Mechanics

CAR IS CHOKING, LOSING ABILITY TO HOLD STEADY SPEED WITH STEADY FOOT ON ACCELERATOR, 55 MPH AT 2100 RPM, DIESEL ENGINE.

1996 Mercedes Benz 300e • 133,000 miles

If I am supplying a heavy foot on pedal and accelerating the car runs fine, but after its shifted and I cruise with a steady supply of fuel at around 50-55 mph on a flat highway, the rpm needle will move from 2100 rpm to 2050 rpm back and forth, cough, go, cough, go, cough, go, with a mild loss/then gain of acceleration and then it keeps cycling like this a couple times a second. If I remove my foot and slow down its fine and the rpm's decrease, if I push the pedal it accelerates cleanly, rpm and speed increase steadily.

I've changed the oil, the oil filter, the fuel filter, and I've Sea Sprayed the engine without any change at all with this problem (although in general, the car runs better).
Avatar
ZoSo
November 2, 2013.



Have you checked the actual fuel pressure?

Mhpautos
Nov 3, 2013.
Thank you for your time.

No. I have not checked the fuel pressure, or the fuel pump voltage/amperage.

I do know that my alternator is working fine 14 or so amps and that my battery is new (months old) and gives me good starts in the cold morning.

I have not cleaned the fuel, that was next. I was going to pour the additive Power Service/32 oz. Diesel Kleen in the tank. I've been told that its a good rule of thumb with diesel's to check the fuel isn't bad or clean the next fill and see how it goes.

But, this problem is specific to limited pressure on the pedal at certain rpms, so I don't think its the fuel. My fuel has been reliable at a local Hess.

Its as if a demon is pulling and pushing a choke lever under certain scenarios of fuel supply/rpms steady

Once again, thank you.

Tiny
ZoSo
Nov 3, 2013.
I would be looking at the fuel filter and fuel delivery system, when if ever were the injectors last serviced?

Mhpautos
Nov 3, 2013.
I don't know how long it has been. Probably a very long time. Years. I don't recall any service to the fuel system except my annual winter Kleen and every other tank fill Cetane Boost, the once a year dump of fuel injector cleaner in the tank, and the fuel filter change every 10-15,000 miles.

The air filter was changed about 12,000 miles ago (15-18 months ago).

Thank you for your time.

Tiny
ZoSo
Nov 3, 2013.
Service the injectors and change the fuel system as a starting point.

Mhpautos
Nov 4, 2013.