fuel issue

[RyanL]

low pressure comes from a restriction or a bad fuel pump. new doesn't always mean good.

if you haven't replaced the high press. pump on the frame rail you should do that. if the truck only has 1 pump and its in the tank then you should replace that.

before you do check for bent lines/hoses. if you can pinch off the return hose and get the pressure to rise you know the pump is weak.

is the fuel pump(s) getting good power and ground? test with the pump(s) running to put load on the wires

That sounds like a test to see if the pump is good, not bad. Blocking the return line will always make the fuel pressure go up (where else is the excess fuel gonna go?). It sounds to me like a bad ECU causing most of his problems (not fuel pressure of course, but throwing random codes is a sure sign). I had an ECU in a different vehicle which acted up only when it was hot out and the vehicle was warm. Failing electronics often only show problems when things get heated up and he can almost rule out bad wiring/grounds (I would still check it all out, but it seems fairly obvious that the ECU is toast). There was a recent thread here where the guy said his ECU relay was going bad and only caused issues when it was on for a little while. I'm sure that this would be very rare for it to happen (relays usually just stop working rather than play games), but it's something cheap and easy to check into.

 

[BRUTUS_T_HOG]

blocking the return line is to see the maximum pressure the pump is able to produce. if not up to spec its clearly bad.

but also if the pump is getting low voltage from a failing relay or bad wiring the pump would be weak.

if a good pump has clean power and ground it will produce plenty of pressure.

like i said.. i've never seen a significant drop in fuel pressure even with the ECM going funky.. there are no sensors to control fuel pump pressure besides the regulator.

 

[Dave90ranger]

one reason im saying its the computer is cause it does it all the time at any temp. also cause it will do it at full throttle and if i let off a little to about half ill start accelerating over 3k and fuel pressure will go up. and the codes thing. now one thing that was strange, earlier today i had charge my ac the rest of the way, since my manifold gauges blew an o-ring and lost half of my freon, and after i had it fully charged or well close enough to full, i went to harbor freight and it drove perfectly fine and i could go up to 4k easy (thats as far as i went cause i dont want to go any higher) and then after halfway there it just started acting up again and not revving up. now at first i thought well maybe having my ac vacuum lines off and low charge could cause it to mess up but apparently not.

 

[BRUTUS_T_HOG]

you've already identified the cause though, low fuel pressure under load... where does the computer come into this?

 

[RyanL]

He already said that it was spitting out multiple random codes in a previous post though. It sounds like he does have some sort of fuel delivery problem, but with these vehicles it seems like an accumulation of problems cause them to run funky.

 

[CraigK]

Just a thought . . .

You indicate you replaced the fuel pump. Which one - in-tank or high pressure? You also state "and a new gas neck since it was broke apart and leaking", which suggests a very high level of rust in your tank/system. Is is possible your in-tank pump has a clogged intake and, especially at high gas flow requirements (high throttle) not enough gas is getting through? Does the truck run any better with a full tank of gas? (might be a hint)
Good Luck .

CraigK

 

[Dave90ranger]

He already said that it was spitting out multiple random codes in a previous post though. It sounds like he does have some sort of fuel delivery problem, but with these vehicles it seems like an accumulation of problems cause them to run funky.

so one reason why ive been saying its not really a fuel delivery problem/clogged line is cause it will run perfect suddenly and then act up a little later. now it will do it at full throttle and less than full and if i go below about half throttle it will accelerate so im positive its my computer. also have no codes at the moment.

Just a thought . . .

You indicate you replaced the fuel pump. Which one - in-tank or high pressure? You also state "and a new gas neck since it was broke apart and leaking", which suggests a very high level of rust in your tank/system. Is is possible your in-tank pump has a clogged intake and, especially at high gas flow requirements (high throttle) not enough gas is getting through? Does the truck run any better with a full tank of gas? (might be a hint)
Good Luck .

CraigK

look in earlier posts about the fuel pump. i only have an in tank pump. and when i did the gas neck i took the pump assembly out and looked at it and the intake filter for the pump was clean and not clogged. and having a plastic tank wont rust so thats out of the question. and your other question on having a full tank of gas it doesn't change it at all acts all the same.

 

[Dave90ranger]

you've already identified the cause though, low fuel pressure under load... where does the computer come into this?

it also does it in neutral, if i rev it a little and get the pedal at full throttle so its under load and no load

 

[BRUTUS_T_HOG]

low fuel pressure is low fuel pressure. engine load is still high and fuel demand is high when you rev it up in neutral

 

[Dave90ranger]

low fuel pressure is low fuel pressure. engine load is still high and fuel demand is high when you rev it up in neutral

if your going to constantly say my fuel pump is bad and not creating enough pressure, or saying my pressure regulator is bad, then just quit trying to diagnose this for me cause its NOT the issue.

 

[BRUTUS_T_HOG]

if your going to constantly say my fuel pump is bad and not creating enough pressure, or saying my pressure regulator is bad, then just quit trying to diagnose this for me cause its NOT the issue.

then whats causing your low fuel pressure and what would that low fuel pressure do to your engine? since you know everything.

10-15psi... im sure thats plenty.

 

[RyanL]

Easy now guys! 20+ year old trucks are hard enough to diagnose in general let alone over the internet. Time can be very cruel to vehicles - hoses and wires crack, connections corrode, things rust, etc, etc. There are so many possible scenarios of what could be up it's not even funny. Maybe the fault is yours, maybe it isn't. I know that I must've inadvertently screwed things up trying to fix other things more times than I can count. To me, it's starting to sound more like an issue which some older GM vehicles have with those evap cannister lines clogging causing something like a vacuum in the tank. Just humor me and see if it does the same thing with the gas cap off. Maybe the issue is much worse with a plastic gas tank than with the factory style ones (maybe the plastic ones have a better air-tight seal or something). Look over your own work one more time. Did you install that gas tank? Maybe it didn't fit perfect and it's pinching a line somewhere. Maybe the lines are crossed (send with return) at the tank. It might actually work somewhat like that since I think that the return goes all the way to the bottom of the tank, but probably not too well since the fuel pressure regulator would then be backwards and useless. I know that it's all fairly comprehensive from the factory on how to hook it all back up, but maybe the lines have been replaced over the years and they're both the same size. Hell, maybe you put the fuel filter in backwards. I don't know how much it actually matters but I have noticed that they usually have an arrow on them indicating fuel flow direction. You should check into possible problems which I mentioned above, but I'm still kind of leaning toward an electrical issue though. A lot of problems can be caused by bad connections, even bad battery terminals. Start at your battery and look over everything. Then check out the connections on your starter solenoid. You should check out that short wire that comes off of the solenoid and goes into that power block right there (I think it's about a 4gauge wire or something) because everything important is fed from out of there. If everything checks out over there, follow your power wires back to the pump and try to check out the running voltage to see if there is a drop (you probably have to back probe the connection which might be a PITA). Anyway, it sounds like you're going to have your hands full. Let us know what you find. It would be something if this was all caused by a bad relay or something.

 

[Dave90ranger]

update and resolved

well been a while since i have updated this page and heres what i found.

now i replaced the computer and that improved the issue some then i went ahead and did the pump and that completely resolved the issue now instead of getting 15mpg in the city i am getting 18 almost 19 mpg

just curious what everyone else is getting mpg wise out of there 2.9?

 

[BRUTUS_T_HOG]

told you.

20 mpg sounds pretty good for the 2.9l

my 2.3l gets 25-30mpg

 

[NewFordOwner]

Seems that your problem has been resolved but I feel the need to hijack your thread now, mostly because I have the exact same problem. However I have the high pressure fuel pump along the rail and the metal tank.

I have replaced the high pressure fuel pump, I've replaced the filter, I've ran my lines dry until I determined there were no clogs anywhere in my fuel delivery. I've chased this problem all the way around my truck and what I've come to the conclusion is: loss of voltage to the high pressure fuel pump.

Not sure exactly where it stems from though, my neighbor and I have been working on this truck since I bought about two months ago and when it first stopped running I replaced the filter(did the lines as stated above) and it began working just fine. Then just the other day it refused to start and I've got over half a tank in it, so I began searching other possibilities(after checking to see if there was a clog, and there is not)

Next I replaced the high pressure fuel pump and still no go, so I began checking the electrical system and found that I had MAYBE 90mV running to the pump. I mean gas is flowing out of the tank constantly because of the low pressure pump in the tank and fuel runs right up to the new high pressure pump but it just doesn't... Well.. Work.

So the neighbor and I bypassed the electrical system entirely and ran some wires from the pump straight to the battery, and sure enough the pump began spitting at me and getting voltage(instead of the mV) and since you cant do that reliably, I'm stuck with where to go from here. Any ideas?