Engine won't start, weird

[96-1500]

Hey folks,
I've been doing some searching and I can't seem to figure out what's going on with my truck. It's a 5.7L vortec in a 1996 3/4 ton. The engine will roll over forever, but will not start, sometimes the odd kick, but not close to starting. Pull off the air filter crap, dump a little gas in, and she'll start right up, but if you jump on the throttle she backfires and dies, the only way to get it to rev up is to slowly get on the gas. It sometimes quits when you put it in gear too. But in park it will idle all day. Then when you shut it off (or when it quits), doesn't matter how long it's been running, you can't get it to go again. Gotta do the whole dump gas in routine again. Quite embarrassing in city traffic lol.

Now of course my first instinct was fuel pump/filter. Replaced the filter, and she's got around 55 PSI fuel pressure. Good spark, replacing the spark plugs and wires anyway but I don't think that has anything to do with it. There is no check engine light on, and the only code my friend's scan tool brought up was a bad o2 sensor.

Anybody have any clue as to what's going on? I'm sorry if this has been posted before, but the search function on GMT-400 was not working for me at all for some reason. Google didn't help much either.

Thanks in advance.

 

[ChrisAU]

Replace that O2!

 

[96-1500]

I guess it's a rear o2 and the mechanic friend I took it to didn't think it should have anything to do with it, but I'll go talk to him tonight about it. I'm not very knowledgeable when it comes to this electronic crap. Thanks man.

 

[JonZ71]

rear o2's don't do to these trucks. Check your ignition module.

 

[96-1500]

Well I just swapped the module over from my old truck, no difference. Dunno if there would be a problem with the injectors or what. I'm at a loss.
Thanks for the suggestions.

 

[tscherbi]

delete

 

[TruckinAin4Sissies]

Does your friends scan tool read live data?
I ask because 55 PSI is ok, but is the truck pulling fuel from either bank?
It sounds like it's starving for fuel.



-Truckin-

 

[DAGEN]

Yea the rear o2 sensor's simply send a code when they read a catalatic converter error, the front's however read the fuel table's. I agree with what's stated above, it sound's like it's starving for fuel.

 

[Aloicious]

is it 55psi with key on engine off, or is that while cranking...with key on engine off what does the pressure peak at, and how long does it take the pressure to bleed off? also watch the pressure while cranking and see if it maintains at 55 psi, or if its fluctuating much...

the fuel system is VERY sensitive to pressure on the vortec engines, even a few PSI can be the difference between running and not...there are situations where 55psi would be considered a little low for these engines, so if you can elaborate a little, that would help out.

the O2 sensor shouldn't have anything to do with the starting. rear O2's dont trim the fuel, and even if they did, they wouldn't be activated during cranking...the O2's only trim while in closed loop...

 

[96-1500]

Ok, this has got me thinking a bit more. 55 was with key on, engine off. I think I need to check out the fuel pressure situation a bit more thoroughly, I didn't realize it was as touchy as it was. Tomorrow I'll borrow the gauge again and see what I can find out. Maybe it is the fuel pump after all.
The guy I borrowed the stuff from lives about 200 metres from my house, and is a licensed mechanic with a shop there. Pretty handy friend to have, especially considering I live about half an hour from anything other than a corner store.

And yes the scan tool reads live data. I don't know much about it, but it's quite the rig. Made by Snap On I think. I haven't the foggiest idea how to use it. I dunno if it's pulling fuel from either bank or not. I'll ask my friend.

Thanks a lot for the insight guys, I really appreciate you spending the time to help me out. I've got 5 vehicles sitting in the yard, and none of them are fit for the road right now. I'm out in the country so this really sucks -- we've all been there, lol.

Kier SG