Ralph77

So tomorrow I would like to do a compression test. Poking around it appears:

Make sure the engine is warm. Letting it idle for about 20 minutes should be good enough.

Pull the fuse for the fuel pump relay.

Disable the ignition.

Remove all the plugs.

Crank 3 to 5 revolutions.

So since it is a '00 it has a coil bar. So thinking removing that will disable the ignition.

Thing is I have a an '01 service manual. It says;

Make sure the battery is fully charged.

Make sure the starter is in good shape.

Disable the fuel and ignition system

Remove the spark plugs.

Rotate the motor for 3 revolutions to get the reading.

It does not say anything about doing it with a warm engine. Wondering what the general opinion is on this?

Also after the fuel pump relay is removed but before you remove the coil bar should you run the Jeep till it stalls to get rid of any fuel still left in the line?

BlueRidgeMark

Hmmm. I don't recall hearing anything about warm vs. cold engine, but I guess it makes sense to have it warm. I doubt that you'll need 20 minutes, though.

If your 2000 has an ASD relay (check your PDC) like the earlier models, pulling that will kill both fuel and spark.

Ralph77

Many posts say to do it warm. My friend doesn't get why either. Just got done Googling the ASD cause it does.

cruiser54

Warm.

Bugout4x4

Warm as Cruiser states with good reason. You want all the parts evenly expanded to their normal size when at operating temp. Rings in the grooves and end gaps closed, Pistons expanded to operating temp size and to prevent tilt from skirt wear, Valves to operating diameter against the seats and length hot, Pushrod lengths, Etc.

Pistons and rings expand a lot more than one would figure and can make quite a bit of difference with a compression check. And there can be substantial variances in size cold because despite popular belief they don't all wear the same or run at the same temp and there can be differences in size and tolerances that tighten up more even when at operating temp.

97grand4.0

I think if you pull the ASD relay and the fuel pump relay you should be ok. Works for a 97-98 that way. Think that shuts off injectors fuel pump and ignition and pump ..

BlueRidgeMark

Makes sense.

Bugout4x4

It blew me away when I found out how much a forged piston truly expands compared to a cast piston. Because of this a fresh engine build with forged pistons feels like they are flopping around in the cylinders and way too loose as you put it together. But they expand up to very tight when warm so a longer warm up time is very important to an engine with forged pistons.

This was one of my first lessons about how much all this metal really does expand as it heats up and how much difference it really can make. As I am sure you already know, it is the same reason why marine engine tolerance specs cannot be applied to an automobile engine Mark.

Ralph77

So this weekend I ran some tests on my engine.
Results from the compression test.
Warm. About a 140 degrees on my temp gauge.

1 165
2 170
3 150
4 155
5 165
6 165

So was curious about what they would be stone cold.

1 145
2 150
3 140
4 145
5 150
6 155

All my plugs looked good. Regapped 3 of them a little. I pulled both the ASD and Fuel Pump Relays. When I put it all back together and started it up I got a CEL. Quick look with the scanner showed it to be a P1389. No ASD Relay Output Voltage. My buddy was hanging out with me and Googled it real quick. So he is seeing posts about people having the P1389 and the Jeep not running. He's like but it is running. I thought it was funny. Maybe you had to be there.

97grand4.0

Of course it set the code; you cranked it with the relays out. Now it runs because they're in, the code will go away on it's own or you can clear it. Clearing it will also reset the obd monitors though.

Ralph77

As soon as I knew what the code was I knew what was going on. I just thought it was funny when my friend started Googling it on his phone and saw post that said things like:

P1389 and Jeep wont start and Truck wouldnt start then threw a P1389. Not knowing it was ASD related yet.

97grand4.0

pfft ROFL yeah

Bugout4x4

Great info Ralph! 20 lbs difference on 1 and 2 and ten different on the rest between hot and cold. did you happen to do 1 and 2 first when it was warm?

Ralph77

Yes. Went right down the line from 1 to 6 when doing it both times.

Bugout4x4

Looks like it started to cool off and the rings shrunk by the time you got to number 3 and the remaining. And unless you started it back up and reheated it, this would make sense with the 20 lbs difference in the first two then only 10 after that going back.

Those ring end gaps and groove tolerances make a lot of difference.

Well done Sir, Thank you and great to see you take the extra effort to experiment with this and do it twice.