CIS Fuel pump relay

jrenfrew · 12-08-2020, 11:35 PM

Hey guys. Looking for some of you other 928 CIS guys to help me with some advice.

My fuel pump relay only runs when the engine is turning over. Some of the other non-928 CIS cars I've owned the pump relay will "prime" for 3sec when the ignition is turned on.
My relay doesn't prime.

Does anyone know how these relays "should" operate for a 78 928?

Thanks!

Danglerb · 12-09-2020, 09:24 AM

The C in CIS is continuous, as in anytime there is adequate pressure differential at the injector fuel will flow, that could get excessive fast if the engine isn't running.

No idea myself what is correct, maybe check the owners manual.

jrenfrew · 12-11-2020, 12:24 PM

Thank you Danglerb. Unfortunately you missed the question.

I'm referring to the relay, not the injectors. I have the owners and a complete set of OEM shop manuals. None of them describe the operation of the fuel pump relay.

Should the relay "prime" the system with activation of ignition or only run the pump when the engine is running? All of the Audi's and VW's with CIS would prime the system when ignition was turned on prior to starting. My 928 does not.

stepson · 12-11-2020, 01:39 PM

What relay part number do you have in the car now?

JK McDonald · JK McDonald's Garage

Hey Jrenfrew – The operation of the CIS fuel injection system relies on Fuel Pump Relay that is energized only when the air sensor plate is off it’s rest position. When you 1st try to start the motor, the air sensing plate is moved as the pistons begin to draw in air. Once running, the air sensing plate is positioned at the idle position. Naturally the fuel pressure is now being regulated from a cold to warm setting by the warm up regulator.

Effortlessly starting a car equipped with the CIS fuel injection system is based on a check valve on the output line of the fuel pump and the vertical positioned control plunger at rest in the center of the fuel distributor housing. These two valves holds a static (residual) amount of fuel under pressure between the fuel pump and the fuel distributor when the key is turned off and power is removed to the fuel pump. When the car is parked, if this static fuel pressure has bled off, there is not sufficient pressure initially available to the injectors (including the cold start injector) and the engine will take an inordenent amount of cranking before it starts. This starting delay is caused by the fuel pump gradually having to rebuild the missing residual pressure needed to open the injectors. The two pin fuel pump relay connector (ground switch) is located on the upper outer rim of the fuel distributor bell housing. The rubber air boot should be removed to see it properly.

Just a comment - What I’ve done to all my (CIS and L-Jet) cars is wired in a ON/OFF toggle switch that allows me to apply a parallel ground to the Fuel Pump Relay and pre-pressurize my fuel injection lines. This is especially helpful on a car that has been parked for a long period of time or is down for major maintenance. Once the car is running, flip the toggle switch to OFF (out of the circuit – for safety) which now allows the factory fuel pump ground switch to control the fuel pump.

Good Luck - Michael

Reanimotion · 12-13-2020, 12:27 AM

The Original 928 Porsche relay does not prime the pump, it simply runs when there is a valid ignition pulse from the tachometer feed, there is no air plate switch on the 928 (there was one on the 911)

The BMW relay I have does prime quickly on turning the ignition on - then runs when tach valid

JK McDonald · JK McDonald's Garage

Sorry If I lead anyone astray. I was thinking that the early CIS 928 originally had the air plate switch that provided the ground for the fuel injection pump relay like the 911. A few years ago I had helped a friend work on his 1978 928 that I could have sworn didn't use the tach pulse signal to an electronic fuel pump relay. Slept a few times since then....

Jrenfrew -If you have the later electronic relay, it should also provide power to the fuel pump for a few seconds when the key is initially turned ON - to prime your injection system.

Good Luck - Michael

jrenfrew · 12-14-2020, 10:46 PM

Thanks guys for the great feedback...
Fuel pump relay = SK6468 (Original 1978 relay I believe)
@JKM, looks like you got your feedback, but I confirmed with the 78 electrical schematic that the relay is controlled by the ignition output pulse generated by the main ignition unit which feeds the TAC and the AC relay if equipped. No, meter switch on 928 CIS.

I've owned the car 28 years now and familiar with the long term park start. I use to have an AUX toggle to allow me to prime the system, but ditched it for the ignition tap. I'll just tap the START position a few times until I hear the pumps load up.

Lately I've been having a hot start issue. Sitting hot for 15-45min and the car doesn't like to restart. My primary suspect is injector(s) leaking, causing engine flooding as they've never been serviced / inspected. Waiting on some parts to arrive so I can do a leak/opening point and spray test in each injector.

@Reanimotion: Which BM'r relay are you using? Having a "prime" would make the car a little less grumpy on longer cold starts I suspect.

stepson · 12-15-2020, 06:47 AM

James,

The correct fuel pump relay number for your car is 928 615 113 01.
You may have a 40+ year old relay in your car. Maybe time to buy a new one to prevent being stranded on the side of a road somewhere.

jrenfrew · 12-15-2020, 08:16 PM

Thanks Stepson. A dead relay wouldn't stop me anyways.

Opelotus · Opelotus's Garage

From what I've seen in the early CIS cars, they do not prime when the key is switched on, and only fire the pump when cranking. Occasionally when the ignition is switched on, we do hear the pump prime, but I suspect it's really due to an intermittent power glitch triggering the relay momentarily, as this occurs rarely.

For what it's worth, if you read the owner's manual for the early cars, it states that when starting the engine hot, the accelerator pedal should be fully depressed. Note that the later L-Jet cars state this isn't necessary. My friend's '79 RoW occasionally has trouble hot starting. I've told him to try holding the accelerator to the floor while cranking, and he has found it does start better when hot. Obviously, you need to be ready to quickly let off the accelerator once the engine catches!

foggy · foggy's Garage

My '79, 5 speed does run the fuel pump when the key is turned on. I don't think it's for 3 seconds but it is more than 1. During that time I don't hear the injectors so it is strictly building fuel pressure. If it has been a long time since last operational, I cycle the switch on-off several times.

On the '79, the fuel pressure regulator on the fuel distributor has a "push valve" on the return from the warm-up regulator to help maintain fuel pressure after shut-down and help prevent vapor lock. This was added some time around 1978 so your car may not have it. That would make the "prime" function of the relay a nice feature. The time delay is performed by a small electronic circuit in the relay itself, I think you would see it if you took the metal cover off.

The relay in my car is not original, I think it may have come from an Audi dealer, but the original did operate the same way.

If this hot-start problem is recent you may also want to see if your residual pressure drops too fast after shut-down and identify the cause. Even without the "push valve" I would expect some residual pressure.

Good luck