Hello Everyone,

About 4000 miles ago I bought an 83 with 90K. Car looks and runs great except I have an intermittent problem - particularly when I drive the car at highway sppeds 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 perfectly except occasionally when I first stop at a stoplight, I get a very light idle surge (from about 650 to maybe 900) nothing big and not always and when it happens - only once when I first stop. Back to the main problem: It happens more often when it's hot. Weather, ie rain and humidity appears to have no effect. Fuel tank level has 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, and dropped and flushed fuel tank. Car gets good mileage no leaks or smells and never smokes.

So far what I've read indicate it could be: fuel pump relay, overload protection relay, reference sensor, ignition control unit, idle control unit, idle control valve, fuel pump check valve, fuel pump vent valve, fuel pressure regulator, or the frequency valve.

Any thoughts or questions? I don't want to just throw parts at it.

Thank you in advance.
Ray

 

Have you checked the fuel distributor? Ours was sticking and was the cause of hard starts.

 

start with the rasy one first put a new fuel filter on and see if that solves your problem

 

start with the easy one first change the fuel filter and see if that solves your problem

 

Well here's an update: the recent heatwave here in the NE got it so it wasn't doing it quite so intermittently anymore - it made the car all but undriveable under any circumstances. So I think we can agree it's heat related. My mechanic said he checked the pressure and concluded it was the fuel distributor. Luckily, since these things are $$$, I had one that had been removed from a frends totaled 500. The mechanic looked at the bottom of it (by the plunger) and said he didn't want to put it on because he could see traces of gas so he already knew it leaked. I told him that the car it came from was working fine when it was removed and I wasn't going to pay him $2041 for a rebuilt part. He put it on. It's still hot here - not as much - but when I picked the car up it worked fine. Idle was a little rougher but not bad and the car had more power. But.. the intermittent problem is still around. It did it the same way as it was doing it before it got really bad with the heatwave - after a nice hilly highway ride. Same symptoms as before - same remedy by restarting the engine. I'm at my wits end. What are the chances both distributors were each bad in the same way? pretty slim I think. It must be something else. Any thoughts?

 

Here's an interesting discovery in my problem: 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?

 

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

 

Had a similar scenario with my '91 560SEC (similar design fuel system). Fuel pump would shut down when cruising on freeway; car would usually re-start fine. My mechanic had seen the problem before - he replaced the voltage regulator that was causing an intermittent electrical problem and the fuel pump was sensitive to it. Problem solved.

 

It sounds to me like your FP is not putting out sufficient volume unless the tank is full, which might increase inlet pressure on the pump.

First thing to do IMO is to disconnect fuel lines at FD. Attach a hose to line. Jump 7&8 in fuse box and examine consistency and volume of flow. Also be sure return line is not blocked.

I'm not "assuming fp and filter are new..."