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-   -   K-lambda CIS & fuel tank vacuum (http://www.peachparts.com/shopforum/showthread.php?t=165479)

Zestieboy 09-23-2006 09:47 PM

K-lambda CIS & fuel tank vacuum
 
Hello Everyone,

About 4000 miles ago I bought an 83 380SL with 90K. Car looks and runs great except I have an intermittent problem. It occurs when I drive the car under acceleration load. Once the car is hot, and under moderate acceleration it will suddenly begin to surge and hesitiate, lose power, etc. If I let off the gas, I'm OK, but only for a minute or less when it will do it at the lowered speed also. Eventually the car becomes undriveable and/or shuts off alltogether. It will do it at city speeds too but since the accereration load is generally less, it's less often. Now when this happens, a quick restart cures the problem. The restart is immediate - no trouble at all. I go about five miles - sometimes more, sometimes less and it happens again. Restart and it's gone again. etc etc. It's gotten so I don't even pull over anymore - I just go into neutral when I feel it beginning, restart, go back into drive and voila.

Here's more info: Car starts and idles fine. Weather, ie rain, heat and humidity appears to have no effect. I tried to remove the gas cap when symptoms exist and it has no effect. I use only premium gas and throw in a can of Jectron every other month. Everything's perfect otherwise - car never goes above 90C. Over the last 10,000 miles either I or the previous owner have: replaced fuel pump, fuel filter, catalytic converter and O2 sensor, coil, wires, plugs, alternator, battery, injectors, distributor cap, all filters, OVP relay, EZL, idle control valve, checked all vacuum lines and dropped and flushed fuel tank. Car gets good mileage no leaks or smells and never smokes.

Now here's an interesting clue: the stalling, sputtering no power problem completely disappears when I fill up the tank. As long as the tank is more than 1/4 full, I'm OK - runs like a dream. At about 1/4 it starts happenning infrequently but then as the gas level in the tank continues to drop, it gets really bad.

Now I know there is supposed to be a certain amount of tank vacuum produced by these MB closed systems. Every other car I have ever owned makes a "whoosh" sound when I open the gas cap - this car - nothing at all. When I got it, it had a generic vented gas cap. Now since I know MB isn't vented, I replaced it with the correct non-vented MB cap. Did I make the problem worse by doing that? Also, assuming this tank vacuum / gas level clue is meaningful, what does this tell me? Have I stumbled upon something here or is this ongoing intermittent fuel starvation issue finally driven me crazy?

Any thoughts or questions? I've already thrown enough parts at it.

Thank you in advance.
Ray

psfred 09-24-2006 10:41 PM

These symptoms sound like a fuel pump relay to me, although that does not explain the problems at low fuel level particularly well.

The tank IS vented, find the vent line underneath and unplug it. Typcially a vented cap is bought a McParts to replace a lost cap by someone who doesn't know MB doesn't use vented caps (no one does these days, but McParts still carry them).

You should also replace the OVP relay if you have not, it will also do this -- engine looses mixture control and stalls, but everything resets when you shut it off and restart.

I would also check the intake system for air leaks after the air flow meter flap -- this can also give you fits.

Peter

david s poole 09-25-2006 10:21 AM

380sl
 
i saw something like this once before and it turned out to be the td signal out of the ignition unit.does your tach drop immediately when problem occurs? fuel pump relay needs signal from td to keep running or you may simply have f/p relay going bad after it gets warmed up.

david s poole 09-25-2006 10:38 AM

380sl
 
just thought jump fuel pump relay and drive car with 1/4 tank .should be good test

gmercoleza 09-25-2006 11:09 AM

I would suspect your FPR. Jumper it and drive the car around to see if your issues are resolved. User a paperclip formed into a U shape, but be careful - it will be VERY hot when you go to remove it! You'll have to use a pair of pliers to grab it.

Zestieboy 09-26-2006 05:23 PM

I've now confirmed that the fuel level is a direct clue on the problem. Three times since my last post I got the problem to start and become chronic as the gas ran down only to disappear when I re-filled the tank. I also tried using the old vented gas cap and using no cap at all and neither one had any effect.

It's difficult for me to explain this as an ignition problem anymore. I'm also not sure how the FPR impacts this anymore either, but I'd like to make sure by getting the problem to manifest itself and then jumpering the relay. What pins do I jumper to test the FPR once and for all?

Assuming the FPR checks out OK and that the FP and filter are new, where do we go? FP check valve, FT vent valve, accumulator, screen, damper, warmup (control pressure) regulator, fuel distibutor?

If this weren't such a PIA, I might actually be fun!

Thank you all,
Ray

Zestieboy 09-26-2006 05:30 PM

Forgot to mention that I still - no matter what the circumstances - ever hear a sound when I open the gas cap.

romansek 11-19-2006 02:17 AM

Your problem should be solved if you change or ultrasonically clean the fuel strainer. I have a similar problem on my 83 280SL and after cleaning it the problem was solved. When fuel level is less than 1/4 fuel flowing surface thru fuel strainer become smaller and the bottom of the strainer is clogged most. You should hear pump making noise from fuel starvation. Good luck.


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