Hi,
I have been troubleshooting the D jetronic on my 914-4 1975, 2 liter engine.
Bought a fine car, but the previous owners messed the D jetronic system up.
Vacuum hoses were missing, throttle switch was new but not adjusted, fuel lines clogged up and temperature sensors not connected.
Here is what I have done so far:

-Overhauled fuel tank.
-Installed new fuel pump and filter.
-Checked fuel pressure at the regulator (2 bar)
-Changed all vacuum hoses (no leaks)
-Adjusted valves
-Checked trigger contacts (looked like new)
-Checked dwell angle (46 deg.) and breaker points
-Adjusted timing
-Adjusted throttle switch
-Compression test showed 121 Psi on all cylinders
-Installed new pressure sensor after the old one failed during vacuum test (membrane cracked)
-checked electric system with Bosch tester
-checked electric system manually

Problem:

Engine is running rich and the idle is high (around 1200 RPM).
By installing a vacuum gage between pressure Manifold and pressure sensor the vacuum changes (-0.65 ~ -0.5 Bar) rather than being constant. Adjusting the bypass screw at the trottle does not change the idle much. The CO value is around 4% and does not change much when adjusting the potentiometer at the control unit.

My question is what is causing the vacuum to rise and fall causing the RPM's to increase and decrease by 400 RPM.

Thanks

A hunting idle is caused a lean condition. You must first adjust the tps or the idle mixture screw on the ecu will have no effect. Disconnect the head temp sensor and check for ohms when cold (over 2K ohms) and then again when hot (under 300). Did you set timing with the hoses on the distributor disconnected and plugged? Set timing at 3200 rpm?

Yes, the vacuum hose was disconnected from the vacuum advance wenn timing was adjusted. Just measured 2.4K OHM cold and 700 OHM's hot.

What Curt said Also, you bought a new MAP or used MAP sensor? If used or rebuilt check it out. Do you know about this site....... http://members.rennlist.com/pbanders/ One other thing......Didn't see a condenser on your list. That's not the problem, though. Two of the most overlooked vac leaks I see are the AAR and Decel valve. Did you change the rubber hoses on intake runners and the injector seals. Be sure the injectors are seated by hand before tightening that 10mm nut down. Just because that gasket is new doesn't mean it sealed. Hang in there, you'll find it. Lean conditions are quite often the result of a vacuum leak.

-Changed the seals for all injectors
-Pressure sensor is new

On this car the deacceleration valve is missing.

I have a new cylinder head temperature sensor, but would have to make a tool to insert it. I am also thinking about the intake runer hoses causing a leak.

If the intake hoses are 40 years old, they probably need changing. Don't forget the Intake Manifold Gaskets too. Pelican Parts sells both. At least you will eliminate this potential vacuum leak source. Sounds like you are on the hunt. Keep it up, you will eventually find the leak.

While you have the air intakes off, sand and spray paint them. It will really dress up your engine. See picture.

All you need is a cheap long 13mm socket then cut a slot in it so you can keep from damaging the wire. All is takes is a grinder and some time.

You don't necessarily need the Deceleration valve to have the D-Jet run well but it does keep you from backfiring like crazy. They tend to get sticky after so many years so I just removed mine when I had D-Jet and it would just pop some while decelerating. No real big backfires unless you step on it a lot.

* No need to cut a notch! All you need to do is buy a deep socket ($5) and bunch the wire inside the socket. Done.

Have you cleaned and tested the Throttle Control Switch?

The throttle switch was brand new. Just wasn't adjusted and was only held in place with only one screw. I just ordered intake manifold hose and gasket. I hope it will finally solve the vacuum issues.

I was thinking of milling a notch into a deep socket.

High idle == vacuum leak or advanced timing.

Most likely a vacuum leak. Keep hunting, you'll find it.

--DD

Where did you get this new tps? Was it a new unit? Or was it a replacement board?. There have been some problems with the new boards, or at least the installation of them.

You have a NEW MPS? If so, nice find. If it's a reman then that is another thing. Let's see a pic of the MPS. Also, be sure it is a match to the ECU.

I bought it in Germany at a Volkswagen dealer for 50 Euro. I also found a Bosch tester at a flea market there. I was also told that Volkswagen Classic intends to support the classic car sceen with original Bosch parts. The problem with the remanufactured MPS is probably the adjustment for altitude and density compensation.

It is a fairly new TPS. The contacts are still shining. The preowner had it installed at a porsche dealer, but it was never adjusted right.

Pardon me if I missed it, but has it been adjusted now?

Yes, it has

Ok, have you tried unpluggingthe tps? It will actually run fairly good without it. If it does run better unplugged, that's your problem. Oh yeah, what color is the circuit board?