Stop throwing parts at them guys and just have the heads rebuilt. It DID fix my truck finally and I should have started there and not wasted the extra dough. Although not an easy job to shade tree, and expensive at the shop, I think this is the root cause of many "mysterious" misfires you can't fix with electronic parts. Dirty, sticky valves will not be helped much if at all by seafoam and treatments etc... and driving the piss out of it only made it worse cause it got hot.

 

unfortunately out of all of the 4 different mechanics, none of them suggested it could be the heads... Well looks like I know what Ill be doing.

 

Did you change the distributor cap?

If you did change the distributor cap, then I would say first swap the #5 spark plug wire with the #3 spark plug wire and see if the misfire code changes. If the misfire code does not change, then put the spark plug wires back and then swap the #5 spark plug with the #3 spark plug and see if the misfire code changes.

If you still have the P0305, see if you can swap the #5 fuel injector with the #3 fuel injector.

Beyond that, it could be a flaky distributor. There was a gentleman on this forum with a V6 engine in his Silverado that caused him fits with chronic misfire codes. It turned out to be a worn distributor drive gear. He put in a new distributor and that fixed it - no more misfire codes.

If it's not the distributor, well, I don't know. It could be a burned valve, weak valve spring, etc.

Good Luck!

 

Agreed,,,,, Id be swapping injectors between cyls next. and checking connector wiring. You have eliminated everything else short of a mechanical issue, or some crazy electrical gremlin.

 

the distributor cap is new, the plugs are new, the button is new, the wires are new. I've already swapped plugs, then swapped wires. No change, always cylinder 5 misfires. The last thing we did was change out the injectors. All of them, the entire spider injector system. they are all new.

 

Surely it wouldn't be a burned valve if they are sealing properly? All 8 cylinders have the same compression, holding good pressure with no bleed down at close to 180 psi. If it was a weak spring or stuck valve wouldn't it misfire all the time? I also forgot to mention the truck does have a slight random stumble at idle, but you really have to be still and really pay attention to notice.

 

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.

 

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.

 

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

 

Thanks everyone I will definitely try the seafoam thing and see if that helps. Probably go ahead and check out that spring too. Any thoughts on whether or not spark plug "type" would matter? I've been using the nice $9 iridium tip AC Delco plugs, but had been wondering if maybe and older style flat electrode platinum plug might be better for this vintage truck.

 

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.

 

Regarding the spark plugs, use whichever plug is factory recommended. If it still misses, then you know it's not plug related.

I'm wondering if there is someway to monitor the distributor to see if it is the problem due to a worn drive gear, internal bearings, etc. I am not sure about your distributor, but I know the ones used on 2000 - 2004 era 4.3L V6s are not made of metal.

 

I noticed the same thing with my 97 Chevy 5.7 vortec c1500 I've replaced the distributor cam sensor crank sensor throttle positioning sensor air position sensor clean the mass air flow sensor all wires are good new new spark plugs new distributor cap rotor haven't touched the spider injectors yet but I came to a conclusion on the downshifting of the transmission I have experience with the TCC I just recently went out and bought one thinking the torque converter solenoid is not kicking it out causing overdrive to downshift in and out I will check that as I'm going to do mine and to give it a good transmission filter and fluid change even flush the transmission out I will let you know how it does on mine I'm pretty positive the torque converter staying kicked in it's actually dragging the motor down and losing power and wants to shut it down whenever you come to a stop, idling goes up when I stop then eventually goes down to low this has led me to believe that the torque converter is staying engaged and for a simple $10 part is worth a try and plus you'll have new transmission fluid and filter into your transmission

 

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.

 

Yes, I did change the spark plugs around and I just put a different kind of plug in last night. It didn't help any, but I did dump an ton of seafoam in the tank and a little in the oil and running the Dog crap out of it every chance I get to see if that helps. I bought another distributor cap with brass terminals and going to swap that again tonight just to make sure the replacement didn't have the same failure. saw some posts saying that may be a common problem with these crab style "correct" caps are prone to. Fingers crossed...

 

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.

 

Yes I agree I actually just replaced the rotor and cap again last night with a "gold" version that has the nicer brass terminals instead of aluminum. Made very sure it was on there tight and not moving around, very solid. It did not help anything it's still throwing the code on the interstate. It hasn't been as bad, some minor improvements around town at 55mph and under, but the issue is still there and it's still not idling the way it should. I'm baffled.

 

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.

 

You ever figure it out man? Did the Sea Foam help?
I have identical issues as you except its cylinder #3/#4 and a P0300 for random missfire.
New parts include: Metal distributor, cap/rotor, platinum plugs, NKG wires, Coil pack, Ignition control module, MAF, Crank Sensor and recalibrate, Fuel pump a few years ago "ITS LOUD"
Fuel filter recently, Reman Spyder injector a few years ago as well as intake manifold gasket, compression tests good, never checked valves yet. Cats gutted, new 02 sensors, tested fuel pressure.
Vac tests and lots of scans, always cylinder 3 and 4 missfires, mostly #3. I DRIVE THE SHIT OUT OF IT and it doesnt help hahaha.
Loses power on the same hills, always starts out good then starts losing speed, downshift, downshift, downshift, starts shaking, flashing CEL, have to slow down to 80kmh for it to stop shaking and then its good until I get on it again and try to maintain speeds above 100kmh on big hills and summits.

 

I've read a good bit about the gear on the distributor wearing out, and being a common issue on these engines. Have you pulled the distributor?

 

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.

 

Years ago, I had a Chev 250 straight 6 that would develop a dead miss on the highway. It would stumble and die when I slowed down. Ran great around town. I replaced all the valve springs and it ran like a top again, the miss was gone. Once the springs got heat-soaked, they would lose their strength.

 

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.

 

I've read a good bit about the gear on the distributor wearing out, and being a common issue on these engines. Have you pulled the distributor?

So what would the symptoms be for a bad distributor? We had it out and inspected it when we were replacing the intake gaskets. I didn't notice any wear but wouldn't really know what I was looking for.

 

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.

 

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

 

I also have a 98 Z71 with the 5.7L engine, and last time I got it smogged it failed due to a misfire. It took my mechanic a week to diagnosis the real issue, along with a factory rep for the analysis equipment they use. Turned out to be worn out hydraulic valve lifters ( along with lower oil pressure due to the miles on my engine). Lifter was not pumping up to correct height. New lifters solved my problem.

It took some very sophisticated analysis equipment to find it, though. hope this helps, if you know someone who can do the diagnosis.

CarloPel

 

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.