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Hesitation, stumble, backfire

6928 Views 5 Replies 4 Participants Last post by  xPosTech

I have a 97 5.7 Vortec 3500 Express
I have been working on this truck I just bought for a couple weeks

It had been sitting for two years, It was billowing pure white smoke, smelt like raw fuel, missing- just like the previous owner! The ad said "run's great"

They replaced most of the sensors and the pcm using a junkyard donor w/ 4.3
It was fun figuring that out, some of the parts look identical; Maf etc..

I've got the miss corrected; bad Autolite double platinum plugs, why would you buy those? and use a junkyard maf?

The Raw fuel was the pcm, When I asked her if she flashed it she handed me the flasher relay! that's for real no joke!

I have replaced or tested almost every sensor on the truck and repaired the wires they messed up. Just installed a new fuel pump over the weekend.

It now starts fine, idles smooth but when cold it stumbles off idle unless I feather the accelerator easy. I let it run for 2 hours and it behaved much better. when it gets hot and I stomp on it it still sounds like it's loading up but doesn't backfire through the tb. It also smells a bit rich with a tiny smoke but still but I don't know how much crap is in the exhaust. That was the longest I had let it run for. It will do a brake stand and WOT sounds fine. It is only when I accelerate just off idle, more pronounced when it's cold

Repaired already
all code issues
raw fuel smell
smoke plumes
cooling system

Problems I need to resolve
I have No codes BTW

Stumbles, hesitates, backfires when cold (but not if i go easy)
Seems to load up if I stomp on it when hot, but does not backfire
A/C does not turn on and is charged; where is the orifice located?
front end needs work; quite loose under there have new idler arms but the upper part on the frame could be bad as well.
Brakes? it's broken all over.

Replaced or tested
PCM, wrong one
EGR, pintle stuck
Knock sensor
MAF, wrong one
MAP, seal torn
Distributer, worn gear, broken screw in housing for cap
New plugs NGK, Autolites don't work out of the box
They had new wires on it so I left them
ECT and waterpump
Fuel pump and harness
tested the poppets they all open and close fine
replaced the fuel reg and gaskets while I was there
sprayed the whole engine down with carb cleaner looking for leaks no changes

Going to test the IATsensor today
tested the IAC already that looks good
I don't think it's the TPS it responds fine
There is a friggin snake hiding in the engine compartment that won't leave.
so some things don't get tested at the same time as you would expect.

MY thoughts right now
The cats could be partially plugged but I don't have a temp gun to check them...yet
maybe it just needs to run for awhile, runs fine at higher rpm

The spider assy is some how messed up, How do you test that?

I do not own a tech2 either but was looking at another program.

anyway that's the size of it. Land vehicle Vehicle Car Motor vehicle Van


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Hey Jackmup, did you ever resolve this hesitation issue? I have a 1999 suburban 5.7 with the same exact problem.

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