Weird Miss??

[1BadBowTie]

Im stumped, even have a few people who are good with cars stumped on this one, a 99 1500 with 5.7 vortec, only misses after full throttle. after u get back on it again, it clears up. i also noticed it doesnt run extreamly smooth at idle.. i threw in a lot of fuel injector cleaner, new AC plugs, good set of carquest wires, cap, rotor, and still it does this... i also experiance poor gas mileage... roughly 12 mpg.... any ideas why this could be doin this???

[jshort74]

Fuel filter?

[mr_mushroom]

timing drift a little off?
seems like that would set off a check engine light though.
maybe hook up to the OBD port and check if there's any recent momentary codes stored in the memory. also make sure the light even comes on when you turn your key to 'on', i had one shake loose once and fail to notify me that the ignition was blowing up.
might be a start.

[MyTOY]

my 5.0 had similar probs after all the usual stuff replaced with
no codes and like your 5.7 was CFI until I had the injectors
replaced with MFI. It runs perfectly now but at the cost of $700
for parts & installing. The dealer has equipment to clean the CFI,
but I hear this can only be a temporary fix for these poppit injectors.
I had already cleaned my MAF & EGR & of course fuel filter first thing
like jshort said and after I had the fuel pressure tested. Most folks
will target the distributor for a miss for a bunch of good reasons
but I think usually a misfire code is involved. If its just an idle prob,
most will say to check EGR system or coolant temp sensor. There are
methods for checking these but assuming you have no codes stored,
I would bet your injectors need to be professionally cleaned. GOOD LUCK

[markqc]

You may want to try a set of GM wires to see if it clears up the problem. I had replaced my wires with Bosch wires (6 months ago) and had problems with misfiring on two cylinders when at cruising speed. I changed the cap with little to no differece. If I accelerated or coasted the misfire seemed to go away. I changed the wires and problem has gone away for now.


Mark

[1968 Pontiac]

Delco wires are pretty good. Next time you change wires check their resistance agianst Bosch, Accel, etc., whatever you have.

[Evsman]

I can't help you. I have the exact same problem. A 1999 K-1500 with 5.7 misses just like you have described. i changed the cap, rotor, wires, plugs, and had the injectors cleaned. It still does it. But it is intermittent. Sometimes it doesn't do it it all and other times it will barely keep running. No trouble codes and the problem appeared all at once. Ran great and then all the sudden one day it started doing this. If you figure it out let us know!

[2000CAYukon]

Originally Posted by 1BadBowTie

Im stumped, even have a few people who are good with cars stumped on this one, a 99 1500 with 5.7 vortec, only misses after full throttle. after u get back on it again, it clears up. i also noticed it doesnt run extreamly smooth at idle.. i threw in a lot of fuel injector cleaner, new AC plugs, good set of carquest wires, cap, rotor, and still it does this... i also experiance poor gas mileage... roughly 12 mpg.... any ideas why this could be doin this???

This sounds like a leaking fuel pressure regulator to me. When you let off the throttle, the vacuum is higher at the fuel pressure regulator and extra gas gets into the intake causing the miss (the PCM sees the extra fuel and tries to lean it out). It would also explain the bad gas mileage.

//2000CAYukon

[1BadBowTie]

wow, sounds like i have a lot of checking to do, but i had the timing checked and it was off, but even set right it still does it.. so im leaning towards a bad injector.. probibly why it doesnt run all that smooth at idle.. but if anyone has any better ideas, let me know hehe... thanx guys!!!