What is the lowest compression a 4.0 can run without misfires or codes?
- Thread starter Hawkeye
- Start date
I’m laughing .
He’s going to do that , then us morons are going to post stuff like ,” hmmm, those cylinders with the low numbers don’t have as much compression as the ones with big numbers”
Then Jerry Bransford will post something actually accurate .
The we will post “x2....I was thinking it should be within 5 lbs of that Jerry”
Then we will log out feeling like we really helped.
Run those numbers man.
He’s going to do that , then us morons are going to post stuff like ,” hmmm, those cylinders with the low numbers don’t have as much compression as the ones with big numbers”
Then Jerry Bransford will post something actually accurate .
The we will post “x2....I was thinking it should be within 5 lbs of that Jerry”
Then we will log out feeling like we really helped.
Run those numbers man.
Compression numbers can get pretty low and still cause no issues. Most cheap gauges aren't even that accurate. You could see an even 110 all the way across and have no problems even though 120 is considered low. Normally you are looking for one abnormally low or an adjacent set low. It's pretty common these days for people to simply screw up the test so be sure to read up and do it right. The 4.0 is nothing special, everything you read about compression tests applies pretty well to this engine too. You don't need a 4.0 expert.
My four banger puts out 100 horsepoer @ 2700 RPM on 80 octane gas all day and night long for hours and hours burning 6 gallons per hour @2300 RPM But my engine is required to maintain 70% engine power at 2300 for hours at a time. This is enough rambling on about this. Oh and my engine had 6.3to1 compression.
I had all fluids out of the jeep so put in a few quarts of oil in and then tried doing the test with the engine cold. All spark plugs were removed. I was using an old tester and then another tester from autozone but the numbers were strange. Two cylinders gave high first readings, around 150. Then they read around 30 on subsequent tests. All the other cylinders read 30-60 on all tests. This engine hasn't been ran in probably two months. Bad testers or dried up rings? Or the common denominator...me? Haha no luck today finding a different tester but I'm going to squirt some oil in the cylinders and try again to see if anything changes.
Follow this procedure:
1. Run engine to operating temperature
2. Remove ASD (automatic shutdown) relay.
3. Remove all spark plugs
4. Starting at the front of the engine (cylinder number one) insert gauge
5. While holding the throttle wide open crank engine five or six revolutions (you’ll see the gauge needle bounce). Make note of the pressure.
6. Do this on the remaining 5 cylinders.
7. None should be lower than 100psi or 25% difference between other cylinders.
1. Run engine to operating temperature
2. Remove ASD (automatic shutdown) relay.
3. Remove all spark plugs
4. Starting at the front of the engine (cylinder number one) insert gauge
5. While holding the throttle wide open crank engine five or six revolutions (you’ll see the gauge needle bounce). Make note of the pressure.
6. Do this on the remaining 5 cylinders.
7. None should be lower than 100psi or 25% difference between other cylinders.