90 454 TBI runs like crap

[zepplinc20]

Hey guys I'm looking for a little direction on this one. I recently purchased a 90 k3500 with 99k miles. This thing runs like dog crap. It's extremely rich and has a slightly rough idle. I've gone through the typical tbi stuff like cts, map, egr...etc. TBI was rebuilt and pressure regulator looked good. I'm waiting on the adapter for a fuel pressure test. All that being said, I've got a hunch there is something going on with the valvetrain. Either timing chain, camshaft, or burnt valve. Timing was verified to be 4 degrees btdc. Also has a slight miss when cold I believe from being so rich.

Every once in a while immediately after shifting and letting the clutch out the thing will buck like crazy until I push the clutch back in and let it out with a gentile application of the gas pedal. Also a few times I've noticed a backfire through the intake. One time that comes to mind was while towing a camper, accelerating up a 6 degree grade from a dead stop.

My question is where to go from here? I have a leakdown tester, and a compression tester. Which one first? I though I might start by rotating by hand and checking the distributor for play to eliminate the timing chain. This type of diagnosis is new to me so I'm all ears. Sorry for the long post.

 

[grampadirt]

Try bumping the timing up 4* to 8* before,I've yet to have a BBC that ran well at stock timing.Check the compression if that doesn't help,might shed some light on your problem.

 

[zepplinc20]

Just wanted to post an update since I hate it when people don't follow up on threads.

Fuel pressure was checked and looked good at 13.5 to 14 lbs.
Timing chain slack was found to be at 8 to 10 degrees. Planning on replacing it soon.
Compression check found 165 to 170 on cylinders 1 through 7. #8 was ZERO.
Leakdown test on #8 with 90 psi in, cylinder only held 10 psi. Air coming out intake side. With leakdown tester installed, intake rocker arm was removed and staked intake valve with a brass drift. Each hit increased the psi on the leakdown tester until 90 / 80 was achieved. Installed rocker arm and adjusted with engine running (with oil deflectors).

RUNS 1000% BETTER.
Used a can of GM top engine cleaner (the old stuff bought on ebay). A slight improvement overall.

So I guess it was carbon. Never thought carbon would cause 0 psi on a compression check, but there ya go. I will be installing a 3 wire o2 sensor for a further improvement. Still seems to have a rich miss at idle when cold. If anything changes I'll post another update.

 

[grampadirt]

I'm thinking a chunk of carbon got stuck on the valve seat and held it open some.Each time you hit the valve stem it broke some loose until it was all gone(or at least mostly gone )

 

[Rusty Nail]

Just wanted to post an update since I hate it when people don't follow up on threads.

Fuel pressure was checked and looked good at 13.5 to 14 lbs.
Timing chain slack was found to be at 8 to 10 degrees. Planning on replacing it soon.
Compression check found 165 to 170 on cylinders 1 through 7. #8 was ZERO.
Leakdown test on #8 with 90 psi in, cylinder only held 10 psi. Air coming out intake side. With leakdown tester installed, intake rocker arm was removed and staked intake valve with a brass drift. Each hit increased the psi on the leakdown tester until 90 / 80 was achieved. Installed rocker arm and adjusted with engine running (with oil deflectors).

RUNS 1000% BETTER.
Used a can of GM top engine cleaner (the old stuff bought on ebay). A slight improvement overall.

So I guess it was carbon. Never thought carbon would cause 0 psi on a compression check, but there ya go. I will be installing a 3 wire o2 sensor for a further improvement. Still seems to have a rich miss at idle when cold. If anything changes I'll post another update.

That is high quality auto repair my man, good work! Most impressive. Thanks for the update/follow through.

(Whoever told you that is a real auto mechanic.) *winky face