[00pooterSS]

My bosses 98 z71 did the same thing for years. Tried everything. He even tried having the trans rebuilt in case it was shudder causing it.

Finally I had him put a good amount (little more than recommended amount) of seafoam in the fuel tank and go drive the dog shit out of it WOT as much as possible. He said that finally fixed it.

My best guess is it was a dirty valve since injectors were new.

Your timing is likely to retard under load at lower rpm, then increase in upper rpm.

We tried distributor, cap, rotor, plugs, wires etc. Nothing would ever fix it. It was one of the weirdest misfire issues I have ever seen. His would mostly start to do it at highway speeds once it got into over drive. If you kept it out of over drive it usually wouldn't do it. Which sent us in weird directions. Since he's my boss he never actually put it in the shop he just tried things here and there and wanted me to try throwing new ignition stuff at it and throw injectors at it. So I never really did diag on it since he didn't want me to. He said it's been great for months after the seafoam and driving it really hard.

 

[00pooterSS]

I just went and asked him, his recollection is different than mine but it's his truck.

He said after I put the new spider assembly in it was worse for a little bit but then all the sudden cleared up and has never came back.

Doesn't make sense to me, but he said it's been good for a year or more.

Drive it hard with the new injectors, maybe try a good amount of seafoam and drive it hard to make the injectors hammer cleaner onto the valves and clean them.

I have gotten several cars over the years to stop misfiring by driving them hard to clean the valves. The added heat helps clean them and the higher fuel flow helps.

 

[00pooterSS]

Also, check or replace the valve springs on #5. One might be funny.

You said it pops? If so that may be a valve sometimes being open when it shouldn't

 

[00pooterSS]

Also, check or replace the valve springs on #5. One might be funny.

You said it pops? If so that may be a valve sometimes being open when it shouldn't

No it wasn't popping, I should have said it was losing power and downshifting. when the misfire occurs on the interstate it gets hard to maintain speed and it shifts in and out of overdrive.

Okay gotcha, that's normal with a misfire.

On the plugs, I've been a tech for a very long time. I've only came across one engine that seemed to not like platinum plugs and I think it was just an anomaly. You can try copper plugs, but I have a hard time seeing that be the issue.

Have you tried swapping the number 5 plug with a new one? Or swapping it with another cylinder? I had a brand new NGK out of the box in my truck that was bad. Had to get another one. Was the first time in 20 years I had seen an NGK bad out of the box.

 

[00pooterSS]

Yes those caps can be temperamental.

And the holes in the distributor strip easily, or break if it's the plastic one. That was one of the issues with my bosses truck, we had to wrap a zip tie around it to hold it down.

 

[00pooterSS]

Just a follow up to what you were saying about the valves on cylinder 5. Logically, if it had a bad valve spring or bad valve, wouldn't I be floating the valves and losing power at WOT and higher rpms? When accelerating hard from a stop with my foot to the floor and letting the truck wind out every gear it usually does fine, or won't throw a code until it shifts to 3rd or overdrive.

Yes. Logically that would be true.

Sometimes things just don't follow logic and that's when you have to try things and see what happens.

Bosses truck did the same thing. Fine at heavy throttle. Never an issue until it got to high gear, low rpm, loaded (light acceleration). Which points straight to ignition break down. But like you, he replaced the ignition components multiple times so we had to just try and see.

Wouldn't hurt to pull the distributor and take a look. We had a hard time finding a quality distributor for another customer with a 92 silverado, got him to get a MSD. Within a year it was starting to lock up, so we just put a cheapo in it since the $300 was junk too.

 

[00pooterSS]

Valve springs are hugely over looked in general.

And can easily cause all sorts of havoc.

And it's rare that they show themselves in an obvious way.

My thought on the springs, if in doubt change em out.

For OP, There's no doubt the springs need to be replaced at this age, whether or not they are causing you problems is hard to say, but there's pretty much no way they aren't weak after all these years.

 

[00pooterSS]

Worn gear. Broke pin that holds the gear on the shaft. Bad bearings (hard to rotate the shaft). Slop in anything. Rusty crusties.

That's about it.

I would also try a new pick up coil and ignition module. Those things can make it fire funky if they are on their way out. And the way to replace those two items is a new distributor. If you go this route, keep your old distributor. I got one the other day that was bad out of the box and needed the old one to confirm the issue.

 

[00pooterSS]

Any suggestions on the best quality brand to get? I've hear MSD and Accel are crap now.

'

OE acdelco if you can even find one.

Edit... I went and looked at rock auto, they have a delphi and ac delco that are $234 and $261, but those should be the best available. I would absolutely be cool with the delphi that's a little cheaper. Delphi makes very high quality parts. OEM level. So does ac delco. However... we put a $80 one in the other day and it's doing just fine.... another however, we put a $80 one in another truck recently and it wasn't fine. So the cheapies are hit and miss. The MSD was a pig with lots of lipstick on it.

 

[00pooterSS]

Probably a stretched timing chain if you couldn't get the distributor to line up like it should using the keyed retainer. If you had to over rotate the distributor something that turns it isn't lined up. Whether it be the cam timing is out or the distributor gear is off or distributor was stabbed wrong. You may want to see if you can get the original distributor to line up on the keyed retainer and be able to hit the + or - 2

If it will and the new one wont. Something is going on there. If neither will then I would suspect the timing chain being stretched.

Still concerned about valve springs too.

 

[00pooterSS]

holy crap that's a lot of codes

The only real one i'd be concerned about at this time is possibly the control module codes and the insufficient engine temp code. Does it run cool all the time?

Should run right around 200

The o2 sensor heater codes and cat codes shouldn't affect anything

 

[00pooterSS]

You can turn the crank back and forth and check for chain slop too.

Here's an article on it that sums it up for me/so I don't have to type it out.

http://www.misterfixit.com/chanslop.htm

 

[00pooterSS]

 

[00pooterSS]

And there's a vid that should be helpful too.

 

[00pooterSS]

Gotcha. Best of luck man, that's a lot to handle there.

 

[00pooterSS]

did you ever try valve springs

 

[00pooterSS]

I haven't been able to. I'll have to hire a mechanic I think my friend is tired of working on this truck with me. lol

They're fairly easy. A basic spring compressor from autozone will do the job. Valve covers are easy to pull.

Just need to fill the cylinder with air or string to keep the valves from falling down while pulling the springs.

 

[00pooterSS]

Son of a gun. Thank you for updating the post.

I thought that was a 99+ thing.

Noted, and seriously thank you for updating the post.