Here it is had my 98 expy 4.6 for 4 years with no issue..Except the norm heater core and alt..Now out of the blue it started running really rough about 2 months ago.I had one code come up Cant remember what it was missfire cyc #4 did a tune-up light went away miss got better but is still there..Ok so when idling it seem fine but not smooth like it use to slight roughness but then all of the sudden slight buck in the motor occurs then back to same idle this happens every few minutes or less.But the worst of it is when driveing the truck if i put it in gear foot on brake holding idle it will do the same as idle in park slight buck in motor but much more noticeable and more often and it is very sluggish leaving a stop or wot on freeway.I,ve changed aic,egr,plugs wires two different sets,both coil packs,fuel filter and I even replaced the intake gaskets with no change.all vacum lines are intaked and working correctly I think I can kinda of here a leak though coming from back of the intake note there are no vac lines there like the 5.4 only thing I think if it is a leak its from the plastic lower intake but think this is uncommon..I,ve done several 4.6 and 5.4 swaps with no issue..This is a miss Ive never seen before..I,m goning to do fuel pressure check and Re do intake gaskets just make sure theres no cracks any where and didnt miss anything..Note done this job several times. ANY HELP IS SO MUCH APPRECIATED!!!I will try to do a video of it..,Thanks

 

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Sounds like a clogged cat, but it would be better to test that part before replacing it.

Also try cleaning the throttle body and testing the throttle position sensor.

Symptoms of a Bad Throttle Position Sensor

 

First I would do the fuel pressure check to make sure you have the correct PSI. Other than that, it seems to me that you've changed or addressed everything but the fuel injectors.

 

yep, bad tps, try changing that sensor its pretty cheap.

 

Thanks for the reply's How do i test the tps..As of today i took the hole top end back off and reinstalled it super slow double checking ever gasket and hose and replaced egr with no change this sucks no codes..Tps was def on my mind but wouldnt that throw a code,but its def a idle issue.Mostly at a stop in gear foot on brake..FUEL pressure holding strong at 40psi.. Thanks again (THIS ONES GOT ME GOING CRAZY)

 

UPDATE!!Not tps I'm outta ideas

 

Can a bad torque convertor cause bucking and hesitation..I replaced the trans when I got the truck 80,000 miles ago with used one 115xxx but had to use another torque which had 220,xxx on it..

 

What was the the fuel pressure reading?

 

No, it will mainly cause slippage, noise, or vibration. Bucking is generally the result of improper fuel mixture, or misfire(s).

 

looks like you check everything, the only thing i can think of is the timming is off, timming chain is loose needs new tensioner and while you in there already change the chain too. hope iam wrong it gets expensive to get to this point. if there no codes this will be it. good luck!!

 

Wow!!! timing chain that's pretty uncommon for mod motor though right..What about the head gasket.?I've been hearing gurgling in the heater core ever scene I changed it' along with a lil bit of milk oil on the cap not always though...which was 2 months before the symptoms..Tried ever thing as burning goes!!!heater core was done by the book.done this on my fords

 

I used a rented on from autozone it was 50psi with vacum hose on regulator and 35 to 45 with out vac on regultor..Think fuel pump is good..I also put the guage in windshield to see if there was drop in reading when it bucked,but psi stayed strong right through the buck..