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Discussion Starter · #1 ·
Suddenly my car has developed a misfire under WOT at 5-5.5k RPM. Redline is at 6k RPM. Power feels great until it starts to hiccup. I can't tell if its in the exhaust or where... Fuel pressure looks good as indicated on the Edelbrock app, staying around 56-60 PSI.

Specs:

383 50k miles
Unknown bottom end, and cam.
882 heads - port and polished
Edelbrock RPM Air gap manifold
Edelbrock E Street EFI - 3.5k miles
Edelbrock E Street Dizzy - 3.5k miles
MSD 8.5 plug wires - 20k miles
MSD 6AL box (red line set to 6K RPM) - 20k miles
AC Delco copper plugs, gaped ~.40 4.5k miles

Timing:
Initial: 16-22.5 played with this, didn't make a difference
all in: 34 @ 3k RPM

Fuel:
Idle: 13.9
Cruise: 15
WOT: 12.7

I figured I would mess with my E-street settings before I would start messing with mechanics. Doesn't look like anything has been working so far... From what I have read, I would guess it could be the spark plugs? I've got a set of NGK plugs I can throw in, but I will have to wait until the weekend. I can't imagine why I would suddenly develop a misfire/backfire out of nowhere.
 

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Discussion Starter · #2 ·
Just took it out for a quick spin. Did about 6 WOT pulls.

I noticed the first 4 it didn't misfire.

The last 2 misfired at the 5-5.5k RPM range.

Thoughts?
 

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Valve float.....
 

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Broken or weak springs aren't able to keep closing the valves the higher the rpm goes. The valves then stay open and the engine misfires. It sounds like the engine starts blubbering when the valves start to hang open. Doesn't happen much with fresh valve springs, but the older they get, the weaker they are.
 

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Discussion Starter · #10 ·
Broken or weak springs aren't able to keep closing the valves the higher the rpm goes. The valves then stay open and the engine misfires. It sounds like the engine starts blubbering when the valves start to hang open. Doesn't happen much with fresh valve springs, but the older they get, the weaker they are.

Could bad valve lash cause this too? I'd describe the misfire similar to the motor hitting the rev limiter early.


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The rocker arms could be almost falling off and you wouldn't likely feel any miss, but soft valve springs do act much like a rev limiter.
 

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Discussion Starter · #12 ·
The rocker arms could be almost falling off and you wouldn't likely feel any miss, but soft valve springs do act much like a rev limiter.


Ok, so it's likely not the valve lash, but just the sprigs if it were valve float. Correct? Thanks!


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2000 Trans Am WS6
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Just took it out for a quick spin. Did about 6 WOT pulls.

I noticed the first 4 it didn't misfire.

The last 2 misfired at the 5-5.5k RPM range.

Thoughts?
This is very similar issue as I was having with the Fitech Fuel Command Center...when it's vent line was filling up with liquid fuel...

I could blast it a good 2 or 3 times WOT and ran great, then the next time it would start doing all kinds of weird stuff...misfiring, nosing over, being the main things.

I'm definitely no valve spring expert, but would have to ask why the car would rev out fine, 4 separate times, then misfire?
If you had a bad/broken spring, wouldn't it misfire every time you ran it out to redline?
 

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Discussion Starter · #14 ·
This is very similar issue as I was having with the Fitech Fuel Command Center...when it's vent line was filling up with liquid fuel...



I could blast it a good 2 or 3 times WOT and ran great, then the next time it would start doing all kinds of weird stuff...misfiring, nosing over, being the main things.



I'm definitely no valve spring expert, but would have to ask why the car would rev out fine, 4 separate times, then misfire?

If you had a bad/broken spring, wouldn't it misfire every time you ran it out to redline?


UPDATE:

I adjusted the WOT ARF to 12.9 and it seems to have gone away. I did about 10 heard pulls yesterday to today and so far there hasn't been any misfire. Thinking back, I never really have tuned the car for WOT. I always had it tuned for cruising MPG. I guess the car likes to be fairly lean for WOT, and the ECU took some time to learn. If it comes back I will be sure to post!


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Discussion Starter · #15 ·
UPDATE: my previous update did not solve the issue. I still think it’s a valve issue. I’m going to look into the valves more closely.


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2 yr old plugs correct? how old are the wires and what brand and size? what are the plugs gaped at?
 
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Discussion Starter · #17 ·
2 yr old plugs correct? how old are the wires and what brand and size? what are the plugs gaped at?

I replaced the wires with MSD 8.5 super conductor and new plugs, gap at .045 about 300 miles ago. Still didn’t help.

UPDATE: I did take a look at the roller rockers and push rods. There were a few that were pretty loose, so I tightened them up on my lunch break. So far no issues past 5k RPM in second gear! Hopefully this fixed it.


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Discussion Starter · #18 ·
Nope! Didn’t fix it. I’m going to look at my fuel pump going to my EFI sump tank. If I recall, it has a very slow full rate, maybe 40gph. I’m thinking it might be starving my sump tank after a couple of WOT pulls...


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May be worth looking at your rotor phasing just to make sure your not getting too far from the terminals at any point in your timing curve. This would apply to both a stand alone system as well as computer controls. Computer for sure needs to be checked and mechanical doesn't hurt to check especially if someone has messed with the curve.

Its easy to check and something you can rule out if its not the problem. Worst case it cost you a cap since you have to drill holes in it.

So whats the deal with working on this in January? Your only time off from work? Like it cold?
 

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Some things I would look at:

Bump the redline rev limiter up a bit. Maybe 6k is too close causing rev limiter to kick in early?

How do the plugs look?

Possible you have a load of bad gas or emissions gas, too much ethanol or that is stale/unstable?

Fuel psi stable throughout rpm range?

Timing stable throughout rpm range?

You mentioned you tightened your loose rockers, did you follow any procedure on that or just take noticeable lash out?

Spark plug gap too large or secondary coil breaking down at high rpm?

Check for broken valve spring

I would investigate further why you had excess lash. You said unknown cam/lifters, assuming hyd. Flat tappet, maybe a collapsed lifter? Probably don’t want to hear this but look for flat cam lobe and or grooved lifters? We’re the roller rocker adjustment nuts loose? Any bent pushrods?

If 1.6 ratio rockers, check for pushrod to head clearance. Some sb heads have interference there.
 
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