96 Fuel Pressue
 

Hello All,

I have read through the fuel system diagnosis thread. I have a couple of questions.

208k miles.
I just replaced the Fuel Pump (Delphi) and the fuel tank as it had a leak in it.

When I was testing the pump, and now the fuel pressure, I notice when I key on, the pump does not run, until you start cranking it. I checked the wires over pretty thoroughly when I had the tank out, and all seemed well for the power supply. I suppose I could have missed something, but is there a place I can start with this to see where the issue might be? I have not tried to power the port under the hood, I will do that the next time I go down to the shop. If it powers on from here should it run when the key is just turned on, or is that a different tissue all together? ***Edit I did connect the power port under the hood and the pump turned on.***

The issue is when cold, it will take a couple times of cranking to get it started. It runs but its rough, and sometimes goes to a high idle, like 2500 RPM. The high idle is not always an issue and normally idles around 1k or so. I did a check for vacuum leaks and replaced many of the lines now, but I was unable to find any leaks around the base

My very quick pressure test shows there to be leak down after you shut the engine off over 10 minutes. I would says its a little more than 10psi taking it around 48-50psi from around 60.

I have not done the pump pressure, as that will be my last check, I assumed it was fine since I just replaced it.

Mainly Im curious about the pump not turning on at all when I just turn the key on. Also any suggestions would be good, if you all have any. I realize im giving incomplete information with the fuel test. If the fuel pump test comes back ok, what am I looking at, a spider, fpr?

Thanks!

The fuel pump comes on for approx 2 seconds in the run position, not continuously. You get no priming sequence at all at key on? If the answer is no you do not, does it always energize when your cranking?

That leakdown test is a fail and you need to figure out if its the plenum or the tank/pump. That is a good quality pump brand and a wise choice but you cannot rule out a failure at the tank. You have to get to solid fuel pressure performance before moving on.

Your poor leakdown results can of course be the pump, plumbing, injectors, regulator, etc but it can also be the net voltage at the pump.


George

I don't hear it come on at all when I turn the key to run. It does pump when cranking, every time I have tried it. Its always started just take a couple tries.

I'd like to hear more of the net voltage at the pump. How would that affect the leak down? When I had the tank out, the voltage was at or slightly above 12v from what I recall when I stuck a voltmeter on it.

I'm going to test the fuel pump in the next day or so, I'm not sure I have the right connection for the fuel filter, I will need to dig around or make a run to the parts store.

**Edit** I forgot to mention, the previous owner had removed the cover under the steering wheel, which makes me think they were in there doing weird stuff. I took a quick look in there and it looked ok, but that doesn't mean its not something I can't see in there. Is the fuel pump wire in there? I can see the purple gray and black with white stripe in there, the same as on the fuel pump, if that means anything.

I would not depend on hearing the pump come on. Watch the fuel pressure. Also get a meter on the priming tab and see if you get 2 seconds of 12v ish at key on.

Low voltage might effect the initial priming pressure or the ability for the pump to start at all but not the leakdown per se unless the starting pressure was low.

Net voltage refers the voltage right at the pump across the power and ground wires, IOW the meter has to see ground through the pump ground wire to frame ground. The best way to measure net voltage is with the load connected and running but that a pain with the fuel pump in place

George

Ok here is full fuel pressure test results.

FP at filter - Initial 100+ (gauge goes to 100) After 10 Mins 95

Key On - None, no voltage at priming tab when key on. No fuel pump sound. My initial thought is ignition switch is bad. Is there a wire in the column I could energize to make sure it gets the pump going? I see a gray wire in there, which is what was at the pump. I in fact see 3 of the wires in there I saw at the pump. I just cant remember last wire color I saw at the pump atm.

Crank - at quick test port 60

Running at quick test port 55-56

Right after shut off it jumped to 58

After 10 mins at quick test port - 48 - Leak in plenum, probably spider injector?

After 15 mins at quick test port - 44

Also has P0122 Throttle Position Sensor circuit Low Input

Thanks for the help!

Your pump pressures are good at the filter so yes your leak down failure is in the plenum (injectors, regulator or tubes).

I was wrong about looking for power on the priming tab at key on. I see now that this tab is only connected to the pump when the relay is not energized so the only verification besides sound is pressure at the schrader at key on.

The fuel pump is controlled by the VCM via the fuel pump relay for the initial 2 second priming at key on. There are a number of wires out of the ignition switch that energize at key on but the two that go to the VCM are orange and pink. If you are convinced that there is no priming by sound alone then onward we go. The first thing that I would do is a simple relay swap. Perhaps the fuel rail has enough pressure to start the truck at which point the oil pressure switch powers the fuel pump. Then check ECM batt fuse 3. If that does not work then we need to pull the relay and see if the energizing coil contact is getting 2 seconds of 12v from the VCM at key on.

Let me know what you get and we can go from there


George

I will head out and check what you posted. There is definitely no pressure when key on, I had the pressure tester hooked up and there was no change at all sorry I forgot to add that earlier, also no sound.

You were 100% spot on with the relay. I swapped it with the horn and it pumped right up at key on, which also caused it to fire up almost immediately. The horn still worked with the fuel pump relay, so I'm not sure if its bad or what's going on there. Key on Pressure was right about 60

Ok the final issue is the leak down. Which I also suspect is causing the rough idle etc. I haven't done a tune up yet, but it is in the works. I want to figure out the leak down issue first.

I also have the P0122 code to deal with, which may cause the high idle 2500 ish sometimes. Ill also continue to investigate for more vacuum leaks as I poke around.

Next step rip the intake plenum off, or is there another step yet?

Sounds good. If you do need to pull the relay and test the relay energizing voltage from the VCM, the relay is controlled on pins 85 and 86. One is ground and the other gets 12v for 2 seconds at key on.


George

I don't feel I need to unless you think I still need to after the relay swap worked, which I posted above. Maybe we were posting at the same time :)