1977 450SLC rough Idle and Missing - HELP PLEASE

[finleymac]

I recently purchase a 1977 Mercedes 450SLC for $1750. It been a great car and really looks nice. The main problem I have had with it is that it misses at idle, sometime from a takeoff from a stop, and while driving at slow speeds (may be there too at higher speeds, but I can't feel it). The car still seems to have plenty of power, but it will shake you to death while sitting in the car at a stop light or in traffic jam.

I have replaced the spark plugs and fuel filter with no improvement on the problem. I also have ran a few fuel cleaners and gas dryers with no improvement. I just purchase new injectors for $275 hoping they'll fix the problem, I'll update when I install them.

Other weird symptoms are it starts no better hot than cold. It really seems to run nicer cold (the first 3 to 5 miles I drive). Also sometime it will decide to idle nice, out of the blue, but it seems when this happens it blows blue smoke out of the exhaust.

Any suggestions?
Mike

[mike_1972]

Hi Mike,
you 've got a great deal on the car!

On the problem.. I would stop buying parts and replace them at will... that can turn out very expensive and possibly only a hose is kinked.....
So before you put the new injectors (you still can bring them back) I would recommend testing a couple of things ....

a) rough idle: what RPM's does the car idle at when it idles rough (do NOT look at the RPM gauge, hook up a real RPM meter).... it should idle at ~750RPM ...

b) fuel pressure. You need to have a fuel pressure of ~75PSI - 80PSI (5.0-5.6 Bar) to the fuel distributor.
After you turned the ignition off the fuel accumulator will kepp the fuel pressure up (so when you hot start it will have proper fuel pressure right away).. measure the fuel pressure after you turn the ign off.. it should drop from 5.0-5.6 to 3.0-3.5 Bar )1Bar ~15PSI)

Next check all the spark plugs, ohm all the ignition cables out (they must measure ~5k Ohm), check the distributor...

After that.. we go from there..

Cheers Mike

[finleymac]

Thanks for the tips. I measured ignition cables today for Ohms. All of them were fine but one. It measured 1500 Ohms. I could see the cable that did this was not stock and was red in color (the others appear to be stock). Would this make a difference? Isn't 1500 Ohms easier for electricity to travel through vs. 5000 Ohms?

The car has 180,000 miles on it. I looked through the records and the injectors have never been replaced or cleaned. Even if replacing them doesn't fix the idle miss, shouldn't I replace them anyhow? I looked online and most places recommend replacing or cleaning them every 90,000.

Thanks,
Mike

[mike_1972]

Hi,
well you have to replace that one cable that is not stock. It will not likely fix your problem but it is definately putting more stress on the ignition coil. There is a purpose for the cables to be 5000 Ohms, that shouldbe changed only if the whole ignition system would be adjusted.
The calbes itself run close to 0 Ohms, the plug is the one that has a 5k resistor in it. The plug runs ~35 and the cable... I have no idea.

Replacing the injectors...sure if you like to do some extra work go ahead and replace them, that is up to you. Possibly they are not all that good anymore... possibly they arejust fine.
However I would recommend starting from the root.
Teky put together a short and nice document
http://www.benzworld.org/publications/pub.asp?id=243
it is quick and easy... should not take to long. Once that is completed post your results here and we should be able to figure out what is wrong (and what is right)

Laters, Mike

[nhparks]

The last post and link to the DIY images were great. I have a 73 450sl that I've had similar problems with. Would this info apply to the 73 as well? It says it covers 75 on. Thanks,

Nicholas Parks

[teky]

i like the old babs best, sounds like the idle bypass valve needs cleaning or a split in the vacuum pipes, blue smoke could be worn valve stem seals, you could adjust the ignition timing to raise the idle problem

[finleymac]

Thanks for all of the help!

I went ahead and replaced the injectors. The idle and overall power did get a little better, but by no means did it fix the obnoxious shake at idle and missing.

So later, I was working on the car around dusk, and I noticed that there was some arching going on around the distributor cap (while the car was running). I had tested all of the wires earlier and all of them were fine (5000 ohms), except for the non-stock one that was (1000 ohms).

Since I saw the arching I wanted to make sure that they all really were working properly. I removed them while the engine was running and held them near the engine block so I could see the arch from the wire directly to the engine block. All of them did this but one, surprisingly NOT the 1000 ohms one.

I borrowed a ignition wire from an old Volvo 240 and connected it instead of the bad one, and everything is fixed!!!!! :) :) The arch on the cap went away and the engine runs just beautiful. It idles great, and really has a lot more power. I did purchase a whole new set of ignition wires, and I plan to replace all of the wires when I get the set in the mail.

The only thing is that the blue smoke is still there at idle. The funny thing is that the car doesn’t burn oil at all. Any suggestions?

Mike

[teky]

quite a common problem and seen better in the dark

[mike_1972]

hmm weird..
if the smoke is blue tha car does burn oil....
maybe not enough for you to see between changes?
What viscosity are you running?
Maybe a 10W50 (or 15W50) would fix your blue smoke problem.......

Mike

[richardr]

I dont wont to sound dum but i use 20/50 . I am seeing blue smoke to would you say try
10/50 maybe that would stop the blue smoke
richie