Good morning folks, I wonder if anyone can help. I am based in the Middle East (very hot) and own a beautiful 1992, 500SL.I have a problem that is intermittent and so far have had no luck with the ‘Mercedes mechanics’ here.I’d be grateful for any suggestions.
After the car has been driven, then parked for a couple of hours and whilst the engine is still warm, if I use it again, often there appears to be a series of power surges as it is running. The engine revs up then back to normal.This is very regular, like a pulse.As you can imagine, it makes parking in tight spaces a little tricky when it is doing this.I’ve noticed also that the temperature gauge climbs significantly when the car is in this condition.
If I drive on a long stretch of open road, this problem appears to go away after a few miles and the temperature gauge returns to normal, but in slow moving traffic, it becomes very worrying.
In this condition, there is also a dull spot when trying to accelerate from a standing start, which can be quite significant and scary especially when fast acceleration is required, at roundabouts etc. If I press the throttle pedal quite hard, the car will surge forward past the dull spot, but this means accelerating much harder than you’d normally expect to do.If I try to move away at a normal speed, the car feels like it is about to stall, before it eventually picks up.
Since all of these problems happen at the same time and only when the car engine is at normal running temperature, and even then, only intermittently, it leads me to conclude that it is something to do with the cooling system.
One of the difficulties I have had is getting to a garage whilst the car is having this problem, so that the mechanic can see and hear it for himself, but as the problem is intermittent this is proving very frustrating. There are also language barriers.
So, can a problem with the cooling system lead to the symptoms above? Or am my on the wrong track here.
I should point out that I have had the injectors replaced and the engine compression tested. Compression was fine and no water leaks in the head.Apparently!
Any suggestions will be warmly received.

Thank you

Rob

 

I'd start with the vacuum hoses and wiring, including connections, first. does it heat up when it idles normally? it should come up some in a hot climate. do your secondary fans come on?

 

Brookings, many thanks for your prompt reply. When the car idles normally, the temperature remains constant. I recently fitted a new radiator because it was overheating. Unfortunately when I have this intermittent problem, the car does not idle smoothly. There is a clear change in engine speed. It rises for a second or so and then drops to normal for a few more seconds, then rises again, and so on. At this point the temperature gauge starts to rise. I don't know how high it would go because I never let it run like that, but it does rise significantly. I'm not sure about the secondary fans. Could you advise how to check this? When the engine speeds up, it sounds as though the fan gets louder. Sorry about my lack of technical knowledge.

 

A quick update. I started the car and let it idle as you advised. I had the same uneven idling speed problem. I let it run and monitored the temp gauge. It climbed slowly to just under 100. It usually sits around 90. I found the secondary fans, but they were not operating. I did read somewhere that these fans do not start until the engine reaches 100. Perhaps that is the reason that they did not come on. Anyway, maybe there is no overheating problem after all, but it definitely seems to get hotter when I have the uneven engine performance issue. I'll keep checking to see if the temperature climbs above 100 and what the secondary fans do when it gets there. Can you advise the operating temperature for the secondary fans?


I took the car out on to an open stretch of road and drove for about 20 miles. The engine quickly settled down and operated normally (no power surges). The temperature came back down to around 90. Still has the dull spot when accelerating though.

This does not of course explain the problem of the uneven engine speed but at least if it does not lead to overheating, that's a weight off my mind.


Thanks again


Rob

 

assuming it is hot there all the time, I think your temp sounds ok. At night, does it cool down to around 70 degrees F? you should be at ABOUT 82 while driving and 90 while idling. on a hot day those numbers can go up another 5 to 10 or more. as long as you don't redline you should be ok. For your idle problem, Check vacuum lines! if hard or cracked, replace. Check around injectors to make sure they were replaced. spray carb cleaner around them and the vacuum hoses while idling. I'd replace the engine part of the ignition system, if you don't have service records. inspect anyway. If it still seems too hot. over heating at idle can be the fan clutch, faulty gauge sensor, maybe even the thermostat or some knucklehead used lots of "stop leak" on an engine leak and plugged it up someplace..like the radiator. none of which are hard to replace, as well as the engine ignition parts. running your car in the dark can reveal problems in your wiring.

 

Many thanks Brookings, I will check all of the things you suggested. I'm sure I'll get to the heart of the problem eventually, however, I wanted to ask, when the car is idling or surging erratically it definitely pushes the temperature up, even though it might not go dangerously high. When running smoothly the temperature remains steady. So, why the rise when running erratically? How does uneven running cause this?

 

all these issues can confuse the computer modules with misinformation, throwing off the timing causing it to labor causing over heating. get it running properly then see how the temperature runs.

 

Many thanks for your patience and advice guys. Much appreciated. I'll take the car to another mechanic at the weekend and see what happens. I will let you know.


Rob

 

I have a subscription to ALLDATA - OEM Auto Repair Information for Professionals. I find them very helpful and the more you can do yourself the more you will able to afford to drive that little treasure. The basic stuff like caps, rotors, wiring, hoses, a four wire temperature sensor, etc, isn't that hard to do. it costs about $15 a year for the subscription and it can save a bunch of money. it even gives what jobs should cost. a mechanic that offers you your old parts at the end of the job without asking for them has a big plus in his his favor. a good one is worth their weight in gold. save them for the job you aren't comfortable with. your sl is a very nice car, they sometimes try patience.