5.3L Wierd compression after sitting for a year

[Why J?]

I recently picked up an 2001 5.3L and have been getting it ready to drop in my project When I purchased the motor last week I was told it came out of a running truck that was wrecked in the rear. I did a vin check and can see that it was salvaged almost one year ago next month. So it sat for a while drained of oil.

I did a compression check this morning after adding new oil and filter, and pouring about half a quart inside of the valve covers. With it still on the engine stand I also turned it over a few times by hand. Hooked up the starter to a battery and My first compression check showed:

1) 120. 2) 180
3) 210. 4) 210
5) 210. 6) 60
7) 210. 8) 60

So I added a little oil to the cylinders and turned it over a few times and then let it sit for about 4 hours. When I rechecked it I got:

1) 150 2) 175
3) 195 4) 180
5) 190 6) 180
7) 180 8) 170

I checked the compression per the service manual, 4 cranks per cylinder. I recheked it 2 hours later with the same results as above.

Rechecked it again this morning:
1) 180 2) 170
3) 200 4) 180
5) 190 6) 190
7) 180 8) 180

My motor is still covered by the yards warranty and I wont be getting it running for a few more months, but I am at the stage where I am getting parts and having the ECU and wiring harness done. So Im not sure if the low compression was a result of sitting and being dry or I should take this one back and get another one?

I am very new to these motors so please excuse my lack of knowledge. I appreciate any insight you can provide.

[Provience]

Sounds like your thought is correct, the rings were not sealing initially and it should function going forward.


With that in mind, i have very little faith in junkyard engines and personally would tear the thing apart for a through cleaning and either a rering and bearings or complete overhaul depending on cylinder, bearing, and other surface wear.

At least then there would be confidence in the internals and it should last for another decade plus.

[Why J?]

I have pulled the intake, valve covers and the oil pan for inspection. It is exceptionally clean. Cylinder wall still have crosshatch. Im not in a position where I can pull this motor down for a rebuild. It would have been much cheaper it I were.

I just find it odd that 2 cylinders right next to one another both pull the same low numbers. Are cylinder # 6 & 8 the last to get oil in this motor?

Edit: I also wanted to add that the plugs looked perfect, there was no soot on the O2 sensors and according to the vin check the truck just passed emision a few thousand miles before being wrecked.

[92 s dime]

take it back and get a new one or fire it up and see how it runs

for all you know the previous owner was dumping a gallon of coolant into it each day because of a blown head gasket

[takotruckin]

When they sit for a long time the valves and seats corrode a little bit and don't seal real well. It should be fine once run. Alot of times a junkyard motor will misfire for 10-30 seconds on first startup and then run fine after that.

[Why J?]

I ended up trading it yesterday for a motor with 45,000 fewer miles. Also I learned that cold compression checks on these motors is a waste of time.