Stumbling 603 - PeachParts Mercedes-Benz Forum

Habanero · #1 10-05-2004, 08:35 AM

Hey guys, I have a problem that has me a little baffled. The 603 in my SDL has developed an intermittent stumble when pulling away from a stoplight. It closely resembles the way an old gasser will act if it is cold and doesn't have enough choke. It sort of falls on its face for a second, then picks up and takes off fine. Car runs great on the highway and pulls hard up through the gears once it gets past the stumble. I bypassed the overboost switch and it didn't help any. Fuel filters have about 500 miles on them and since it pulls so good after the stumble I can't hardly believe it is starving for fuel. I haven't hooked up my boost gauge yet, but there again, since it runs so well up through the gears it sure seems to be getting boost. Once in a while I notice after this stumble there is a puff of smoke, but I drive mostly at night so I don't know if it does it every time it stumbles or not. Also this doesn't happen at every stoplight, although it seems to be getting a little more frequent all the time. The car just had a new head put on about 2500 miles ago, and started doing this stumbling thing about a 1000 miles ago. I did have to put new injector lines on after the head was replaced and in the process knocked some vacuum lines loose. I think I got them all back in order, but I know a couple of the connectors need replaced. Could a simple vacuum problem be a culprit? Any ideas from the collective brain trust would be greatly appreciated.

TX76513 · #2 10-05-2004, 08:55 AM

Try something simple first - Look at the line from the intake manifold that runs to the valve on the firewall and over to ALDA. Clean it out -make sure there are no leaks from a deterioated fitting/line.

Habanero · #3 10-05-2004, 09:24 AM

Sorry, should have mentioned that I did do that already. As far as I can tell, the alda should be seeing the boost pressure just fine.

jcyuhn · #4 10-05-2004, 10:26 AM

I doubt it has to do with boost or alda compensation. From your description it sounds like the stumble happens before there's any boost being generated.

Is your EGR and/or air bypass valve disconnected? You can get some very funky and unpleasant behavior when the air meter (in the air filter housing) goes south and the engine computer starts activating these two thingamajigs at the wrong time. Simply disconnecting the vacuum lines to the EGR and boost valve can prove/disprove that theory.

Other idea is air in the fuel lines. Inspect the clear (probably formerly clear) fuel lines to/from the main fuel filter. Look for air bubbles running to and fro within them. It may be impossible to do so due to darkening of the lines, but give it a shot. These injection pumps are self bleeding and can deal with a surprising amount of air. But at lower engine speeds the air can make them hiccup, while running fine at higher speeds.

My $.02,

- JimY

Habanero · #5 10-05-2004, 11:24 AM

I wondered about the air thing, but the fuel line is too dark to see if there are any bubbles. I haven't disconnected the EGR, although I have the kit. I will try that route.

Brian Carlton · #6 10-05-2004, 02:01 PM

See if you can get one of the clear primary filters and install it. Watch the filter for a bunch of very tiny bubbles. One large bubble that remains fixed is normal. I, too, think that a bit of air is getting in the line, somewhere. The engine is stumbling at all times, but, it is very noticeable at low rpm's.

You will also need to look for a tiny air leak between the primary and the the IP. An air leak here will not show in the primary.

Habanero · #7 10-05-2004, 02:08 PM

The fuel should be pressurized between the primary filter and the IP, right? I thought a leak there would show as fuel leaking out, not air getting in. I did just change the spin-on, so I thought maybe it didn't seal correctly, but then I decided that would show as a fuel leak, not as an air entry.

Brian Carlton · #8 10-05-2004, 02:18 PM

IIRC, the primary is ahead of the fuel pump, so a leak between the tank and the primary would introduce air.

Yes, downstream of the pump, it is far more likely that fuel will leak out rather than air getting in.

Habanero · #9 10-05-2004, 02:45 PM

I'm sorry, I read your original post wrong. I thought you were referring to the spin-on as the primary. Stupid mistake. Guess that is what happens when you spend hours looking at GC data, with the only break being an occasional check of the MB forum.

Brian Carlton · #10 10-05-2004, 02:52 PM

Not a problem. I wasn't thinking too clearly, myself, with regard to pressure downstream of the fuel pump, between the secondary and the IP. Very unlikely that air will be introduced without a visible fuel leak in this section.

Far more likely that there is a small leak between the tank and the pump.

psfred · #11 10-05-2004, 07:47 PM

Check the suction lines -- if they have fuel on them, they are porous and leaking air. You will also have too much air in the pre-filter -- look for bubbles entering the filter.

Otherwise, I'd suspect the air recirc valve or EGR.

Peter

Plantman · #12 10-05-2004, 09:39 PM

My Sdl has a very slight miss at idle. I'd say it does it 10-15% of the time if that much. When it does do it, it is pretty noticeable.

I had rebuilt injectors installed, the mechanic installed a rebuilt IP after he ruined the old one so I am a bit stumped.

BTW, this irregular miss present prior to the aforementioned things being done to it.

I will check out the prefilter for tiny air bubbles tomorrow.

Any other ideas appreciated.

BC, I still haven;t gotten around to installing your plate blocking kit. No time. Looks real good in my glovebox though.