Sick of hunting down this issue on my own - Please Help! :)

[Evil Trailblazer SS]

So I decided a video would work a lot better in describing the issue I am lost with right now!

I have fully tuned up the truck and I am just plain out f ideas right now...so I am enlisting the help of all my friends on the site!

1987 Chevrolet TBI 454 Issue - YouTube

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[Goldfinger]

Electric fuel pump or manual? I had an f250 that did this on the rear tank. it was a bad pump.

[Evil Trailblazer SS]

It has dual electric pumps and both pumps, pick-up screens, all soft lines, filter, etc. are all new...Does it on both tanks.

I have an extra throttle body on the shelf and even swapping out the TPS and IAC yield no results, it still does this...
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[Goldfinger]

Bad spark plugs. such as to much or to little gap. not enough current bad wires.

[Evil Trailblazer SS]

The spark plugs were my first thought...like i may have broke the porcelain insilator... But they are brand new NGK replacements gapped to stock specs with brand new MSD 8.5mm wires. All tested out perfect even after re-checking them twice.

I used the NGK-2771 plug as alled out for my truck...
http://www.summitracing.com/parts/NGK-2771/

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[Goldfinger]

But that's odd when its idling it sound like every cylinder is firing but increase the idle and it stumbles almost as if the cylinders are misfiring. I would assume it was a fuel problem but it seems to get plenty. electricity would be another possibility. However air doesn't seem to be an issue. could it be an internal problem like a sticky valve?

[Evil Trailblazer SS]

Sitcky valve would (or should) get worse with RPM, it nearly clears up before 2,500rpm...And when you let off the throttle, it clears up like a switch has been flipped and everything is perfect almost instantly at idle...

[Goldfinger]

Are you sure it clears up or could it be masked by a smoother operating speed.

[Evil Trailblazer SS]

It does have a slight miss at upper RPMs but its not debilitating like it is from 1,000-2,000

[Goldfinger]

It almost sound like a faint tink at idle. could it be a loose lifter prior to the point at which it hits the ticking sound.

[Evil Trailblazer SS]

I highly doubt it...the way it switches in and off with throttle, it's not likely a lifter...
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[Goldfinger]

I'm all out of ideas

[Evil Trailblazer SS]

Maybe a 454HO crate and Howell EFI unit will fix it...
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[68Timber]

What kind of exhaust is on the truck? More specifically, does it have the original cat?

[Goldfinger]

I'd say a new motor would fix it, how many miles does it have and has it been rebuilt?

[Evil Trailblazer SS]

The truck has 52,500 on it...it's verified. But last week I found the GM Reman decal plate on the side of the block, so I have no idea how many miles are on the engine. It's a .030" over 454 now...so 461.
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[Goldfinger]

when did it appear?

[Evil Trailblazer SS]

The exhaust does have a newer aftermarket cat, not the original...

The stumble issue has been present since I brought the truck home. I spent over a month correcting everything, tuning it up, replacing vacuum lines, fixing all the wiring problems, etc. The truck would not idle down below 1,600 when I picked it up and it ended up being the coolant sensor for the ECM having really crappy splice connectors that broke and corroded.
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[1LowToy]

I'd pull the codes and see what you have going on. I'd take a close looke at cap and rotor for arcing or other defects. It also sounds like it's leaning out as it lowers rpm which may be the stumbling problem. Where is the O2 sensor located? Are there mufflers on it at all? Exhust system back pressure may also be an issue.

[Evil Trailblazer SS]

That is the strange thing, the ADLD connector seems to be dead...I get no response when I jumper the pins...I am really not liking the hack work performed on this truck in the distant past! >

The new o2 is in the stock location of the driver side manifold and the exhaust is stock y-pipe and 3" Catco converter with a 14" case 5x8 Magnaflow split to dual 2 1/2"...it's how the truck was done when I bought it.
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[irishman999]

Hello from Reno, if your sure the fuel delivery is good and want to check the ignition I have an extra HEI distributor you can slap on if you have doubts about your ignition. Let me know.

[Desert1957]

Check the top bushing in the Distributor, Iv'e had a lot of 454's go bad. Take the Dist cap off and wiggle the rotor , If the top shaft has ANY play, that's whats wrong. Higher RPM's seem to make it spark scatter.

Desert

[lha1992]

I had a problem with my '89 C1500 with the 350TBI where it would idle sort of yet it would increase and decrease increase and decrease in idle. But when you would give it a little gas it would stay constant. Though as you would increase the RPM as it got past 2800 and 3000 it would stumble and the RPM would go up and down.

I had change the TPS, distribitor cap and rotor, spark plugs/wires, fuel filter. Nothing changed it. Turned out it was the fuel pump. Had low PSI at idle yet good PSI when you gave a little gas but at wide open throttle it sucked.

The pump was the original pump in the truck and my mechanic said it should have gone out 10 years ago haha.

[craazy22]

Check the dist. and the rotor with out cap for play. I just finish rebuilding my 454 and it was doing the.same thing due to bad shaft
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[Evil Trailblazer SS]

Originally Posted by irishman999

Hello from Reno, if your sure the fuel delivery is good and want to check the ignition I have an extra HEI distributor you can slap on if you have doubts about your ignition. Let me know.

I will surely let you know! Thank you for the offer!

Originally Posted by Desert1957

Check the top bushing in the Distributor, Iv'e had a lot of 454's go bad. Take the Dist cap off and wiggle the rotor , If the top shaft has ANY play, that's whats wrong. Higher RPM's seem to make it spark scatter.

Desert

I will check this out ASAP...

Originally Posted by lha1992

I had a problem with my '89 C1500 with the 350TBI where it would idle sort of yet it would increase and decrease increase and decrease in idle. But when you would give it a little gas it would stay constant. Though as you would increase the RPM as it got past 2800 and 3000 it would stumble and the RPM would go up and down.

I had change the TPS, distribitor cap and rotor, spark plugs/wires, fuel filter. Nothing changed it. Turned out it was the fuel pump. Had low PSI at idle yet good PSI when you gave a little gas but at wide open throttle it sucked.

The pump was the original pump in the truck and my mechanic said it should have gone out 10 years ago haha.

Since both pumps and all related parts of the fuel delivery system are new, I doubt this is the case. The truck is getting plenty of fuel at idle, we think it may possibly be getting too much...we are going to check some things out and I will report back.

Originally Posted by craazy22

Check the dist. and the rotor with out cap for play. I just finish rebuilding my 454 and it was doing the.same thing due to bad shaft
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That makes three that say it may be the distributor. I will check it when I get a minute...
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