380SL Fuel Pump Replacement Causes Rough Idle & Other Problems? - PeachParts Mercedes-Benz Forum

ezrider · #1 03-08-2004, 06:16 PM

380SL with 106K miles needed fuel pump replaced due to low output, cold start problems and it was very noisy. Fuel filter had metal bits in it too. Engine had excellent idle and tranny kickdown at moderate speeds worked fine prior to the pump replacement. After the pump was replaced the idle is now very rough and the car does not downshift at moderate speeds like it did before the pump replacement. I have looked for a vacuum leak and can't find any.

What could be causing these problems, it is directly related to the new fuel pump as that was all that was changed

Thank you.

txstinger · #2 03-08-2004, 07:50 PM

Could be burning a little rich. The old pump was weak - did anyone adjust the mixture? If so it sound like it needs a little leaning.

rudy100 · #3 03-10-2004, 05:18 PM

I had a problem with a rough idle, after I had some work completed by a repair shop. They replace the fuel injectors, fuel filter, and fuel pump, etc.
... I ended up having to replace (rebuilt) the Fuel distributor...the only conclussion that I could reach is that dirt must have worked its way into the distributor?

ezrider · #4 03-10-2004, 05:48 PM

Rudy,

The fuel distributor was replaced 25K miles ago. The tech that replaced the fuel pump tried to patch crumbling fuel lines before finally replacing them so it is possible trash made it to the engine, is there another filter on the engine end?

Thank you.

rudy100 · #5 03-11-2004, 02:39 PM

I do not believe that there is any fuel filters located any where else in the fuel line, except at the fuel pump
...anyway, the repair shop ran some tests and concluded that one cylinder was lean and therefore poor distribution of fuel was the reason for the rough idle. They also concluded that this indicated the need for a rebuilt fuel distributor, ....but they would never accept financial responsibility for the demise of the original fuel distributor. After a rebuilt distributor was installed, the car ran great....

ctaylor738 · #6 03-13-2004, 07:24 PM

This certainly sounds like a vacuum leak. Is the cabin vacuum line on the fitting on the back of the manifold? Is the metal line from the intake manifold well-connected to the modulator on the transmission? There is a rubber connector that might have gotten knocked loose.

ezrider · #7 03-13-2004, 09:38 PM

Chuck,

What you say is making sense because the kickdown downshift stopped working too. Mechanic has the car now and is scratching his head. The problem acts like a vacuum leak to me too because it runs good at higher RPM's than idle. I will try to find the vacuum connections you are referring to. I have looked all over the top of the engine and can't find a vacuum leak there. It also does not run well when first started in the morning and gets better after the first minute or so of running.

Could either of the connections you are referring to possibly have been contacted when replacing the fuel pump or fuel filter?

Thank you.

ctaylor738 · #8 03-14-2004, 12:07 PM

Not likely.

The cabin/transmission vacuum fitting is on the very back of the intake manifold, near the firewall. If it was knocked off, you would get no reading on the vacuum guage in the cluster.

From that same fitting, a steel line runs to the left side of the trans, and connects to the modulator with a rubber piece.

There are other connections on the intake manifold as you know, including the big line for the brake booster. If he did a volume test, he might have messed with that one getting the return line unhooked.

ezrider · #9 03-14-2004, 12:12 PM

Chuck,

The mechanic has the car now so I can't check it. The cluster gauge does have a vacumm reading, off the good side of the peg a little bit at idle.