K-lambda CIS & fuel tank vacuum - Mercedes-Benz Forum

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post #1 of 3 (permalink) Old 09-23-2006, 06:56 PM Thread Starter
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Date registered: Apr 2006
Vehicle: 1983 380SL
Location: Jersey Boy
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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.
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post #2 of 3 (permalink) Old 09-25-2006, 08:24 AM
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Date registered: Apr 2005
Vehicle: 1985 300CD
Location: St. Petersburg, Florida
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First of all, great description of problem, that should peak the interest of some of the DIYer's (do it yourself ers). Did you try the vented gas cap back on to see if the helps. Maybe, ( I am not sure on SL's) there is a filter or sending unit of some type that is immediatly following your tank that is hindered in some manner (just athought).
Did you try posting this in the SL forum, or posting it in a forum that also uses the 380 model engine (ie. 126 chassis).

Good luck

1985 300CD
Black on Palomino
@ 158,000ish miles
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post #3 of 3 (permalink) Old 09-26-2006, 03:17 PM Thread Starter
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Date registered: Apr 2006
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Good suggestion - the 126 is a good choice!

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. And no matter what the situation or circumstances, I NEVER hear a sound when I remove the gas cap.

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 for your excellent suggestions, time, and interest in my problem.
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