Worked on 300E, now it stalls with idle valve connected! - PeachParts Mercedes-Benz Forum

whipplem104 · #1 12-04-2008, 01:53 AM

This is really odd. You may try with the air cleaner off pushing down on the fuel door and seeing if the idle goes up. This would indicate that the fuel mixture is too lean.

Ron in SC · #2 12-04-2008, 07:03 AM

I had same problem which was cured when I changed the the fuel computer.

babymog · #3 12-04-2008, 09:00 AM

The fuel pump is kept running by a signal from the ignition, if the engine dies the fuel pump shuts off. That is normal.

Quote:

Originally Posted by whipplem104

This is really odd. You may try with the air cleaner off pushing down on the fuel door and seeing if the idle goes up. This would indicate that the fuel mixture is too lean.

I believe that this would indicate the engine was running too rich, an easier way to check would be to unplug a manifold vacuum line to introduce more air and see if it runs better = too rich. Running worse with extra air indicates too lean. Gasoline engines are sensitive to air leaks unlike your diesel, and to temperature-dependent mixture settings.

It is difficult to diagnose completely from symptoms, often a sensor out-of-spec will cause mixture issues, would need to be diagnosed with an ohmmeter/DMM vs temperature using your FSM. Can be the fuel computer also, they do go bad, I've had two replaced under warranty from driveability issues.

It is unlikely IMO that it is the coil based on your symptoms.

If you can retreive the fuel computer PN, I will look to see if my spare is the same, which would be a 5minute swap, and one way to eliminate that from the mix.

I don't know what years the wire-harness was bad, my '91 was only 5y/o when I sold it, wiring was fine, others here very likely can tell you if yours is a suspect year.

Hot-wired fuses/relays are never a good thing, did the fuse remain good after replacing it? Only the fuse was bypassed, not the relay?

Check also the wiring to the O2 sensor, an unplugged O2 sensor will cause the engine to default to mechanical-injection settings. I don't think it'll affect idle though.

How do the engine grounds look? When it doubt, clean it. Weak grounds can cause you to chase symptoms forever, a couple of millivolt leak to a sensor ground can signal a whole different parameter. Ditto with the fuel computer's ground.

pawoSD · #4 12-04-2008, 09:37 AM

The grounds and wiring connectors all look ok, the car is a desert car, so there's no corosion on anything...

Only the fuse was bypassed....I haven't checked to see if the new relay-fuse has blown. The relay itself looked old/original.

I don't have a FSM for the W124....I have been going by what it says for the 1990 300SE W126.....I'd assume the engine/mechanicals are the same, it looks to be. I have a good multimeter/ohm meter. I guess the next step is to verify all sensors to be in spec....

I am pretty sure the cold start injector is working, as there was some fuel residue on its tip when I removed it. After the engine has been running/warming up for about 3-4ish minutes, there is a "click" sound and then it seems to idle a little slower (only slightly) but its noticable. What is occuring when it does this?

I will try to get the part number off the fuel computer, that is the larger unit in the back right? Does the fuel computer control the idle?

I'll also try disconnecting a vacuum hose with it running to see what happens.

Also, when I finally got it started the exhaust was smokey and smelled like unburned gas. Running way too rich I'd assume??

When just sitting there idling with the valve unplugged the exhaust smells normal and clean.....and it runs/accelerates great. If it starts good cold then maybe I will just leave the valve unplugged.