99 yukon 5.7 vortec misfire HELP

[lifted_yukon22]

99 yukon 5.7 vortec 215k new plugs, wire, cap, rotor, fuel filter. Drives fine when taking off, but after it shifts into 2nd gear it's just gets weaker as it shifts. The fuel pressure was checked today under throttle it's about 63-65 psi about 55 idling. The your driving and you hold it above 2500 rpm for any sustained amount of time it will go to misfiring and bucking then after the torque converter locks up of its still misfiring the torque converter will unlock. The fuel pressure is above 60 psi while doing these symptoms, the only code it's thrown was cylinder 5 misfire I replace that injector, same problem it is the orginal poppets. The intake gaskets was done about 8 months ago with the fel pro gaskets. I was having the problem before the intake gaskets was done but the more i drive it the worse it does it, the first time it done it it was 6 months before it done it again. It's getting to the point it does it every time I drive it. I'm at wits end with it any help will be appreciated.

 

[SunlitComet]

get the misfire history first.

 

[lifted_yukon22]

It's only had the p0305 once....I've had a mechanic with a scan tool hooked to it on it showed nothing...All I have is the handheld scan tool from napa. I even drove it while it was doing it with him hooked to it and he said it wasn't showing nothing.

---------- Post added at 07:10 PM ---------- Previous post was at 07:07 PM ----------

When it threw the p0305 that injector was replaced it seemed to smooth out slightly when it would misfire. I'm think I need to get the upgraded mpfi but don't wanna sink 350 into it unless I'm sure that's what the problem is.

 

[SunlitComet]

that is where the misfire history comes in not just a p0305 code.You will likely only get a single cylinder code if that cylinder is just dead all the time. the history tells you a lot more. So you mech was looking at the history and it showed notta while it misfired?

 

[OR VietVet]

Spider fuel injection problem? Rare but maybe ignition coil load problem?

 

[lifted_yukon22]

Yes exactly he was hooked to it while it was doing the misfire problem and it showed nothing. I thought it may be fuel pump that's why I went and bought a fuel pressure tester but by gm specs it seems to be spot on. It seems to be starving for fuel when it gets in a strain around 3rd and 4th gear. I've read that the 4l60e is designed to unlock the torque converter if it detects a slight misfire as true that is idk. When it first did it I was doing 75 on the interstate and the torque converter just unlocked. You could feel as very slight misfire. Now it's tons worse.

---------- Post added at 02:56 PM ---------- Previous post was at 02:51 PM ----------

When taking off from a dead stop I couldn't ask for 1st gear to be any stronger. It's just 2nd gear and after you can hear a different tone in the engine that it weak.

 

[SunlitComet]

if misfire HISTORY shows nothing and converter unlocks then the scanner is not working correctly or your pcm is outputting bad data. misfires just don't happen and not get recorded some where.

 

[lifted_yukon22]

Ok I talked to the mech....his scanner doesn't show history would I have to go to a gm dealer and get them to scan it? Also I replaced the throttle position sensor after this started happening and when the intake gasket was done the distributor gear look great barely any wear on it. I just wonder if this could be the notorious spider problem I've heard about with the vortecs.

 

[ccapehartusarmyINF.(ret)]

okay correct me if im wrong but from what i got out of yer description is the higher the RPM for a sustained persiod of time the worse it gets?
if thats the case its possible you have a floating valve typically is caused by a faulty valve spring and at higher RPM since the valve isnt closing all the way or when its supposed to it will cause a misfire or in some cases a bent valve
with a 5.7 with that many miles on it id change all the valve springes while yer at it

---------- Post added at 03:36 AM ---------- Previous post was at 03:32 AM ----------

is that 5.7 a flat tappet cam or roller cam
what year did GM replace the flat tappet cams with roller cams

 

[SunlitComet]

forgot the year. Would need to reference it but all vortecs are hydraulic rollers.

Lifted all kinds of scanners do history readouts he may have just had a basic code reader. try any shop that has a handheld snap-on or otc scanner. more then those actually just can't remember the names. Not tho if you end up needing another poppet injector replaced later, eat the monetary damage and get a redesigned delphi unit right then. at this point now on you will only benefit from it no matter what. Maybe the mech might just find a bad wire or plug. and you could save a bit for now. Post up any results of the testing here for further analysis.