Random misfire at idle 454 vortec

[elevatorman]

We chased a random multiple misfire at one point and the culprit ended up being a new-ish cap. It was crossfiring in the cap itself. They're cheap enough that it might be worth trying that. This is where it would be nice to be able to check the plug wires to make sure they're actually firing at the right time and with the right energy. I'm sure that's possible but I don't know how.

Ive checked the cap twice and have not found any signs of arcing. But def worth another shot

 

[Davin Reckow]

Following thread. I have a 99 454 with same issue. Been through ignition, fuel system and cylinder heads. I recently found idle changes with the oil cap off. Are the intakes prone to warping. I didn’t think about checking for flatness when I had mine off.

 

[Schurkey]

Following thread. I have a 99 454 with same issue. Been through ignition, fuel system and cylinder heads. I recently found idle changes with the oil cap off.

Inspect the PCV system completely--PCV valve, suction hose, vent hose, etc.

Are the intakes prone to warping. I didn’t think about checking for flatness when I had mine off.

Not that I know of, but anything can happen. And the gaskets can fail even if the manifold isn't warped.

 

[Davin Reckow]

Inspect the PCV system completely--PCV valve, suction hose, vent hose, etc.


Not that I know of, but anything can happen. And the gaskets can fail even if the manifold isn't warped.

Pvc system all checks out.

Can’t see it being the intake gaskets as it has done it constantly before the bad gaskets were changed and still after having the manifold off to put fresh heads on.

 

[Schurkey]

If the idle changes when you remove the oil cap, it has to be due to relieving crankcase vacuum, or crankcase pressure.

The crankcase is not supposed to have vacuum OR pressure because there's a fresh-air inlet for the PCV system which normally allows air into the crankcase to relieve PCV suction, but if there's pressure, it allows the pressure to escape.

If you have vacuum or pressure in the crankcase that you're relieving via the oil cap...something is wrong.

Vacuum in the crankcase would be more-likely to cause an idle difference.

 

[elevatorman]

If anyone is able too find a wiring diagram for the injector harness it would be greatly appreciated having a hard time finding any for a 454 vortec

 

[Spareparts]

Man, I feel your pain with the misfire issue. I never did figure it out.
Before i finally said fk it i had replaced every sensor with either Delphi or AcDelco, 2 new distributors, caps, rotors, 2 new sets of plug wires, 2 new sets of spark plugs, 4 new AcDelco O2 sensors, 2 sets of injectors, New injector harness, 2 ecus, 1 0411 ecu. Several compression tests, Several Leak down tests.i don't even wanna think of what else.
Nothing fixed it.
I really do hope you figure it out!

 

[Schurkey]

If anyone is able too find a wiring diagram for the injector harness it would be greatly appreciated having a hard time finding any for a 454 vortec

Download the service manual set for your vehicle from the links in the sticky section of this forum.

It's in there.

 

[elevatorman]

Man, I feel your pain with the misfire issue. I never did figure it out.
Before i finally said fk it i had replaced every sensor with either Delphi or AcDelco, 2 new distributors, caps, rotors, 2 new sets of plug wires, 2 new sets of spark plugs, 4 new AcDelco O2 sensors, 2 sets of injectors, New injector harness, 2 ecus, 1 0411 ecu. Several compression tests, Several Leak down tests.i don't even wanna think of what else.
Nothing fixed it.
I really do hope you figure it out!

I have recently replaced distributor and cap and rotor i would say its slightly better bank 1 trims are below 14 percent now.
I pulled the injector harness and have found some exposed wiring near the plug for number 5 which appears too share a common with number 1 and couple others.
Pulled the plugs too computer and found a black wire with white stripe that ohms too one of the wires at injector harness the insulation was missing going info the pin connector. I am going through the whole harness .

 

[Spareparts]

New ones are available and not to hard on the wallet.

1998 Chevrolet K2500 Harness Assembly - F/INJR Wiring - 12163080 | Wholesale GMC Parts Online, Louisville KY

Harness Assembly - F/INJR Wiring. 1998 Chevrolet K2500. Genuine Chevrolet Part - 12163080. Ships from Wholesale GMC Parts Online, Louisville KY

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GM 12163080