I have a '77 6.9 with 42k miles on it. The car has developed a problem running while cold. It will start right up and fun for a few seconds and die. You can keep doing this over and over and finally you can keep it running by feathering the throttle or slowly pour some gas into the throttle body.

Has anyone run into this problem with a CSI injected car, I believe all the V8's used the same basic system, all that wern't EFI I should say.

It's probably something simple I over looked but I've check just about everything I can think of....Help!!

Tox!

It sounds like just increasing throttle position doesn't work. This leads me to believe you are running to lean. Does the thing pop back and die if you try and rev the motor?

If so, you probably have a bad control pressure regulator (warm-up regulator). This device regulates the control pressure for the system. It's job is to enrichen the mixture during cold running. It also gives enrichment during sudden acceleration by vacuum control.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Yes, the engine will puff through the intake if you apply the throttle.

So what is the location of this device? On the drivers side there is an injector that is part of that big plastic idle screw assembly and on the passenger side there is another device that also has fuel lines to it...would it be one of these?

I have another 6.9 which I could swap parts before buying a new one, which one should I try?

Thanks

Tox!

The warm-up pressure regulator is located at the front right of the engine. It has two steel lines and one vacuum. It has an electrical connection and is rectangular in basic shape.

Although trying another regulator is a choice, I would much prefer that you got a pressure reading off of it first. The best running cars have had their warm-up pressures modified to suit the conditions of the engine. I won't get into adjusting them untill you have some pressures.

The pressure that you should read is the one in the line that comes from the center of the top of the fuel distributor and runs to the warm-up reg. This pressure, which is a re-regulated partial of system pressure, opposes the movement of the metering piston working off the airflow meter. The higher the pressure the less fuel for the same airflow.

The pressure at cold temps is much less than at operating temp.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

I tried to check the voltage on that while both cranking the car and while running. Neither times did I get a reading, but frankly I'm not sure if my connections were proper considering I didn't have the right pig tails to tap the connector properly.

I thought about applying voltage but I seem to remember that most injectors with those connectors use 6 volts, so I resisted. I also checked the car that was running properly and didn't find voltage there either so I wasn't sure how the system actually functioned.

I used to me a GM tech and was a service manager for 10 years but a fuel pressure tester is not one of the tools I still have.

A few questions I have are, how does the system work? Considering its for cold warm up mostly there must be a temp sensor that tells it the engine is cold. Is this something I should be checking and also considering it functions with WOT is there a controler that reads the signals from the temp and vac sensor and tells the warm-up compensator what to do.

There is one thing that I noticed and not sure what it means but when I pull the vac line off the car thats running properly the idle increases, when I pull it off the car that's not running right the idle decreases and the car stumbles...

Sorry to be a pain but where I live there is only one benz dealer and not only didn't they know my 300SL has a 5 speed they've never seen a 6.9 or want to.

Thanks

Tox!

No controllers, no electricity. The only thing electrical to the operation of the CIS (K-Jetronic) fuel system is the fuel pump. The warm-up regulator and the cold start valve also have electrical connections but they do no have a function to the warm motor.

The cold start valve sprays extra fuel ONLY DURING CRANKING!!! for a length of time up to 12 seconds depending on the engine temperature (thermal time switch in cooling system). The electrical on the warm-up pressure regulator is to heat up a bi metallic inside the device so that the pressure drops faster than the time it would take for the heat of the engine to do it. If the electrical is off, the enine will stay on the enrichened state much longer than necessary. Once the motor is hot the device stays at the higher warm pressure even if the engine has been shut down for some time. Cold starting mixtures are not needed at those times and the heat of the engine keeps it that way.

There is absolutely nothing that can be diagnsosed on this system without pressure gauges. On your car basic mixture can be adjusted with an exhaust gas analyser, but if it is wrong at any other time like cold runnning, high speed running, full load running, etc. diagnosis will require a gauge. Setting up and verifying pressures are necessary.

In all engines the speed is determined by the air not the fuel. In the case where you openned the vacuum line and the engine speed increased, the engine had enough fuel to go with that air and the engine did what all engines do it went faster. On the other engine adding air must have caused the mixture to be so lean that poor running overcame the increase in engine speed and the engine slowed At any given airflow the engine speed will be maximum with the proper amount of fuel; too much or too little will cause a drop in speed.

If your observation was done to a warm motor there also is the possibility that one engine liked the enrichment that would come from the removing of the vacuum line and the other didn't. When the vacuum is reduced as in full throttle the warm-up pressure (control pressure in this case) goes down for full load enrichment.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

a 6.9!?! does that mean 6.9 liter? man that is a big engine

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1996 E320
2000 C230 Kompressor
1988 190e 2.3 - 225K miles, owned for 7 years. I cannot say enough good things about this car. Very well built, even at 225 it ran like new.