Hesitation/misfire at revs

[rob walker]

An old problem seems to have reoccured on my 280SL.
After running at speed for some time there is a hesitation at around 3,500 rpm which slowly gets worse causing the car to struggle to pull more than 4,000 rpm.
It has a reconditioned injection pump, new filters and has just had tappets and timing reset.
Car runs very smoothly until this problem occurs after a motorway run.
Spoke to Roger Edwards in London they feel it could be siltation in the fuel tank, can the group advise their thoughts or experiences of anything similar as this means I have to take a flight back to the UK to take her back into the garage prior to my next vacation where we are going to cover a lot of miles and I would like to be fairly sure this will cure the problem. Thanks

[glennard]

How full is the tank?  Above the 'Shroud of Fuelin'?  If the level is below the top and the makeup gap around the return line is blocked by crud and rust, then the quart in the shroud is eventually depleted.  With slowed makeup thru the crud, the engine is partially starved.  Could be the problem------------  Open top of tank, turned car upside down, shake out crud-----you get the picture--

[rob walker]

Level in the fuel tank seems to have no bearing on the hesitation. I had half a tank when it started and filled up and this still occured.

[jameshoward]

Rob,

You said that the timing had been reset. It could be worth checking if you have a strobe. Does it stumble with no load on the engine, i.e. in neutral when you rev it after a run?

Does the fuel filter on the FI pump shut off when warm? I think I had that problem for a while and it made for bad running whilst warm. An easy check to perform.

If the car was running well before the timing and valves were redone, I would tend to assume first that someone had maladjusted something in error. Mistakes happen no matter who does the work. It might be worth checking the timing, linkage and valves again before turning to anything more drastic.

JH

[Cees Klumper]

Things I would check:

- gas linkages (throttle body butterfly closes to the point of almost binding, FI pump against stop at idle, minimal play and everything tight)
- ignition timing using the strobe light, at between 3,000 and 4,000 RPM
- replace spark plugs, check wires for resistance
- check coil, ballast resistor
- fuel delivery: pressure/volume per minute
- check the vent lines on the fuel tank

Probably there are (far?) more sensible things to do, but this is what I can come up with given these symptoms!

[ja17]

Hello Rob,

Running too lean will eventually short out the spark plugs and cause severe missing at high rpms. Remove the spark plugs and read them, if they are snow white you are probably too lean.  Try a simple and cheap fix first........ install a fresh set of spark plugs. NGKs will usually cure the issue they tend to handle the heat a little better.

As the others have suggested, fuel delivery can be another issue and a bit more complicated to fix.

Even a set of ignition points out of adjusment can cause issues like you are experiencing. A new set of ignition points will sometimes seat themselves and close. I always check the setting after a few miles have accumulated. Check this also.

[scoot]

I am suspicious of the dwell.

[rob walker]

Interesting most talk about the dwell. Very difficult for me to check things as the car is in the UK and I am in Turkey and have to fly back to get the car into the garage for work.
However, the car has a reconditioned fuel injection pump and electronic ignition.
The misfire was present about 2 years ago and it went into a specialist (who has since gotten an awful reputation in the UK) to be sorted. They installed the new FI pump and electronic ignition. This seemed to solve the problem but it did reoccur on occasions but then ran well for about 1 year although the use of the car is sporadic but when used its for a reasonable mileage. The car went into the best UK specialist for some other work. They noted difficult starting which was correct as the engine always was sluggish to initially turn over.
They checked the dwell and stated it was very advanced. When they reset it the car would not run. they adjusted all the mixtures and linkages and reset the timing to the correct point. It started easily with none of the sluggishness and ran more smoothly and delivered power better until I had run it for about an hour at about 4,000 rpm on the motorway when the misfire reoccured. Roger Edwards still maintain it sounds like a fuel problem and most likely due to sediment in the fuel tank. Seems I am going to have to fly back over to take the car back into them for them to recheck everything and probably strip out the tank and flush.

One other item they noted was that the ballast resistor has been disconnected but as they were not sure on the type of electronic ignition fitted they were reluctant to alter anything to do with the ballast resistor.

[rwmastel]

quote:Originally posted by rob walker

Roger Edwards still maintain it sounds like a fuel problem and most likely due to sediment in the fuel tank.

Very well could be.  We've seen fuel delivery be the problem on many problems that occur after the engine has been running for a while.

quote:Seems I am going to have to fly back over to....

Sounds like you need a trusted car nanny to take care of it while you're gone.

[rob walker]

What impact would having the ballast resistor disconnected have when running electronic ignition?