98 E300 TD stumbles at idle and deceleration - PeachParts Mercedes-Benz Forum

chago · #1 02-23-2007, 05:44 PM

Just had IP shut off valve and fuel filters and all clear fuel lines replaced, car has a stumble when it decelerates at lower rpm and at idle when in park or neutral. Chago

Parrot of Doom · #2 02-23-2007, 07:19 PM

Remove the black injector cover from the top of the engine, and check that all the injector lines are clipped securely to the side of the engine (clips are near the intake manifold bolts). While you're there, check the condition of the diesel return pipe (its a braided fabric pipe running from injector to injector, and back to the fuel filter)

Check the top of the injector pump - the delivery valve seals may have gone, I know mine are on their way out and need replacing. You'll know as the top of the pump will be damp with diesel (presuming you've wiped it dry first to check).

Also check the 2 vacuum lines to the EGR valve - they're easy to see, just look at the base of the intake manifold, close to the windscreen washer bottle. One pipe to the left of the valve, one pipe to the right.

Stumbling at idle, if it feels like a misfire, is likely to be the EGR valve - the spring inside it gets weak with age. Replacement tends to be the only cure, but seek further advice before you get your wallet out. Other symptoms of a weak EGR valve are poor response when cold, and surging when cold.

TMAllison · #3 02-23-2007, 09:24 PM

Stumbling from ~1200rpm to 600rpm idle is also a known IFI/ECU issue that most cars do to a greater or lesser extent.

Stumbling AT idle is not.

chago · #4 02-24-2007, 01:33 PM

I checked all that Parrot of Doom suggested. Everything looks ok. Rough idle is only when engine is warmed up,if I put heat on it idles ok. While driving it is ok.

chago · #5 03-04-2007, 03:34 PM

Took car back to shop who performed the work they think it is a ecu problem. When engine is off the fuel line to prefilter drains down about 2". When I give it gas about 10,000RPM I could see air in the 3rd from top fuel line and the very bottom one. Any suggestion.

Chago

scott19_68 · #6 03-04-2007, 03:52 PM

That line will appear to drain a little - I think 2 inches or so is OK. Are you seeing air bubbles in the fuel line from the pre-filter to the shut-off valve and/or the line from the shut-off valve to the lift pump? Either one is not good and points to an O-ring leak. Odds are that it is the one under the pre-filter - it has to be perfect to work properly. I slightly bent the tab that holds down the fuel line into the top of pre-filter to get a little more force on that O-ring.

benzforlife · #7 03-04-2007, 03:54 PM

ur saying that ur diesel benz will rev up to 10,000rpm?

Matt L · #8 03-04-2007, 04:07 PM

I would bet that he means 1K rpm. I can't picture the lines of which you speak; I may have to go out and look at the car. However, if you have an air problem, you'll see lots of tiny bubbles running around in all the clear lines. Two inches of air gap at the prefilter when sitting may not be bad. Mine always has at least an inch there.

Hopefully you can get by without a new $1300 IFI computer.

chago · #9 03-05-2007, 05:14 PM

Scott I checked the prefilter and that line oring they are ok . Their is no air in line at idle only if I give it some gas, its the line just below the prefilter line that goes to the shut off valve and the very bottom line.

Chago

TMAllison · #10 03-05-2007, 05:38 PM

Normally air at the fuel line entering or exiting the pre-filter portion of the fuel filter housing means a leak preceeds the pre-filter.

Try a thin film of silicone grease on the pre-filter oring, pre-filter fuel line oring and the oring on the opposite end attaching to the fuel heater/thermostat.

scott19_68 · #11 03-06-2007, 08:24 AM

Quote:

Their is no air in line at idle only if I give it some gas, its the line just below the prefilter line that goes to the shut off valve and the very bottom line.

I am assuming that you are also seeing air bubbles in the line from the bottom of the lift pump up to the spin-on filter, correct? The fuel delivery system in these cars can handle a small amount of air bubbles but not much as the bolt that holds the spin-on filter has a small passage through it to feed the IP and it only extends about an inch down into the filter. Any air in that line WILL make the engine stumble.

I think the next step is to try some simple experiments:

- push down on the fuel line fitting that enters the pre-filter with your thumb while revving the engine and note any impact to the location or rate of air bubbles
- try revving it with the gas tank cap off and see if you are seeing less air bubbles
- try a new pre-filter O-ring (they're cheap - I always have a backup in my emergency road-side kit)

chago · #12 03-11-2007, 05:32 PM

Terry & Scott i replaced prefilter and line orings. The line no longer bleeds down.
I tried it with the gas cap off on differents. Still have bubbles in the line at bottom of prefilter and the very bottom line.

Chago

TMAllison · #13 03-11-2007, 06:30 PM

Is because you still have bad a o-ring, a bad fuel line or a poor connection at a fuel line.

There is a oring behind the shutoff valve too.