!!!5.7L compression test need advice!!!

[tybic]

Need advice on the dry compression test below.

Here is the history, motor does have 229,000 miles on it.

So I purchased a new MPFI spider due to the old one causing misfires on Cyl 5 & 7.

I also thing the fuel pressure regulator was not working correctly due to the amount of gas on intake when upper was removed and no pressure to release from fuel lines.

Before I started I thought I would go ahead and change the Lower intake manifold gasket since most of it is already off.

Borrowed tools to do compression check and here are the results (Test was a dry test).

 

Cylinder Result Cylinder Result

8 195 7 165

6 110 5 150

4 170 3 175

2 165 1 145

 

Cyl 8 and 6 have a large difference. What does that mean?

Would a lower intake gasket cause this or is a head gasket?

 

I guess what are my next steps?

Never went down to head gasket as of yet.

[grampadirt]

Since you had no response I'll take a shot ......

 

Here are the specs for your engine.Note the factory psi & recommended psi difference between cylinders.Some of the psi values on your engine seem high.#6 might have wear on the rings,valve guides or Did you prop the butterfly wide open when doing the compression test?

 

On a side note I recently checked the compression in my big block...all eight had 150-155,not bad for 180,000 miles.

[8.1HD/ALLISON]

I would squirt a lil oil in cyl 6 and try again. If compression comes up, rings are worn. Could have valve issues, may want to do a cylinder leakdown test. 8 does seem high, possible head gasket between the 2? I would try wet compession and leakdown ne.xt

[LeoS41280]

Manifold gaskets don't affect compression. I would retest it with the butterfly open. Take all the plugs out. Let each cyl hit the comp cycle 5 times. With that many miles things that can cause low comp are head gaskets, the head itself, valve and or seat, lobe worn off the cam, lifter, push rod, and rocker arm.

[tybic]

Thanks Guys.

@ granpadirt I actually have the upper manifold off for the test, planned on replacing the Fuel spider and decided to run a compression check. Then found this.

@ 8.1HD/Allison and LeoS41280 I plan on doing the leak down and wet compression this week and will get back to you for the results.

@ LeoS41280 Just so I can learn how does all the plugs out affect compression, just trying to learn.

 

Thanks I will keep you up to date as I find out more.

[8.1HD/ALLISON]

You want all the plugs out whe doing compression test so there is minimal drag on engine. Also be sure to have a battery charger hooked up on low/ medium setting to be sure a low battery doesnt slow down cranking speed.