Engine/Transmission (1998.5 - 2002) fuel pressure on startup?

[Nemies]

Okay, so I finished my new fuel pressure gauge install, and everything is fine on the road. All the PSIs are good, etc, etc.



There is a really weird behavior, however, right after I start up the engine. I go from "off" to "on", and the gauge jumps to 15, and then slowly falls back to 0.



I was assuming that the VP44 is returning fuel, but that the lift pump isn't on yet. Is that correct?



Then, after I start, it hovers around 5psi, both pre and post filter. I was really freaked out, so I checked the senders, but nothing is leaking. Could this be a grounding issue? I know the system cycles the fuel heater, etc, and that draw a lot of current (I can see the changes it in my headlights).



The other thought I had was that I had air in my sender's hoses, even though I did my best to make sure the air was out.



Once I'm on the road, it comes up to the right PSI levels and stays that way the rest of the time.



Am I seeing a freaky lift pump, air in the lines, or electrical trickery? (I can check the ground... )

 

[dresslered]

What gauges are you using? Initially, during startup, the lift pump is modulated with a 25-50% duty cycle. This explains the first part of your observation. As far as air in the line, this isn't a problem either (I purposely left mine full of air for additional dampening). Do your senders have their own ground wires or are they relying on the grounding between the threads? Good luck.

 

[jweir]

Nemies,



My fuel pressure does the same thing on start-up. . when I turn the key to on, the lift pump will run for a few seconds then stops and the fuel pressure drops. Right after start up, I will generally see lower fuel pressures but not as low as yours. I usually see 9-11 lbs then when it warms up I will see 12-13 at idle. I can drop my pressure below 4 if I tun the Comp up and floor it guess I had better change the lift pump.



Hope this helps.



Jim

 

[Nemies]

I'm using Westach 0-15, with the transducer senders. They're mounted under the hood, grounded there. The gauge is grounded in the cab.



The 5psi makes sense if the pump is run at 20-25%. When does that short duty-cycle stop? Is there a time limit?



As for the 15 to 0 in the "on" position, I can only guess it's the VP44 sucking all the fuel away. Which seems weird, if that's when the "WAIT TO START" is on, heating the (lack of) fuel in the filter.

 

[Steve St.Laurent]

What you are seeing on the initial key turn on is normal. The lift pump operates for 2-3 seconds and pressurizes the system and then turns off and it will bleed off to zero within a couple more seconds. There is no check valve in the system to stop the pressure from bleeding down. The low duty cycle is only when the starter is cranking. Once the engine starts it should come right up to your normal operating pressures. The VP44 isn't drawing any fuel until your are cranking the starter or the engine is running. When do your pressures come up to normal numbers and what are they? BTW, what are the outside temperatures when your heater grids are cycling? Anything above 65 degrees and they shouldn't turn on at all.

 

[Gary - K7GLD]

Looks like you are grounding the meter and sensor separately, and then counting upon the resulting "common ground" to complete the metering circuit? It's better to run a direct ground from the meter to the sensor, and then to chasis ground - less chance of erratic varying ground returns that way - especially critical in metering circuits...

 

[Nemies]

My comment about heater cycling is just a memory, and grabbing at a possible answer. I don't think it is cycling (how can I make sure in daylight?)



My normal pressures come up either after I've jumped out of the cab to make sure the senders aren't leaking, or after that, and I've backed out of my driveway. Either way, I wasn't looking at it. I think the gauge is shy. Running, I get about 14 idle, 12-13 regular driving.



And, yes, I've used separate grounds, figuring they'd both be ground. I'll rig a quick ground wire just to eliminate that as a possibility.



Thanks everyone!

 

[Amianthus]

From what you are describing, the first short run is to prime the VP-44 for starting. Having the lift pump running while trying to start the engine can cause a hard start condition. It just runns a little while to help avoid this.



As for your running pressures, they seem just right. Although you didn't post your WOT pressure. It should be above 8psi. If it isn't, you need a new lift pump. Based on the behavior that you are describing though, I am leaning towards the idea that your lift pump may be on the outs. It's too bad you moved so far away from us knotheads, we coulda looked at it and surely found the problem.

 

[Bob Cochran]

Nemies ~ In daylight watch the voltage gauge and you can see it dip if the heater is cycling

 

[Nemies]

Mostly, my WOT pressure holds at 8. I can get it down to 6. 5-7 at near red-line, though.



The pressure at the start, though... I'll check my ground and post back.

 

[Nemies]

Okay: an update.



My heater isn't cycling, and I checked the grounds, they're the same. (0. 001 volt between my two ground points. )



So, if the Lift Pump is running (since the truck has started), and air in the line wouldn't change the reading, then what I'm seeing is a dying pump?



This appears also to only happen in the morning on my first start. If I go somewhere, shut down, and then restart, it immediately jumps up to 15psi. The morning starts show the pressure wavering around 5psi, sometimes spiking up to 10, and then after between 5 and 30 seconds, it finally rises all the way up to 15.



I'm worried about my LP, but I'm thinking I won't be able to convince the stealer that I have a problem.

 

[Amianthus]

Sounds like a dying pump to me. That's exactly what mine did just before it died. It would spike, then drop off and hang around 2-3 psi, then jump back up. Every so often it would do the same thing, warm or not. Highway or not. But it always did it in the morning. Take an mpeg of the gauges behavior. That is what sealed it for my dealer to replace it (actually they replaced it with no fuss whatsoever). Good luck.

 

[Nemies]

I can record an mpeg, but how exactly did you show it to your dealer?



I'm gonna have to find some BOMB-friendly dealers out here. My previous one didn't mind (or notice) the boost and pyro. The fuel pressure is hard to miss. And I'd rather not take the EZ out.



I'll ask around...