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Old 10-31-2005, 09:41 PM
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Join Date: Aug 2001
Location: Missouri Ozarks near MountainView MO
Posts: 107
Very hard Cold Start, 1980 SLC 5.0

This car has been sitting for the better part of 2.5 years. Work on it has been sporadic. I finally got a transmission (4 spd) installed and began to drive it a little.
At temps above 55F the car can be coaxed into starting with a little tender footwork. At temps between 30 and 45F the car startes instantly and stops just about as quickly. After about 20 of these cylcles it appearently warms up enough to start. I also have used a hair dryer left under the hood for about 20 minutes and successfully started the car. This still requires some help from the throttle, gentle jostling.
On many domestic and japanese cars with fuel injection and some sort of 5th, 7th, or 9th cylinder cold start help, when this happens it is often a tempurature sensor that is the culprit.

1. Does the MB have such a sensor that governs or is integrated with either the cold start unit or the warm-up unit (or both) that the would cause the problem described above?

2. Is there another problem here and how can we test of trouble shoot this sytem to nail down the "bad guy"?

Any help I can get from the forum I would greatly appreciate.
Thanks. Lee

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Old 10-31-2005, 09:48 PM
page62's Avatar
Registered User
Join Date: Sep 2005
Location: Luckenbach, Texas
Posts: 436
You're dealing with Bosch CIS mechanical fuel injection (also known as K-Jetronic). Nothing like electronic fuel injection we're all so familiar with.

Given the cold temperatures you're dealing with, my initial suspects would be the warm-up regulator and cold start injector. Perhaps the cold start valve. Hopefully some folks with more expertise than I will chime in...

Haynes has a fuel injection manual that explains the system. Make sure you get the one that covers early systems, up to 1985.
'01 SLK320, '79 450SL & '01 C320 -- What? 3 Mercedes? I am DEFINITELY crazy!!!
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Old 10-31-2005, 10:56 PM
t walgamuth's Avatar
Join Date: Apr 2005
Location: Lafayette Indiana
Posts: 34,887
those are the early

system i bet.

only two things i remember ever doing to those:
1. replace the injection points that reside under the ignition points. they send electronic signals to them to squirt.
2. cleaned the rotary points with a pencil erasor.

that is all my expertise from running a 75 opel sportwagen and a 74 saab 99. my saab 99 (80) had the later style i think. more reliable but less guts.

tom w
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.
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Old 10-31-2005, 11:14 PM
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Join Date: Apr 2000
Location: Falls Church, VA
Posts: 5,316
I have never worked on a 5.0, but I assume it is like the 4.5.

First of all, this is K-Jet, not D-Jet, so don't go looking for trigger points.

The car is starting better when it's cold because the cold start valve starts to fire at 15 deg C. So you are getting a shot of gas to get it going, but that only lasts for a second or so at 40 degress F, and then the mix is too lean to stay running. So the good news is that the valve and the relay are probably OK.

There are three things to check. First, the warm-up compensator richens the mix at low temps. Unplug it and put a voltmeter on the contacts on the wire and make sure you are getting 12V to it. Measure the resistance on the unit, and make sure you don't have an open circuit.

Next is the auxiliary air valve. It bolts to the water jacket and allows extra air during warm-up, which raises the idle. As the coolant warms, it slows down and eventually closes and the idle air valve supplies all the air. So if the idle isn't up over 1000 RPM, this may be the problem. You can try some PB Blaster - it may be stuck shut.

Last is the basic mixture. If it is really lean, the engine simply won't run when cold. You might also have crud in the fuel distributor in ijectors, making the mixture lean. Go no more than a 1/4 turn clockwise and see if that helps. If it doesn't, put it back.

Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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