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'90 420 SEL Black Smoke, misfires

6K views 18 replies 4 participants last post by  Reid 
#1 ·
Car was running alright. In a 45mph zone, at 40mph I floored it for the first time to kickdown position for maybe two seconds..damn it went like the devil! 2-3 seconds of joy. Then I notice a real lack of power going up the next hill .. black smoke, misfires, limped home, very low power.

Car history: I just purchased June 29. Car had been sitting for 4 years or so. I put in a new battery, fuel pumps & filter. It started and ran pretty well. Less than two days and 60 miles later...I floored it for the first time to kickdown position for maybe two seconds..

I was able to get a used CIS E (KE jetronic) and just swapped it out, made no difference (still a good deal for $39 USD). I know it could probably use new plugs, wires, etc. but perhaps the cold start valve needs looking at..I have searched the forums and found many references to the cold start valve, but no idea where it is, or how I (without fancy tools) can test it.

My plan B is to call a friend who works at a high end independent MB shop.
 
#2 ·
Damn-- that's not a pleasant outcome...

First things-- how many miles on the car? Service history before sitting?

Black smoke-- running rich/ incomplete burning. Could be several things, but I'd have a compression check done to rule out jumped timing chain and/or rings. Any issues such as oil in the coolant, or water in the oil? (blown head gasket) Have a look under the oil fill cap-- any milky residue?

The fact that you floored it and it ran like a bat outta hell but did not make any weird noises when you took your foot out of it makes me want to think that the issue is not catastrophic. On the other hand, it sat for several years, which might mean something catastrophic happened. Did you change the oil before this happened? Might be kinda bad to have old oil clogging passages.... Definitely have the compression check done first. Then go after spark, fuel, and air issues. Lotsa things are possible, let's start by ruling out some big ones.
 
#3 ·
Well, I just tried pulling the wires off the cold start valve. It started right up, virtually no black smoke (not like before). So I suspect the cold start valve just dumping fuel (black smoke, running rich). Shopping online for one now.

174K miles; The car was service religiously; I have the fully stamped service booklet.
 
#4 · (Edited)
Well, that's pretty damn interesting-- I had not thought that the cold start valve would do that. It does make sense. Now before you go and replace the cold start valve (which should have been inactive when you were running warmed up), I'd suspect the Fuel Pump Relay (FPR), which has a circuit that controls the CSV. Reading up on this (there's been a rash of FPR issues recently) makes me think that you could be throwing money away on a new (or used) CSV. I happen to have a spare (working) CSV and a few FPRs lying around, due to my junkyard addictions, and you might check out a local yard or two in your area.

Here's a recent thread on FPRs:

http://www.benzworld.org/forums/w126-s-se-sec-sel-sd/1532489-fpr-function-article-fyi.html

As an additional question, can you drive the car and put your foot in it when the CSV is disconnected? Everything runs nice?
 
#7 ·
When I flatbeded it from the PO's, maybe 1-2 gallons, very empty. I put in 3 gallons fresh from a can; then after I got it running, I filled it about half, about 10 more gallons and a can of fuel inj cleaner. Then I put about 40 miles on it before I hit the gas hard and the black smoke started.

I am checking out the cap,rotor, wires, plugs this morning. Maybe its not enough spark as opposed to to much gas.
 
#11 ·
Boy , this has been a roller-coaster! Might indeed be the FD-- hard to check though. How soft does the air plate rebound feel (when the engine's off)? Might be worth pulling it and cleaning up the built up varnish that accumulates on the plunger. Don't forget to install a new gasket at the base of the FD -- about $2 bucks at Autohaus.
 
#13 ·
maybe my fuel injectors...

Thinking more..concerning the spark plugs I removed & cleaned..some were quite thick with fresh black carbon, some were nearly normal looking. I'm thinking now this might be some dirty/bad fuel injectors, so I think I'll clean them all first. At 175,000 miles and sitting for 4 years, it's probably a good bet.

I hope my wife doesn't notice the injectors in her ultrasonic jewelry cleaner, or smell the sea-foam.

I will order new caps/o-rings. I was thinking a few clips might be good to have on hand, but I can't find a good place to get them [1990 420 sel US].
 
#15 ·
So the cap/rotor/wires were all very old I guess, W. German markings. Replaced and runs much better. A nice, consistent black smoke from the exhaust. It seems I have multiple issues like many others around here. Even though the plugs looked good (after I clean the carbon off) I believe they were also W. German markings, and are cheap enough to replace, advanced has them in stock (bosch 7905) and opens soon.
 
#18 ·
Glad to see you're getting somewhere on this. Poco a poco, as they say (little by little). Hope she continues to improve!
 
#19 ·
And yet another update for the (morbidly) curious. The shop is doing more than I could have hoped (I asked him to be gentle as my wallet is mighty thin these days) the manager/owner pokes at it a few nights a week, after the phones stop ringing.

Last evening's update was that he found the air flow plate (as measured) was no deflecting enough; so he is checking the cheap stuff first (vacuum leaks,etc) before heading to the expensive stuff (FD, etc)

..and the beat goes on..
 
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