Several months ago, I accidentally put regular gasonline into my 81 300D. I had the tank drained and even the fuel pump replaced. The car runs very well around town. If I take the car on the freeway, it will tend to run a little hot. As soon as I return to surface roads, it will idle very rough and especially at stoplights. I even had to put the car in nuetral to avoid a stall. I have been to two mechanics and it has not been correctly diagnosed. Any thoughts????

 

Thank you for the suggestion. That may be beyond my skill level. I had a shop do the fuel pump and drain the tank. I wouldn't know what to look for. I was frustrated because I had taken in to two shops already. It will dirve well for nearly 40 minutes until it starts to run rough or nearly stalls out. If I let it cool, it will run fine again.

 

When you say Fuel Pump are you meaning Fuel Injection Pump?
Of the 2 times I have seen (customers Cars brought into the Fuel Injection Shop I used to work at) Mercedes that had Gasoline put into the Tank only the one that the guys Wife drove until it had obvious problems suffered Damage.
The damge was to the tips/Pintels of the Fuel Injectors. It it burned them.
After the Injectors were rebuilt it was fine.

In both cases there was no apparent damage to the Fuel Injection Pump.

With the one who suffered no damage at all; he realized at the Pump he had put Gasoline in the tank and he dumped several 1Qt cans of Motor Oil (Oil came in Cans back then) into the Tank and drove his Car to us. We were the ones who drained his tank and the gas-diesel mix; changed the filters and sent him on his way.

 

Yes, the injector pump was changed.

Isn't it strange that it will run for 40 minutes or so just fine before before it starts to idle rough? The idle will become so slow it nearly stalls out.

Once it cools down, the car will run fine again. Are injectors or injector tips expensive?

 

Well there is a subject here that the brought up when I was in Trade School. It has to do with Heat and Heat Transfer and time.

When you are first driving you Car the heat from the Combustion Chamber is being transferred through the Engine Block and of course the Coolant but also to the Transmission Housing and so on.
It takes a bit before everything gets hot enough so that the rate the heat is transferring slows down. When the heat transfer slows it starts to have more of an effect on the parts because the heat is no longer leaving them at as fast.

An example given was a bad Axle Bearing that is not bad enough to make noise. The Guy drives it around town for years with no apparent Failure. But, when he goes on long Road Trip to Grandmas House the Axle Bearing Fails completely.
The reason in driving around town heat did not get a chance to build up enough to finish it off.; and ever time he parked some where or was stopped at a Traffic Light it got a chance to cool Down.
When he got out on the High way the bad area on the Bearing that was causing the
excess friction started to create more heat than could be transferred away fast enough.

So it could be when you pull off of the Freeway the heat build up is causing the Pintels in your Injector Nozzles to stick or hesitate under idle conditions. Maybe you could have pull off of the road and have some one check your exhaust for smoke.
When the Injector Pintels stick the stick in the open position and when they do that the Diesel Fuel does not get atomized. So I would suspect to see some sort of smoke.

Unrelated to the above;
You might also check your Fuel Inlet Hoses. I had a small Air leak that only showed up when the Car was hot due the old hard Fuel hoses that did not seal well around the little plastic filter.

Do not do this on a full tank but try driving without your Fuel Fill Cap on. If the problem goes away most likely your Fuel Tank Vent is restricted.
On the Engine see if loosening the Oil Fill Cap returns the idle to normal. If it does it may be your Crankcase Breather System is restricted.

Also is your idle speed within specifications.

 

It is very nice of you to take the time to write such a great post. Thank you.

I can try the gas cap off. The tank was most likely removed so the gasoline could be drained. I could also try to crawl under there and look at the connection.

I will try the oil cap too, otherwise it sounds like the injector pintels. Is that an expensive repair? I think that is beyond my ability unless I can just replace them easily enough.

The car was just in the shop. The valves were adjusted and it seemed like the idle was sped up. I would not know if it was within spec or not.

 

Has raising the idle speed solved the stalling when hot issue?

On US Cars there is a emissions sticker up on the Brace in front of the Radiator; it has the idle speed.

I do not know if your year and model have a Tachometer. If it does just look at that and see if the idle speed is set close to spec.

The Pintel is part of the Injector Spray Nozzle.

To test you need to remove the Injector and put it on a Nozzle Tester.

To If you need a New Spray Nozzle the Injector is taken apart and the precision surfaces are Lapped flat and a completely new Spray Nozzle is installed. The whole thing is assembled and tested. If the opening pressure is not correct that take apart the Injector and change the size of a shim that changes the tension on a spring inside of the Injector.

One rebuilt Injector bought from one of the internet parts places is around $45 each (you would need to pay shipping and also shipping to send the old ones to them to recover your core charge). The New Heat Shields that go under and seal the Injectors in the Engine are in the $2-$3 range and you need one for each Injector.

It is not difficult to remove and install Injectors. But, you will need a Torque Wrench and a deep 27mm or 1-1/16 of an inch deep Socket to tighten them properly.
Also you would need some of the skinny Injector Fuel Return Hose.
And, when you put the Fuel Injection Lines back on you need to leve the Nuts loose. You crank the Engine until you see Fuel coming (this bleeds the air out of the lines) out and tighten them.