I know the basics of a compression test of just taking the plug out and threading the tester in, but have never done one myself (had mechanics work on my truck until a few years ago when I took an interest into doing engine work myself). I have a few questions about it that I couldn't find searching here. Do you need to get the engine up to operating temp first or can it be done with a cold engine? Pros/cons or either? Online I read where someone said that you have to take all spark plugs out before doing it and to me that sounds like a lie. Do I pull the fuel pump fuse/relay and also disconnect the ignition plug connector deal? I kind of thought there might be a DIY on this but didn't find one. Maybe there should be one for people like me? Thanks guys

 

Doing it on a hot engine gives you more accurate results.... but..... (we will get to that.)

taking out all the plugs helps the engine spin easier. (and also makes it quicker to change from cylinder to cylinder. If you are doing a 'hot' test, this is important)

Hot vs. Cold: Cold is easier, hot is more accurate. Although, hot is hot..... and pulling the plugs and such on a hot motor really isn't much fun. I don't think the difference between doing hot, and doing it cold is going to be more than 5%.... I generally do them on a 'warm' engine. (let it cool some before sticking my hands in there to yank plugs.)

In any event.... warm the engine or not, your choice.

Yank ALL the plugs.

Disconnect the electrical connector from the coil. (the power feed, and ground wire. this way, the coil simply does not fire at all.)

Pull fuel pump relay.

Prop throttle wide open.

Start with cylinder number one. Screw the gauge in, crank the engine five or six times, read the gauge. Write down the number.

Repeat that process for all cylinders, then, post them here. I wanna see what they look like.

According to chrysler, anything over 100 pounds is "acceptable", all cylinder should be within 20% of each other as well. Anything over 150 is pretty good. over 160 is outstanding. Anything below 100 means it's rebuild time. Even if it is only one cylinder.

 

thanks hey you, I'll try to get on it tonight or tomorrow night. Sounds as easy as I was thinking it was going to be, I think I'll do it on a warm engine as you said you do and I'll report the numbers.

 

you may want to get an oiling can and some oil and do a wet test after you do the dry test. to do a wet test you squirt a bit of oil in the cylinder before you do a compression. it is good to compare dry and wet readings. the wet readings should always be higher but if the rings are in pretty good shape then there should not be much of a increase in compression.

 

Thanks. Forgot that tidbit. This is a VERY good idea. Do the dry test first. then, go back around and do the wet test. Significant jump in compression means the rings are shot.

 

I feel like i'm going to have pretty good numbers as my engine runs pretty solid, no oil burning or coolant issues. But numbers will bring out the truth I guess.

 

New sticky thread worthy.

 

At least a DIY.....

 

maybe an ATTABOY.

 

Well I finally got around to checking the compression in the ol girl. Although i didn't get to do the wet test with the oil as I could not find my oil can to squirt some in there. But the numbers are as follows:

1 - 124
2 - 111
3 - 85
4 - 105
5 - 110
6 - 130
7 - 121
8 - 85

Two of them are significantly lower than the others which was a bit disheartening, but expected I guess for how old it is. If I can find my oil can or buy a new one, I'll retest those cylinders, or all of them. But for now this is what I got! What do you think?