Odd issue
 

1983 300D 260,210 miles. Anyway, last week at 260,000 miles I did an oil change, a diesel purge, and changed both fuel filters, and the car ran great. Fast forward to yesterday. I filled it up and noticed it was a tad bit more clattery than usual (or else I was just being overly anal) anyway, the car still ran great, but then about 27-28 miles into my 38 mile commute, running about 55-60 mph, I felt a very slight "loss of power/surge" feeling for a 3-4 seconds, then it went away. A couple minutes later it did it again, but ever so slightly ( I don't think a passenger would even notice it, but the driver would). I brushed it off, but it did it again on my way home, but only after running flawlessly for about 26-28 miles after a cold start. Today, it did the same thing, ran perfect for the first 25-30 miles, then a very light loss of power/surge.

When it does it there is no change in motor sound or increase/decrease in RPM like if the transmission was slipping. If you floor it, it downshifts and accelerates nicely with no miss or stumble (like a partially clogged fuel filter would cause). So, what the heck could cause this? I checked all the fluids today and they are fine, and I pulled and drained the primary (in-line) fuel filter and it was fine. I do have an injector that nails a touch at startup but it quiets down after a minute or two. I'm going to pull the banjo bolt on the back of the manifold and at the ALDA and clean them out but outside of that, I am a bit stumped. I doubt it is bad fuel as the car runs fine until about 25-30 miles, then the light, intermittent power fluctuation starts.

Many thanks for any help

Glenn

this sounds like water in the spin on filter. runs well, then acts slightly starved for fuel, then runs well... I'd change the spin on. inspect the primary filter as well, I had some slime that would randomly plug up the primary...

Or it could be a plugged fuel tank vent. That usually shows up after driving XX number of miles.

I'll check the spin on filter, but it only has about 200 miles on it as I changed it when I changed the in-line. I'll also clean the fuel tank vent.....

Ok, here is what I did. Checked and cleaned the tank vent. Checked the new in line fuel filter (it was ok) and removed the new spin on fuel filter and emptied its contents into a clear jar. No signs of crud or water droplets. Next I cleaned all the ALDA lines and banjo bolts. Finally I used some compressed air and blew out both the supply and return lines to the fuel tank (Yes I could hear it bubbling in the tank :-) I also replaced the rubber fuel supply line from the steel line to the in line filter and also from the in line filter to the IP.

Starts, idles, and revs fine. On the road it seems to have better pick up, but once it is warm, holding a steady speed around 50MPH, you can feel it surge a bit, almost like you are playing with the accelerator a bit. RPM and engine sound stay the same while it is surging. Applying more power or letting off will make it go away. Also when it is surging I have started to hear a slight buzzing behind the instrument cluster.

Any ideas?

Glenn

Try driving with the vac line to the shut-off disconnected.

Ok, but bear with me when I ask why??

Also the vac shutoff is the brown one, correct?

I would pay attention to the buzzing you hear everytime it does this. Is the sound like a relay problem? Some way there is cruise control interaction even if not active?

Barry,

I was kind of wondering that myself, but the cruise has not worked on this car since I have owned it. However I WAS messing with it last week before this started. I think I will disconnect the cruise linkage rods tomorrow just to be sure it is out of operation.

Immediately after a diesel purge? Sounds like loose crud/carbon.

Possible, but where?? In the IP? The filters were both changed after the purge and they are still clean and clear. Also, it runs for several miles until it starts randomly and lightly surging. I have half a tank of fuel right now, once it is pretty much used up I'm going to check the tank screen again

Read this: http://www.peachparts.com/shopforum/tech-help/141723-italian-tune-up.html

Yes. The buzzing in the dash could be the vacuum control on the ignition switch. The engine may be trying to shut off due to some vacuum related issue. Simple test to eliminate one possibility.

Check all the hoses and clamps

I will check the hose clamps in the morning when I disconnect the vacuum shutoff line, but when I started it this evening and let it idle, I did not see any air bubbles in the clear fuel lines. Interesting info on the Italian tune up....worth a shot for sure!