560sl continously missing & low power - PeachParts Mercedes-Benz Forum

DavidSoine · #1 10-05-2004, 09:16 PM

Yes you read that right. We've had an occasional miss at idle all summer, slowly getting progressively worse. Now the car appears to have one cylinder missing all the time...with continuous vibration, exhaust rumble, and slightly down on power.

Please help me diagnose the problem, or determine what to look at first.

The car starts right up, cold or hot.

I just noticed on a downhill grade, the engine would appear to stall for a fraction of a second during engine braking.

There is no "flatulence" when I remove the fuel filler cap, none at all when I checked after my drive tonight. The car used to have plenty.

I put a bottle of Techron in the fuel tank just before the car started having this problem - I have only run 70 miles or so since then.

I have looked at the vacuum lines, spark plug wires, etc. under the hood and see no cracked lines (replaced a vacuum elbow in the process) or other problems.

1987 560sl, 215,000 miles.

Thanks in advance for your thoughts and ideas,

Dave

Strife · #2 10-05-2004, 10:03 PM

The "flatulence" thing is fairly temperature-dependent and it is getting cooler. How did you check the ignition wires? Mine "looked" alright when I got the 83K car. Last tuneup? Generally, when you go to a dealership with a problem of this sort they won't touch it until _they_ tune it up...with good reason: eliminate the obvious. I was worried my car would have problems I wouldn't figure out in a million years, especially after readong boards like these - until I did the plugs/cap/rotor and replaced the ORIGINAL 18 YEAR OLD WIRES! The whole thing was a whopping 250 in _parts_ - but I've been running great since.

Your post seems to lean towards thinking that it's fuel related...I don't know, but if those are the original fuel pumps, they must really be good!

BTW I was looking at your previous posts to get an idea of what work you might have done in the past. Did you ever replace the timing chain tensioner?

airedale · #3 10-06-2004, 07:41 AM

It started quicker in my '84 380 SL and soon sounded like 2 or 3 cylinders were missing but only turned out to be the distributor rotor. It was amazing that it ran at all as the end 1/2 inch of the rotor was completely burned away. Wish I would have looked at that before having it towed in.

DavidSoine · #4 10-06-2004, 10:41 AM

Alright! I appreciate everyone's help...

A new chain tensioner and timing chain were one of the first things I did on the car, so they are OK. I have not done much to the engine lately, but in addition to the timing chain, valve stem seals were replaced, as well as ball studs replaced and shimmed properly. One spark plug cap was replaced last year - the wires & ignition parts look great...but of course looks aren't everything.

I hadn't thought of the distributor - but now that it's been mentioned it's an obvious first stop - probably needs replacement anyway, along with all the wires, etc. It fits the "started suddenly" nature of this problem.

I was leaning toward fuel because it is an area I've "neglected" with the idea that eventually I will get to it, and when I do I'll renew the whole area - filter, lines, etc.

Any other ideas?

best,

Dave

Pili380SL · #5 10-06-2004, 11:59 AM

If you have a miss at idle, you can try pulling plug wires off at the distributor one at a time, using insulated pliers, until you find one that doesn't affect the idle. This isolates the missing cylinder. To accentuate the miss, if you have a helper, do it with the car in drive (with the brake on, of course

) You can then check the spark plug in that hole, and while you have the plug out, check the compression as well.

DavidSoine · #6 10-08-2004, 10:14 AM

Bad cylinder is #8, identified by troubleshooting in the manner described in the previous post.

Running "character" also appeared to change with movement of the spark plug cable.

Cap and rotor inspected - Definitely some wear on the rotor, and pitting of the contacts on the cap. Looks like it's time to replace them all & hope it solves the problem.

Now all I have to do is find the $300 for the parts...

Dave

Strife · #7 10-08-2004, 12:40 PM

The parts are expensive - and there really isn't any "cheap" source. I bought wires that cost about $150 (not the most expensive, but up there) and I was very disappointed with their tailoring - not exactly the same lengths as the original Bosch wires. But they definitely solved the problem. Hope I'm not insulting your mechanical know-how, but don't forget to use silicone grease at both ends or they will be "disposable" - they are so delicate that it's really difficult to take them off without damage WITH the grease, never mind without.

Tom McMenamin · #8 10-08-2004, 01:40 PM

PinPoint the problems and then fix them as you can. If you start throwing parts and money at it you will have just that. a whole bunch of parts, not much money and a car that still isn't right!

RandyA · #9 10-08-2004, 01:43 PM

Quote:

Originally Posted by DavidSoine

Bad cylinder is #8, identified by troubleshooting in the manner described in the previous post.

Running "character" also appeared to change with movement of the spark plug cable.

Cap and rotor inspected - Definitely some wear on the rotor, and pitting of the contacts on the cap. Looks like it's time to replace them all & hope it solves the problem.

Now all I have to do is find the $300 for the parts...

Dave

I am very impressed with the KingsBorne wires. Perfect fit and angled connectors where they needed to be.

Kingsborne Wires

DavidSoine · #10 10-08-2004, 05:26 PM

Quote:

Originally Posted by Tom McMenamin

PinPoint the problems and then fix them as you can. If you start throwing parts and money at it you will have just that. a whole bunch of parts, not much money and a car that still isn't right!

Yes - I still intend to do at least a compression test on #8, since I don't have a leakdown tester. Is there anything that would be caught by a leakdown test that wouldn't show up first as poor compression???

Thanks,

Dave

DavidSoine · #11 10-09-2004, 05:54 PM

Perhaps this could be the problem...from cylinder #8.

DavidSoine · #12 10-09-2004, 05:59 PM

In addition to Plug #8 being totally fouled, I noticed plugs 2,4,6 were wet and slightly fouled (8 was wet too.) Remaining plugs looked _good_.

Turns out wires were not a problem - I figured it out by swapping wire #8 with another.

So what is going on here?

BTW running very nicely with 8 new plugs...

best,

Dave

Pili380SL · #13 10-09-2004, 11:38 PM

Dave, the plug looks oil fouled, but it's hard to tell by the picture. Did you notice if the wet plugs smelled like gas? Four leaky injectors on the same side sounds uncommon. If it is oil, it sounds like the valve seals are worn. Hopefully the guides aren't worn, as that would require removing the head. The leakdown test would serve to show the cause of low compression. When compressed air is applied to a cylinder at top dead center, you listen for a noticeable hiss through the intake (intake valve leaking), exhaust, or crankcase (rings leaking). Hopefully the new plugs will stay clean.

Paul

Tom McMenamin · #14 10-10-2004, 01:53 PM

What is a Leak Down Test?

Thw search above gives you all you should need! Copy into your Browser!

droederer · #15 10-10-2004, 01:57 PM

Paul, can you tell me how to conduct a leak down test?

David
86-560SL