Compression test on an engine that's not in a vehicle


#1

I am considering buying an engine that’s sitting on a pallet . Supposedly the engine came out of a wrecked vehicle & has less than 20,000 miles on it . The owner said he would sell it to me for $500.00 with no warranty or $1,000.00 with a 30 day warranty . Needless to say , I don’t want to do all the work to install it & have to take it back out regardless of the price .
I have a heavy duty 1/2 " drill & am considering chucking an extension in it with a socket that will fit the large bolt in the front of the crankshaft & spinning the engine & doing a compression test on it on the pallet . I would likely remove all spark plugs first . It is a V6 engine . The owner doesn’t have a problem with me doing this .
It is a 2005 engine & I don’t know how long it’s been sitting . The owner says a salvage yard closed its doors & auctioned off their inventory & somehow missed selling this engine at the auction . The engine has tags on it telling what it is , year model & mileage .
What do you think about my approach to doing a compression test on it ? Should I do a dry test first & then a wet test ? How would you go about this if at all ? Input will be greatly appreciated .


#2

I would take a chance on the $500 engine. I would not pay twice the price for a 30 day warranty.


#3

I doubt very seriously that a 1/2" drill is going to spin the engine over. It will take a very serious 1/2" air wrench with a lot of grunt behind it or a 3/4" air wrench.

A compression test on a palletized engine can certainly be done and yes, both a dry and wet test should be done.

Ideally, one would like to bolt a transmission and starter to the engine, connect a battery with cables, and run a compression test that way.


#4

Even if the impact wrench could turn the crankshaft a meaningful compression test could not be made. The most likely result from making the effort would be breaking the crankshaft bolt.


#5

The idea with the 1/2" drill is NOT going to work

I wonder if you meant a 1/2" impact wrench . . .

I also don’t think that would work, because the tool is designed to run down bolts, not turn over an engine

I think @ok4450 has a good idea . . .

If you can’t carry out his plan, just buy it for $500

First, you’ll want to make sure the engine isn’t seized, from sitting for who knows how long

A 1/2" breaker bar, with an extension and socket, will allow you to turn over the engine


#6

BTW, what kind of engine is it? The starter on most GM engines bolts directly to the block and it is quite easy to spin them on the bench… or floor.


#7

Aren’t the documented compression test “acceptable” ranges meant for an engine spinning at the RPMs that a starter can deliver?

If the OP uses something else like a drill to drive the engine, unless the RPMs are close to what a starter would provide, it may limit how much can be read into the results.


#8

If you’re unable to perform a compression test on the engine, you might consider doing a leak-down test on the cylinders.

A leak-down test will provide more information on the condition of the engine than a compression test will.

Tester


#9

It’s a 3.7 V6 for a jeep liberty , actually a Chrysler engine . I have a Milwaukee right angle drill that will break your arm . With the plugs out & no compression except 1 cylinder at a time you don’t think the drill will turn it ?
I would think drill speed would be somewhat comparable to starter turning speed . I’ve turned many engines with the plugs in with a breaker bar & socket & never broken a crankshaft bolt , do you think it would be different turning it with a drill ?
Not arguing or being hardheaded just wondering . I read a thread or two about people using an impact gun to do this .


#10

I’d probably first remove the spark plugs and make sure the crankshaft rotates by hand turning it with a ratchet/socket, and without any weird noises. If it passes that test, I’d use compressed air to pressurize the cylinders positioned at tdc on the compression stroke one and by and makes sure they aren’t leaking. Look up “leak down test” to see how this is done.

Before turning the engine at all, even manually, make sure the cylinders are properly oiled.

I think if you try to crank a crated engine to get a compression reading without the proper fixtures to do it, you may end up damaging something which might prove expensive to repair. A leak down test accesses the same information, the only risk is that something dynamic wouldn’t be caught. Like maybe there’s a broken valve spring.


#11

I don’t have a leak-down tester but could possibly rent one from one of the parts stores that rent tools . When performing a leak-down test the instructions say to ensure the engine is warm . I wonder how critical that is ?


#12

A 3 foot extension would be needed to hold a drill that had enough power to turn the crankshaft on an automobile engine against compression and an impact wrench hammers the socket and would likely break the crank bolt while turning it clockwise or stretch the bolt and weaken it and cause it to break when turned counterclockwise to remove it. Maybe one of the more mathematically astute here can calculate the torque required to turn the crankshaft against compression but my best unSWAG is 400+ foot pounds.


#13

If you can’t use the engines own starter, you could TRY your 1/2 drill idea. With the plugs out it MIGHT be able to turn the engine over. You could check the oil pressure too…Be sure you spin it in the right direction…


#14
When performing a leak-down test the instructions say to ensure the engine is warm . I wonder how critical that is ?

Not critical. You’re looking for differences among the cylinders.


#15

@Sloepoke

Thank you for telling us what engine you’re thinking of buying

Why do you want to buy this engine?

Please don’t take this the wrong way, but that 3.7 liter V6 in the Liberty is NOT known for being a good engine. Major top end problems, which cause hard to diagnose misfires. Hard to diagnose for some guys, anyways

Are you buying it as a spare?

Is the engine in your own vehicle already toast?


#16

You’re right about head problems with the 3.7 . They are notorious for dropping valve seats & I have read the V8 4.7 has the same problem . I have 2 liberty’s & one has high mileage & valve problems & the other 1 has had the heads rebuilt & now has a rod knocking .
Both have excellent bodies & interiors . I have located a wrecked liberty with low mileage that I’m buying & plan to install the engine , transmission & transfer case into the 1 I have with high mileage . The wrecked 1 still runs , it was hit behind the drivers door .
I am considering the 1 on a pallet for the other liberty . I bought both these liberty’s as projects & neither are or have been used as my personal vehicle . I am retired & enjoy doing things like this as a hobby . I have people wanting to buy them when they’re running . I was looking for an engine for the 1st liberty when I ran across the 2nd one , the price was right so I bought it too . Sucker for punishment I guess .
I think I’m going to try the drill on the one that has the rod knocking & see what happens .


#17

@Sloepoke

Well, that certainly puts things in perspective

In that case, I’d just go ahead and buy the pallet engine for $500, as long as it isn’t seized

Install it, and if all turns out well, sell quickly to those “people wanting to buy them when they’re running.”

Good luck, and please keep us in the loop!

This sounds pretty exciting, by the way

:sweat_smile:


#18

I have fun with it & I guess in the long run that’s all that matters . Anyone interested in a nice Kia Sportage ? I know where one is cheep , just needs an engine . lol


#19

A caution about model year selection, the engines look alike but the tone wheel on the crankshaft changed for 2004. A 2004 and later engine won’t work in a 2002-2003 Liberty and the early engines won’t work in 2004 and later vehicles.


#20

A leak down test, like Tester suggested, is exactly how aircraft engines have their compression measured. Kinda hard to do it with a spinning Hartzell prop right next to you.

A shop will use a differential compression pressure gauge set (normally) at 80 psi. A second gauge indicates the pressure held in the cylinder. Most A&Ps and I/As insist on 60 psi to “pass” a cylinder (Airplane cylinders are removable from an engine).

Most nerve wracking test for an owner. Been there, done that.