Low compression on 5.3 swap

[hewittca]

Hi everyone. I've been working on swapping a 2004 5.3 into my truck and I am finally at the point where I can power everything up and start to check electrical components. With that, I decided to test the starter, turn the motor over, and do a compression test. This motor supposedly ran well before the previous owner sold it to me, but I never heard it run. It sat for two years before I could get to this point of turning it over and it is still completely stock. The inside of the motor looked great under the oil pan and valve covers. When I crank it, I get 30 psi oil pressure. When doing the compression test I have all spark plugs out, throttle body held open, and PCM keyed power disconnected. My compression results are kind of all over the place, ranging from 60 psi up to 170 psi. On cylinder #2, after squirting oil in the cylinder the compression jumped up to 150 psi.

I'm wondering what you all would do at this point. Do you think this may just be a symptom of the motor sitting for so long? Or is it a sign that the engine needs more attention? I do not have the fuel system figured out yet so I can't fire up the engine. I'm somewhat curious to get the engine started and see if these compression values improve after the motor is fully lubed up again.

[RustyPile]

If adding oil to a cylinder with low compression, raises the compression, that's a sure sign of worn or dry rings..If the compression doesn't go higher, you probably have burned or rusty valves and or valve seats.. Did you add oil to #8 ??? If so, what happened there??? Have you bore scoped the cylinders?? Doing so will reveal wear progression and perhaps the accumulation of rust on the walls.

You have two routes here.. Finish the install, get the engine running, and drive the truck a few hundred miles and hope the condition gets better... Or, turn the engine over to a reputable rebuilder..

[EDIT] The compression tolerance between cylinders is 20%.. That does not mean +/- 20%, it means 20% between the highest and lowest..

[hewittca]

Ah yes, I forgot to mention cylinder 8. For that one I could not get the fitting I had in the spark plug hole due to low clearance against the firewall. I tried to use a different adapter fitting and still couldn't get it tight enough, so I don't fully trust that reading.

[Jason Banks]

I would do exactly what you say. Get it running and see how it goes. As painful as it sounds, it could always be pulled out later and fixed.

I'd say it needs to be good and lubed up....oil on the cylinder walls, to get a good reading. You probably won't get it coated well just by cranking with the starter. Some fogging oil might help - https://www.amazon.com/STA-BIL-Rust-...dp/B000H7CKAY/

You might get a borescope in the cylinders and see how the walls and valves look. Here is an inexpensive one that has multiple cameras on the tip so you can see different angles. There are many other on the market, this is just the first one I came across:
https://www.amazon.com/VEVOR-Triple-.../dp/B0BVMFYQNS

[AussieinNC]

IMHO,

I would pull the heads and check the bores...can almost guarantee you will find surface rust in the low result cylinders.

I would also check the heads for cracks and or corrosion.

Even if the engine was "running" before purchase, sitting around with the intake and or exhaust valves open on certain cylinders, surface rust is a certainty.

Again, if it was mine, the heads would have been pulled before it even went into the chassis....IMHO....

[hewittca]

Thanks all for the insight. I definitely plan to, at some point, go back through the motor in more detail and freshen it up. I was just hoping to start with a baseline first and get the motor running since this is my first swap. I think for now I will keep working on all the auxiliary stuff that needs to be completed regardless of whether or not I pull the motor. That will give me some time to ponder what to do next with the engine. I will continue this thread with any updates so it's not left open ended.

[Joyridin]

You may not like doing this, but we have had the rings stick quite a few times on LS/LT engines. My suggestion is to squirt some transmission fluid in the cylinders. Crank the engine over a few times to make sure it is all around the cylinder, and let it sit about 2 days. This usually frees them up.