Rough running except idle

[Rhinkel]

I've finally got my 280 sl project back on the road running beautifully. I found a great mechanic that has 40 yrs experience with these cars but he's expensive! I purchased a rebuilt fuel pump and had it tuned to the car with him. However, the pump developed a few leaks so I returned it was replaced with another rebuilt unit. Everything worked fine upon replacement. I took the car out last weekend for a long cruise and it started to develop - what I will call a miss- at all rpm levels but idle. So if I was running along at any level of acceleration other than a very very light throttle, I could feel the shake. Even at hwy speeds, smooth at a very light throttle, then a constant almost shutter under any load. My pagoda is a 1970 and has a new factorytransistorized ignition. I have new plug wires, and tune up parts ordered and coming in hoping that Its ignition related.plugs are bright white, slightly brown and dry.

It almost feels like once I go past a very light throttle, I'm working against a rev limiter.  I've read about almost similar issues here on this board with the fuel shut off solenoid. The throttle linkage has been adjusted by my mechanic.  The car ran perfectly then about an hour in, the shutter began. Any ideas of things I should look into other than ignition? Thanks. Rob

[Tyler S]

When you say "Pump" are you refering to the injection pump or the fuel pump at the rear?
Start basic. Assuming the ignition components are ok and the linkage is properly adjusted. Was the fuel filter replaced? Could be fuel starvation. If the injection pump was replaced it is likely the main rack adjustment needs to be enrichened. This can be done by shimming the baro or adjusting the main rack adjustment screw. Most rebuilt/replaced IP's will need to be tuned to the car they are installed on. Keep in mind the thumbwheel on the back of the pump only adjusts the idle mixture. If your car does have a fuel cut-off system, disconnect the wire going to the solenoid and see if the problem clears.

[ja17]

Have your mechanic check the fuel pressure while the problem is going on. A fuel delivery problem will show low fuel pressure during these episodes (tank, screen, filter, elec fuel pump prob.). 

[Bonnyboy]

I echo the fuel check - check volume from fuel pump before the injection unit and volume going back into the fuel tank.

But also - check your advance/retard unit on the distributor - mine was loose missing one securing bolt and it had weird symptoms too.

[Rhinkel]

Off the car the pump output was tested at 112 % of spec. The tank was opened, sandblasted, a metal flower pot was designed and welded in. I've run two full tanks through the car and it delivered fuel from full tank down past reserve. The line to the pump has an new inline filter and the fuel filter up front was changed 600 miles ago. I'm leaning towards ignition but the posts are thinking more fuel delivery. In the 600 miles since back on the road, the last 40 were the erratic ones.  The only change was the pump. I guess  I should check output on the car first, then move to ignition. Outside of something restricting flow, is there any other component in the fuel delivery chain that could be causing the issue? The car sat for 20 years before I got it going again. Anything electrical that might cause this other than normal ignition parts? Appreciate the comments. Rob

[Rhinkel]

Ok, so I checked the plugs, all good. Checked the resistance on the plug wires, found one questionable and had an extra that was well with in. Cleaned off the carbon tracks on the distributor cap and rotor tip. Reinstalled and it ran better but still a little rough on acceleration. Do backfiring, plugs clean. I dropped the fuel filter up front and the gas was very cloudy. Pulled the inline filter I installed in line before the pump and the gas I collected back there was cloudy. I'm wondering if I have bad gas OR the guys who did the tank put in a coating that's stuffing off.   They have done tanks for 50 years! I'm going to run this tank through and see if it gets better. Could it be as simple as bad gas? Never had that happen to me before?

[ja17]

You might want to check the screen in the electric fuel pump again. Also for diagnostic purposes only, use a jumper wire to by-pass the ballast resistor. If the car runs better you may have an ignition issue. Do not operate the engine long this way or you will burn your ignition points. Only do this test if your car does not have the factory electronic ignition.

[Rhinkel]

Thanks Joe. Unfortunately, I do have the transistorized ignition. The issue is much better after the checking and clean up of ignition parts. But it's still there. Changing the fuel pump definitely changed something in the chain. Of note, it definitely runs richer but no black smoke on acceleration. I'm still going to run this tank thru and see. If not it back to the mechanic. Should changing the pump make any difference in driveability?

[Tyler S]

You may want to omit the extra in-line filter or make sure it is for a return style system. A standard in-line filter you get at auto parts stores arent big enough to handle the volume our pumps produce. Cloudy fuel, especially fuel with ethanol, usually means water contamination. Take another fuel sample in a clear container and cover it up. Let it settle for a few hours and check for separation. 

[Rhinkel]

Thanks S Tyler.  I kept the fuel I drained ina c Lear bottle testersay and it all settled out. No water in it. I put the inline filter prior to the pump as a suggestion from the Mercedes source guy Ken.. he offers filters that handle the flow and don't impede.  I drove the car today about 50 miles and it runs good, it's just rough- almost like a vibration at low rpms and anything other than a light throttle but has good power and no pinging.  My plan will be to burn thru this tank and then refill, and take back to a very capable mechanic. I've turned wrenches for years on my classics, but these cars definitely require tribal knowledge beyond me. 

[Tyler S]

If the ignition is sorted, and fuel pressure and flow are correct, then start looking at fuel injectors. A partially plugged injector wil idle fine but be more pronounced as load increases. If you can isolate the cylinder/cylinders causing the miss it might help you pinpoint a direction.

[Rhinkel]

Ok ! I think I found the problem....don't laugh at me though..laugh with me..I installed a time valve  stainless system a few months ago. I was noticing that under hard throttle or very light throttle, the shuttering would go away. I took my wife out for a ride and her first comment when I asked her if she can feel the vibration she says " ..it sounds like a hole in your muffler! "  Sure enough, I get under the car and tighten up everything and start hitting the exhaust with a pry bar and the muffler in the middle of the car- the first one- I think it's a resonator, rattles and vibrates inside. I'm almost sure that I was feeling of whatever has broken loose inside and was resonating with certain pulse of the engine exhaust. 

Leave it to my wife of 26 years! Great job babe... now can you barter with time valve and get me another one? JK, I'm sure they'll honor their work.

[Tyler S]

Ha! Too funny. You actually may be feeling the effect of the issue if whatever is broken inside is blocking off the exhaust. There may be higher backpressure on 3 of the cylinders than the other 3 causing uneven power delivery.

[Rhinkel]

Yes! My hope exactly. It's just a coincidence that it came at a time when I replaced the fuel pump. At least that's what I hope.  Already shot off a note to time valve. If this wasn't the issue I'll be surprised but it's definitely AN issue!