D Jetronic, engine dies above 1500 rpm - PeachParts Mercedes-Benz Forum

knarf · #1 02-14-2006, 07:49 PM

Greeting Forum Members,

Happy Valentine's Day.
I Just started up my sitting (3 months) 350sl after installing a new battery.
It idles beautifully, but when given throttle the engine dies right at 1500 rpm.
I know that the throttle valve controls the fuel delivery up to 1500 rpm.
What componet(s) takes over over the 1500 rpm mark?
The answer to this question should help me troubleshoot the problem.

Thanks,

Knarf

Duke2.6 · #2 02-14-2006, 11:50 PM

"I know that the throttle valve controls the fuel delivery up to 1500 rpm."

This statement doesn't jibe with my understanding of D-Jetronic, which I have researched in great detail.

My Cosworth Vega has a Bendix electronic fuel injection system. Bendix was not able to sell their system to the Detroit OEMs in the sixties, so they licensed the patent rights to Bosch and Bosch produced the D-Jetronic, which is generically a speed density system with analog electronic control. The first prototype Bendix speed density EFI system was built and installed on a Buick V-8 circa 1953. It used a vacuum tube control unit about the size of a small AM radio of the era.

My CV has both Bosch and Bendix components.

There are two critical sensors that this system MUST have to function. The MAP sensor and the speed sensor. I can disconnect the coolant and air temperature sensors and the throttle position sensor and the engine will still start and run.

So check both your speed sensor and MAP sensor using whatever test specifications for your particular components that you have or can find, and be sure to carefully inspect the connectors to these components. It's possible that engine vibration at 1500 is causing an open.

On any old car, connectors are the most common source or electrical anomalies.

Duke

MDF · #3 02-15-2006, 11:58 AM

The throttle position sensor, unless it is totally losing contact at a certain place along it's range of movement, is probably not the problem. After verifying that the ignition system is set right, including your timing and dwell, put a gage on the fuel rail and make sure the pressure is correct (~30 psi). Is the pressure constant during idle and also at higher RMP as you approach the stalling point? It could be that you need to change the fuel filter. How's the inside of your fuel tank look?

Also, make sure that you are not running too rich. What is your regulated system voltage? Is the hose well connected between the manifold and the pressure sensor? And is that hose in good condition?

Mark
'84 300D
'69 VW Fastback (D-Jet FI)

knarf · #4 02-15-2006, 08:39 PM

Duke2.6 & MDF,
Thanks for the feedback.
Tomorrow I will try to find time to troubleshoot the problem based on your input.

knarf

knarf · #5 02-17-2006, 11:59 AM

Greetings Forum Members,

After adjusting the timing, it was only slightly off, adding a little more gas to the tank (it was near empty), playing with the throttle and tapping on the switchgear I got the engine to rev over 1500 rpm.
I'm now attempting to adjust the idle speed but can't get a pick up signal/voltage from the negitive lead on the coil to my engine analyzer yet the engine is running.
Thinking there might be a problem with the analyzer I hooked it up to another car and it read out the rpms fine.
Does this simptom sound familiar to anyone?

Thanks,

Knarf

Matt L · #6 02-17-2006, 01:50 PM

Check your ignition condensor. Failure of this part will make the engine run horribly, as there is no ringing in the coil primary winding when the points open. It could also be fooling your analyzer.

knarf · #7 02-17-2006, 07:59 PM

MattL,

Thanks for the advice.
I will check it out.

Knarf

Greg in Oz · #8 02-20-2006, 12:34 AM

Quote:

Originally Posted by knarf

I'm now attempting to adjust the idle speed but can't get a pick up signal/voltage from the negitive lead on the coil to my engine analyzer yet the engine is running.

You won't get a signal on the negative side of the coil on the ignition system of a D-Jet engine. The negative side of the coil is permanently grounded with the electronic switchgear switching the positive side of the coil. If you need a signal that switches to ground, use the green wire between the switchgear and the breaker points.

Greg