Hello everyone. My name is Bas. Wanted to say hi! Just bought my 500sl about 2 months ago.
I have a 1990 500sl. It takes awhile to start from cold. Will start up no problems when warmed up. Starts to run hot at about 105 degrees plus when in traffic cools off on highway speeds. Also accelerates sluggish when hot. I have replaced all temp sensors, hoses no more vacuum leaks, new fan clutch, aux fans come on. No smoke from exhaust.
Idle is goes up and down. 600-650rpm. could it be fuel related. Spark plugs look good. Seems to run worse when outdoor temperature gets hotter above 80 degrees. What should I look for? Thanks in advance
hard to start
surging idle
runs rough
smell of unburnt gas

Check cold start injector system - be sure it is working, don't just replace parts.

And electrically test temperature sensors at the other end of the wiring from the sensors - be sure there is no problem with the wiring or the sensors and that the sensors are giving the correct readings for what the engine temperature is. (Use a separate thermometer to determine engine temperature.) Be sure those are the correct temperature sensors.

And check/replace the radiator cooling system mechanical thermostat. Be sure it is opening when it should.

Be sure electric fans (if you have those for the radiator) are working when they should. And/or mechanical fan clutch is working properly if you have that.

Overheating can also be due to a clogged radiator (replace) and/or wrong temperature indication on your instrument temperature gauge [due to wrong or faulty temperature sensor]. Verify with another thermometer the actual temperature of the engine.

Thanks for getting back to me. Yes the fan clutch is new, thermo is working too. Could I use a laser temp reader aimed at the radiator to make sure its the right temperature? How do I check the cold start valve?

The car sometimes runs good. Other times when it heats up and outside temp is high it would lose power.
Do you know of any places to buy cheaper radiators?

Yes any secondary thermometer can be used to double check the temperature - laser temp, wife's meat thermometer, etc. (could get in trouble with the wife if using the latter! )

And I just noticed you are in Tucson. There used to be all sorts of independent radiator repair shops there. They can actually take one apart, repair it, then put it back together! But these days you should be able to go to any auto parts store and get a new radiator for a reasonable price.

Also just seeing the name "Tucson" suggests getting a new radiator anyway - for any older car! You need the engine cooling system to be in tip-top shape to work in the summer there. And even then they don't always work too well with the high summer heat. Stop-and-go city traffic requires a well working electric fan/fan clutch system.

So far as checking the cold start valve, read everything you can find (factory service manual) on how it should work - then see if it is doing so when it should be.

You may find information on that in a search or in a youtube video. Mercedes has similar gadgets for various years, so sometimes searching for [mercedes cold start injector] (leave out year) might find some good videos or information. Searching for a specific year/model will remove a lot of good writeup's.

Also the injection system is called a Bosch KE Jetronic mechanical fuel injection. Other cars like Rolls Royce, Porsche, and others used the same system. So a search for KE Jetronic cold start injector might find some troubleshooting/operation/testing information or videos.

Learn all you can. Read, read read!

FYI - Following is a picture of the inside of an old clogged radiator. Coolant is supposed to flow through the holes, but if they are blocked like in the picture, then very little flow and very little cooling of the coolant/engine takes place...

http://www.bernardembden.com/xjs/radconvert/clogged.jpg

Thanks for the advise. I will probably buy a new radiator as sl still has the original. Yes Tucson can get super hot in the summer 100 plus for months. Most of the components in the cooling system have been replaced except for the radiator. But could the hot running condition be attributed to the engine not running as it should be? Idle just goes up and down.

BTW I don't know if previous owner used distilled water or tap water. It looks like that radiator had lots of deposits from the water. Tucson water is hard.

P.S. Another thing to check for cold starting problems is an "internal fuel system leak". That is a "fuel damper" or "fuel accumulator" or check valve leak in the fuel system.

Normally the fuel system will retain pressure with the car off. But if there is an "internal leak", then there is no fuel pressure when you go to start the car, thus poor cold starting.

Here is an example of a service manual write-up for testing that - (maybe not for your specific car, be sure to get the factory service manual for your specific car).

07.3-120 - Testing fuel pressures and internal leakage...
http://www.benzworld.org/forums/attachments/w126-s-se-sec-sel-sd/1597065d1441366173-hello-orange-county-ca-fuel-pressure-gasser-126-073-120.pdf

So far as that goes, and being as you just bought the car...

I would first be sure the engine cooling system is working as it should.

Then check out the cold starting issue as well as fuel pressures and internal leakage.

Then next I would do a general tune-up, replace filters, fuel filter, air filter, oil filter, change oil, replace spark plugs* and distributor cap/rotor.

*And BE DARN sure to use the proper spark plugs as specified for your car. My car requires the non-resistor type and is much happier with those. Don't listen to auto parts guys and note the previous owner may have installed the wrong plugs!

Search on the internet forums for this. Find what people have to say by including the word "forum". Like

forum Mercedes W126 spark plugs

Using the word forum will weed out all the auto parts stores selling spark plugs, etc.

W126 is another name for the model years of your car...
https://en.wikipedia.org/wiki/Mercedes-Benz_W126

(Again, read, read, read.)

If you still have the idle up/down issue after the above, THEN start troubleshooting other things for that problem.

You are awesome Elektri. I will check everything you stated. That's what I was thinking about the fuel system. Thanks for all the tips. I will keep you posted

Thank you! FYI- Harbor freight has a fuel pressure test kit which has an adapter for a Bosch KE Jetronic fuel distributor (Adapter #17 in set [8mm - 1.00]). And for a reasonable price!

http://www.harborfreight.com/master-fuel-injection-pressure-test-kit-97706.html

Also I was just thinking about your "new car"...

Another thing to do since you just bought the car is to check for vacuum leaks. And clean every electrical connection you find when you come across them.

The vacuum hoses will get brittle with age and cause vacuum leaks. And I suppose especially in dry Arizona! Note the economy gauge on your dash is actually just a vacuum gauge. And that should be all the way to the left pointing to the white line at idle. Like this...

http://www.peachparts.com/shopforum/attachments/diesel-discussion/53230d1205607397-got-myself-economy-gauge-w123-lmn-215000miles.jpg

(And of course move to the right as you step on the gas.)

There are vacuum hoses running all over the place under the hood. The black rubber hoses are the ones which get brittle and need replacement.

When I bought my car, I just bought several feet of each size vacuum hose at the auto parts store (smaller sizes) and kept that on hand. Then when I would be working somewhere on the car and came across a brittle line, I could easily replace it. I don't replace the tiny hard plastic lines as those are in good condition on my car.

Anyway these cars are notorious for vacuum leaks if those hoses have not been kept up to par. Many Mercedes owners are good at maintenance too, so you could have all new hoses.

There are also vacuum hoses under the dash and are used for the air conditioning system.

Note the door locks are also vacuum operated - by a vacuum pump in the trunk. That is TOTALLY SEPARATE from the engine vacuum system.

Then the other thing is the electrical connections get poor with age. There are electrical connectors for this and that ALL OVER THE PLACE!

Anyway I keep a can of electrical parts cleaner on hand and clean each electrical connection I find as I work on the car. That can solve problems and also prevent problems.

Note that the factory service manuals will be in English, but the wire color abbreviations on some wiring diagrams are in German! Here is a translation...

BL blau blue
BR braun brown
GE gelb yellow
GR grau gray
GN grun green
RT rot red
SW schwartz black
WS weiss white
VI violett violet

Thanks again. I had no idea that the MPG guage could tell you about Vacuum. Shows I have a lot to learn about this beast of a machine. One question everytime I accelerate hit the gas hard my climate control blower stops, then I let off the gas its back on? Does this have to do with the vacuum? I will tell you it seems like you are a expert on these cars. Thanks

I can imagine the air is being redirected say from the front vents to the floor/defrost vents and that could be a vacuum leak problem (not enough vacuum to properly operate climate control). And the fan, which is electrically controlled would continue to run at the same speed.

But if the fan is in fact stopping...

First I would look at the factory wiring diagrams and see if there is any "energy saving" feature for your specific model which would do this intentionally. On many cars, WOT [Wide Open Throttle] will shut down the AC compressor to give the engine more power. I've not seen one which also turns off the inside fan?

But that can be tracked down with a voltmeter and factory wiring diagrams. Learning how it is supposed to work and then determining if this is a feature or a malfunction.

Yes, finally I have the weekend to figure all the faults. I appreciate all your advise! I will try to figure out the wiring and fuel trim. I will keep you posted.

Regarding the cold start issue, search for a thread started by me about a year ago "fuel accumulator failure analysis," and run the simple test to check yours. The cold start injector valve is not activated unless the ambient temp is below about 32F.

Chances are the radiator has a lot of built up deposits if antifreeze changes have been inadequate or "green" antifreeze has been used, which can cause a lot of deposit buildup due to all the silicates. These deposits are insulators and will drastically reduce heat transfer.

There are some shops that claim they can remove the plastic side tanks, rod out the aluminum core, and then install new plastic side tanks and seals with no leaks, but you are probably better off in the long run buying a new Behr radiator. If it's the OE radiator the side tank seals will eventually leak and the hot side nipple has been known to break off when changing hoses or even driving down the road. Radiators don't last forever and getting 26 years out of an original is not bad.

Use Zerex G-05, which is the original BASF patented HOAT blend that was OE in that era - a 50/50 blend with distilled water, and change it at least every five years regardless of mileage.

Duke