fast951

I'm new to the 928S4 world. However, I'm learning quickly.. I own a MY87 928S4 Euro car that I keep overseas. Problem is, no one overt here knows 928S4, parts are hard to get. So I'm trying to diagnose the problems from the US so I can send the parts that might be needed.

It appears that for some reason, when the ignition is turned on, the injectors go wide open and fill the cylinders with fuel. The car wouldn't start.

My friend, mechanically inclined, but not familiar with the 928S4 tried few things, that I will describe next.

- According to him there are 2 fuel pumps by the fuel tank area. However according to PET I only see one fuel pump. Assuming there are 2 pumps (please correct me here), when one of them is disconnected, the car starts & idles. When power to 2nd pump is connected the injectors flood the cylinders.
This leads me to believe that when 2 pumps are operating, the system build higher pressure than 1 pump.

I'm thinking of 2 possibilities, and I appreciate your input on it.

a) Fuel regulator is bad, causing excessive fuel pressure to force injectors to stick open. According to PET there is only 1 fuel regulator and 2 dampers

b) The signal to the injectors from LH is ON all of the time. With less fuel pressure the injectors flow is lower and car idles (but badly)

Looking at the LH connector I see that pin#18 is the signal to injectors.
Few questions here:

1 - Does LH ground Pin #18 to switch injectors? (if so, a short to ground in the wire, would cause the injectors to open all of the time).

2 - Is there another LH pin# to control injectors?



Any input on this will be appreciated. It's impossible to diagnose something from far away, at least I'll have an idea what to look for.

Thanks.
John
fast951@comcast.net

fast951

Quick Update. I misunderstood my friend's description. There is only one fuel pump, however the car runs with the power to the pump disconnected..

This is very weird!!! Nothing makes sense... Any one with a possible theory?

ErnestSw

Does the car idle at all? When the problem occurs does the car stall, or run badly? Exactly what is he disconnecting?

fast951

When he disconnects power to the fuel pump, the car starts and idles. However Idle is lumpy and runs rich...
When fuel pump is connected, car will not start at all.

This sounds very weird, I'm thinking that the car idles long enough to get the fuel out of the lines... I'm totally puzzled...

Kevin in Atlanta

The fuel goes from the tank to the injector rails and then back to the tank. The pressure regulator and dampners pressurize the fuel system. The computer opens and closes the injectors. If your injectors are staying open and filling the cylinders as you say, then the injection brain is faulty. Find another brain and test it.

Steve Cattaneo

When the ignition is on, engine not running does the fuel pump (one) run without turning off IF it does you could have a bad fuel pump relay, (XX) stuck on. To test it substitute it with one from the board that is the same.

ErnestSw

Have your friend turn on the ignition without starting the car.
Does he hear the fuel pump at the back of the car?
Does he hear the injectors clicking?
Exactly which connector is he disconnecting?

WallyP

Stop. Take a deep breath and back away from the car.

There is another possibility that makes more sense. Your car has a pressure regulator and two pressure dampeners installed on the pressurized fuel rail. Each of these units has a flexible diaphragm exposed to fuel pressure on one side, and manifold vacuum on the other side.

What happens if one of those three diaphragms splits?

Yep - fuel pumped directly into the intake. Instant flooding.

If the pump is disconnected, manifold vacuum pulls fuel into the intake and the car runs (poorly).

Pull the vacuum line off of each of the three units and check for traces of fuel. Replace the one that leaks.

fast951

I appreciate the help, I'm learning the 928S4 but the hard way!

Wally, I think you are on to something! My friend told me that the intake manifold was full of fuel (lots of fuel). I'll have him check all of the components..

Initially He jump started the car to get it running. So I'm wondoring if he fried the computers...

John Speake

Steve,
With a fried ECU and flooding, you will normally hear the injectors clicking whenever the ignition is on.

Lets hope it's a pressure regulator or damper !

fast951

Had my friend do some more testing...

- He disconnected vacuum lines from Fuel pressure regulator and both dampers.. There is no sign of fuel. Even when ignition is turned on.
Question: When ignition on, the fuel pumps should be running. So pressure will build at the fuel rails??

- When ignition on, injectors are not ticking

- When power to the main pump (by fuel filter) is disconnected, car idles poorly.

- When main pump is turned on, engine stalls due to flooding..


Few items that might be causing this:
- VERY high fuel pressure, due to a faulty FPR or a blockage in fuel the return line. (Fuel pressure will be checked).
- Injectors are stuck open (never seen it before).


Any suggestions?

John Speake

Few items that might be causing this:
- VERY high fuel pressure, due to a faulty FPR or a blockage in fuel the return line. (Fuel pressure will be checked).
- Injectors are stuck open (never seen it before).
>>>>>>>>>>>. Try disconnecting the fuel return pipe to the top of the fuel tank (under a round cover under the RHS trunk floor), you should see fuel returning freely there.............
John

Ted928

How are you controlling whether the fuel pump runs or not? Are you pulling the relay? With the relay pulled, will the car still run for more than a few seconds? Do you have to jump the fuel pump relay to make the pump run? You need a fuel pressure gage to determine whether if it is a mechanical problem (too high of pressure). In the meantime, check for return fuel flow as JDS suggested.

fast951

In the process of checking fuel pressure. Thank you for the input, will report back soon..

Thanks,
John

fast951

I feel like a daily news reporter LOL

Fuel pressure is normal... My friend pulled the injectors and is having them tested/cleaned.. That's what he found:
- One injector was stuck closed
- One injector is normal
- Six injectors are stuck open

I guess that's what happens when the car sits for 9 months. Bad quality fuel did not help...

Hopefully this will fix it.. Will find out tomorrow. I'm hoping the LH is fine...