3.8L cold compression test on engine stand - Jeep Wrangler Forum
Jeep Wrangler Forum

Go Back   Jeep Wrangler Forum > JK Jeep Wrangler Forum > JK Tech Forum

Join Wrangler Forum Today


Reply
 
Thread Tools

Please support our sponsors and let them know you heard about them on WranglerForum.com
Old 05-17-2017, 11:26 PM
Thread Starter
  #1
Jeeper
 
nueveibarra's Avatar
 
Join Date: Aug 2015
Location: Menifee, CA
Posts: 15
3.8L cold compression test on engine stand

Hey everyone. So I know its pretty non-standard to do a compression test on an engine stand. However, I didnt feel like installing a possible bad engine since it has 75k miles and was from a new source. I purchased the engine from a reputable jeep shop and the ad claimed their compressions numbers as 130-140 across the board.

I know I cant expect full PSi on each cylinder from a cold engine, but my question is this; If I do a compression test while an engine is on the stand and crank it by hand, should the numbers stay consistently low? or should they be all over the place?

The numbers I got were as follows with dry and wet values respectively. 1=110/150, 2=40/60, 3=30/30, 4=70/80, 5=60/70, 6=50/55

What strikes me as weird is the 1st cylinder. If all the numbers were low, I would definitely attribute that to the cold engine test. But that first cylinder is the curve ball. Havent done anything aside from taking off the valve covers and intake manifold. But before I invest more money, I would prefer to know what im getting into. Thanks everyone.

__________________
2012 JKU Rubi
2.5" RockKrawler Suspension
33" KO2
nueveibarra is offline   Quote
Old 05-18-2017, 09:49 AM   #2
Jeeper
 
SoK66's Avatar
 
Join Date: Dec 2014
Location: Colorado
Posts: 820
3.8L cold compression test on engine stand

I would disregard those numbers. The only valid static test you can perform in that situation would be a cylinder leak down test, where you set #1 at TDC and put compressed air into the cylinder via the plug hole. The leakdown tool will have a manifold with gauges. Depending upon the engine's compression ratio you'd asses the percentage loss via the tools gauges. You'd then set #2 at TDC and repeat, etc. for all 6 cylinders.



You are wise to check this before installing the engine. On 3.8L high oil consumers the oil is either going past the rings, or past worn valve stem seals and down the valve guides. If I can find it there's a You Tube video on the procedure that shows setup, and how to determine cause(s) of leakage.

__________________
Jeeps: Where my 401(k) went to die!
SoK66 is offline   Quote
Old 05-18-2017, 10:15 AM   #3
Jeeper
 
SoK66's Avatar
 
Join Date: Dec 2014
Location: Colorado
Posts: 820
Here you go. You can do this very easily on your engine stand and although the engine iscold you'll get a good idea if what you have is sound or scrap. You can robably rent a tester from the Zone, etc.

https://www.youtube.com/watch?v=PqffrhIQL-M
__________________
Jeeps: Where my 401(k) went to die!
SoK66 is offline   Quote
Sponsored Links
Advertisement
 
Old 05-20-2017, 10:38 PM
Thread Starter
  #4
Jeeper
 
nueveibarra's Avatar
 
Join Date: Aug 2015
Location: Menifee, CA
Posts: 15
Well I did the leak down test and discovered a head gasket blown.Thanks for the advice SoK66. Luckily I have the headgasket and head bolts so it shouldnt be too big of an issue pending valves and pistons being kosher. But my old engine still has to come out. I hear that the torque converter has to stay with the transmission, is that the case? And is there any other tips or tricks that may be useful for this engine swap? First time doing this on a jeep.
__________________
2012 JKU Rubi
2.5" RockKrawler Suspension
33" KO2
nueveibarra is offline   Quote
Reply

Tags
3.8 engine , compression , rebuild

Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off






All times are GMT -5. The time now is 09:36 AM.


Powered by vBulletin® Version 3.8.8
Copyright ©2000 - 2020, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.1
User Alert System provided by Advanced User Tagging v3.1.0 (Pro) - vBulletin Mods & Addons Copyright © 2020 DragonByte Technologies Ltd.
vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2020 DragonByte Technologies Ltd.

Jeep®, Wrangler, Liberty, Wagoneer, Cherokee, Gladiator, Mopar and Grand Cherokee are copyrighted and trademarked to FCA US LLC.
Wranglerforum.com is not in any way associated with FCA US LLC.