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Posted by: ThePaintedMan Jul 7 2016, 07:36 AM

In an attempt to broaden my horizons as a diagnostician (and mostly because I need the money to keep the lights on), I'm helping a neighbor with his 1983 Mecedes 380SL. The car is a V8 with the Bosch K-Jetronic mechanical injection system, similar to the CIS on the 911.

I have studied the system to have a pretty good understanding of how it works, or at least the physics behind it. What I'm dealing with at the moment is a no-idle condition, both cold and hot, but it's far worse when cold. The neighbor claimed he can get it to run by dumping gas in the air sensor plate orifice for a few minutes and then if he jams a screw driver in the linkage, it'll idle (poorly). Above idle, it revs up and down pretty well.

Here's the data I have gathered:
- All 8 injectors are brand new
- The Idle Air Valve has been replaced relatively recently
- The Idle Air Control Unit appears to have been replaced recently

A couple things - if I "prime" the fuel distributor by pressing down the sensor plate, I can get the car to fire to life right away, but it will not idle. If I have someone else start the car, but I hold the sensor plate just barely down, it will idle almost perfectly.

To me, this tells me that the car isn't getting enough air, or fuel at idle (duh), indicating that the idle control system itself isn't functioning.

Here are a couple things I still need to check:
-The functioning of the Overload protection relay (OVP) which I heard has an impact on the idle system
-Need to test for voltage at the idle air control valve (I think it's supposed to be 12+ volts)
-Need to test for voltage at the cold start injector (but my understanding is this is only functional when the car is being cranked anyway)


Otherwise, I'm open to your troubleshooting suggestions and/or workflow. Thanks!

-G

Posted by: Mueller Jul 7 2016, 07:49 AM

Make sure the boot has no leaks. Had this issue with my Audi years ago.

Posted by: dlee6204 Jul 7 2016, 08:01 AM

Check for air leaks

Posted by: ThePaintedMan Jul 7 2016, 08:54 AM

10-4. To be honest, I can't even figure out what the bottom of the intake even looks like - I.e., how the throttle body lets air into each intake runner. I haven't been able to find a good diagram with the air sensor and fuel distributor removed to get an idea how it all works.

Posted by: Mueller Jul 7 2016, 09:08 AM

QUOTE(ThePaintedMan @ Jul 7 2016, 07:54 AM)

Posted by: Shadowfax Jul 7 2016, 09:18 AM

Had K-jet on my '82 E21 323i. Miss that car...
Anyway, agree with the others on air leaks but could also be a improperly adjusted throttle plate.
http://www.k-jet.org/articles/information/k-jet-debugging-guide/ is for Volvos but has good troubleshooting.

Posted by: rgalla9146 Jul 7 2016, 11:07 AM

You need to check the boot between the sensor plate and throttle.
Beyond that you need to know system pressure and control pressure which is measured with a special fuel pressure gauge.
Fuel pressure regulators are an issue as well.
I have no experience with the system as used by MB.
I was used on 911s from 73.5 to 1983

Posted by: infraredcalvin Jul 7 2016, 11:54 PM

Bathroom reading...


http://jimsbasementworkshop.com/CIS/

Posted by: flyer86d Jul 8 2016, 04:01 AM

Check the cold start injector for voltage and if you have that, pull the injector out and see if it sprays. You need that spray of fuel to get the engine to fire and pull the mass flow sensor plate open to supply fuel to the injectors. Also, I have seen idle air valves seize. Pull it out and see if it swings freely.

Charlie

Posted by: rick 918-S Jul 8 2016, 04:32 AM

QUOTE(rgalla9146 @ Jul 7 2016, 12:07 PM)

Posted by: Dave_Darling Jul 8 2016, 03:04 PM

System pressure is the primary control signal for K-jet. It is critical that you know what the pressure is.

--DD