help with fuel sticky situation, please

[enochbell]

The car has run so well for so long, until...

Went for 30 minute drive last weekend, mixed town/highway.  Car started fine, but I noticed it ran a bit rough under light load at anything under 2k rpms.  The problem at idle was barely noticeable, with just a tiny bit of stumble.  But the car would splutter until the engine was above 2k.  Above 2k there was no issue, ran on highway at 70 mph with no issues.  But back in lower revs the problem was still there and persisted for all 10 remaining miles of the trip.  I parked the car for about 45 minutes, started it up for the return trip and the problem had disappeared!  The car ran perfectly for the entire 30 minute return trip.

Went for a drive today and the problem was back, exactly as last weekend.  I drove about 20 minutes, then shut the engine off, let it cool 20 minutes and started up again, hoping the issues would resolve as it did last weekend.  No such luck, the problem is still there.

Now, I don't want to send anyone down a blind alley, but I have also recently noticed that the whine of the fuel pump has a different quality to it, it is sort of a two-tone whine where as for the last 10 years it has always been a steady, single high-pitched whine.  I assume the pump is getting tired (bearing worn?), but I can't imagine how it is related to the stumbling issue, given that the car runs great at full song.

I assume something is sticking, but don't know where to start to diagnose, would really appreciate some advice.

Thanks,
greg

'64 230sl, fully sorted out...ooops, spoke too soon

[Paddy_Crow]

It could easily be the fuel system. If your pump isn't making enough pressure, your air/fuel ratio will be off. This will be more pronounced at idle and off idle when the pump is turning slower. The fact that you've noticed a change in the fuel pump would lead me to start there, although it is always tempting to do the easier things first.

As you well know, the problem could be unrelated to the fuel system. The symptoms are also consistent with how an engine behaves when it's time for a tuneup.

The hardest part of fixing a problem is determining the root cause.

Democracy is two wolves and a lamb voting on what to have for lunch...

[J. Huber]

Hi Greg. It wouldn't be a bad idea to check the points. Or did you already?

James
63 230SL

[Cees Klumper]

Check the ignition components: wires, plugs, points, timing, vacuum etc. Also I would suggest checking the throttle linkages.

Cees ("Case") Klumper in Amsterdam
'69 white 280 SL automatic

[George Des]

You might want to check the inlet screen on the fuel delivery pump. If it is clogged it could cause the pump sound to change a bit. This pump turns at the same speed and delivers at the same volume and pressure no matter what the speed of the car is.

George Des

[enochbell]

Thanks all, I will check the plugs and the inlet screen.  The car is in fine tune, checked timing (electronic ignition) and it is correct and I  pulled the plugs to check their condition, all OK.  I will also check linkages, unfortunately all this will have to wait for two weeks as that is the next time I am in the same city as my car. (but don't feel sorry for me, I commute from/to Atlanta/S Florida)

Thanks for the advice, I will report back mid-November.

Best,
greg

'64 230sl, fully sorted out...ooops, spoke too soon

[Cees Klumper]

Hey Greg - last time I had a rough running problem (stumbling and hesitation at low RPMs) it turned out to be the plugs that looked ok, but apparently (one or more) really weren't. You can swap out your current set with new ones (need those at some point anyway); if that does not cure it, you can put the old ones back in and mark that off the list as an easy check. Probably won't address your problem, but then again it just might.

Cees ("Case") Klumper in Amsterdam
'69 white 280 SL automatic

[enochbell]

Cees,

Thanks, I agree and will swap out the plugs, even though they have only 3k miles on them, just to eliminate them as a possibility.  But the thing that has me stumped is the fact that the issue went away COMPLETELY on that drive last weekend when I let the car sit for a while.  Made me think it was more likely a "sticking" situation that corrected on restart.  Maybe the IP rack, some solenoid, or whatever.  The "two tone" whine on the fuel pump has me worried, but not discouraged.

Thanks,
g

'64 230sl, fully sorted out...ooops, spoke too soon

[jeffc280sl]

Just had a similar situation with my car.  After the car was warm there was a hesitation under light load in the lower rpm range. Also the idle when warm was rough. At higher rpms the engine ran great. The plugs looked looked like the ideal set from the Haynes manual.  Changed plugs and the car runs much, much better.  I switched from the Bosch 9wdc to the NKG 5eps (I think those are the correct numbers)

I had no idea plugs could make such a difference.  Thanks to those on this site.  I think maybe my old plugs had some sort of thermal degredation failure, not sure.

Jeff C.
1970 280SL 4-speed

[J. Huber]

Hi, Jeff you were close -- I believe the NGKs are BP5ES. Those are what my cars likes as well. And for what its worth, I used the gap right out of the box.

James
63 230SL

[enochbell]

Pulled the plugs, all evenly very light gray, probably burning a little lean but not enough to cause the stumbling, I think.

Pulled the FP, went to clean the inlet screen.  There ain't no inlet screen on this very early pump.  

I hesitate to put the pump back in, as I know something is wrong or about to be wrong based on the sound of the running pump, it has an obnoxious two-tone whine that just started in the past month or so.  I will check with George Des about rebuilding, but I fear that is not my only problem.  Oh well, nobody said this ownership thing was going to be all "Sunday rides with the top down".

Any suggestions would be appreciated.

Best.
g

'64 230sl, fully sorted out...ooops, spoke too soon

[Mike Hughes]

Pull the distributor cap and check the point gap.  I had a little bit of a stumble and occasional stalling when shifting the automatic from forward to reverse, or vice versa,in the days before my car quit altogether.  It ws behaving just like it was being starved for fuel. After checking the fuel pump and screen, draining the tank and checking the screen inside the flower pot, and confirming that the fuel tank return line was, in fact, returning fuel, I finally discovered closed up points.  Rough running and stalling cured!

- Mike Hughes  -ô¿ô-
  1966 230SL Auto P/S
  Havanna Brown (408)
  Light Beige (181)
  Cream M-B Tex (121)

[Naj ✝︎]

quote:Originally posted by enochbell

  I will check with George Des about rebuilding, but I fear that is not my only problem.  

Any suggestions would be appreciated.

Best.
g

'64 230sl, fully sorted out...ooops, spoke too soon

Have a look here re pump rebuild:

http://www.sl113.org/wiki/pmwiki.php?n=Fuel.FuelPumpRebuild

naj


68 280SL

[enochbell]

Thanks all, the help from this group is really amazing.  George has offered to pull together a rebuild "kit"; Michael offered to lend a replacement IP to help me isolate the problem, but I am holding out for someone to just show up and fix the thing for me!

Kidding aside. The only thing I know is that the pump recently started running loud and weird, so I am going to fix that first by rebuilding.  I opened it up last night and it looks clean (no gunk), so I am guessing it is a bearing or brush that is noisy.  If that is the case I can't really see how that is related to the rough running, but it needs to be fixed.

Then I will begin to knock off the rest of the suspects until I solve.  The electronic ignition and coil are about 8 years old, the IP was "calibrated" (years ago I had made the foolish mistake of removing the throttle arm, necessitating a trip to Pacific), but my money is on something amiss in the IP that has cropped up since then.

There is a "law of parsimony" that says that when you have a problem that is apparently multi-facited or symptomatic it almost always has a single cause.  Now to find it.

Thanks for all the advice and help,
g

'64 230sl, fully sorted out...ooops, spoke too soon