First compression test

[ilikefunkymusic]

Need to "turn off" fuel delivery before conducting this test, as I understand it.

No fuse cover on my truck, and not seeing a fuel related fuse listed where i've done my reading. 

Do I have to dive in and unplug the pump relay on the pax side?

I've read that it's best to test on the exhaust plug holes.

Just looking for guidance. This is the first time I'll have done a Comp test, ever.

 

Cheers

[weldingrod]

For some reason

Haynes manuals cover very little of the fuel system other than carburetor and some notes on the pump. You can unplug the fuel pump close to where it attaches to the frame rail.

[datzenmike]

3 hours ago, ilikefunkymusic said:

Need to "turn off" fuel delivery before conducting this test, as I understand it.

No fuse cover on my truck, and not seeing a fuel related fuse listed where i've done my reading. 

Do I have to dive in and unplug the pump relay on the pax side?

I've read that it's best to test on the exhaust plug holes.

Just looking for guidance. This is the first time I'll have done a Comp test, ever.

 

Cheers

 

No need to shut the pump off. The plugs will be out so no intake vacuum to draw any fuel out of the carburetor. If you had an engine with a mechanical fuel pump what would you do?  Don't worry about it. It's common to hold the throttle fully open to let the air in unopposed so even less able to draw in fuel. Take the coil wire(s) off so it doesn't try to start. The exhaust side are simply easier to get at, no other reason.

 

Screw in style testers are the best. Record your numbers on all four. I do on cold engine realizing that is good numbers they will be even better when warmed up. Before wasting time on a compression test make sure that the valve lash is checked. I tight valve can often not close properly causing a false low reading OR if you have a low reading check that cylinder's valve lash. Two adjacent low readings could indicate that the head gasket is failing or failed between them. On any low cylinder(s) try a couple of squirts of engine oil in the plug hole. This will help seal in compression. If the readings jump up then the rings are tired. No change could be bad valve sealing. No compression at all could be bent valve or hole in piston.

[ilikefunkymusic]

Always appreciate the thorough replies Mike.

 

I was finishing up my test as you were typing that.

I wound up disconnecting the positive wires to the coils (whatever the two things are called upstream of the distributor)

Ran the test from the exhaust side, because it sure as hell is a lot easier to access those plugs.

 

I'm at 4,800 feet

Engine warmed up

No throttle applied during testing (I forgot)

 

Dunno cylinder #s so I'll list em as front (1) to back (4)

 

1: 133/134

2: 137/138

3: 136/137

4: 116/117 added a shot of oil and got 128

[Charlie69]

Front (radiator) is #1.

[datzenmike]

2 hours ago, ilikefunkymusic said:

Always appreciate the thorough replies Mike.

 

I was finishing up my test as you were typing that.

I wound up disconnecting the positive wires to the coils (whatever the two things are called upstream of the distributor)

Ran the test from the exhaust side, because it sure as hell is a lot easier to access those plugs.

 

I'm at 4,800 feet

Engine warmed up

No throttle applied during testing (I forgot)

 

Dunno cylinder #s so I'll list em as front (1) to back (4)

 

1: 133/134

2: 137/138

3: 136/137

4: 116/117 added a shot of oil and got 128

 

You can see that 1-3 don't improve much so rings are probably fine.#4 jumped a lot with oil added. 

 

130-140 us down but if not burning oil will run just fine. The Z24 is 8.2 compression anyway so they will read a bit low. Under 120 you'll feel as a chugging while giving the throttle. If running ok I would start saving for a rebuild.

[ilikefunkymusic]

Damn, my factory service manual arrived in the mail today and sure enough on the back cover is the compression pressure:

Standard is 171 PSI

Minimum is 128 PSI

 

Any chance that if the valves were slightly out of spec that it could boost compression a bit after an adjustment?

[datzenmike]

As long as they have the slightest clearance they will be closed and opening up the clearance won't change that.

[ilikefunkymusic]

This thread is veering into rebuild territory now...

 

Could I get away with simply replacing the piston rings to rebuild compression?

Or is the right way to go about this to pull the engine and go through it completely

[datzenmike]

Done properly you would then have one cylinder around 170 and 3 in the 130s. If apart for one, best to do all. Depends what you want. A few gaskets and a quick hone and ring will get you by but it's not going to be near as good as a proper over bore with new fitted pistons, rings, rod and main bearings. Then you have basically a new engine. 

[Stinky]

Did you prop open your butterflys?

 

My mech says that at our elevation, that 165,  on a Volvo, is all that you'll ever get and that 150 is plenty.

 

117....that is low.  But, 100 is not bad, and if I were you, I'd run it for awhile...meaning that although it is low, @ 100, it should run awhile.

 

Edited August 20, 2021 by Stinky

[datzenmike]

I never open the throttle. Have tried it, saw no difference really. May take an extra turn or two. Gives the same reading.

 

A worn timing chain or performance cam will also lower the readings.

[ilikefunkymusic]

Wish I knew what a healthy z24 felt like. This thing won't climb mild to medium extended grades at highway speeds. I wind up at 45-50mph frequently on the interstate.

Anyhow... I have a lot of reading to do regarding the rebuild. Gonna run the thing as is until i've got the knowledge and the dough.

 

I don't want to go crazy on a rebuild. Just do it right and maybe boost the compression slightly over the standard.

So minimum kit would be?

Bore 1mm over?

new pistons (ka direct swap?) and rings

new main bearings

new rod bearings

[datzenmike]

The Z24 is NOT a screamer but it has really good low and mid range torque for pulling a heavy 720 truck. It should easily break the speed limit.

 

 

1mm oversize KA24E pistons and rings will bump you to 9.78 compression. Take the new pistons to the shop so they can accurately measure them and bore to the proper clearances. No piston is exactly 90mm and the piston to cylinder clearance is only 1 to 2 thousandths. New rod and mains bearings and a gasket kit including new valve seals.

[Stinky]

Mine is my DD.  I climb the hill on 32 RD, by Corn Lake in 4th.  Going to WW, on 50, I run the whole way in 5th.  Coming from WW to town, same thing, in 5th

 

Gonna send you a PM w/my # and we'll set up some coffee, or ?

[ilikefunkymusic]

On 8/24/2021 at 10:56 AM, Stinky said:

Mine is my DD.  I climb the hill on 32 RD, by Corn Lake in 4th.  Going to WW, on 50, I run the whole way in 5th.  Coming from WW to town, same thing, in 5th

 

Gonna send you a PM w/my # and we'll set up some coffee, or ?

Just saw this and your pm, gonna shoot ya a text.

[captain720]

Did you ever check you valve lash? I have gone from 97 compression to 140 compression just be doing that on a Z24. Before adjusting valves it had so little compression you could just push it while the truck was in gear, afterwards it had more power and 140 psi ran it for awhile longer that way before eventually swapping it out

[datzenmike]

The exhaust valve is cooled by contact with the valve seat. If the exhaust valve clearance is too tight and it's not tight against the seat it will over heat. Combustion pressure will escape and burn a path across the valve face. A tight intake may randomly backfire through the carburetor and upset the intake vacuum by pushing air back into the intake.