Hesitation and rough idle when warm

[Han Dagith]

Hello,
I'm a relatively new owner of a 500sl (1992). The car starts fine, runs fine, and looks great (it only has 35,000km), but once it gets fully warmed up (at least 45 minutes of driving), it starts to hesitate. There is a noticeable lag when I hit the accelerator, and it almost seems to cough before it kicks in. Also, it begins to idle really roughly although it doesn't stall.

I've already replaced the fuel distributor and rotors, I've checked the plugs and they're fine, and I've run a bottle of injector cleaner through it--all to no avail. Can you suggest anything?

Thanks,
Han

[tecqboy]

Well, the engine coolent temperature sensor should tell the ECU when the engine is warmed up and adjust the mixture leaner. Sounds like you are running a bit rich when warm. Do you smell gas at idle? But, it seems odd to me that it would take 45 minutes to get warmed up. Even 10 minutes would be a long time. Also, possible O2 sensor. Any other opinions from the group?

[ctaylor738]

Did you mean "distributor caps and rotors"?

[Han Dagith]

Yeah. The mechanic found a fuel leak around the rotors and replaced them along with the distributor caps (apparently there are two?). This seemed to fix the problem, but I think all it did was made it show up much later (after the car was quite warm). Because it's still giving me trouble. He said the next step would be to replace the injectors, but I'm wondering if there may be some other ideas. I hate throwing parts at a problem.

Thanks,
Han

[ctaylor738]

The rotors are an ignition part. They turn inside the distributor caps I don't see how there would be fuel leakage around them.

Anyway, it's interesting that you report the problem at around 45 minutes, because anything warm-up related is long over by then. Try this - when the car starts to stumble, find a safe place to pull over, shut it down, and let it sit for 5 minutes. Then start up and drive away.

If the car runs OK away for a while, you probably have a fuel delivery problem. The engine is slowly running out of fuel. This actually happened to me with a 380SL that sat for a long time - at about 25 minutes after starting, same symptoms as you describe.

To fix, you need to drain the tank, remove and clean out the strainer, and replace the filter. You should also replace any fuel lines that get unhooked in the process.

Do keep us posted.

[aam]

Originally Posted by Han Dagith

Yeah. The mechanic found a fuel leak around the rotors and replaced them along with the distributor caps (apparently there are two?). This seemed to fix the problem, but I think all it did was made it show up much later (after the car was quite warm). Because it's still giving me trouble. He said the next step would be to replace the injectors, but I'm wondering if there may be some other ideas. I hate throwing parts at a problem.

Thanks,
Han

We need more details in order to help you.
There is no fuel at the rotors, oil maybe?
The part with many small pipes, going to the injectors, behind the air filter is your fuel distributor did he work on it, It is expensive.
The rotors have been upgraded to heavier kind with new adapters.
You must install the new, if you don't have them you will have problems like
you are describing.

[rocky raccoon]

See my post of 17 November. Check your engine temperature sensor.
Rocky Raccoon

[O-Zone]

Hello, I,ve been searching for a solution to a problem with my newly aquired 82 380 Sl, same issues, car starts immeiditly, but as soon as starts to reach temp it runs very rich and has no power on hills, or to pull away. Stop the car ,and let sit and it runs well for short time, then the problem returns.... fuel problem or electrics HELP.......

[ctaylor738]

Whenever I hear that a car starts to run badly after a more-or-less predicable time period, then is OK after it sits for a while, I recommend checking out the fuel supply. I would start by removing the tank strainer to see if it is clogged. If it is clogged by rust, then you will need to remove the tank and have it boiled and coated.

Put it back together with new fuel lines to the pump, and a new filter.

Sorry, not a pleasant job, but IMO it's the place to start.

[O-Zone]

Thanks for the reply, I agree this is a good place to start, how big a job to take the tank out,and does it come out through the trunk. Can you steer me to info on how this is done.... Thanks Mike

[bozemanbiofuels]

I am almost sure my 87 300D is fuel starving, as the cheap plastic filter was full of gunk and even after I change that it still starves. I think there is a little filter sock over the fuel pickup in the tank but don't know how to access that.

That's what high concentrations of biodiesel will do - and I know it's a diesel fuel problem b/c I can switch to svo and it runs fine.

So more info on draining the tank and cleaning the strainer would be great!
Actually, here is the link for cleaning/replacing fuel tank screen:
W123 Fuel Tank Screen Replacement

Thanks, Paul

[ctaylor738]

The basic drill is to remove the carpet and the panel at the front of the trunk compartment, unhook all the vent lines, the gasket around the filler neck, the tank outlet and return lines, and then to move the tank to the left to clear the filler, and remove it toward the rear. A lot of work, but no rocket science.

Here is a post that we did on some of the finer points.

107 Fuel Tank Removal & Install Lessons

[rocky raccoon]

Please look at my post of 11-17-2007 "560SL rich running after warm-up". There are of course differences between the 380SL and the 560SL but I believe the fuel injection is very similar. I hope it helps.

Rocky Raccoon

[O-Zone]

Thanks Mr. Racoon, I think I will replace the engine temp sensor, the car seemed to work worse as I went north, Virgina doesn,t have the temp changes we do, probally never changed, I like the prices in the U"S better than ours, wish I had picked one up on the trip home, I live in rural Nova Scotia, every thing is hard to get...... Thanks Mike

[ctaylor738]

Checking out temperature-controlled enrichment is always a good idea.

But don't look for the 560's big temperature sensor on the 380 - it doesn't have one. On the 380's, warm-enrichment is controlled by the warm-up regulator, which uses a bimetallic piece of metal pressing against a diaphragm to change control pressure in the fuel distributor.

This is one of the major differences between CIS K-Jet and CIS KE-Jet, which Mercedes started using in 1986.