After a several month hibernation, I tried to start the car a few days ago. It ran for a minute and then stopped. In the course of troubleshooting, I found the fuse to the fuel pump was corroded, so I cleaned it. Now the fuel pump would run, but still no start. I then removed the air duct from the fuel distributor to the intake manifold and tried lifting the air flowmeter plate. This should be quite hard because fuel pressure wants to keep the plate down. I could lift the plate easily and repeatedly and each time it got off it's rest position I could hear the fuel pump relay click and the pump started running. But, no build-up of fuel pressure. The only culprit I can think of is the fuel pressure regulator. Is there any other possible fault, or is there a foolproof way of isolating this to the pressure regulator? I just want to be as sure as possible I replace the right part.

Thanks,
Kåre

Mike!

Remember that when the engine is cold the control pressure is reduced by the warm-up regulator, and the plate will feel somewhat easier to move. Pull the injectors and then move the plate, if the pressure is in fact too low, it will not be enough to "break" the injectors to produce flow. That means the pump is weak.

Thanks for replies. I agree I should check that the pump is capable of delivering enough pressure (this reminds of my s-l-o-w wipers - bad motor). I get confused though with fuel pressure low/high etc. When the engine is cold, you want to enrich the mixture. So, for a given position of the air metering plate, you want more fuel into the engine, right? That means the fuel pressure must be higher, no? My understanding of the control pressure regulator (or warm-up regulator) is that it increases the restriction in the return line to the tank when the engine is cold, thus increasing fuel pressure. As the engine warms up, the restriction through the regulator decreases, a larger portion of the return fuel passes through the regulator and fuel pressure drops. I'll do some more troubleshooting on the pump first, though.

Thanks,
Kåre

Yes you need more fuel getting to the cylinders when the engine is cold. K-jet accomplishes this by reducing the pressure on top of the plunger which is moved by the air flow sensor plate. This allows a greater deflection of the plate for an equivalent airflow, hence a richer mixture. As the regulator increases pressure, the plate deflection is reduced, and the mixture strength reduced to normal hot values.
So have pressure gauges installed and check the pump for both pressure and flow. Accurate diagnosis is not a guessing game - it gets too expensive.

It's more likely to be not enough pressure from the fuel pump. The pump also has a relief valve built in to it, although it is unlikely to ever operate. There is a check valve in the pump too, if it doesn't hold pressure it makes it hard to start. Before you do anything, attach a pressure gauge to the cold start hose & see what the pressure actually is.

Cheers,
Paul.

I remember it being easy to lift (rather push down) the flap on my 760 with K-Jet. But that might have been with the engine off.

It did run for a minute, so I would guess that the fuse connection got worse during that minute while current was flowing. Or perhaps the fuse corrosion was not the cause of the problem, and only corroded on the outside.

Being in storage for a while, I would guess you've got rust flakes in the tank, and now the fuel pick-up is blocked. K-Jet is very reliable, and I would not think it will die like that due to a faulty component. After all, it did run fine for a minute.

Have fun...