Poor Compression After Rebuild

[Quasimondo]

Before the rebuild, the compression on my engine was 135-135-110-135. For my rebuild I did the following:

Replace stem seals on cylinder head.
Resurface cylinder head.
Resurface block.
Replace 1G pistons with new 2G pistons (w/ new rings)
Re-hone cylinder walls w/ deckplate
Arp studs on block
Cometic head gasket
Copper spray on head gasket

After the break-in period, I checked the compression. 150-130-110-150. I haven't done a leakdown test yet, but this has me completely baffled. I've done everything short of boring out the cylinders and that wasn't done after the shop told me that the cylinders were still within their tolerance for roundness and taper.

Where could I have gone wrong? Feel free to ask any questions to help me get to the bottom of this.

 

[Mackzero]

That is interesting that #3 is significantly lower than the others before and after... Maybe one of your valves are not seating completely on that cyclinder... Did you have the machine shop check that out?? Just a thought.

 

[92awddsm]

I agree. I would have done a valve job on a fresh rebuild. It is probably leaking past the valve seats causing the low compression.

 

[Quasimondo]

The shop had the head tested for leaks after the seals were replaced. They saw no leaks. I forgot to mention that this is the second time the block was assembled. It had to be reassembled after I ate a rod bearing. The compression numbers I had then were 150-150-130-130

 

[92awddsm]

That is still extremely low compression. With the 2g pistons, compression should be around 180-185. Only thing I can really see causing the problem is either too much piston ring end gap or leaking valves. Have you discussed this with the machine shop that did the labor? Thats where I would go first and see what they have to say.

 

[dnhieu]

are you compression testing on a cold motor? a cold motor will give you lower compression numbers.

 

[Zex4g63]

So has the head had a valve job. If not, that's where you went wrong.

 

[defrag010]

It doesn't matter if the machine shop "didn't find any leaks" in your valve/seat contact, there are Tons of other things that can cause bad valve/seat contact. If your gudies are worn out, that can cause leaky valve and it will kill stem seals in no time.
There is Always leakage on anything but a new valve job no matter how many leaks you don't physically find.

Plus, it matches up nicely with your pre-rering compression numbers, so Your problem is Definatley a valve job.

 

[PSI2HI]

Check your lifters and make sure none of the are froze solid.

 

[jltcat]

sounds like a valve seat problem to me.

 

[sleepyvr4]

just because the shop said they checked it for leaks doesn't mean they did a vacuum test on the valves. They check to make sure the head was flat, checked the water jackets for leaks, etc. I was confused as I had my head at the machine shop last week and wondered the same thing when they said it checked out perfectly, but I had 110-120psi on all 4. He said re-cutting the valves and seats would certainly up the compression, and to check the quality of the valve to seat seal would require a vacuum test. My friend ported my head while it was out, and the valve job cost me $150. I can't wait to put it together w/ arp's and cometic headgasket. I'll post my comp when it's together.