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just for ***** and giggles since my truck went from 382 to 317 on the dyno the other day i did a compression test on all the cylinders..the motor was warm and i unplugged the coil packs and open the throttle and crank it several revolutions like ive always done on my race cars...

the motor is a 6.0 LQ4 with p&p 241 ls1 heads with stock 6.0 head gaskets

#1 175 psi #2 150 psi

#3 164 psi #4 140 psi

#5 180 psi #6 170 psi

#7 155 psi #8 115 psi

 

Ouch..Maybe try a Cyl. leakdown test before ya do anything too drastic! That Really Sux though dude, hope it works out for ya!

 

i think im gonna go ahead and pull it and build a 370..

 

put about a tablespoon of oil in there and see if it changes it. If it does it's the rings

 

Pour a very small amount of oil into the cyl and see if it gets better. If it does you could have 2 problems. Cyl running very rich or bad rings.

 

Leakdown..

 

What was the needle doing when you were turning the engine over? There's about 8 different things you can find out just by that alone. I also second the leak down test. If you have air coming out the tail pipe then you have a burned valve. If it comes out the intake, it's the intake valve. If it comes out the dip stick tube, rings. If it comes out the adjacent spark plug hole, head gasket.

BTW it's not just cylinder 8 you should be worrying about, two cylinder side by side should not have more than a 15% difference from each other, I'd do some tests on #4 and #7 as well.

 

im pulling it out today and my other half said i can build a 370..took alot of talkin for her to let me spend the $$$

 

Good info!

Can you expand on what you were talking about when watching the needle?

By the way, having never done a compression test, all you do is unplug all the coil packs first, then take one plug out at a time, each time thredding your gauge down into the hole, and hitting the key for it to turn over a few times?

Won't it flood by the time your done?

Thanks!

Wish I had enough money to do a shortblock rebuild (and a new clutch and a tranny rebuild, and a 9", and a torque arm and lower control arms, and a lowering kit, and wheels and tires... ).

 

" Won't it flood by the time your done?"

unplug the fuel injector pack, 1 per side

 

i disconnected both fuel pumps...im pullin the head off now

 

Is there a fuse that you can pull to disable all fuel pumps? If so, where/which one(s)?

 

I'm sorry, forget what I said about the needle, that's referring to a vacuum test. It was late when I posted that.

CRANKING COMPRESSION TEST PROCEDURE:

1. Make sure engine is at operating temperature.
2. Disable fuel or ignition system. Check what the manufacturer recommends for the proper procedure, it does vary from manufacturer. (I usually pull the fuel pump fuse)
3. Remove spark plugs.
4. Thread the compression tester into the spark plug hole of the cylinder being tested.
5. Block the throttle plate open.
6. Crank the engine at least four compression strokes. Crank the same number of strokes for each cylinder.
7. Record readings.

I'm willing to bet that it's either your valves or rings that are bad as a blown head gasket would give to adjacent cylinders low but equal readings.

 

This is on a chevy truck and i have a fuel pump relay

 

there is an inj fuse one for each bank they kill the coil packs and the injectors.

 

 

****, it's got to be hard to count the compression strokes...

About how many seconds would that be on an LS1?

 

sorry to cloud this thread with my own questions but I recently did a compression test on my ls1. I didnt block the TB open and the engine was cold. How bad would that effect the results? Would they read higher or lower?

 

A cold engine might be slightly lower. I never worry about the T/B and just crank the engine until the needle does not increase with a compression stroke. I usually have to crank it 5-7 compression strokes. I figure having the T/B open might save you one stroke or so. No biggie.

Remove the Fuel pump relay and you will be safe to perform the compression test.

Or..............You can do what I accidently did and have the engine start on you while a frined is holding the compression gauge. I thought he was gonna **** him self. he didn't find it as humerous as I did.

Re'