sportybob

Please help! I have a 1982 928 that seems to run fine from anywhere from 15 minutes to 3 days and then just quits. The engine will turn over and I have spark, but the car will not start unless it sits for at least 3 hours? I have already changed the Fuel Filter and Fuel pump and Yes, this is the L-Jetronic system. Is there a easy way to monitor the Fuel pressure? Since the problem is not consistant, I'll have to troubleshoot in the breakdown lane! By the time I get the car towed to a Service station, it starts up and I drive away without knowing why! Please, help!!

Randy V

Sounds like the fuel pump relay Bob.

Jim V

Or the fuse; I've got one that no matter how I
clean it or how often I replace it it just loses
contact on occasion. A quick spin of the fuse
and she fires right up.
Last time it did this I had just gotten strapped
down to the dyno with everyone watching.

Ern

I had the same problem. I left the wood cover off the electrical panel and made up a little jumper wire. When the car failed to start, I pulled the fuel pump relay and plugged in the jumper wire into pin 30(12V all the time) and pin 87 (to fuel pump. The car always started. That jumper makes the fuel pump run all the time. If this always solves the problem, then you know your problem is the relay or signals to the relay. You can easily pop the top off the relay and clean the contacts. There are three other pins on the relay: pin 31 (ground), pin 30 (12V when ing. turned on) and pin 31b (tack pulse from dist). This is a safety design that will only allow the fuel pump to run when the engine is turning. If you plug in that jumper and you don't hear the fuel pump run, then you have a problem with the pump or the pump getting 12V from the source. I first replaced the $50 relay and gues what, it wasn't the problem. I checked and cleaned all the fuses and the problem has gone away for now. Just to be sure that I can catch it if it happens again, I made up a little block of wood with five nails. I tacked little wires on each pin of the relay and attached each wire to a nail. Each nail is labeled and my VOM is lying ready for trouble shooting if it happens again.

sportybob

Excellent suggestions! After reading the others who also thought the fault is with the relay or fuse, I was going to try and monitor voltage right at the fuel pump. The pins at the relay socket make this much easier! Thanks a million!

Randy V

Ern, I believe you've invented the first 928 electrical gremlin trap. Most excellent!

Ern

Oh, I forgot to include, I also unplugged the connectors and replugged them in. I have not had the problem since, maybe two weeks now. When it was running good, I got out my oscilloscope and made a diagram of the waveform from the tach. in case that changes on me. When I first turn on my ignition, the power antenna goes up and I can't hear the fuel pump turning on for that first spurt before starting the car. I pulled the fuse on the antenna so I can hear everything. I have an aftermarket radio. It is my guess that if you have an orginal radio, the antenna does not go up until the radio is turned on. If you don't have a fuse chart, I can tell you where to find the info on the Internet.

WallyP

The fuel pump normally does not start when you turn the ignition on. It normally runs for about two seconds when you first hit the starter.

As an aside - one of the fairly common problems in cranking a 928 is the loss of fuel pressure as the car sits. The system should hold pressure for up to a few hours, but it is rare for a system to hold any significant pressure longer than that. The CIS system on the earlier cars simply won't inject any fuel if the pressure is not up to a minimum, and the later L- and LH-Injection cars will not work well at low pressures.

That means that the best way to start a 928 that has been sitting for any period of time is to hit the starter for several short bursts initially, rather than one long grind.

Brett Matthews

Funny thing Wally,
Everyone sez the fuel pump should run 2 -3 seconds then quit. Since we bought our car, when the ignition is turned on, the fuel pump runs. Never stops, until the key is switched off. Three wires additional come off the POS (+) terminal of the battery. They look to me like they are add-ons. I have already taken remnants of three add-on systems to the car out, and the pump hasn't changed it's actions.
I have heard there is a relay in the engine compartment, where is it at, what does it control? I found one in there, but it was jumpered, traced the wire harness, which was cut! But still connected to power.
The car runs great. Embarrassed a recent black Mustang today.....
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WallyP

Either your car has been incorrectly rewired, or the fuel pump relay (XX) is stuck on.

To have the pump operating as yours does is a safety hazard. If the car is left idling and the engine dies, the pump will continue to run. Any leak in the injectors or dampeners can result in an engine full of fuel, and possible serious damage. In case of a serious accident, the fuel pump can continue to run, possibly causing or feeding a fire.

The only relay involved is Relay XX in the central power panel. It is triggered by a line from the LH ECU. The power to the fuel pump comes from Bus 15 (the ignition bus), goes thru relay XX, then fuse #42, then straight to the pump(s).

Brett Matthews

Thanks Wally,
I'll check relay XX out, may arbitrarily change it out.
Any idea if those three wires on the POS (+) terminal are stock or not?

sportybob

Here we go again! I just had the car towed, it's 11:30pm and it just quit without warning. I tried placing the jumper to pins 30 and 87 of the fuel pump relay, but it still would not start. I do have voltage there, but I really couldn't tell if the pump was running or not? It's too late to do anything tonight, so I hope it won't start tomorrow! Maybe then I can fix the thing? I guess I need to verrify spark again and if it's Ok (which I really think it is) then I'll need to check for voltage at the Pump? If the pump has power, is there a simple way of checking the Fuel pressure?

JKelly

Hello,

For the last several weeks I have been reading a book entitled "Gasoline Fuel-Injection System L-Jetronic". It is a technical instruction book published by Bosch. Under the heading "Electric Fuel Pump" it says: "The electric fuel pump delivers more fuel than the maximum requirement of the engine so that the pressure in the fuel system can be maintained under all operating conditions. A check valve in the pump disconnects the fuel system from the fuel tank by preventing return flow of fuel to the fuel tank.
The electric fuel pump starts immediately when the ignition and starting switch is operated and remains switched on continously after the engine has started. A safety circuit prevents fuel from being delivered when the ignition is switched on, but when the engine is stationary (eg. after an accident)".

If I am reading this correctly, then the fuel pump begins to run when the ignition switch is turned on and it continues to run, to provide fuel line pressure, until the ignition switch is turned off. If you jumper the relay, then the ignition switch control would be by-passed and the pump would run with the ignition switch off. The "safety circuit" would prevent fuel from being pumped if the car were in an accident and the ignition switch was still on. The check valve would prevent flow of the fuel backwards to the tank if the pump were to stop working.
So, to check the operation of the fuel pump, you would use a jumper in place of the relay and the fuel pump should turn on. If there was a pressure leak, then I assume you would smell gas fumes, see gas leaking, or maybe even hear it.

Sportybob, I'll warn you now that I have absolutely no experience with anything that I'm saying. I'm just going by what I've been reading and hoping that it helps. What I have read is this: You need a balance of air, fuel, and spark at all moments to keep your car purring and roaring. If this were my car, I would check to see if my fuel pump was running, if I had spark, if my injectors were spraying, if my air intake and filter were clean and clear, and if all fuses/ relays were working. A fuel pressure gauge would be nice to have also (which I would like to get for myself). After that, I would start checking voltages and maybe start replacing things.
The most important measured variable is Air Flow, which is registered by the Air-Flow Sensor. The Air-Flow Sensor, along with other sensors, sends signals to the ECU (electronic control unit). The ECU then evaluates the signals and generates the appropriate control pulses for the injection valves. These pulses open and close the valves for a determined length of time. If your Air-Flow Sensor or one of your sensors is faulty, then you might be able to start the car, but then it doesn't stay running because of wrong input. Since I haven't read anything about wiring yet, I can't suggest places to check for possible grounding of wires etc... which also may make the car turn off. If you find out what's wrong with it, please post the answers here. I am curious.

Brett, it sounds like your fuel pump operates just like the book I've been reading says it should........?????? Either the book is wrong, Wally is wrong, it's a different injection system, or I'm wacko and reading words that aren't there.
HTH


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JKelly

Can't edit your own message in the forum anymore???

"arn't" should read "aren't"


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---duh,, I must have hit someone else's paper and pencil icon. thx Randy. All fixed!


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Incendier

Bret:

If you're talking about the wires from the under-hood + terminal, you're right - those three wires do look like add-ons, but they're not. They provide power to three points on the central electrical panel.

If they come from the battery - either it is an add-on, a replacement for the ones I'm thinking of, or I'm just clueless...all possible.