compression test

[Craig 94 TA GT]

I know that compression test questions have been beaten to death, but I have a few questions, that might be unique for my situation.
My only goal is to look for variances between cylinders 1,3,5,7 - to find a possible blown head gasket. I'm going to remove the PCM fuse and open up the throttle.

Firstly, do I still need to warm up the engine? If so, would I have to remove all 8 spark plugs? It seems really hard - to remove all 8 when the motor is hot, with the super hot headers, heads, plugs, etc.

Secondly, what kind of compression would I see for a cylinder that has a blown head gasket to a bolt hole, or to another cylinder? I'm hoping I can get away with not warming it up, nor removing all 8 plugs just to find one or two low cylinders.
Thanks in advance

[fastTa]

Craig, I would do both a compression check and a leak down test. A leakdown test would likely be the better way to look for a blown head gasket, ring seal, and valve sealing. But there are so many variables like ring gap, piston to cylinder wall clearances, valve seat pressure, etc. that you could show a large amount of leakdown (10% or more) and still show a good compression number (180-190 or more psi). That would indicate the rings are sealing well and could be a headgasket or a valve not sealing. There are also many thing that can cause a valve not to seal adequately such as; worn valve guides, cracked valve stem boss, carbon buildup under the valve, etc. the majority of the leakdown normally occurs past the rings. A leakdown test is a good indicator of where to start. Just like compression checks, a variance in cylinders is what you are hunting for.

You need to remove all 8 spark plugs. That will allow the starter to spin as freely as possible. And yes the throttle blade should be wide open and the motor should be completely warmed up to normal operating temperature if possible. Also remove the radiator cap so the cooling system cannot build pressure. Some builders also believe that you should loosen the rocker arms to make sure the valves are fully seated, but that won't show you real world conditions in my opinion and if your valve lash is properly adjusted anyway, you should have the valves fully seated at TDC.

With the engine fully warmed up you should perform the leakdown test at TDC first. Some guys will do a leakdown test at BDC as well to check for cylinder concentricity. The thrust side of the cylinder wall takes more of a beting down low in the cylinder since that is where the rod is at it's greatest angle on the compression and exhaust strokes. For now I would just worry about doing a TDC leakdown test.

First get yourself a leakdown tester. I think NAPA and Autozone will let you borrow one as long as you give them a deposit to hold. It screws into the plug hold just like a compression gauge does. First, set the air pressure to 10-20 psi and then turn the crank or flexplate by hand until you are at TDC. Then bump the pressure back to 90 psi on the regulator and look for the variance between the 2 gauges. Some will have the 2nd gauge just show a percent.

On an LT1 with a stock longblock and normal wear, I would say anything under 8% is acceptable. Anything over 10-11% indicates a problem.

[Craig 94 TA GT]

Thanks for the reply, Kev.
Hmm...a leak down tester is gonna run me another $70. I already bought the compression tester and used it to test 1 and 3 preliminarily - so I can't return it. But it looks like I'll need to remove the headers in order to get the plugs out quickly after it's been warmed. And since I'm doing that, I might as well change the motor mounts then.

A couple more thoughts. I'm pretty sure that the rings are still in decent shape...no oil consumption at all, and no smoke out of the tail pipes. I'm also not losing any coolant, nor overheating. Oil looks great after changing 4000+ miles. So hopefully, I'm only looking for a blown head gasket to a bolt hole, or an adjacent cylinder. Or possibly a bad valve. Well, whatever would cause a split in BLMs (elevated in the driver's side). It could still be an exhaust leak causing this, but I've already ripped my hair out trying to fix that problem (and think it's fixed, but not sure).

I'll still try to do a leakdown test when I pull stuff apart, but I wonder if a compression check would show any preliminary results that could indicate the problems mentioned above

[fastTa]

Thanks for the reply, Kev.
Hmm...a leak down tester is gonna run me another $70. I already bought the compression tester and used it to test 1 and 3 preliminarily - so I can't return it. But it looks like I'll need to remove the headers in order to get the plugs out quickly after it's been warmed. And since I'm doing that, I might as well change the motor mounts then.

A couple more thoughts. I'm pretty sure that the rings are still in decent shape...no oil consumption at all, and no smoke out of the tail pipes. I'm also not losing any coolant, nor overheating. Oil looks great after changing 4000+ miles. So hopefully, I'm only looking for a blown head gasket to a bolt hole, or an adjacent cylinder. Or possibly a bad valve. Well, whatever would cause a split in BLMs (elevated in the driver's side). It could still be an exhaust leak causing this, but I've already ripped my hair out trying to fix that problem (and think it's fixed, but not sure).

I'll still try to do a leakdown test when I pull stuff apart, but I wonder if a compression check would show any preliminary results that could indicate the problems mentioned above

A compression check can be a good start but it doesn't tell the whole story. It only shows the cylinder's instantaneous pumping/compression capacity and not how well everything seals and holds the pressure. That's why leakdown tests help show what's going on a little better.

For instance, if you were to have a cylinder with a washed out #1 compression ring from running excessively rich, a preliminary compression check might show good results, but then when you perform a leakdown test, it might be a very high and unacceptable percentage.

[Craig 94 TA GT]

Thanks again Kev. Since I already have the compression tester, I'll probably just do that first. If I see a cylinder or 2 way low, then I might as well just pull the head. If I don't, then time for a leakdown test.

It's just hard to imagine doing a compression/leakdown test on these cars -

- Do it when the motor is up to normal op temp: scald your hands
- Remove all 8 plugs after the motor was warm: by the time all 8 plugs are removed, it's no longer warm, unless you can somehow work through scalding metal and remove all 8 that quickly
- insert compression tester hose into plug hole: extremely hard to do without "elasti-arms", or removing the headers

Well, I guess there's a first time for everything

[fastTa]

Thanks again Kev. Since I already have the compression tester, I'll probably just do that first. If I see a cylinder or 2 way low, then I might as well just pull the head. If I don't, then time for a leakdown test.

It's just hard to imagine doing a compression/leakdown test on these cars -

- Do it when the motor is up to normal op temp: scald your hands
- Remove all 8 plugs after the motor was warm: by the time all 8 plugs are removed, it's no longer warm, unless you can somehow work through scalding metal and remove all 8 that quickly
- insert compression tester hose into plug hole: extremely hard to do without "elasti-arms", or removing the headers

Well, I guess there's a first time for everything

Trust me I hear ya. Some might consider it pu$$y, but wearing mechaninc's gloves while working on an LT1 is a must! I don't care what they say ! Screwing the fitting into the LT1's spark plug holes is no doubt a PITA, but hey, we are stuck with that inevability. Do you want me to send you my leakdown tester; it might be cheaper even with shipping.

[Craig 94 TA GT]

Trust me I hear ya. Some might consider it pu$$y, but wearing mechaninc's gloves while working on an LT1 is a must! I don't care what they say ! Screwing the fitting into the LT1's spark plug holes is no doubt a PITA, but hey, we are stuck with that inevability. Do you want me to send you my leakdown tester; it might be cheaper even with shipping.

Thanks Kev but I can just add it to the order when I get the motor mounts from Summit. I'm still trying to figure out though...it takes a while to remove all plugs, so to the motor would be much cooled down by the time it's done. I guess the headers help in plugs removal, but it's still no easy task to remove all 8 plugs within a decent amount of time.