01 Burb 5.3 ~290K P0303 Bad Engine?

dogememe

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https://m.imgur.com/a/eJWcY4G Compression test results are at the link above. Bad valve spring isn't ruled out yet. Decided to change all the plugs and wires, got 7/8 done before I had to go to work. 41-110 came out and r44ltsm went in. Some of the old plugs were quite loose. Some were quite oily on threads but otherwise looked ok. Problem is the same, still have misfire at high load. And it actually seems to shake a tiny bit more at idle, or perhaps it's just that I'm paying more attention now? Trying to find my scanner that will read live misfire data to see which cylinders are actually missing at this point since it's gone from a p0303 to p0300. Also changed PCV valve because when I checked it a ton of oil was passing through into the little hose. For $1.50 why not change it lol. Unrelated but I also replaced the alternator this morning. I was having flickering and the battery was draining overnight. Bad diodes. It was a rebuild about 6 months old. Put in a brand new one this time.
 
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That 130-170 spread isn't good. Your at the upper variance threshold if not just past it. Try a cylinder leakage test. That will likely give you more specific results. Cylinder needs to be at TDC compression for this test, or just remove/loosen both rocker arms for that cylinder. Obviously valve covers would have to be removed for the latter but you could also check the springs at this point. I have done many 4.3 valve guides (valves hanging up in guides) for causing misfire under high load. Granted a 4.3 is not an LSx engine but still worth noting. Make sure you clear the codes to reset the misfire counters.
 
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dogememe

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Originally Posted by mattd
That 130-170 spread isn't good. Your at the upper variance threshold if not just past it. Try a cylinder leakage test. That will likely give you more specific results. Cylinder needs to be at TDC compression for this test, or just remove/loosen both rocker arms for that cylinder. Obviously valve covers would have to be removed for the latter but you could also check the springs at this point. I have done many 4.3 valve guides (valves hanging up in guides) for causing misfire under high load. Granted a 4.3 is not an LSx engine but still worth noting. Make sure you clear the codes to reset the misfire counters.
What would cause the valves to hang up in the guides? And what do you think of my fuel pressure? After I made the video I tried unplugging the fuel pressure regulator and it went to a solid 55psi https://youtu.be/HfeqYsBAppE
 
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Sorry my post was meant to say piston needs to be at TDC compression stroke not cylinder... As far as what could cause the valve to hang up in the guides it would be wear/too tight of tolerance. There is a glare over the gauge so I can't see over 10 and less than 80 but it looks like it increases on throttle snap which is good.
 

dogememe

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Update: misfires are on cylinder 3 and 4. Not sure how to continue diagnosis. I can't imagine that both of these cylinders have a mechanical issue and it's not a blown head gasket since they aren't next to each other, rather they are across the engine. While I had this fancy scanner I did a crank relearn and nothing changed. Still, the misfires are only at high rpm under high load. Now I'm thoroughly confused smirk any help folks? https://youtu.be/BHNBjFXkT70
 
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If they are at high rpm and high load I would either look at a fuel delivery issue or valve float or valve sealing issue. What are your upstream oxygen sensor voltages at full throttle?
 
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dogememe

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Originally Posted by mattd
If they are at high rpm and high load I would either look at a fuel delivery issue or valve float or valve sealing issue. What are your upstream oxygen sensor voltages at full throttle?
Thanks, as per above fuel pressure seemed OK and fuel trims aren't super positive. Perhaps injector? Is it possible for an injector to work fine except at high load? Is it possible for one misfire to cause a phantom misfire in the cylinder that fires after it since my misfires are in 3 and 4 and they fire next to each other in the firing order?
 
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Your 02 voltages should read 800-900mV at WOT. If they are, you have sufficient fuel delivery. You can do an injector balance test with your scanner to make sure each injector is flowing enough fuel and are all balanced.
 

dogememe

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Originally Posted by mattd
Your 02 voltages should read 800-900mV at WOT. If they are, you have sufficient fuel delivery. You can do an injector balance test with your scanner to make sure each injector is flowing enough fuel and are all balanced.
Unfortunately I don't have this scanner anymore, I cout only borrow it for a short while. I have one coming that can read gm specific pids like misfire data etc but I don't think it can do an injector balance test. Should I change the injector on 3 or what other test can we perform? Thanks! PS: I finally removed the valve cover, and everything looks ok! https://youtu.be/MOV43mASN2w
 
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I would do a cylinder leakage test while you have the cover off. I wouldn't replace anything yet without more data
 
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Originally Posted by dogememe
Originally Posted by mattd
If they are at high rpm and high load I would either look at a fuel delivery issue or valve float or valve sealing issue. What are your upstream oxygen sensor voltages at full throttle?
Thanks, as per above fuel pressure seemed OK and fuel trims aren't super positive. Perhaps injector? Is it possible for an injector to work fine except at high load? Is it possible for one misfire to cause a phantom misfire in the cylinder that fires after it since my misfires are in 3 and 4 and they fire next to each other in the firing order?
Yes.....Sympathy Misfire. #4 didn't count near as high & didn't save it as misfires. Fuel Trims do not update at WOT......The reason to watch actual O2 voltage. A leakdown requires compressed air.....Swapping #3 Injector for another cylinder might be your next move after a leakdown. I didn't catch the Squeak part of your original post.....Along with the ticking! A bad lifter roller can do that, But it looks like you have good valve lift on #3. Just thinking out loud!
 
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Ive seen some higher mileage 5.3s develop intake gasket leaks which could cause a vacuum leak on certain cylinders intake runners, but that would probably present its self more at idle or low speed. Still something worth looking into if you dont find any valvetrain issues. Spraying a little carb cleaner along the intake while idling and if the idle picks up speed suddenly, you found the problem.
 

dogememe

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Originally Posted by cronk
Also seen fuel pressure regulators with torn diaphrams that leak fuel into the vacuum line and cause a rich mixture.
smile I checked the regulator and it didn't seem to be leaking any gas, although since there was no suction I can't tell for sure. Intake gasket is supposedly new. I'm in the process of changing the injector, I'll get back to you tomorrow smile
 
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Originally Posted by cronk
Ive seen some higher mileage 5.3s develop intake gasket leaks which could cause a vacuum leak on certain cylinders intake runners, but that would probably present its self more at idle or low speed. Still something worth looking into if you dont find any valvetrain issues. Spraying a little carb cleaner along the intake while idling and if the idle picks up speed suddenly, you found the problem.
Usually when stone cold....Once they warm up, The intake expands enough to seal up the vacuum leaks. Unrelated, But.....Replace the bolts/grommets when doing intake gaskets on these! The grommets collapse over the years & many intakes are replaced because of it. It's not Ford 4.2L V6 level notorious, But still happens.
 
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Originally Posted by clinebarger
Originally Posted by cronk
Ive seen some higher mileage 5.3s develop intake gasket leaks which could cause a vacuum leak on certain cylinders intake runners, but that would probably present its self more at idle or low speed. Still something worth looking into if you dont find any valvetrain issues. Spraying a little carb cleaner along the intake while idling and if the idle picks up speed suddenly, you found the problem.
Usually when stone cold....Once they warm up, The intake expands enough to seal up the vacuum leaks. Unrelated, But.....Replace the bolts/grommets when doing intake gaskets on these! The grommets collapse over the years & many intakes are replaced because of it. It's not Ford 4.2L V6 level notorious, But still happens.
And it's usually not a bad idea to replace the knock sensors while the intake is off.
 
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dogememe

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Sooooo I think we got it! Changing the injector made a big difference. It runs a lot smoother and while I'm not 100% sure since I couldn't hop on the highway I floored it a couple times on the way home and no flashing check engine light! I'll update in a couple days if it's not fixed but I so appreciate everyone's input and I'm really happy the old girl doesn't need a new engine smile Thanks to everyone who chimed in!
 

dogememe

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NOPE it's not fixed smirk it does run better than it did before, the injector made a difference... but it still flashes the check engine light and sets a P0303 above like 4500rpm at high load.
 
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