My 99 GMC truck needs a miracle

[Redrocket1045]

Hi guys. As a newbie to this forum - I'm hoping you all might be able to help. Here goes. I have a 1999 GMC 4x4 Sierra with the 5.3L w/ headers and a cat back system. About 4 1/2 years ago, I installed a Powerdyne supercharger with 39lb injectors and a custom Diablo sport tune. It ran fine till late January. The CEL came on and I erased it. 3 days later it came back and I erased it again. This time the truck was running like **** and missing and it fouled out the plugs. I installed a new set of 41-985 plugs and it started but it was missing and running rough with sluggish throttle response. I removed the Diablo tune and downloaded a Hypertech tune to see if that would help. It didn't. I sent my programmer back to Diablo for a new download and that didn't help. Now comes the weird part. Upon start up - if I hold the rpm steady at 2000 rpms for approximately 30 seconds - the motor will shoot up to 3100 - 3200 rpms. However - if on the open road and I want to shift from 4th to 2nd - it bogs and sends black smoke out the tailpipe. I know it's running super rich(LTFT is -16). This truck has seen 5 local mechanics in 4 months and the Chevrolet stealership had it 1 week and nobody has come close to fixing it. All - and I mean all sensors have either been replaced or a known good sensor has been tried. It has new intake gaskets, new knock sensors and new catalytic converters. Also another PCM has been installed and it runs the same. Short of burning it down - what can I do? Anybody have this problem?

[stumpuller454]

can`t say i`ve had that problem, i know intake housings can have bad seals and/or be warped and not throw codes. have any of the mechanics driven it with the scanners in to see exactly how its having problems?

Plus have you ever considered having a custom tuner a hand at it? thats the way i go, if theres ever problems i just go visit him, he hooks up the laptop we go for a drive then he makes corrections,

P.S. don`t burn her down, its probly a simple overlooked problem man, you feel great when you find it!

[leolkfrm]

What did the compression test show when you changed the plugs?

[bradrhale]

for curiousity sake, what was the code for??

[Redrocket1045]

Originally Posted by bradrhale

for curiousity sake, what was the code for??

If I recall, the first code it threw was a code for the knock sensors(replaced w/new). Then it threw the famous or rather infamous PO300(random miss)
Remenber Guys - this truck ran fine for 4 1/2 years with the Diablo program. And also keep in mind - Why does it run well after running it at 2000 rpms for 30 seconds? It seems to clear up - but on the road it bogs down bad. Oh, and if I shut it down for a couple of seconds and do a restart, I have to again keep the rpm at 2000 rpm for approximately 30 seconds and then it shoots up to 3100-3200 rpms.
Thanks guys.

[Redrocket1045]

Originally Posted by leolkfrm

What did the compression test show when you changed the plugs?

I'm kinda of **** on this - but what does the compression test tell me? It ran fine for 4 1/2 years till the CEL light came on. Then it started running super rich. Will bad rings or a valve cause this? Wait - can't be a valve cause it doesn't miss fire - once I run it through the 30 second cycle.
I have noticed a low engine vacuum at idle and one mechanic said that he restricted the amount of vacuum to the brake booster - and the engine's idle was very smooth.
What are your thoughts on this now?
TIA

[scavanger]

Check ALL ground connections, to frame etc.
Bad grounds do strange things.


Scav.

[Redrocket1045]

Originally Posted by scavanger

Check ALL ground connections, to frame etc.
Bad grounds do strange things.


Scav.

Yes, this I know. All grounds were checked(6) and all were OK.

[stumpuller454]

Originally Posted by Redrocket1045

I'm kinda of **** on this - but what does the compression test tell me? It ran fine for 4 1/2 years till the CEL light came on. Then it started running super rich. Will bad rings or a valve cause this? Wait - can't be a valve cause it doesn't miss fire - once I run it through the 30 second cycle.
I have noticed a low engine vacuum at idle and one mechanic said that he restricted the amount of vacuum to the brake booster - and the engine's idle was very smooth.
What are your thoughts on this now?
TIA

I read this many times, just trying to get this right, so the engine vacuum IS low? and a mechanic then somehow adjusted the vacuum to the booster? So it could possibly be a ruptured diaphram in the booster, and it causes engine vacuum to be low, restricting it would have gained a little vacuum. are your brakes operating okay?

Heres what i would do, pinch of the supply line to the booster and see how she runs, carefully go for a drive (realising of course you will not have brake assist...) but just to see if that changes anything.

I`m still feeling your problem is vacuum related. highly possible in a charged engine, and it will certainly cause the problems you describe. The engines MAP sensor will be "fooled" to think your throttle body is open (lower manifold vacuum) and be forcing more fuel in however there is not actually any available causing a rich condition. It may not be much of a problem when your engine is idling or runing below 2000 rpm, but i know my charger starts making boost about 2000-2500rpm so it could be at this pressure were the problem leak gets worst.

Let us know what you think....

[Redrocket1045]

Originally Posted by stumpuller454

I read this many times, just trying to get this right, so the engine vacuum IS low? and a mechanic then somehow adjusted the vacuum to the booster? So it could possibly be a ruptured diaphram in the booster, and it causes engine vacuum to be low, restricting it would have gained a little vacuum. are your brakes operating okay?

Heres what i would do, pinch of the supply line to the booster and see how she runs, carefully go for a drive (realising of course you will not have brake assist...) but just to see if that changes anything.

I`m still feeling your problem is vacuum related. highly possible in a charged engine, and it will certainly cause the problems you describe. The engines MAP sensor will be "fooled" to think your throttle body is open (lower manifold vacuum) and be forcing more fuel in however there is not actually any available causing a rich condition. It may not be much of a problem when your engine is idling or runing below 2000 rpm, but i know my charger starts making boost about 2000-2500rpm so it could be at this pressure were the problem leak gets worst.

Let us know what you think....

Yes, brakes work OK and what you say makes sense but remember that when it's started - it will run rough, misses and has no throttle response. I'll try to restrict the vacuum hose to the booster and see what happens. Funny though, you don't hear a vacuum leak anywhere in the engine bay. Just plain weird!