Strange air ingestion problem on the '98. - PeachParts Mercedes-Benz Forum

KarTek · #1 12-09-2008, 09:01 PM

I'm continuing to work to get my car running right and it's down to air in the fuel lines.

Here's the strange part: The fuel lines will be completely air free then, suddenly, the fuel just drains back out of the line leading from the fuel heater to the pre-filter. Completely empties! No fuel flows for about 4 seconds then the engine starts running rough from lack of fuel. Then, just as suddenly, the fuel rushes back into the pre-filter, the engine smoothes out and all the lines are air free again.

The process repeats itself every 30 seconds or so just like clockwork and is not effected by the speed of the engine.

Is there a tank screen or something else in this car that would clog, cause a cavitation and then release?

Any other ideas what's causing this? It never did it before I replaced all the o-rings.

Thanks!

muleears · #2 12-10-2008, 05:51 PM

I'm sure you checked all this but I'll ask anyway. Did you change the O ring on the prefilter with viton? Could it have been nicked in the process? Is the prefilter all the way in and snapped in place? When I got my viton kit a couple of the O rings were slightly smaller than the others, could they maybe have gotten mixed? That line on my car has a 2" bubble when stopped but it disappears when started.

Come to think of it I swapped my O rings for viton when I changed the SOV, wonder if that is contributing to my problem...

HTH,
Cal

Us 606 people need to support each other!

TMAllison · #3 12-10-2008, 06:54 PM

Evan - Havon't heard your complaint before. Wonder if that is what happens with a sticky or failing SOV since when that valve closes at shutdwon that line drains back? More likely could be losing lift pump suction and draining back when the SOV oring begins to fail to seal?

KarTek · #4 12-10-2008, 08:28 PM

Thanks for the replies guys. Here's a short history of what I've done to the fuel system:

1. Early on this year, I replaced the fuel heater to pre-filter line - no problems.

2. A couple months later, I replaced all of the o-rings that I could reach from the top with viton - again, no problems though, I was somewhat suspicious of the viton o-rings as they look a little thinner than the regular ones. At this point, I didn't see any extra air in the system.

3. In Septenmber, I replaced the remaining o-rings on the lower fuel lines and the SOV. This is when the trouble started. It took a great deal of cranking to finally get it started and in addition, it began doing the little rev before shutting down.

4. So far, I've pulled and re-seated the prefilter and inspected the o-rings. Also, I pulled and re-seated the SOV then pulled it again and replaced the viton o-ring with a conventional one.

So far, the only thing I haven't done is switch all the o-rings back to the originals. It's a little irritating because it ran perfectly before I messed with it and I also threw all the old o-rings in the trash...

I'm thinking that it's possible that one of the o-rings is weak and can hold a certain amount of suction then it just fails for a second and then builds vacuum again in cycles.

Next step for me is to buy a new set of o-rings and swap them back out. I have to order a bunch of stuff from Phil anyway so I'll add them to the list...

TMAllison · #5 12-11-2008, 12:06 AM

The slight rev and delay at shutdown is usually going to be the SOV oring; occasionally a bad valve.

I helped establish the counts and sizes of the viton orings and have all of them except the DV's installed. No prob on mine for a couple of years and ~60k miles. When old they do develop an oblong profile; the new (round) profile and firmer rubber coumpound has cause some folks to have difficulty getting a good seal.

KarTek · #6 12-11-2008, 09:35 PM

OK, how dumb am I? Go ahead, tell me... I'll save you the trouble - I'm REALLY dumb!

I went back at it this evening. I thought I'd try running it from an alternative fueling source - my diesel purge bottle. When I pulled the return line to run some fuel into the bottle, the engine quit and would not re-start! Then, I looked down and I saw that the stinking SOV was completely disconnected from it's electrical connector. In short, I've been driving around for a couple days with it offline!

So for sypmtoms I had:

1. Fuel system acting strangely - bubbles moving back and forth in the lines and engine intermittantly starved for fuel.

2. Engine won't run without the return line connected.

3. Engine runs with the SOV disconnnected.

4. Engine revs and runs on a little on shut down.

5. Normal combustion knocking is abnormally loud.

So this just defies all logic and I figure I'll go back to basics - check the lines and make sure they're all connected properly and in the right places. Well guess what, the lines on the lift pump are backwards! I didn't think it was possible because of the way they're made but it is. In fact, they almost fit better when they're backwards then they do when they're correct...

So, lesson learned - always check the lines against the diagram upon re-assembly if you remove more than one at a time! Right now, the battery's dead from trying to crank it so I'll know in the morning if it's all back to normal.

Go ahead kick me, I deserve it...

KarTek · #7 12-12-2008, 08:33 AM

Update: She is back! Cranked it up this morning and drove it to work. After 4 months - pulling and replacing the broken glow plug, engine mounts, viton o-ring swap and horsing around with the fuel lines, everything is again working like the symphony they were designed to be.

Next on the list is to get the heat working again. I think I have a bubble or something since I had to dump a bit of coolant out. Oh, and I also need a new washer resorvoir to replace the existing broken one.

TMAllison · #8 12-12-2008, 11:41 AM

Its not the first time someone gotten the lift pump lines on reversed.

Am surprised you were able to start it w/o the SOV connected; guess I'm more surprised that you could shut it down after you did so?

KarTek · #9 12-12-2008, 01:17 PM

The car runs actually quite well with the SOV disconnected. Now, if I didn't hve the fuel lines reversed, it probably wouldn't run at all.

I believe that the reason it has no problems shutting down is that the IP rack slams closed when you turn off the key. It's that "TOK" sound you hear on shutdown and also when the ECU cycles the rack just prior to startup. The IP doesn't supply quite enough fuel in the closed position to keep the engine running so it just kind of grinds to a halt.