1991 500SL dropped 4 cyl

[chiwanga]

My SL started missing really badly. I smell unburned fuel, and the check engine light came on. I suspect it is a bad cap, so I ordered a new bosch unit. I am usually working only on diesel MB, so I do not have a whole lot of experience with gas. If you guys can give me any other pointers, I would greatly appreciate it. Thanks

[bwheitman]

It seems like you are going down the right road with looking at the ignition. If the CEL came on it is likely due to misfire. Don't run it as you will ruin your converters and those are a lot of dollars to replace. After the cap replacement check the wires and the plugs. It could be an arc that caused the limp mode to kick it, that is a safety mode that the computer will shut down half the engine to protect it. On 95 and newer it will actually shut the fuel off too, but not on earlier models with the fuel distributor, just the sparks get shut down. So check all the ignition out, if no problem there, look at your wiring and coils, then get the computer tested. There are plenty of diagrams on our website for you to review on engine controls.
Good luck!
Brian

[Mike Miller]

My 92 500 SL started misfiring some time ago. It would do it mostly when it was at full temperature and not all the time. They traced it to a bad rotor. Actually two bad rotors. The caps were fine. Car runs like a bear now. The early 500's were noted for bad rotors and the caps were not much better.

[chiwanga]

Ok, I did some diagnostics, and this is what I have found so far. I knew one bank was not firing. I pulled the center plug wire from the cap and found the car would not start when Pull the drivers side, so I know pass. side bank is the problem. I switched rotors, and still the same. I then switched caps, and still the same. I drove the car over to a shop, and on the way it had full power at times, and then it would start missing again. We will see what the mechanics says this week.

[chiwanga]

Finally. It was a bad coil. what a PITA. Thanks for all of your help. Now the belt tensioner has failed and the car squeels like a little girl. Any suggestions other than paying $150 for a new part and replacing it?

[Mike Miller]

Replace it.

[turbodiesel]

Interesting. Your problem was intermittant? How did you determine it was a bad coil?

[chiwanga]

My mechanic was going nuts, and so was I. I did my diagnostics, and could not find the problem. I eliminated the cap/rotor. He found that the car would still run when he cutt of spark to individual banks. he then checked the fuel pump and that was not it, so he replaced the coil and that fixed the problem. I think I will stick with my reliable diesels for now.

[Daytonaman]

See my post on spark control module. When it is intermittent or shot, one bank does not get spark. DON'T run it except for a short test; my car almost burned.

[LeaUK]

I would have diagnosed a tracking distributor cap and/or rotor too as with my 500 I have to treat these as service parts. Every 6 months I have to pull the caps, clean and reassemble - at 100GBP each, cleaning is the way ahead!

Just so happened this is to be carried out next week so she's be puring again shortly

I'll bear your bad coil experience in mind...

Lea

[LeaUK]

For those interested in distributor issues for cars upto '95:

Recently the car showed the classic signs of one bank misfiring so I carried out an inspection this evening.

Here's what I found:

After removing the caps several times over the last two years (after replacing both caps and rotors), I have found tracking from the outer contacts to the centre. The actual contacts and rotor look reasonably clean.

My cap cleaning process has been to spray with WD40 and remove the tracking with a rag. Reseal. Repeat next 6 months!

However, upon the last clean, I experimented on one of the caps and heavily roughed/scratched the surface directly beneath the tracking.

After investigation this evening (around 6-9 months later) I found this modified cap to be fine but the other non-scratched cap showing heavy tracking again and the responsible party for rough running.

Tonight I repeated the clean and scratching process on the remaining cap and I expect the car will be fine for significantly longer periods - only time will tell!

Lea

[roadodg]

I have a 1991 500SL and my mechanic just told me I should upgrade the coil, caps and rotors....$975. Otherwise I might damage something else much more expensive (I'll find out what that is tomorrow when I go pick up the car). I asked him to hold off on that right now, just paid him $1,600 last week for work and another $1,500 this week for stuff. Getting this old dog up to speed, I bought it a month ago and lots of this work is preventive maintenance some is stuff like new fuel lines, filter, seat back adjustments, door latches, etc.....we're getting there.

What do you think/know about upgrading the coil, caps and rotors? $975?

[deanyel]

Originally Posted by roadodg

What do you think/know about upgrading the coil, caps and rotors? $975?

Upgrading, or just replacing? Is this guesswork or diagnosis? Even if it's good solid diagnostics you could probably justify replacing the mechanic just based on the price.

[roadodg]

Originally Posted by deanyel

Upgrading, or just replacing? Is this guesswork or diagnosis? Even if it's good solid diagnostics you could probably justify replacing the mechanic just based on the price.

Well I find the mechanic off Benzworld.org forums...one of the three best / most recommended in Phoenix....he recommended upgrading the items as he says they are known to be a weak system on this year car.

What price is reasonable?

[deanyel]

Parts should be about $350-400, maybe a couple hours to install. Actually I think you could do it in a couple of hours. I'd first want to make sure that it all needed to be done. You have a PM.