine but has a bad missfire nit seems like on one cylinder
but sometimes seems like multiple cylinders. The missfire
is very easily noticed at stop when reving up the engine
and can also be easily felt when driving the car. It will
continue to miss even after hard accellerating for up to 2
minutes. Then it will smooth out and be fine for the
duration of the drive. This situation will repeat itself
always after a 10 minute HOT SOAK after driving in hot
weather. The coils and injectors and wires etc. have all
been tested. I cannot see the missfire on the scope. No
codes set during the short misfire period. What can be
happening during this period when the engine compartment is
so hot with no air moving that corrects itself after 2
minutes of driving???

___________________ TEST AND REPAIR DATA ___________________

Checked for codes and none were found.

TESTING AND RESULTS

fuel pressure - ok
coil output - good

RELATED REPAIR HISTORY

New O2 sensor. New spark plugs. 1 new injector #5. New
hydraulic pressure regulator..was leaking. Wires 7k old.

rocky raccoon · #2 06-05-2008, 08:28 PM

Sounds like a possible hairline crack in distributor cap. The crack expands with high temperature. If so, you will probably find carbon tracking inside the distributor cap also.

Pete Geither · #3 06-05-2008, 09:04 PM

Quote:

Originally Posted by rocky raccoon

Sounds like a possible hairline crack in distributor cap. The crack expands with high temperature. If so, you will probably find carbon tracking inside the distributor cap also.

What he said,,,,, distributor caps are a very big problem on the 119's.

lymlyt · #4 06-06-2008, 08:47 PM

thanks, mechanic replaced both caps/rotors-no luck. Any other ideas?

lymlyt · #5 06-12-2008, 06:02 PM

Update:

My indy replaced 8 plugs, both caps & rotors, wires, both coils,
all 8 injectors and three days trying to isolate/correct the problem but no luck-
he is referring me to Mercedes. We're both frustrated and disappointed.

Mechanic is excellent and a great guy, no charge for time and parts-
all returned back to stock.

I could really use some expert advise.

thanks in advance!

-andy

nt-j · #6 06-20-2008, 09:15 AM

Try the fuel divider, that was the problem in my 91 500sl, this after replacing all plugs,leads and dist.caps etc.
Very expensive if from a main dealer, got mine from a breakers yard.

rsmcmahon · #7 07-07-2008, 03:07 PM

Did you find a solution for your problem. Same problem for me now

After running for 30
minutes or more in hot weather..over 75 degrees.. if the
engine is shut off and sits for 10 minutes, it will restart
fine but has a bad missfire nit seems like on one cylinder
but sometimes seems like multiple cylinders.

400se 1992

rocky raccoon · #8 07-07-2008, 07:02 PM

I guess my distributor cap notion was wrong but you still have an apparent heat related problem. I had something similar on my 88 560SL. I isolated the fault to the engine temperature sensor that provides temp input to the fuel injection computer. I see that your mech has been mainly chasing ignition issues rather than heat related driveability issues.

The sensor location on a 560SL is at the front of the intake manifold, just under the front edge of the air cleaner housing. It has two wires attached. It is easily testable but I don't have the data in front of me. It can be found on ALLDATDIY or on your CD tech manual if you have one. The sensor is not very expensive (about $40 as I recall). Replacing it and fixing a vacuum leak made my car run like new.

lymlyt · #9 07-08-2008, 10:27 PM

Sorry for the delay in responding but lots has been going on.

To recap, my local indy couldn't locate the problem, 2 weeks in the
shop, chasing ignition/fuel delivery, replacing numerous parts, leaving
some on, taking some back, no charge by the way, but he gave up.

Instead of taking it to the dreaded dealer, I took it to a different
indy that specializes in Mercedes.

Summary:
Condition on way to different indy: Cold star rumble, 2-3 secs, after warm up, car would occasionaly mis fire, in drive, at idle , waiting for red light, hesitate on accelleration, but eventually (15 mins later) run fine. Noticed oil guage drop below one bar, and idle would "hunt" in park/nuetral.

New indy had the car for a week and ran numerous tests and concluded that
the fuel distributor was lthe likely cause. Once replaced the car idled fine, no mis fire, and no hesitation. I tested the car for about 30 mins, got it hot, and all seemed fined although I had feeling the car wasn't as "quick" as it was before. After paying the bill and the car sitting for 10-15 mins, I started to drive off and the hesitaion was back. Indy drove and confirmed
hesitation. Indy replaced fuel accumultor with used spare, problem was worse. Replaced with the original and problen was the same: worse. Indy kept the car for a couple of more days. In the end, I was told: "fuel distributor and fuel filter was replaced and air/gas ratio was set to address idle and hesitation problem. Tomorrow will be two weeks and so far so good but I haven't had some real long drives but I haven't experienced the problem like I normally would have based on my "normal" driving habits.

Current status: initial "rumble" at cold start for 2-3 secs. then stable idle, idle also stable after warm up, no hesitation, oil guage still drops beow 1 when hot, performance is strong.

I know this is not specific but the owner/mechanic has left for Italy and I haven't talked with him directly. This is what I got from his son.

Comment/ questions welcomed.

thanks

-andy

LeaUK · #10 07-12-2008, 03:01 AM

Upon reading this thread I would have instantly said distributor caps and inner cracking. I have exactly the same issue in terms of running fine from start, then 15 minutes after misfire, heightened by engine off for 10 minutes then restart.

The cause is definitely tracking in the distribs as once this is cleaned no problems for 4-6 months. The reason why the issue is exacerbated after leaving the engine off for 10 minutes is presumably the build up of moisture in the cap feeding the tracking issue!

However, you story adds yet a further dimension - and sorry I can't offer any advice.

Lea

lymlyt · #11 03-15-2009, 10:54 PM

Well, my problem came back in late fall, too far into the winter season to address, here's a summary now:

Cold Start: 1-2 sec crank, 3-4 sec stumble/mis, then OK at 900rpm

Drive-650 rpm

Scenarios:

1. Temp at 80 +, 20-30 mins of driving. Let sit 10-15 mins (hot soak),
starts up, 1 sec of stumble mis, 10-20 sec hesitation/mis, then OK.

2. Temp at 80 +, 45 mins driving. Let sit 45 mins. 3 sec of stumble mis, 1-2 minutes of hesitation/mis, then OK for 7-8 mins, then hesitation is
back but 8-10 mins later its gone.

This is driving me crazy. I've spent alot of time here researching the posts and I'v come up with a list of suspects:

1. OVP: I've checked mine and fuse was good but it was not mounted-
it was dangling free....any chance of damage...?

2. Crank Position Sensor

3. Fuel accumulator

5. Engine temp sensor

I could really use some expert advise. Update on what's been replaced
from previous post:

My indy replaced 8 plugs, both caps & rotors, wires, both coils,
all 8 injectors and three days trying to isolate/correct the problem but no luck-
he is referring me to Mercedes. We're both frustrated and disappointed.

Mechanic is excellent and a great guy, no charge for time and parts-
all returned back to stock.

My last entry on this post:

Sorry for the delay in responding but lots has been going on.

To recap, my local indy couldn't locate the problem, 2 weeks in the
shop, chasing ignition/fuel delivery, replacing numerous parts, leaving
some on, taking some back, no charge by the way, but he gave up.

Instead of taking it to the dreaded dealer, I took it to a different
indy that specializes in Mercedes.

Summary:
Condition on way to different indy: Cold star rumble, 2-3 secs, after warm up, car would occasionaly mis fire, in drive, at idle , waiting for red light, hesitate on accelleration, but eventually (15 mins later) run fine. Noticed oil guage drop below one bar, and idle would "hunt" in park/nuetral.

New indy had the car for a week and ran numerous tests and concluded that
the fuel distributor was lthe likely cause. Once replaced the car idled fine, no mis fire, and no hesitation. I tested the car for about 30 mins, got it hot, and all seemed fined although I had feeling the car wasn't as "quick" as it was before. After paying the bill and the car sitting for 10-15 mins, I started to drive off and the hesitaion was back. Indy drove and confirmed
hesitation. Indy replaced fuel accumultor with used spare, problem was worse. Replaced with the original and problen was the same: worse. Indy kept the car for a couple of more days. In the end, I was told: "fuel distributor and fuel filter was replaced and air/gas ratio was set to address idle and hesitation problem. Tomorrow will be two weeks and so far so good but I haven't had some real long drives but I haven't experienced the problem like I normally would have based on my "normal" driving habits.

Current status: initial "rumble" at cold start for 2-3 secs. then stable idle, idle also stable after warm up, no hesitation, oil guage still drops beow 1 when hot, performance is strong.

lymlyt · #12 03-19-2009, 03:20 PM

OK, I'm schedule for the mercedes dealer next Tuesday. I'll post results. Hopefully this will do it. Any predictions on the cause of the problem...?

-andy

Cal Learner · #13 03-19-2009, 05:25 PM

Sorry for intruding on the SL forum, but I'm intrigued by your symptoms, having tried in vain with what might be a similar problem in my W124. Check this thread posted in the Tech Help forum:

103 Runs rough after prolonged hi-speed driving

Does any of this sound similar? I wouldn't call my symptoms mis-fires, they're really rough idling and terrible stumbling on acceleration from a stop. But some of it may be traceable to the same electrical or fuel components. I'll be watching for you report on your "fix". Hope the dealer can get it right for you.

lymlyt · #14 04-17-2009, 12:21 PM

Finally Fixed:

My INDY solved the problem:

Bad Fuel Pressure regulator and fuel injectors.

Eugene · #15 07-26-2009, 11:01 AM

I have the same problem with my 94 500SL.
It sounds like you had the injectors replaced without solving the problem prior to the dealer.If this is correct,do you believe replacing the fuel pressure regulator solved the problem? or replacing both.
Thank you