I don't believe/trust the 2 diagnosis on my '98 jimmy

[redron]

My '98 jimmy 4.3 vortec has problems taking gas right after start up, let me make this clear she starts right up 100% of the time with one turn of the key so it's NOT a starting problem. Once started and put in gear you press down on the gas to go and the car just spits and sputters sometimes stalling out and sometimes smoothing out . either way I just work thru the sputtering for about a few minutes and then the car runs GREAT nice and smooth. Also if I try to PUNCH the gas, say to beat a light while she's running good she also doesn't seem to take the gas smoothly , but again only when I really punch it ,under normal driving she run fine.
Now for you Mechanics that are still reading , qoute from the mechanic looking at car.
" fuel pressure is 52 lbs. should be atleast 60 lbs."
" Noted very high fuel trim values 160-185 normal is 128 +/- 10% "
"very low 02 sensor readings below 100mv normal varies 200-800 mv "

Both signal a very lean condition which COULD be caused by poor fuel pressure. BUT all signals return to normal as soon as MAF is disconnected which indicates possible MAF problem.
BUT BUT BUT he still wants to replace the fuel pump first which of course is way more expensive than a MAF.
I've been going crazy trying to figure out what to do HELP !!!!!

[tblake]

you unhook the maf and all the problems go away? could you have the fuel pressure checked with the maf unhooked? Maybe its just dirty. I think you can clean these out with some carb cleaner. Just spray the screen sort of things thouroly, and let it dry. When you go to put it back in, check the wiring, and maybe spray the plugs with some wd40 and make sure its seated securly. Dont know what else to tell ya. I myself wouldnt think the pump is a problem if it can pump up enough fuel pressure if the maf is disconected. Maybe just replace the MAF yourself. Or heck, maybe get a second opinion from a different mechanic.

[94 Jimmy]

Believe it.
Take a peek at:
http://www.asashop.org/autoinc/feb2000/techtotech.htm
Their talking primarally about the Central Port Injector problems that some Jimmy's had, but these are the injector folks and they know their stuff about fuel flow and delivery. And I quote:
"We say the system running pressure should be slightly lower than KOEO(Key on-Engine off) (normally between 59 to 61 psi), but never go below 57 psi. There is a 30 percent fuel delivery loss at 56 psi, which can cause hard starting and driveability problems."
If you get a 30% fuel flow loss from 60 to 56 PSI, how much do you lose at 52 PSI? It acts lean, measures lean, it's damn well lean. These fuel pumps are a problem and yours has been running for over 7 years. Unhooking other sensors just fouls up the computer program with unknown results except a check engine light and failed smog test.
The fuel pump from:
https://www.rockauto.com/
is about $300. If that's less than the mechanic wants to charge then get the part and take it to him, this can often save a bunch of $. Or get one from a junk yard and change it or the whole tank, it's doable.
If you have'nt changed the O2 sensors in the last 50K Mi, do this also. These things have a tendency to drift with age, and can cause lots of problems.
Good luck, let us know what happens.

[94 Jimmy]

Believe it.
Take a peek at:
http://www.asashop.org/autoinc/feb2000/techtotech.htm
Their talking primarally about the Central Port Injector problems that some Jimmy's had, but these are the injector folks and they know their stuff about fuel flow and delivery. And I quote:
"We say the system running pressure should be slightly lower than KOEO(Key on-Engine off) (normally between 59 to 61 psi), but never go below 57 psi. There is a 30 percent fuel delivery loss at 56 psi, which can cause hard starting and driveability problems."
If you get a 30% fuel flow loss from 60 to 56 PSI, how much do you lose at 52 PSI? It acts lean, measures lean, it's damn well lean. These fuel pumps are a problem and yours has been running for over 7 years. Unhooking other sensors just fouls up the computer program with unknown results except a check engine light and failed smog test.
The fuel pump from:
https://www.rockauto.com/
is about $300. If that's less than the mechanic wants to charge then get the part and take it to him, this can often save a bunch of $. Or get one from a junk yard and change it or the whole tank, it's doable.
If you have'nt changed the O2 sensors in the last 50K Mi, do this also. These things have a tendency to drift with age, and can cause lots of problems.
Good luck, let us know what happens.
94

[redron]

Ok guys I'm impressed with your knowledge. 94 Jimmy I think your on to something , that makes sense to me. With your input I've deciced to go ahead and put and orginal GM fuel assemmbly. I figure on keeping the truck till death do us part,so I'll spent the extra for a hopefully quality part. Again thank you guys for your time and input I'll be sure to tell you how things work out

[GMMerlin]

By looking at your fuel trim and O2 sensor readings your engine is running lean and the PCM is compinsating for the condition.
The fuel pressure is to low..needs to be between 60-65 psi..anything less and the engine may start, but will not accelerate.
Disconnecting the MAF tricks the PCM, but will not solve the problem.
Fuel pressure is critical on these engines!

Do not use a junkyard or used fuel pump...only new