1999 454 Vortec misfire issues

[Saab84mw]

K2500 suburban 454 vortec misfire mainly under a load and worse with the A/C on.

Here is what I’ve done so far just recently with in the past week I put a MSD cap rotor and coil on the truck trying to fix the misfire issue was told that the original caps were made out of a different material and MSD cap would be better.

Truck ran Better for the next three days then started misfiring again.

Within the past five years have replaced the injectors with fiveomotorsports injectors also replace distributor crank sensor, fuel pressure regulator, fuel pump and recently replaced map sensor And O2 sensor that was determined to be bad by a scan tool From a friend who had one.

The catalytic converter‘s have been removed and the O2 sensors are in the stock location in the pipes.

Seems like every time I work on this truck and replace parts it will run good for a few days and then start running bad again truck has 216,000 miles on it when it is running good it runs pretty strong.

Today I noticed that the misfire starts at about 1500 RPMs up to about 2500 rpm’s when accelerating.

I have been fighting this issue for a while I’ve had this truck for about 14 years bought it when it had 144,000 on it I was running strong when I bought it.

I just want to try to figure this issue out before I put a remanufactured engine in it and the transmission and transfer case has already been done.

Thanks and any help would be much appreciated.

 

[L29Sub]

Distributor bearings? Vortec BB distributors will wear out. And the driven dist gear will wear out. Cause the timing to rock. Improper plug wire routing?
GM caps are good items.
Primary ignition wire feed intermittent?
Fuel pump?
Does it run well above 2500?
Too large injectors? The stock ECM won't manage much more than 20 lb injectors. Bad injectors? I had good results with Accel injectors. YMMV..
Long shot, cam chain or cam lobe wear? That would be more noticeable in the torque range.
A short in the ignition wiring. Does the tach jump around when the the miss starts up?
Loose ground wires?
Correct spark plugs and gaps?
Any work performed on the truck just prior to the miss presenting?
Have you considered a Q-jet? HEI distributor?



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[Saab84mw]

Distributor bearings? Vortec BB distributors will wear out. And the driven dist gear will wear out. Cause the timing to rock. Improper plug wire routing?
GM caps are good items.
Primary ignition wire feed intermittent?
Fuel pump?
Does it run well above 2500?
Too large injectors? The stock ECM won't manage much more than 20 lb injectors. Bad injectors? I had good results with Accel injectors. YMMV..
Long shot, cam chain or cam lobe wear? That would be more noticeable in the torque range.
A short in the ignition wiring. Does the tach jump around when the the miss starts up?
Loose ground wires?
Correct spark plugs and gaps?
Any work performed on the truck just prior to the miss presenting?
Have you considered a Q-jet? HEI distributor?



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[Saab84mw]

Replaced distributor stock replacement distributor 20,000 miles ago.
Running tailor-made wires have checked at night time no sparking underneath the hood.
Replaced the injectors about the same time with aftermarket injectors from five0motorsports one step up from stock injectors I do believe they are 19lb injectors.
Fuel pump holds at 55 to 60 PSI. The fpr Was replaced same time as injectors.

Double platinum spark plugs about 5000 miles ago.
Tachometer stays steady does not jump around.

The only thing that I have not replaced so far is the ignition module.

I have considered putting a MSD style distributor in it.

 

[L29Sub]

Pull the plugs and see if any are darker or lighter than average. Keep up with which plug came which cylinder. If black, too rich. A proper fuel delivery will be rather light colored. Bone white is too lean. If any wet, problem.
Do a compression test. Verify healthy engine...to a point.
Aftermarket injectors are scary. I forget whether the 99 PCM will Id a specific cylinder misfire. Some will with good scan tool. Run a live scan and see what 02 sensors are saying. See what's out of range.

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[Saab84mw]

Pull the plugs and see if any are darker or lighter than average. Keep up with which plug came which cylinder. If black, too rich. A proper fuel delivery will be rather light colored. Bone white is too lean. If any wet, problem.
Do a compression test. Verify healthy engine...to a point.
Aftermarket injectors are scary. I forget whether the 99 PCM will Id a specific cylinder misfire. Some will with good scan tool. Run a live scan and see what 02 sensors are saying. See what's out of range.

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I haven’t done a compression check in a couple years so probably a good time to do that, it was good when the last time I did it.

I have noticed one thing the misfire or stumbling it seems doesn’t happen in a mechanical manner like at the same time same place all the time sometimes it will run good for a few minutes and then it will start to stumble under acceleration or just cruising down the road.

When I changed the cap and rotor and coil about a week ago for the first three days and seem to run very well then start back to stumbling again.

When I used to race cars SCCA I would see some of the ignition systems started breaking down it seems like my truck is doing the same thing just having a hard time figuring out we’re it is failing.

 

[L29Sub]

Is the distributor getting wet? Moisture will cause what you are experiencing. As will broken wires and bad fuel, fuel pum failing, and the day of the week...

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[Mangonesailor]

Pull the dizzy cap. The rotor screws probably loosened up and is flopping around in there.

I've ate 2 MSD caps so far. 1 to the rotor screws backing out after a week's worth of driving (use real loctite to hold them in, not just the mud on the screws) and the second to the terminals corroding. I've had a summit cap/rotor that I was using for a bit but the aluminum terminals oxidized and blew oxides all over the cap (camshaft offset code got thrown and value was all over the place), now using a Duralast gold.

Idk what I'll use next.

If it ain't there then it's likely the dizzy drive gear is worn out. Mine did at 100kmi and my 0411 picked it up as a random misfire at idle. Maybe that could cause you issues. You CAN pull the dizzy with the upper plenum installed and oil pressure sending unit still installed.... I've done it.

 

[Saab84mw]

Pull the dizzy cap. The rotor screws probably loosened up and is flopping around in there.

I've ate 2 MSD caps so far. 1 to the rotor screws backing out after a week's worth of driving (use real loctite to hold them in, not just the mud on the screws) and the second to the terminals corroding. I've had a summit cap/rotor that I was using for a bit but the aluminum terminals oxidized and blew oxides all over the cap (camshaft offset code got thrown and value was all over the place), now using a Duralast gold.

Idk what I'll use next.

If it ain't there then it's likely the dizzy drive gear is worn out. Mine did at 100kmi and my 0411 picked it up as a random misfire at idle. Maybe that could cause you issues. You CAN pull the dizzy with the upper plenum installed and oil pressure sending unit still installed.... I've done it.

Distributor was replaced about four years ago.
The MSD cap and rotor I just put on this cap has brass in it.

Also installed tailor-made wires that was replaced at the same time I did the distributor what is the likelihood going bad or life expectancy on those tailor-made wires?

thanking about upgrading the distributor to an aftermarket distributor any ideas? And maybe doing something to tune the truck for the non-stock injectors I have an it I’ve read some things about the 0411 ECU.

maybe I just need to upgrade my whole ignition system.

 

[Mangonesailor]

Distributor was replaced about four years ago.
The MSD cap and rotor I just put on this cap has brass in it.

Pull the cap.