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2004 Trailblazer LS 4.20 L
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Thanks! I think you’re supposed to cut off ignition and fuel to the cylinder being tested. Ignition is easy. But how do I cut off the fuel for one cylinder only? (Except maybe unplugging the injector, which is a lot of work to get to, there has to be an easier/quicker way?)
yeah dont bother with any more compression tests. or new parts. its something simple and probably a damaged wire. its almost certainly not mechanical. those motors run forever as long as the oil is clean and such
 

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`03 Trailblazer LTZ
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Discussion Starter · #22 ·
I’d honestly be surprised if it would be a mechanical problem, because it only has just above 80k. I’m doing the spark test next, hopefully it’s that easy, thanks so far guys
 

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Discussion Starter · #24 ·
Ok so I checked for spark and the result is pretty, disturbing I guess. First I checked if I can see a spark on the bad cylinder with the new plug I recently put in. Didn’t see anything.
Checked on a good cylinder if I could see a spark. Didn’t see something either. Checked with the old set of plugs, didn’t see a spark. Am I doing something wrong here?
 

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`03 Trailblazer LTZ
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Discussion Starter · #25 ·
Also it sounded really rough when I had the coils and plugs out on one cylinder. Was way smoother when I had everything in, so I have to have a spark right?
 

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2004 Trailblazer LS 4.20 L
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Also it sounded really rough when I had the coils and plugs out on one cylinder. Was way smoother when I had everything in, so I have to have a spark right?
only crank it over to look for spark. i usually sit the plugs up against something black and grounded and metal. maybe the wires are broke inside the insulation right there by the connectors or the wire harness has a break or rub before the coils. those springs can be picky about making contact also. make sure there's no oil or anything on the connections. if you have to much oxgaurd on there it will insulate your connections aswell.
 

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2004 Trailblazer LS 4.20 L
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only crank it over to look for spark. i usually sit the plugs up against something black and grounded and metal. maybe the wires are broke inside the insulation right there by the connectors or the wire harness has a break or rub before the coils. those springs can be picky about making contact also. make sure there's no oil or anything on the connections. if you have to much oxgaurd on there it will insulate your connections aswell.
i use cotton swabs and electrical connection cleaner to shine up as much as possible
 

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`03 Trailblazer LTZ
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Discussion Starter · #28 ·
Ok I had the coils just sitting on the valve cover, maybe there was no spark because they weren’t grounded. Trying it again
 

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`03 Trailblazer LTZ
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Discussion Starter · #29 ·
Checked again and I’m definitely getting a spark. Unfortunately...so we’re back to valves possibly?
 

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Discussion Starter · #30 ·
Something else I checked:
I used a infrared thermometer to check the temperature at the exhaust manifold.
cylinder 5 was a lot hotter than 6 or 3.
up to 30°F hotter after 30 seconds of idling. The others had 120°F and the faulty one had just under 160°F.
 

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`03 Trailblazer LTZ
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Discussion Starter · #33 ·
Oh and a running compression test showed no signs of cylinder 5 being worse than others. Every cylinder around 60psi running
 

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All good tests so far.

It may be a single broken valve spring - but the idle would surely be messed up and you'd likely hear a funny cranking rhythm when starting it.

Try removing the fuel pump relay and crank for a bit and listen to the rhythm.

The colder cylinder MAY be because of 1) lower compression, relatively 2) too much fuel (again relatively) or 3) flat cam lobe --- WHICH I HAVE NEVER HEARD OF IN THIS ENGINE - so put that 'way off outta your mind for right now.

You couldda spit out one of the valve/cam followers though! Hmmmmmmmmm?!?!
► This would affect the performance, but not the actual cranking compression test, by very much.
► I've seen engines soo-o-o gooped up with condensate {NOTE} that the rollers have seized for lack of oily lubrication 'cause water won't lubricate a bearing --- and stopped rolling.
► It then momentarily welded itself to the cam lobe and got ripped off the valve end.
► This results in NO lifter action on that particular valve --- and since we have two valves on the intake and two valves on the exhaust - missing either one will not kill the cylinder --- not completely.

If you have the scanner with a relatively fast refresh rate, then try the dynamic running/compression test at 2000 RPM. You'll have to be tricky 'cause that throttle position motor is going to try to compensate for all it's worth against your test!

Has this engine had a history of burning or "mystery oil loss" problems? I'm just spitballin' here for a badly coked exhaust valve - or two.

So far -
Compression is OK
Cylinder temp is somewhat cooler
Plugs have at least been switched around and no problems there
Coils have been also (like above)

{NOTE} If you want to see my expostulation upon "Defective PCV Systems And The Result Of Condensate In The Lube Oil" - I can post the link. (Psssst --- that's not a link - not yet - you hafta ask)

Let me know.
 

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`03 Trailblazer LTZ
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Discussion Starter · #35 · (Edited)
Well I don’t know much about the history of the engine, since I’ve owned it for just about 5.000 miles, but your theory sounds plausible!
If this is the case, I’d be able to actually see it once I lift off the valve cover, right?
If what you’re suggesting happened, I should be able to see it.

Also, cylinder temp was higher than the other cylinders, but I guess that’s 'normal' for a misfiring cylinder and would back up your theory that one exhaust valve stays shut all the time, thus having exhaust gases not being extracted properly and staying inside the cylinder -> heating it up.
So let’s just say this really did happen, it SHOULD be possible to repair it without having to take of the head, because the valve itself should be intact, right?
I can repair basically everything myself as long as that head can stay on.
I drove it for about 50 miles since the misfire first happened, so I guess it shouldn’t have damaged the closed valve.


Unfortunately I’m going to be on a work trip for 2 weeks starting tomorrow, so can’t check anything on the car, but I’ll just list the possibilities that are left and properly diagnose when I get back
 

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IF the problem is the possibility of a non-opening valve - and it is the roller-follower got spit out - then yes - it will be obvious.

As to what peripheral damage it did - I can only speculate because I've never been inside the Atlas engine before.

This same problem used to happen on racing Renaults when they'd spit a follower and it would kill the only exhaust valve on that particular cylinder - and be really obvious. This was also an early Kawasaki Z1-R 900cc engine problem when it over-rev'd and spit a shim.

I'm torn about this "almost-but-not-quite-a-dead-cylinder" problem and the only thing that turns up is that one of the followers got damaged - how I do not know.

But it's a profound problem wrapped up in a conundrum.

FWIW - I'm fighting a 4.3 GM V6 with an occasional dead cylinder on #5 - but only after a drive and restart after the hot engine gets hot-soaked.
► Start it once - drive it to New York from LA and it won't miss a beat.
► But shut it off for 30 minutes and #5 is not cooperating with the rest of the engine and throws a YOU'RE GONNA DESTROY THE CAT IF YOU KEEP DRIVING" engine code and a blinking SES.

This wouldn't be so bad if it was a customer - then I could tell him it's haunted and he needs to burn it down at a crossroad in a cemetery at midnight - but the van is my wife's and there's no living with her when she says:

"You're the big head mechanic and you can't fix it? You're fired!
 

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2004 Trailblazer LS 4.20 L
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Well I don’t know much about the history of the engine, since I’ve owned it for just about 5.000 miles, but your theory sounds plausible!
If this is the case, I’d be able to actually see it once I lift off the valve cover, right?
If what you’re suggesting happened, I should be able to see it.

Also, cylinder temp was higher than the other cylinders, but I guess that’s 'normal' for a misfiring cylinder and would back up your theory that one exhaust valve stays shut all the time, thus having exhaust gases not being extracted properly and staying inside the cylinder -> heating it up.
So let’s just say this really did happen, it SHOULD be possible to repair it without having to take of the head, because the valve itself should be intact, right?
I can repair basically everything myself as long as that head can stay on.
I drove it for about 50 miles since the misfire first happened, so I guess it shouldn’t have damaged the closed valve.


Unfortunately I’m going to be on a work trip for 2 weeks starting tomorrow, so can’t check anything on the car, but I’ll just list the possibilities that are left and properly diagnose when I get back
i would get it up to speed and see how it goes a couple times. been sitting for awhile and that is hard on stuff. never know a little heat and pressure may unstick whatever is hung up. really isn't much to loose you wont clog that cat in 50 miles even if its barely running on one cylinder. plus its easier than tearing down the top end. you already replaced alot of it, let it work a bit. there are four valves a cylinder, two in two out. id blow the dust off it on the highway.
 

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`03 Trailblazer LTZ
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Discussion Starter · #39 ·
So I drove it for quite a while over the last few days, but the misfire is still there, although it didn’t throw a code yet.
I’m trying to think of any possibilities that could cause that misifire other than one exhaust valve not opening properly and I’ve hit a wall. Guess I really have to open the valve cover to see.
Is there a way to test if a valve works correctly? Can I try to turn the camshaft some way to see if it lifts the valves?
 

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Once the cover is off, visuals will usually find the culprit quickly. Look for equal height lobes, no flat lobes, followers under (or over) each lobe, springs intact, equal valve stem height, etc.,
 
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