1983 928S No voltage to FI

[Sirendancer]

Hello Everybody! It's been a long time since I posted anything to in this forum, over 8 years! I have a question about a problem I'm having with a recent 928 purchase. I can't get it to run from anything but a shot a starting fluid down the intake. I've replaced all of the following due to discovered problems: fuel pump, filter, all relays that have anything to do with ignition/fuel delivery, and several fuses. I ran a few tests here and there based on what I'm learning on these threads and have discovered there is no voltage at any of the injectors. This is done both with the key at the on position and cranking. I've gone through all the injectors with cleaner and really found nothing that you'd call a problem. I have good pressure at both fuel rails. I did notice that there is a moderate amount of fuel in my oil. I don't know if this is a recent deveolopment or it's been that way for a while. I'm guessing I did it with all the cranking and pedal pumping I've been doing. I'm also having a hard time understanding the power routes for the injectors or what other systems effect them. I'm starting to lean towards the temperature sender(s) but don't understand how these really come into play. I thought maybe the computer was bad and again don't know where to begin testing it short of swapping it with a known functioning one. Just so you know, I can fix anything on any car from a mechanical or electrical standpoint but I'm an idiot at diagnosing the problem. If somebody can explain why my FI's would be without power as though I'm 5 years old I should be able to physically resolve the problem.

[Danglerb]

Year and model of the 928?

Type of alarm semi disconnected lurking in the bowels of the wiring?

[JK McDonald]

Hey Sirendancer - Read over a similar suspected fuel injection problem on a previous post "When cold - starts fine and dies ". The difference sounds like you have no start in all temperatures. Always remember to stick to the basics first before chasing rabbits through the brush. There is lots of help on this site. If you do have a fueling issue -

Relay XVI supplies a parallel B+ path to the Fuel Computer and to all 8 fuel injectors. The fuel injectors are fired in pairs and looking for a pulsed millisecond ground from the Fuel Computer. Each basic pulse width is calculated using a set of pre-determined 8 cylinder Porsche parameters plus…Fuel Pump Running (yes/no)/Engine RPM(Tach Input)/Air Temp/Water Temp/Oxygen Sensor/Air Flow (83 Model) - Throttle Position Sensor and tweaked by the Throttle Closed Switch or the Throttle Wide Open Switch. As each of these inputs are plugged into an analog formula, out pops the correct fuel injector firing point. All of this hocus-pocus is pretty bullet proof – and you will normally find that the fuel computer is either operating or not. The exception is if someone has accidentally shorted one of the pulsed output signals headed for the injectors to a hard ground during troubleshooting. This can generate a couple of fuel starved dead cylinders from the output driver transistors burning open or even kill the fuel computer outright.

When troubleshooting a (yes/no) run related problem - always stick to the basics. 1) Fuel Pump/relay 2) Fuel Computer /relay 3) Ignition Unit / B+ power source.

Good Luck, Michael

[Landseer]

good reasoning above. On the 83 and 84 there seem to be a lot of things that can cause this condition.

I'd clean the ground points, esp. the two above the fuse panel, the cable linking block to frame under pass side, the fuel injection ground on the right cam cover and the battery ground strap.

Then I'd clean the red small battery connection.

Then, I'd jumper the fuel relay. 87 to 30 (careful, hot wires, you should immediately here fuel pump running)

Then, jumper the fuel injection relay with a y shaped wire. 87 to 87a to 30

Don't leave jumpers in when you walk away from the car, they are powering up stuff.

That's an empirical way to quickly knock-out the easy stuff. It might help. If not, then there are host of more in-depth things to work through.

Lots and lots of threads about this at rennlist . com. I've been rescuing 2 84's and one of them seems like lightning struck the electrical, what a mess, so if its been a no-start root cause, I've dealt with it.


Maybe spend lots of time reading there, I think, and posting here.

[Sirendancer]

Thanks everybody! In response to all this, I have in fact replaced and confirmed fuel pump operation both from the ignition switch and jumpering the two leads on the relay board. Pump runs fine and brings fuel all the way to the fuel rails. Oh yeah, this is a 1983S US. I have replaced the red power wire to the battery post and cleaned the brown ground wire points under the dash above the fuse panel. I also found the ground block on the pass. side front engine bay connected at the block but not attached to the body, I've since resolved that issue. Based on these threads I've looked for and found most of the ground points to make sure all had good connections. I still haven't found, or maybe it's just not there, the ground wires that are supposed to connect from either cam cover to the firewall? There are no wires back there at all and none connected to the engine in any way in that area. Based on your suggestions it sounds like I've found my problem? If there is supposed to be a ground back there for the EFI computer to work then I think that may be the problem, because there isn't one. I'll do some research today and let you know. I love a challenge but I can't thank you guys enough for sharing your intellect!

[Landseer]

Please say more about connecting the ground block that wasn't connected and how you connected it. A picture would be helpful. You don't mean the 14 pin hot wire connector or 12V jumpost, right?

The block is grounded to the body underneath. Specifically, there exists a braided cable that should bolt to the passenger side of the block and also bolt to the frame on the passenger side.

There is no direct cam-cover linkage to firewall ground point. What makes you think there should be?

The ground connections for the 83 + 84 are the same. The 85 and beyond cars have some differerent ground connections, so don't use the typical late diagram that seems to get offered up to everybody.

On the 16 V 83 USA, the fuel injection harness grounds to the cam cover, which contacts the block, which is then connected to the frame underneath as-noted.

Post pics, tell us what you discover.


BTW, sounds like you have some electrical knowledge. You need to have a copy of the wiring diagrams, though. Combined with your intution, the wiring diagrams will expedite your solution to the problem. And the next one. And the next one.