Need a little help with compression test diagnosis

[Z28chris]

Hey guys, I have a 2001 Tahoe LT with the 5.3 L Vortec LM7 engine ("T" VIN code) and about 150000 miles. Two weeks ago the engine started misfiring with the P0308 code. I also heard a loud ticking from the engine bay, so obviously I supected, that something with the lifters has gone bad. Also the ticking came from cylinder 8.
After taking off the valve cover and fiddling with the rockers, I saw, that the second rocker from the back was moving about a 1/4 inch up and down. So I'm guessing the lifter for this valve has collapsed.
Before taking off the cylinder head I wanted to check the compression and here are the results:

Cylinder	dry	wet	difference dry to wet
1	130 psi	135 psi	5 psi
3	137	145	8
5	132	137	5
7	131	140	9
2	130	150	20
4	115	135	20
6	127	150	23
8	125	137	12

This is the first time, I checked the compression on an engine, so I wanted to ask you guys for help. Do you think I should replace the cylinder heads, according to these numbers?
Since I already have to take them off, I wouldn't want to reuse them, if they were bad. Of course I still have to check them, when I have taken them off, but I thought the compression test would be a great help here.
From what my Haynes repair manual tells me (minimum compression pressure 100 psi; maximum variation between cylinders 25 % from the highes reading), I would be guessing, that these values should be OK-ish. The lowest value is 115 psi for cylinder 4 (so 15 psi above the minimal pressure) and the variation from this to the highest reading would be about 16 % (22 psi difference from the 137 psi reading of cylinder 3).

What do you guys think? Would you recommend getting new cylinder heads, by looking at these numbers? Or would you rather save the money and reuse the old ones?

 

[OR VietVet]

Welcome to your first post on the forum.

I am not a machinist and if I take heads off, they get checked/tested by a trusted machinist that I would have already located. The heads off, IMO, always means a resurface and valves inspection and guides/seals refreshment and check heads for cracks. I would also deal with new lifters and while I have seen some here say, "the cam looked ok", I personally replace it. New lifters and new cam. But I love spending other people's money.

When you did the compression test, did you have all plugs removed? Did you get info on how to do the test, dry and wet?

 

[Z28chris]

Thank you

Yes I removed all the plugs first.
I got info mainly from my Haynes repair manual and from youtube. I had to test on cold engine, because I already had removed the coil packs and valve covers (and I did't want to put them back on, just for the test).
For the dry test I did 5 cycles for each cylinder. Same for the wet test, but with a few sprays of WD40 into the combustion chamber. I think engine oil might be more ideal and the usual choice. But the small WD40 can with the long nozzle was just way more practical. And I figured, that might still be better, than not doing a wet test at all.

 

[rockola1971]

Compression test looks ok. Nothing to worry about. Why would you replace the heads? The heads are only a small part of building compression. Compression comes from your rings, piston and cylinder (walls). Your valves in the head need to seal of course but usually when you have a valve sealing problem your compression numbers drop like a rock toward zero. Your numbers improved when the rings got wet which is normal. If you had seriously low readings that fell out of the min spec but not zero then that would mean you have worn/stuck rings. And you ALWAYS resurface a head once it is pulled, especially an aluminium one.

 

[OR VietVet]

I have never used anything but oil for a wet test. Get a little squirt can and squirt in the hole.

 

[Z28chris]

A guy that runs a shop nearby told me, that the heads would need to be replaced.
OK, so the numbers should be fine, nothing to worry about. But I'll get the heads checked and resurfaced, before reinstalling it.
I'll also think about the new cam, as OR VietVet mentioned. And I think I'll be replacing the lifters + trays, while I'm at it. It's not that much money after all. And if it saves me from doing it all over again in a few months/years, it would be money well spent imo.
And next time I'll use a squirt can with oil

 

[strutaeng]

I would recommend you also do a leak-down test to see where you are losing air pressure: valves, piston rings or head gasket(not likely.)

You can take the heads to a machine shop to get the valve stem seals replaced and checkup if there's no issues with them. It shouldn't be a huge cost for that.

If you are losing a lot of pressure through the rings, then maybe a different approach. You can reconsider then.

 

[nonickatall]

I think there is something wrong with the diagnosis here.

If you have code p0308 that says misfire on cylinder 8, then you should be wondering where the misfire is coming from.

These can come from the injection, of the ignition, or the compression.

The compression is measured and okay, sofrom my point of view this cause is already gone.

So check ignition and injection.

There may be an additional ticking there, of course, but that should be a separate issue, as long as the compression is right.

I would first change the spark plugs and swap the ignition coils of cylinder 8 and another cylinder.

Then I would check whether the plug on the injector is seated correctly, I had the problem with missfire, because it had come loose.

If the code stays, I would just switch injectors of two cylinders with each other and see if the misfires switch to another cylinder.

And for the ticking:
Is the oil pressure shown on the gauge ok?
If not, swap the oil pump o-ring.

If its ok, I would just put a liter of diesel in the engine, drive 20 to 30 miles and do an oil change.

That removed my liftertick.

And concerning resurface of cylinder heads,. You definitely don’t do a resurface every time you disassemble the cylinder heads, because this wears off material and you then actually have to use a thicker cylinder head gasket. You only do a resurface when the surface is no longer straight, which you normally measure with a straightedge before.

Good luck...

 

[swathdiver]

A nice 3 angle valve job is good for horsepower and torque with no downsides other than a little labor cost.

Use OE lifter trays unless you are getting them from Summit. There's is ok.

 

[Z28chris]

...

If you have code p0308 that says misfire on cylinder 8, then you should be wondering where the misfire is coming from.

These can come from the injection, of the ignition, or the compression.
...

I maybe should have mentioned this in my first post. I had checked the ignition and fuel injection before. Basically I did all what you suggested, except switching the injectors. As far as I could tell, ignition and injection weren't the problem.

But I'll take into consideration, what you said about the resurfacing being necessery or not. I will take the heads to a shop, to get them looked at and resurfaced if need be. Maybe they can also hook me up with the 3 angle valve job swathdiver suggested.