Compression on a 4.0L

[GosaGT1988]

What are the spec compressions for the 4.0L? Mine has 243,000 miles on her, she still runs pretty damn good.

[GosaGT1988]

Nevermind, I found it in the Haynes manual. The inline six should have anywhere between 120 to 150 psi with a maximum variation beween cylinders being 30 psi.

[summerinmaine]

Nevermind, I found it in the Haynes manual. The inline six should have anywhere between 120 to 150 psi with a maximum variation beween cylinders being 30 psi.

 

Hmmm . . . my 1987 4.0L has 155# across the board (with no significant variation), with 296K miles. Don't think it's been rebuilt, but who knows?

[james750]

summer in maine. is that camper in ur avatar on ur mj. it looks like a cool setup. what kind of suspension mods did u have to make?

 

 

 

PS sry for the off topic rant

[GosaGT1988]

Did the compression test and got some pretty impressive numbers.

From front cylinder to back I got: 130 - 135 - 140 - 141 - 135 - 137.

I may have a bad spark plug or a bad fuel injector however. My 5th cylinder from the front is getting compression but it's burning extremely rich while the others are burning fine. I took the spark plug out and hooked up a spare spark plug to the ignition wire and tested it externally. It has spark, I got shocked so I know it has it, but I'm not sure of the one that's in there. I'm going to replace the spark plugs anyways with the Champions instead of the AutoLite ones. So basically I have a 5 cylinder engine for right now. I don't mess with fuel injectors whatsoever. I did notice however that my fuel injectors are kind of loose, how can I tighten them?

[jtdesigns]

Nevermind, I found it in the Haynes manual. The inline six should have anywhere between 120 to 150 psi with a maximum variation beween cylinders being 30 psi.

 

Hmmm . . . my 1987 4.0L has 155# across the board (with no significant variation), with 296K miles. Don't think it's been rebuilt, but who knows?

 

That could be gauge variance too. I need to do a comp test on my engine, but I don't know if the gauge will fit. I bought it for my Honda, but isn't the 4.0 plugs standard??

[summerinmaine]

Nevermind, I found it in the Haynes manual. The inline six should have anywhere between 120 to 150 psi with a maximum variation beween cylinders being 30 psi.

 

Hmmm . . . my 1987 4.0L has 155# across the board (with no significant variation), with 296K miles. Don't think it's been rebuilt, but who knows?

 

That could be gauge variance too. I need to do a comp test on my engine, but I don't know if the gauge will fit. I bought it for my Honda, but isn't the 4.0 plugs standard??

 

Could be! Test was done by the mechanic I use for my Tii, so you would expect the gauge to be precise. Accuracy is always a question, however.

[summerinmaine]

summer in maine. is that camper in ur avatar on ur mj. it looks like a cool setup. what kind of suspension mods did u have to make?

 

Thanks. It's a Scamper pop-up camper that weighs @950# (empty but wet).

The mj has a D44, 4" total control lift, skyjacker shocks, and a JKS adjustable track bar. I've got a set of ProComp 31/10.5/15s ready to go on it, but I haven't decided whether to switch to 8" rims.

 

Even with the added weight and wind resistance, it still gets @16mpg on the highway.

 

I use it for desert camping and my pit crew work for the Baja500/1000 races.

 

Here's better pics:

 

 

[Randy in Maine]

Did the compression test and got some pretty impressive numbers.

From front cylinder to back I got: 130 - 135 - 140 - 141 - 135 - 137.

I may have a bad spark plug or a bad fuel injector however. My 5th cylinder from the front is getting compression but it's burning extremely rich while the others are burning fine. I took the spark plug out and hooked up a spare spark plug to the ignition wire and tested it externally. It has spark, I got shocked so I know it has it, but I'm not sure of the one that's in there. I'm going to replace the spark plugs anyways with the Champions instead of the AutoLite ones. So basically I have a 5 cylinder engine for right now. I don't mess with fuel injectors whatsoever. I did notice however that my fuel injectors are kind of loose, how can I tighten them?

 

When you do a compression test, keep in mind that you have to do it to give you the information that you seek.

 

I like to loosen the torque on the plugs and then start the engine and warm it up, not hot just warm.

 

Remove all of the plugs. Put the testing in #1. Put a brick on the accelerator pedal and pretend you are at wide open throttle position.

 

Using a fully charged battery and a remote startter switch, I let the starter go for 10 compression cycles and write down the number. Then I do it again and write down that number and average them out. I call that the "dry test".

 

Then I remove the compression tester and squirt in about a tablespoon of oil from my oil can, and repeat the test for 10 cycles and write down the number for the "wet test". I repeat the testing (without injecting oil in there) for another 10 cycles and write that number down and average the 2 "wet test" numbers.

 

If the wet tests raise the compression by more than 10-15 pounds, the piston rings are "iffy". If it doesn't, and one of them is still low, the valves are "iffy".

 

A leak down tester will confirm where compression is leaking out of the cylinder also.

 

Just my 2 cents....

[jtdesigns]

When you do a compression test, keep in mind that you have to do it to give you the information that you seek.

 

I like to loosen the torque on the plugs and then start the engine and warm it up, not hot just warm.

 

Remove all of the plugs. Put the testing in #1. Put a brick on the accelerator pedal and pretend you are at wide open throttle position.

 

Using a fully charged battery and a remote startter switch, I let the starter go for 10 compression cycles and write down the number. Then I do it again and write down that number and average them out. I call that the "dry test".

 

Then I remove the compression tester and squirt in about a tablespoon of oil from my oil can, and repeat the test for 10 cycles and write down the number for the "wet test". I repeat the testing (without injecting oil in there) for another 10 cycles and write that number down and average the 2 "wet test" numbers.

 

If the wet tests raise the compression by more than 10-15 pounds, the piston rings are "iffy". If it doesn't, and one of them is still low, the valves are "iffy".

 

A leak down tester will confirm where compression is leaking out of the cylinder also.

 

Just my 2 cents....

 

That sounds like a good breakdown,, I never did the two wet test and took the average.. Next time I will do that.

I like using the remote switch too.. I get so frustrated when someone (usually Steph) gets in there and just starts cranking away and never listens for me to say,, "OK" but keeps it spinning...

:headpop:

 

One more thing you should do is disable your fuel pump.. either remove the fuse or wire. you forgot that one....