HELP! '95 SL500 backfiring, stalling

[raftel]

Decided to take my mother's '95 SL500 with 53K miles on a short run. It hadn't been started in six weeks. Clear weather, 60 degrees F. For the first ten minutes, running through light traffic, it was fine. Then traffic started getting heavier, and it started to stutter. It got to the point of being labored leaving a light. For the last part of the trip, it was backfiring, felt like not all cylinders were firing all the time, and would not pick up from a stop. It also smelled awful - a rich/sulfur smell.

No CEL, oil pressure/water temp were fine. Is this just a question of old gas and not using the car enough, or do I have other issues to look into? I'd really like to get this car ready for the season.

Thanks!

[elau]

Sounds like stale gas and moisture in the system. Then again I was not there to get first hand information. Try to run it on a highway with someone following just in case. Sometime the 'ole girl may just need to stretch her legs a little and blws the coweb out.

[raftel]

Thanks elau, I sincerely hope you are right. I recently bought this car for my mother since she still has her faculties and can drive well. But she is apparently wiser than me, and refuses to drive it. If I can't get her interested in using it when the weather warms, I may end up having to post it here.

I took the car so I can get it to an upholstery shop to get the top fixed (typical plastic/top seperation). When I got the car to my home, it was so weak it could barely climb the minor hill up to my garage, and once I stopped it, it refused to start again, although it did manage to backfire a few more times.

I've heard mention of moisture in the fuel system, and the use of dry gas to deal with it. Has anyone here tried using Seafoam on these cars?

I'll top it off with fresh super tomorrow, and give it a 30 mile highway run tomorrow.

[bwheitman]

Sounds to me the engine went into limp mode. If the engine computer senses any arcing or misfire it will go into limp. Check your plug wires and make sure none are lose, also check the distributor (I think the 95 had a distributor) and make sure there is no moisture in the cap. Bad gas could have clogged the filter. Have you replaced the harness on that engine yet? If not it is something you will need to do. Good luck!
Brian

[bwheitman]

Oh, if your harness is original I would recommend not driving the car until it is replaced. If you incur arcing or grounding it will ruin your computers. It can also ruin your idle control valves which are very expensive. A friend had a 94 SL600 where the harness failed and it cost him $3500 to replace the idle control valves, as well as the computer. There are 2 in a 600.
Brian

[raftel]

bwheitman -
That's a pretty scary diagnosis, but wouldn't the computer going into limp mode trigger a CEL? I've pulled the air cleaner and looked at whatever wiring I could, and it looked fine. The car was from Pennsylvania, and I understood that the fried harness problem was more of an issue in hotter parts of the country.

The cost of electronics is one reason I went with the 500 vice the 600. Consider it a different circle of hell

Ignition given the miles on the car is probably original, and I could probably stand to replace the caps and rotors for a start.

I should mention that when this has happened before (and it has the few times I've driven it), letting the car sit for ~30 minutes and restarting seems to clear things up. Unfortunately for diagnosis, it tends to get laid up for a few more weeks at that point and the cycle repeats.

[elau]

I think the symptons are more fuel and/or plugs related. Before going to the major expense, you should start with the obvious. As bwheitman mentioned, there is a chance that your distributor cap has a hair line fracture that cannot be seen with naked eyes and mositure can collect. If the car has not had a tune up, it properly won't hurt to get one. As far as wiring harness is concerned, you will probably see other weird things. Good luck and keep us posted.

[Pete Geither]

Mine did the same exact thing and plugs, caps amd rotors fixed it fine. Inside of the caps were covered with green crap.

[raftel]

Drove the car to work today - 45 F, clear and sunny.

Car started fine, with no trace of stumble or smell. 30 mile drive to work, mostly at a 65 mph cruise. No problems. Even going up fairly steep grades at speed in top gear, no hesitation or stumble.

Warm idle is still very low - running between 4-500 rpm. As a consequence, it is a little rough at idle and the oil pressure at that rpm is just above 1 bar.

Filled the car up with fresh 93 octane - the car was 3/8 full. Perhaps having a close to empty tank sit around in an unheated garage while the temperature and humidity are zinging around led to massive condensation problems and subsequent misfire. Perhaps a good dose of dry-gas if the car is going to sit around in the future?

I'll try to get a look at the caps and rotors, I presume they have never been off.

[Pete Geither]

My caps and rotors were shot at 32K.

[raftel]

The car continued to run well when driven home, but just to be sure I pulled to distributor caps off this weekend. Sure enough, lots of pitting on the rotor and distributor cap contacts, lots of green corrosion. The caps were all cracked at the base where they bolt to the head. From the dates of manufacture stamped on the OEM parts, I know they were never changed. I will order new ones tomorrow, as well as plugs. The wires look fine. I haven't checked them for resistance, but at $200+ for replacement, I'll wait to see if the problem continues.

In getting the parts off, I ingeniously managed to strip the head on one of the 3 mm bolts that secure the rotor. Any good ideas on extraction? Thanks!