Why is my 450 stumbling and shaking?

I am at the point of total frustration with this car.

History-
I purchased in January for $5000 from the original owner w/172,000 miles with all service records and a bad transmission which I replaced(lots of thanks to stevebfl and M.B. DOC)

symptoms-
Hard to start. terribly rough and fluctuating idle. In P somewhat smooth in D shakes and stammers or dies. Hesitates awfully up to ~2500 rpms (25-30mph) after that smooth as glass.

what I've replaced(sorry for the long list)-
fuel distributor - set to 5.0 bar system pressure
air slide valve, cold start injector, control pressure regulator,injectors w/ seals, holders and o-rings, plugs, wires distr. cap & rotor, coil, fuel filter, distributor vac. advance, thermo vac valve, catalytic converter. All vac. hoses and checked against diagrams from my CD.

What I've adjusted-
timing to 0 degrees (runs much better if advance 20-30 degreees)
checked that distributor was pointing to #1 plug wire when engine pulley is at O on crankshaft across from timing pointer.
set CO to 3% to make it keep from stalling in D
cleaned throttle body

I've spent a lot of money on this car and would like top be able to drive it someday. I know throwing parts isn't the correct way to fix or diagnose problems, but with the car being 25 years old, a lot of them were safe bets for peace of mind if it ever gets running and WE can travel in it.

I would apreciate any suggestions,
Help me renew my faith in MB

jmdunn


__________________
77 450SL
Med. Blue

[stevebfl]

You will need to do some forms of testing to get results. Does the car drive properly at 40mph? Does it accelerate well from that speed?

I would bump the timing to 15 deg BTDC (with the vacuum lines disconnected), at least till we have fixed the problem. The factory set-up is based upon having vacuum on the retard side of the distributor - many causes for this setting the timing too low: no vacuum, low vacuum, A/C on (no vacuum), vacuum retard unit leaking, corroded, malfunctioning, etc.

Once in this condition, determine if there is a single cylinder misfire or are all cylinders equally poor running. Effective cylinder balance testing can be done by cracking the injection lines loose one at a time (wipe spilled gas - be carefull) while noticing the difference in speed and vibration. Cylinders that are poor will give little if any change. This same test can be done by pulling plug wires. I personally would rather deal with the fuel than the spark.

Let us know what you find.

Just got back from thanksgiving break,
I messed with the car all day on Friday and checked all the fuel lines from the fuel distributor they all seem to be getting fuel(75psi). I checked system and c ontrol pressure again and they are respectively 75 psi and 50 psi after warm up cycle. It ran last week but was hard to start and stumbled and idled rough in D up to ~30mph after that smooth as glass except under medium to hard acceleration. It still guzzles gas. probably because I set the CO at about 3.7% to get it to idle at all.
Well, now the rest of the story on Friday after checking all fuel pressures with the Fuel pump jumpered. I get no fire form the ignition coil at all. The coil is brand new. So I look at the ballast resistors on the fire wall and noticed a crack in the blue one. Could this have been causing my problems all along and finally just gave out. My ohmmeter shows broken continuity. I remember that the car would do much better if I advance the timing quite a bit(20 degrees or more) but there was never any pinging that I normally associate with timing being to far advanced?????
Well, the resistor is on order I wait and see. Are there other suggestions maybe spark control box???

thanks for all your efforts ,
I won't give up

jmdunn

[Doug Wilson]

I know this sounds silly, and you have probably checked it without knowing it. However, I had a similar problem the other day and I found that my battery cable had come loose on my 75 450. It ran like crap unless it was under acceleration, then plain wouldn't start (or course). Worth a try, what can it hurt?

[hill]

"I know this sounds silly,"

I had the same thing happen. I cleaned up the ground strap connection and it ran better after looking at everything else. The car would start and run at 11 volts but perform badly.

Battery strap good and clean. Correct me if I'm wrong but if the cart starts doesn't the rest of the electronic system pull fromthe alternator? Also ran compression test on the cylinders all seem to be within 5 psi of each other the lowest was 127psi highest 131.
Has anyone replaced the resisitors before?

jmdunn

[ctaylor738]

My .02 - sounds like a vacuum problem.

Have you tested to see how much vacuum you are pulling at idle? I have been told that the intake manifold gaskets deteriorate with age and start to leak.

[hill]

"doesn't the rest of the electronic system pull from the alternator?" I am really not sure. I do know that with a good well charged battery and a bad alternator it will run well. But not visa versa.

Battery is less than a year old and alternator has less than 20,000 mile since replacement.

jmdunn

[vutown]

Have you tried looking at your fuel injectors yet?

I had similar problems with my high mileage benz. Once it awhile, it would run ok, but most of the time it was sutter and rattle. I believe it is a cheap fix.


vu

Replaced them. Anyone had experience/advice about how to test ignition control box or resistors?

thanks
jmdunn