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

 

Hi Ray,
Good problem description.
I have no specific information about your MB but your description is such that i will hazard an informed "instant solution"!!
Fuel injection systems of your vintage used something called a swirl pot in the tank. This is a saucepan (4" dia) like structure which fits around the fuel pickup. The purpose of this pot is to ensure that there is a constant supply of fuel at the pickup point once the tank level goes below lets say 1/4 full. This is required since the pumps on your type of system recirculate VAST quantities of fuel around the system in order to ensure there is no air etc in the fuel supply and also to keep the fuel relatively cool at restart etc. More specifically the swirl pot ensures that fuel is present at the pickup under acceleration, braking and cornering.
If the tank is low then the pot is kept filled by fuel RETURNED from the engine. The bootom of the pot has small flap valves in it to allow some fuel in for initial startup at low tank levels but shut under cornering etc. Alternatively some installations used small holes rather than flap valves.

So is the problem in the tank? Possibly but are you sure you are returning "large" (ie gallon/minute or similar) quantities of fuel to the tank?

This URL from Google seems to have a bit of additional? info. I attach so you can read at leisure. http://www.fortunecity.com/silversto.../swirlfaq.html

Hope this helps. Stuart

 

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.
Ray

 

Ray,

So why aren't you looking where I suggest?

Purpose of pressurised gas cap systems is linked to evaporative (gas fume) loss controls. I don't believe it has anything to do with this problem.

Stuart.

 

I see you said the tank was removed and flushed. Was it checked as Stuart suggests? He has a goot thought about the return flow too. Maybe the flow is restricted.