hoytpr

Okay, another quick dumb question... Can I adjust the fuel pressure regulator to help?

Chris_1

iTrader: (1)

No. It's just a simple vacuum operated open / close valve.
The idea being that in high vacuum situations (idle, coasting) full pressure is too much and doesn't give the right spray pattern from the injectors.
At high vacuum the vacuum opens the regulator and reduces pressure at the rail by allowing some of the fuel to return to the fuel tank.
Here's an example of the pressures you're looking for - it's what my truck does and it's operating properly:
key to 'on' - 42 psi immediately. Start truck, idling - 38 psi. As I start to open the throttle a little, pressure climbs back up to 42 psi; let it go back to idle pressure drops to 38 psi.
I'm fairly sure that the culprit here is your regulator. I think you should grab a new one, it's the most likely cause of your symptoms.
That's the kind of thing you can tear your hair out trying to track down but it becomes fairly obvious with a pressure gauge.

hoytpr

Makes sense. I bought a fuel pressure gauge yesterday, I'll grab a regulator today. To be exact, when I first hook up the gauge it only reads about 10psi (but there is air in the gauge line), then I turn on the engine and it goes to 30psi (I quickly bleed the air from the gauge line at this time). If I push up the RPMs, the pressure very briefly jumps ~4psi then goes back to 30psi. Then I let off and go back to idle... no change in 30psi. Turn engine off and key back to "ON" and the pressure goes to 38 and slowly climbs to 40-41 over a minute or two.

ToddS

if regulator is bad, why would fuel pressure go higher with engine off key on, then when the truck is running? Wouldn't that indicate a supply side problem like fuel filter or pump?

hoytpr

You're right. Of course I already replaced the regulator and found out it made no difference.

So it's probably the fuel pump, unless the fuel filter was so dirty it needs to be replaced again.

Below are a few things I want to mention in case they mean something to somebody:
1. I can't fully depressurize the fuel lines by pulling the fuel pump fuse and running the truck until it stalls. In fact the truck was willing to keep running (stumbling, gasping) forever without the fuel pump fuse. It finally did stall. But the lines were not depressurized. Ask me how I found out.

2. When I have the fuel pressure gauge hooked up, and I increase the RPMS to about 2500, the fuel pressure needle is essentially vibrating back and forth almost too fast to see. Is that normal?

3. I can hear the fuel pump turn on when I turn the key to the "on" position. If I do this about five times in a row, the pumps sounds different... a weaker noise. Is that normal?

4. I pulled a couple spark plugs tonight, and they looked great. It did remind me that I put hotter plugs in the engine last time I changed them. Is that a bad idea? NOTE: I realize now that I used hotter plugs because I was having a slight uphill "miss" problem at least two years ago and hoped hotter plugs would help. So this has been going on a long time.

5. I can smell gasoline sometimes, outside the cab standing next to the truck, usually when the tanks are both full. I replaced the gas caps, but no joy. I can't tell which tank the smell is coming from, and although there are some small signs of gasoline running down the sides of both tanks, I've never seen any and I've looked many times. I assume this is a hole in the system somewhere. But this would likely be outside the pressurized system correct? and how do you find such a small leak?

Sorry for the long post, just want to cover everything before pulling tanks off and spending big bucks.

Pete

R.low.

I'm now a week out from finding anything on my end. May end up with no results. I do seem to recall testing the newly installed fuel pump back then and it was just a cunhair under spec for running pressure. I didn't have this problem then. Many others. A miserable job.

ToddS

Pete,

It's been many years since I've deal with 5.0 engines. I was really into fox body's and just recently got a 96 f150. I'm having odd bucking issues too.

I seem to remember using the a header valve to de pressurize the line, it did make a bit of a mess. 😒
Is your truck still drivable? If you run full throttle does it seem to have power? If so I find it hard to believe lack of fuel pressure is the problem at light throttle. It would fall on its face at full throttle.
Have you measured vacuum?
Is there anything in the trans that could cause it?
My truck has all new plugs wires cap rotor icm. Vacuum lines are helping and I think 1 and 3 were cross firing sometimes.

ToddS

The other thing I notice with mine and I'm not sure if it's related to yours or not.

When I left off the gas to slow down I feel vibration, not touching brakes yet. Seems like something in the drive line when the engine is slowing the truck. Also when it does the bucking the RPM will jump 100-200 rpm.

When I'm on the throttle hard it shifts great, no sign of slipping at all. I wouldn't think it's slipping while just cruising around. It also doesn't seem like a big enough change to be a gear change.

It's difficult to diagnosis these problems. I'm wondering if we have similar problems if maybe it isn't the fuel system at all but something in the transmission? Could the converter be sticking or something?

R.low.

I doubt it's the transmission. More likely the transmission mounts have loosened. I've often suspected it as well as the diff, never had any issues.
Front end bushings, I beam or x beam bushings (I can't recall the name).
Wheels and tires have especially given my truck shake. I can't get rid of it without buying one or the other.
If you have four-wheel-drive make sure that the front axles are disengaged when you're driving around town if you still have the automatic 4x4 wheel locks you need to drive in reverse 15 ft or so to disengage them.
There are a lot of things, but if you've gotten lean codes you might want to start there. I'm not sure what the pressure needs to be.

ToddS

I have manual hubs but now that you mention it I'll make sure they are unlocked.

I only had one lean code and that was when the big vacuum line for the gas system was unhooked. Fixed and it's never been back.

Being OBD2 if it was a mis fire wouldn't it throw a check engine light?

Okay hubs are not locked, I'll have to make sure axles are not turning.

The trans mount which bolts to cross member was loose but I just tightened it the other day.

rear suspension is all new and everything on drive line seems tight, nothing flopping around.