Compression leak test blowing out of exhaust

[dean1roberts]

Follow up-I have a 98 k1500 with low compression it is a 5.0. With an air compressor I did a leak down test the compressor was set at 150 per the average compression on each cylinder to make sure I couldn’t over compress the engine. Took the valve cover off as well as the rocker arms on cylinder 3 (the low compression cylinder) and used the adapter to blow the air through the valve system. I had air coming through the exhaust as a product of the test. Now with this cylinder still able to hold 60 psi could it be anything other than the valve leaking pressure? Are there any common problems with the exhaust ports between the valve train and the headers?

 

[k1500 97]

Follow up-I have a 98 k1500 with low compression it is a 5.0. With an air compressor I did a leak down test the compressor was set at 150 per the average compression on each cylinder to make sure I couldn’t over compress the engine. Took the valve cover off as well as the rocker arms on cylinder 3 (the low compression cylinder) and used the adapter to blow the air through the valve system. I had air coming through the exhaust as a product of the test. Now with this cylinder still able to hold 60 psi could it be anything other than the valve leaking pressure? Are there any common problems with the exhaust ports between the valve train and the headers?

That I could not help with but there will plenty of good members that will chime in soon. Good luck.

 

[tayto]

did you try whacking the valve stems to make sure the valves were fully seated. maybe have a burnt exhaust valve

 

[Schurkey]

There's ALWAYS leakage past the rings. You'd hear that as noise at the oil fill cap...if the valve cover was still on.

Noise at the tailpipe indicates the exhaust valve isn't sealing. Yes, I'd tappety-tap-tap the valve tip (NOT the retainer!) with a soft-face hammer. IF the problem is a chunk of carbon on the valve seat/valve face, you'd crush it and maybe the valve seals properly. If not...the head comes off to inspect the valve and seat. Valve may be bent, or burnt. Potentially burnt seat.

Things would be the same if you hear noise at the throttle body--you'd have to open the throttle body to hear it better, although you might hear it with the throttle plates at normal idle--noise coming up the idle air control passage, or past the slightly-open (idle) throttle blades.

Make sure you look in the radiator, if you see bubbles there it's a head gasket or cracked casting.

Listen--or poke your finger over the spark plug hole(s) "next door" to assure you don't have a head gasket blown between two cylinders.

"Most" folks test at 100 psi. Nothing wrong with more pressure, but it's also more-likely to blow the piston down the hole which will be very inconvenient if you get sliced-open by the fan blade and have to get stitches.

 

[Road Trip]

Follow up-I have a 98 k1500 with low compression it is a 5.0. With an air compressor I did a leak down test the compressor was set at 150 per the average compression on each cylinder to make sure I couldn’t over compress the engine. Took the valve cover off as well as the rocker arms on cylinder 3 (the low compression cylinder) and used the adapter to blow the air through the valve system. I had air coming through the exhaust as a product of the test. Now with this cylinder still able to hold 60 psi could it be anything other than the valve leaking pressure? Are there any common problems with the exhaust ports between the valve train and the headers?

Your test has conclusively identified a burnt exhaust valve, especially since you removed the rocker arms too.

1) If there's no history of coolant use (ie: cracked head) then a normal valve job will get you back on all 8 cylinders.

2) How do all the spark plugs look? (7) good + (1) bad from #3? Were you getting P0300 and P0303 codes prior
to troubleshooting? IF it's only a single cylinder, I'm wondering if the burnt exhaust valve is a
victim of that cylinder running too lean because of a partially plugged injector?

3) This is optional, but if you want to gain additional confidence in your test setup, apply the compressed air
to one of the good cylinders while it is at TDC. (No need to remove the rocker arms on this cylinder.)
My assumption is that you will not hear any air escaping into the exhaust. It's always OK to cross-check
your test setup to prove/disprove/refine/calibrate your expectations using your approach.

EDIT: I just had a reply collision with @Schurkey . He makes good additional points, especially with
the additional check of removing the radiator cap & verifying no bubbles. Also thumbing the #1 & #5
spark plug holes while pressuring #3 in order to check for a blown head gasket between adjacent cylinders. Good additional data
for no extra charge!

Let us know what you find. Sounds like a good troubleshooting start to a successful fix!

 

[GoToGuy]

Have you ever done a compression check before this? Are you doing dynamic, total cylinder compression, or are you doing a differential compression check? From what you said your doing neither one.
You kinda start with one style but halfway through it changes to something else.