Bikeartist1

I'm trying to bring back the life in my 79 928 that's been sitting for 10 years I've installed new fuel pump 75lbs at fuel distributor. It appears to me that after reading up on the system I have too much control pressure hence the sensor plate isn't opening to allow air into the plenum. The car will start and immediate dies, if I open up sensor plate it will run for a couple minutes. What is causing too much control pressure and what steps can I take to correct it? Ty for any help or advice. Ps Roger at 928 r us has been extremely helpful in regards to helping me on this pia delema!

blown 87

So what is your control pressure?
Is the 75 LBS your supply pressure?

jpitman2

We need to know control pressure - in the line from distributor to WUR, with tap open. Basic test for where there is a blockage, with pressure gauge showing control pressure, fuel pump relay bridged, - open a union (carefully, with extinguisher handy!!) eg outlet of WUR - if fuel comes out, and control drops, the blockage is further along the line - move to next union in direction of fuel flow; if little fuel comes out and control does not drop, the blockage is behind the test point (eg in the WUR). Most money will be on the WUR, but it could be the return line junction at the distributor, in the return line to the tank, at the return connection into the tank (fuel gauge head). I use a 2 core flex of 6+ft, with male spade tags at one end, and a switch at the other, so you can turn the pump on/off easily in the engine bay, plugged into the pump relay sockets. Some have had succes by loading the tank with a solvent (berrymans), and bridging the pump relay to circulate fuel and dissolve as much crud as you can....
jp 83 Euro S AT 54k

Bikeartist1

Yes 75 lbs is supply pressure I haven't measured control pressure which I'm assuming is taken between wur and distributor? I just know it's too much, hence the amount of pressure on sensor plate,because when I relieve the pressure in distributor the sensor plate travels downwards easier. Since I'm back to work tomorrow I'll have pressure reading answer sat. Question by lengthening or decreasing spring in primary pressure inversely proportional to control pressure?

jpitman2

DONT play with springs in there. Changes to system pressure are done with shim washers . Look for a copy of Ben Watson book on Bosch injection - very good coverage of K-jet/CIS.
jp 83 Euro S AT 54k

Bikeartist1

Up thanks although I haven't gauge tested for blockage I have opened most of the fittings and have had some form of pressure I'll hook up a gauge on sat and see what I can come up with

Bikeartist1

I meant thru means of shims on springs does the primary pressure have relevance on high control pressure?

jpitman2

There will be some relationship between system and control, but its not really relevant. 75psi system is on the money, so you dont need to look at that setting. If you did change it, the main mixture control would need adjustment. Best bet is get a gauge on control (dist->wur line, closing the tap on the gauge set up will show you system), and find the problem with that reading, and fix it. Check out some data about WUR insides on ferrari400.com also. You might also have blockages in some injectors, as these can clog with sitting .
One step at a time - control pressure, localize blockage if there is one, fix it.
jp

Bikeartist1

Thanks jp I'll let you know what I find

Jadz928

Yes. Anywhere in that loop.

There should be fuel pressure on the barrel in the FD, which is pushing against the AM plate. This is common.

SMTCapeCod

Are you talking about the system pressure regulator assembly here, threaded into the bore in the FD?

9two8

Is this info of any use ?
http://www.diagnostic-assistance.co.uk/mech_inj.htm

Ken
80 928 S
5 Speed

UK

karl ruiter

You will want to buy or borrow a guage to measure the control the pressure at the input the the WUR. There is a very good chance that you are right and the control pressure is too high. One likely way this can happen is that the WUR is plugged. You can test for this without any special tools by disconnecting the return line on the WUR and turning on the pump. Fuel should trickle out of the WUR. If this does not happen your WUR is plugged. Of course there are other things that could cause the control pressure to be too high, and also that might not be your problem at all, but this is where I would start.

dr bob

To the OP--

I want to make sure that you are absolutely clear on the function of the pancake mechanism in the metering unit. It is used to MEASURE airflow, not to meter it. As airflow increases, the plate moves the support arm, whichin turn moves the piston in the metering unit. That movement, and based on fuel pressure and biased by control pressure, determins how much fuel is metered into each injector.

From your description, it sounds like you are expecting fuel pressure and control pressure to move the pancake somehow to meter airflow to the engine.


I apologize if I made a bad interpretation of your original statement.

Jadz928

That's right Bob. The only thing thing that makes the AM plate move is engine vacuum via opening the throttle plate, countered by FP (control & system pressure).
...Or your finger.