ALPOC Message Board

FelipeT · Posted: Sun Dec 06, 2009 10:35 pm Post subject: Fuel Pump Not Priming.

OK, I've got an interesting problem. I've recently figured out that my fuel pump is not priming on my '95 during the initial key-on, engine off conditions.

So far, I've determined that the computer controls the fuel pump relay during all conditions. What I've been trying to figure out what conditions those are that makes the computer kick on the fuel pump for the first two seconds that the key is on and the engine isn't running.

The inputs that the PCM uses in relation to fuel pump operation are the ignition switch, the crank sensor in the distributor, and the throttle positions sensor. Of all of those, I think that the TPS, or its adjustment might be the culprit.

Any ideas?
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FelipeT · Posted: Wed Dec 09, 2009 5:40 am Post subject:

Update: Quick test indicates that TPS is not out of adjustment, and there doesn't seem to be anything wrong with the way it's reading. Unfortunately, I didn't have time to test it thoroughly. Nobody happens to know what the resistance range for the TPS should be, do they?
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BarryG · Posted: Thu Dec 10, 2009 12:08 am Post subject:

From '94 shop manual:
1. Use Breakout Box Adapter T92DX-6000-AH to connect Rotunda Breakout Box 014-00322 to the powertrain control module.

2. Turn the ignition switch (11572) to the ON position.

NOTE: Measure the voltage between the Breakout Box pin 47 and ground.

3. Rotate the throttle linkage by hand and observe the voltage readings.

4. With the throttle valve fully closed, the voltage must measure 0.1 - 1.1 volts.

5. With the throttle valve fully open, the voltage must measure 3.1 - 4.4 volts.

6. If the voltage is not withing specifications . . . . <skip>
a.
b.
c.
d. Tighten the TP sensor set screws to 2 N-m (14-20 lb-in).

Does that help?

Barry
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FelipeT · Posted: Mon Dec 21, 2009 9:14 pm Post subject:

Thanks, but not really. I have a 1995 factory service manual. My FSM references another book (powertrain performance, maybe) which is what I think I need. I'm trying to figure out what signal it is that the computer uses to determine if it should prime the fuel pump or not. Either the computer isn't getting that key-on, engine-off signal, or the computer is faulty.

As far as I can tell, everything is fine with my TPS.

I may just break down and get a Walbro. At least if I had a fuel pump with a working check valve, it would be a lot less noticeable that it wasn't priming.
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BarryG · Posted: Mon Dec 21, 2009 11:38 pm Post subject:

Sorry I can't help. I just don't have the knowledge needed.
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FelipeT · Posted: Tue Dec 22, 2009 8:07 pm Post subject:

Thanks anyway. It's a tough one, I know. Does anyone else have a '95 on which they can confirm that the fuel pump does, indeed, prime for about two seconds with the key on and the engine off?

I'm about 99% certain that my car did for about the first 14 years of it's life. Now, it's gotten really hard to start, however, with both the check valve failure and the pump not priming. I'm afraid that if I keep going like this, the start will burn out.

Sure, I could get a fuel pump out of a parts car, but it just isn't worth the trouble with what a PITA it would be, and how (relatively) inexpensive the Walbro pumps are.
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--Felipe I Torrejon
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