hello, I have been all over the forum and finally decided to join and ask for help.
I have a 95 z71 4x4, 5.7L engine.
when it is cold it drives fairly good, but when it is warmed up and especially when hitting higher speeds it seems to misfire or surge. not sure what to call it maybe hessitation? this happens for a few seconds then stops, drives fine for a bit then does it again.
I have changed the plugs, wires, cap/rotor, fuel filter, air filter, and new oil change.
the truck starts and Idles strong, but it is when I put it into drive that my symptoms occur. the other day on the highway it seemed to bog down, then blow a shot of black smoke (first smoke I have seen from it) then drive normal and do the bog thing every now and then. no other smoke coming from it, not when starting or driving now.

PLEASE HELP!! this is my baby

 

Have you tried cleaning the throttle body?

 

yes throttle body is nice and clean

 

Dont rule out a bad cat or muffler either. If its partially clogged, the low exhaust volume at idle wont do affect it, but at speed or under load it may not be flowing well enough, and creating too much back pressure in the engine.

 

Tap on your Cat and see if you can hear a rattle.

 

CEL(check engine light) on?

If yes, then have the codes checked, if it's an obdII system. But i'm pretty sure OBDII systems didn't start until 96 on gm vehicles. so it's most likely a OBDI computer, you can access the codes yourself on the obdI system. Even if the CEL is not on, check the codes anyway, it may have stored the trouble code, with no further CEL warning.

how to check codes--> http://www.extreme-check-engine-ligh...%20Decoder.htm

 

I will check the cat and muffler on Friday.

Yes it is OBD1 system, and no the engine light has not come on. It did a while ago but has not come on since. Also the Battery has been changed so I understand the codes are gone but I will try

 

OK so it is not the cat or the exhaust system.
It seems to idle a little erratic when first started. Again starts fine. WHen I get on the gas it boggs down a little and when at a higher speed it boggs down more. seems to drive pretty good then does the hesitation thing, and if you push the gas when it is hesitating then it comes out of it and goes.

GGGRRRRRR

any other suggestions?

 

Check your throttle body. They are known to have the rod that the throttle plate rides on go bad and it will give an erratic idle and other symptoms like your describing.

 

ok I will take a second look at the Throttle body.

Today is a hot one out and I noticed it is running alot worse today, exact same symptoms, just hesitating more often.
IF tht helps at all