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

 

Normally, i would hook up a fuel pressure gauge and watch both upper and lower chamber pressures with something like this, but he's a shot at your problem- using your suggested parts

Fuel pump relay- really not an issue- it would shut the car down, not impact driveability so much
Overvoltage relay- could do it- an open circuit, that opens with heat could act like this- usually, though, when they fail, they impact cold driveabillity
Reference sensor- no- usually no- start/stall hot when they act up
Ignition control unit- probably one of the most likely- prone to fail and very, very expensive ..
Idle control anything- NOT- they do nothing when the car isn;t idling
Fuel pump check valve- not- this would impact cold start after sitting
Fuel pump vent valve- not really sure what you are talking about here
Pressure regulator- not probable- most likely to show up as a cold complaint
Frequency valve- also, not likely- its used to control lambda- so massive driveability problems don't usually come from this.
I do this- it can get waaaayyy expensive if these era cars aren;t diagnosed right- i would suggest looking for someone that knows the cars and you can trust to look at it.

 

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. He 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 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 - 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 problem is still around. I did it the same way as it was doing it - after a nice hilly highway ride. Same symptoms as before. I'm at my wits end. 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?

 

With all the parts that have been replaced, seems likely that you may have a bad ignition module. If it comes down to wanting to replace it they are very expensive. I have two that are good. PM me if you need one.

 

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

 

Normally one of the fuel pumps and in line filter will cause your fault.

malcolm

 

I have a 1982 380sl with the same problem. I haven't checked it out yet but by the pictures of the fuel outlet screen in the tank, it could be partially blocked with debris as it mounts through the bottom of the tank. I will be checking it out in the next few weeks and will post my findings.
When my ingnition module failed I had irrattic tachometer readings along with rough running during acceleration. after a week or two of this it died completely.

 

I have weird and somewhat similar problem. 82 380sl.

Thanks!

 

I have a 1982 380SL with similar problem. Recently, I started having problems that appear to be fuel starvation under load and also at idle. I replaced the fuel pressure regulator and fuel filter. That didn’t change the lousy performance. Then, I decided to top off the gas tank. It was sitting at about a 1/4. The 380 runs great when the fuel tank has lots of fuel in it. Absolutely runs ‘great’ with the fuel level up. I read about the gas tank screen clogging up, and thought: “aha! This must be it… It has to be clogged”. I ordered a new screen from the dealership, and while waiting for the part, I removed the old screen. DANG! The screen was perfectly clean. It looked like it was changed yesterday. This is a puzzle! Anyone figure this problem? I’m about to replace the fuel pump and relay, but I want a little more information on this specific problem before I continue to throw parts at it.

 

After testing the 380SL various times with different amounts of fuel in the tank, I became certain that the engine would run perfectly with more that 1/4 fuel quantity. I replaced the pump with a new “Bosch”, (Napa had it for $150, dealer $250). I have tested the car various times… running it to the point where the low fuel light comes on. It hasn’t skipped a beat—it is running perfectly every time.
I think someone mentioned that perhaps the pump was weak and with plenty of gasoline on top of the pump inlet, it provided enough pressure to run the engine normally. We’ll, the guy was right!
thank you!