Fuel Distributor Question

[wnmimms]

Hey guys,

In short...
1979 930, 45k miles, Euro.

Rebuilt:
WUR (I completed)
Fuel Distributor (I completed)
CDI (************)
Coil (New from ************)
Plugs (New)
Beru Spark Plug wires (New)

Pressures:
System Pressure - 6.7bar
Cold Control Pressure 20C - 2.0bar
Warm Control Pressure - 3.6bar
Pressure on boost .5bar: 2.8bar

Timing:
Idle (Z)
4000 RPM: 29 BTDC

The car runs and sounds good.


https://youtube.com/shorts/GcFaCgGi5pc?feature=share


I'll spare you all the backstory but in short, I have corrected all the previous issues after completing the above and now have the car running!

Here's the issue.

I am only able to get the car started if I disconnect the two pole connection at the fuel distributor.

Thoughts on this? After the car is running I can plug the two pole connector back in and it continues to run fine.


Any help, suggestions would much appreciated!

Bill

[Alan L]

So, with key ON and that airplate sw disconnected, are you getting the pump noise/whirr?
Good job on the refurb. Well done.
Alan

[wnmimms]

Hey Alan! How you been? Key on and the air plate sensor disconnected both pumps are activated and running. When I plug the sensor back in the pumps stop...

So I went out to confirm exactly what was happening before replying... The only way to get the pumps to run is when the sensor plate plug is disconnected.

Any bets on what you think it is???

I'll save you the mental gymnastics.

When I was initially running through the car I found that the thermo-time switch, which is a NO (Normally Open) switch was not grounding out once the engine got up to temperature. Given access to this switch is somewhat difficult... to say the least I put a ring terminal on it and grounded it to a stud on on the charge air bypass valve.

Apparently, with all that I was messing with in there I managed to cause a short at that ring terminal. I put a new terminal on and now all is running as it should!

Once the new tires arrive I can finally take it out for a spin. It's taken me a year to get to this point... I have learned a ton and developed even more appreciation for the car along the way.

Really appreciate all the advice you provided early on in the project!

I'll send you a link to a video once it's on the road!

Hope all is well with you and your family.

Bill

[mark houghton]

Sounds like perhaps the microswitch to the air plate may be toast, which is kinda rare. One way to find out would be to plug the connector back in, and manually depress the air plate to see if the pumps start running.

[Alan L]

Yes, I am not sure if the problem is solved now or not - but sounds like it might be?
Otherwise - I was heading down the track Mark is suggesting. That airplate switch provides a 'grounding' for the relay in the engine compartment (when the plate is depressed). That relay is wired 'backwards' so when you break the circuit by opening the airplate the solenoid moves to the rest position - which is wired so that can then supply power to energise the front pump solenoids. I think there is an adjustment on that switch - I don't have access to my manual right now - but I think there is a correct process to set it so it activates at the right amount of plate depression.
The fact that the car will run when you plug it back in suggests the switch is working OK. Maybe just not set quite right.
I am not quite totally convinced how your diagnosis will have solved the problem. If it has - fine, enjoy the beast. If the problem comes back - go back to this thread.
Good luck.
Alan
Edit - the Thermo time switch only operates the Cold Start Valve. It is a circuit(ie closed) to the CSV when below 35 deg C. It is providing +ve 12V from the starter solenoid. The body of the CSV is the earth point. After 35 Deg it opens and there is no circuit to the CSV when you crank the car. That is it's only function.

[wnmimms]

Okay, a coupe of things, you guys were right on the thermo-time switch, I misspoke in my excitement at the idea of finally driving this thing! The wire I grounded went to the charge pressure safety switch.


With the air plate switch plugged it it activates the relay in the engine compartment and it sounds like the fuel pumps kick on but I’m not hearing any “chirping” from the fuel injectors. Conversely, if I unplug the air plate switch, the relay is switched, the pumps come on and I hear the injectors “chirp”.

It seems to me that if the relay is tripped when the sensor plate is moved and sensor plate switch is connected the pumps should come on… not sure why I’m not hearing anything from the injectors.

Is it possible I’m getting more pressure out of the pumps when the sensor plate switch is unplugged?

When I was setting my cold/warm control pressures I was doing it with the plug disconnected at the sensor so the pumps would run continuously.

I can hook my gauges up with the switch connected and move the sensor plate down to see what I’m getting.

I feel like I’m so close to getting this running.

I can hook my gauges up later today and report back.

Sound like a good plan?

[Alan L]

When the key is on and engine not running, the pumps should not run. Until you disconnect that plug. Doing that is the same as depressing the airplate and breaking the circuit on the switch. Same process - break the circuit by tripping the micro switch or pull the plug.
With key on and you depress the plate, if the pumps are running, you must hear the injectors squeal - they are quite loud. The question would be - how far do you have to depress the plate for the rear relay to operate - by breaking the circuit.

"With the air plate switch plugged it it activates the relay in the engine compartment and it sounds like the fuel pumps kick on but I’m not hearing any “chirping” from the fuel injectors. Conversely, if I unplug the air plate switch, the relay is switched, the pumps come on and I hear the injectors “chirp”."

This is a bit confusing. You seem to be saying the pumps are running irrespective of whether that plug is in or not. In other words, continuously, but the engine won't start if the plug is in.
Just to confirm what is going on here - you could hook your CIS gauge in, turn key on, plug in on back of airplate. What pressure do you have? This would be your cold pressure. Now let the car sit for an hr or so - until the rest pressure drops off. Repeat with plug out and report the pressure.
If all is right - you should not be registering any pressure with the plug in. So do that test first.
Alan

[Alan L]

So here is another test you can do - just for my benefit - so I can understand what is happpening.
Pull the plug and start the car. You seem to be saying this is the only way it will start.
Now, with the engine idling, put the plug back in.
What happens?
Alan

[wnmimms]

Engine continues to run and will respond to throttle…comes up to 4k without a problem all seems normal.

[wnmimms]

Sorry, Alan, I missed your first reply. I’ll work through it once I get back.

[Alan L]

Originally Posted by wnmimms

Engine continues to run and will respond to throttle…comes up to 4k without a problem all seems normal.

So if the enigine ONLY starts with the airplate plug disconnected, but then continues to run with the plug back in place, that suggests one of 2 things;
The airplate is not opening enough on cranking - to trip the switch and allow pumps to run. Which would suggest it is not moving freely at that end of the cycle - ie sticking.
Or, the switch is slightly maladjusted, ie should trip earlier.
I don't have my manual right now - but I think there is some adjustment on that, and a process for setting it just right - trips at a certain gap with the airplate cracking open.
My best guess so far.
Alan

[wnmimms]

The first video is with the plate sensor connected.

https://youtube.com/shorts/dbDfaDz8NRQ?si=LfW_6itbDFbJlU_X

Disconnected:

https://youtube.com/shorts/i1FHi7wFoUI?si=JdtxZPBbUQD8hr12

When the plate sensor is plugged in and I depress the sensor plate both fuel pumps kick on. However, itt seems that it’s taking a few seconds to build pressure before the injectors start chirping.

When I unplug the plate sensor both pumps kick on and the injectors start chirping immediately.

That seems to be the difference. In the first video you can see I’m barely depressing the sensor plate before the pumps kick on but takes a few seconds before you hear the injectors. You would think that if I just continue to turn the engine over it would start (giving the pumps time to build enough pressure to open the injectors) but it won’t start.

If I then disconnect the sensor plate switch I immediately hear the pumps and the injectors and it starts immediately.

Bill

[wnmimms]

Got it. I have the full set of shop manuals and the Bosch K-Jetronic manual. I’ll look through both of those… unless your referring to something I don’t have.

[Alan L]

See if you can find something in the K jet fuel system about the setting for that switch.
I will check the videos.
Alan

[Alan L]

You don't seem to be holding any Rest pressure?
When you did your CIS tests, did you have residual pressure after shutting off?
I don't think that gauge should be dropping to zero once you flow fuel thru the system.
Have you previously noted a residual pressure - usually starts at 2bar and drops away after an hr or so to around 1+bar.
Alan

[Alan L]

Both those vids seem to be behaving like they should.
As soon as you pull the switch, the WUR pressure rises - to the cold pressure.
The one without the switch disconnected needs the airplate to move, and then the pressure climbs.
But in both cases there is no residual pressure. Your system is bleeding fuel pressure.
This makes for difficult Hot starts. Typical symptom of Hot start issues - no residual WUR pressure.
Cold starts would/should be difficult too - except the CSV fires a slug of fuel down the intake as you crank the engine - while the fuel pressure builds.
How are the Hot and Cold starts?
It may simply be a bad Fuel Accumulator - or a couple of other things allowing fuel pressure to bleed off as soon as the pumps shut down.
Which in turn would make it slower for the pumps to supply enough pressure on cranking.
Have you previously measured any Rest pressure?
Alan

[wnmimms]

I was messing with things a little more a few minutes ago.

Way back when I was troubleshooting... about a year ago, I checked my thermo-time switch and noticed it was always open and never closed to send power to the CSV, it has been disconnected ever sense.

I used some patch wires and put 12V to the CSV. With the Air flow sensor switch connected and 12V to the CSV it started right up.... no problems.

I can't let it run that way for more than a few seconds or it goes rich and dies. As you mentioned above, the CSV provides a "slug" of fuel, the engine fires, fuel pressure builds since the engine is running its pulling enough vacuum to move the sensor plate to the point that the injectors open and it continues to run...

Sound about right?

I read in the forum where someone had bypassed the thermo-time switch and wired the CSV directly to the starter so that it provided the slug of fuel while the engine was cranking and then stopped after it fired. Might be a solution vs... going through the gymnastics and expense of removing the faulty one and replacing...

I'll check the resting pressure in the am... and report back.

Thanks so much Alan!

[Alan L]

Connecting the CSV to the starter directly is not really a good plan - only 50% of the time. Your hot starts will also fire fuel down the intake.
I have the same issue with the TTS on my 930. For now I have disconnected it. It still starts 'OK' cold without it.
I think if you have no Rest pressure, and a faulty CSV, we are getting close to the issue.
Alan

[wnmimms]

Understood… I’ll let you know what I find out regarding my resting pressure.

[wnmimms]

Hi Alan, here are my residual pressures:

6.8bar starting
3.6bar @ 10min
3.4bar @ 20 min
3.2bar @ 30 min
3.0bar @ 45 min
2.8bar @ 50 min
1.8bar @ 3:40 min

Given these numbers I’m guessing the fuel accumulator is okay.