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 tank 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

 

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?

 

Hi,

Excellent description of your proble,

Unfortunately your model is only partially electronic and I'm not very familiar.

However if you want to add my input to your existing understanding I would move the following items up your list of priorities.

OVP (Overvoltage not Overload Voltage Protection). This is a small relay (replaceable) which ensure that supply voltage is cut to various electronic modules should the Volts go to high for whatever reason. They are known to give problems due to manufacturing issues. (Board soldering) Although this device is basically ON or OFF it can also fail in a sort of half way mode due to the relay contacts being dirty. in this case regular supply voltage is not available to the ECU's and they don't like this. I don't think OVP's are very expensive but to avoid cost I would risk jumping the contacts for the duration of a test run to see what happens.

EHA (Electrohydraulic Actuator) located on the fuel distributor. This is the device which makes real time corrections to the fuelling.

I suspect that your problem is electronics based since a reboot (restart) resolves the problem. Your driveability issue is likely the car going into limp home. Since the problem clears on a reboot you must be suffering from a transient problem. the system thinks all is OK at startup but something triggers Limp during your drive.

Diagnostics on these early part electronic systems is challenging due to the lack of captured fault data.

It is not surprising that changing your fuel distributor has made a difference since I suspect your fuelling is now all over the place due to different spec etc. Since it hasn't solved the problem I suggest you revert to your previous part.

Is your MIL/CEL (Check Engine) light coming on?

For info the system will go into limp if the EMS detects a misfire which could be due to almost anything.

Try the OVP as a starter.

Stuart

 

Stuart,

Thanks for your time in replying to my post and problem. I'm hitting two electrical issues next based on my reading and everyone's suggestions. One is the OVP relay as you've suggested and the other is the ICU. I've tested the coil and it appears to be OK and I plan to jump out the fuel pump relay to see if that helps. Everything I'm looking at now is electrical as you can see.

As to fuel, I'm reluctant to swap out the fuel distributor again since all I have going on negatively is an idle speed that's a little lower than it used to be but on the upside the increased power is exhilerating. Nice and smooth too, like I always imagined these cars should be when I admired them but couldn't afford them. Mileage appears to be about the same or a bit better so I think I'll hold on to the new distributor a little bit more.

This car is pre-EHA so I can rule that out. No check engine light - nothing on the gauges even remotely hinting I may have a problem. Temp rarely exceeds 100C.

On the investigative side, I did find two vacuum connections beside the warm-up regulator that should be both connected to a red 50 degree thermo valve and both are off thanks to broken connections on the valve. I have ordered a new TCV to repair that issue but I doubt that's going to cause me massive drivability issues like I have. I also founf that the air cleaner housing lid did not fit properly at the back - something I had just never noticed - which created a large gap between it and the housing itself. I found that after remedying that situation that the car ran much smoother.

It amazes me that with their reputation as bulletproof tanks capable of sustaining endless miles that even the slightest disruption to their delicate balance causes a myriad of problems.

Once again, thanks for your time and imput.

Best,
Ray

 

Hi,
Sounds like your old fuel distributor was really shot.

Pre EHA?I don't question this statement but if correct what actually controls fueling? The EMS must have something to act on?

Air leaks are always important especially when pressure signals are used to control emmissions etc.
The air cleaner element causes a pressure change thus signals up and downstream of it are different. If the filter casing is incorrectly fitted these signals are incorrect and consequently the systems which use the signals get "confused"

If you have signal tubes which are not correctly connected then this needs connecting. These signals do something...... even if it is not part of you main problem.

Stuart

 

Hey Stuart,

I'll admit to being a COMPLETE novice on these cars until this puppy began acting up so I may have the whole EHA thing completely muddled. Here's why I said what I did about it being "pre-EHA":

When I first began to read about the EHA, I thought gee, there's a likely culprit. First thing I did was to instinctively check online for how much the part cost. Although it's listed for later years, my car (1983) does not list the part on any online parts store. Then I read articles which give a fine summary of the evolution of the Mercedes CIS and CIA-E and have concluded that my car has K-Jetronic with lambda not KE-Jetronic which is where you find the EHA. I thought OK, mine must be somewhere in the middle of the technology - more than completely "mechanical" but less than fully "electronic". Finally, after seeing of picture of the EHA and what it looks like on the fuel distributor whilst mounted on the car, I went and took a look at mine and no EHA is to be found.

Anyway, that's why I responded the way I did. Hey - maybe that's my problem - - someone forgot to mount the EHA on my car?

Thanks again. I'll keep you and everyone "posted" on my results after installing the OVP and Ignition Control Unit. And if I'm wrong about the CIS version on my car, please enlighten me.

Ray

 

hi .. gerry from nr san antonio ...

i have nothing helpful to offer yet, but i have an '84 300SL with essentially the same problem ....the dealer has it now, and they're stumped ... i do agree that it seems to be air temp related .... and thanx for the electrical relay clues, which i'll pass along to the dealer tomorrow ...

i'll let u know if the "pro's" find anything out, and read your progress with interest...

best regards and thanx for your help ... gerry

 

Hi Zestieboy.
Good article on K Jetronic at http://www.volvoclub.org.uk/tech/fue...etronic3.shtml

Does your car have O2 sensors? If so these MUST input to an electronic circuit which subsequently acts on a valve in the fuel distributor. This valve weakens/enriches the mix (like the EHA valve I referred to).

If you do not have O2 sensors i.e. purely mechanical then the primary fuel pressure is very important since this is the reference against which fuel is metered.

I suspect you have O2 sensors and therefore the article at the above URL explains how it all works.

I remain concerned whether your existing and previous fuel distributors are interchangeable even if the replacement one appears to work somewhat better than the original.

I believe the key symptom is that an ignition restart clears the problem. This suggests that an electronic circuit (O2 Lambda control) resets itself. This circuit has a temperature input into it which might help to explain why the problem is engine (and possinly ambient) temp sensitive.

However other causes are possible like lack of fuel supply as consumption increases above idle. I've even known the ignition coil break down once warm but this is unlikely to clear with an ignition restart. Anyway you state that you have checked out and replaced many components.

Keep us posted.

 

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