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How to test for compression??

G

gone postal

NAXJA Forum User
Location
St. Augustine, FL
Alright, just bought a gauge so that I can test out my '91s 4.0. I'm going to pull all plugs and tie the throttle body plate wide open. Do I need to do anything w/ the ignition or fuel? I heard that you can ground out the coil wire going to the distributer, but didn't know if that was necessary. Also, for fuel, is it okay that it's spraying in the cylinder while you're cranking? Not sure if it'll get in teh gauge and affect readings at all.
 
Well, IMO, it won't affect the gauge and you won't be cranking it that long to matter. If you want, you can pull the fuel pump relay.

Test each cylinder. If one reads low, squirt a little oil into it and try again. If there's a big jump, that indicates worn rings (or you squirted too much oil).
 
no need to tie the throttle body open. Remove ALL spark plugs, put the tester in, and crank. Record readings. Industry standard is all cylinders withing approx 20% of each other.


The squirting A LITTLE oil is a good tool as well.
 
Alright, what about the spark? Just leave the plug wire laying off to the side okay? It won't arc out of the boot and damage anything will it? Is there a fuse or wire I could pull for that too?
 
just leave them off to the side, or disconnect them at the source. If you disconnect them at the source, be sure they don't arc to any nearby fuel lines (had this happen on a mminivan once, terrifying)
 
Dustin_Z said:
no need to tie the throttle body open. Remove ALL spark plugs, put the tester in, and crank. Record readings. Industry standard is all cylinders withing approx 20% of each other.


The squirting A LITTLE oil is a good tool as well.
Click to expand...
Yes, you do need to block the throttle open. I have seen this make a HUGE difference. The reason that you need to do this is that the engine needs to be able to breathe in before it can breathe out. I know that renix (87-90) if you block open the throttle it cuts off spark and fuel, but I don't know about HO (91+) maybe an HO expert can chime in here.
 
88 Wagonman said:
Yes, you do need to block the throttle open. I have seen this make a HUGE difference. The reason that you need to do this is that the engine needs to be able to breathe in before it can breathe out. I know that renix (87-90) if you block open the throttle it cuts off spark and fuel, but I don't know about HO (91+) maybe an HO expert can chime in here.
Click to expand...


Oh, thanks for the tech info! I'm still new to Jeeps, but am very experienced in vehicles/farm machinery in general. I'm thinking off general theory.
 
Compression testing needs to be done at FULL throttle.
 
Okay, just did the test. It is a '91 HO w/ 166k miles. It has a knock to it that I haven't been able to diagnose - even w/ multiple videos posted on this site. It has solid oil pressure, but figured I'd do this too since it's relatively easy.

I just read online that the vehicle should be at normal operating temp when the test is performed - ooops!! It last ran a few hours ago, but has been sitting in the sun in low 90* weather. I guess the colder thicker oil will skew my results some?!?

Anyway, here are the results: (two tests - no oil squirted for either)

Cyl 1 - 145 / 137
Cyl 2 - 139 / 139
Cyl 3 - 145 / 146
Cyl 4 - 144 / 144
Cyl 5 - 136 / 135
Cyl 6 - 139 / 144

What is compression suppose to be? I looked in my Chilton's, but could only find directions on how to test (duh, should've checked that earlier), but not what it should be - just that it should be within 20%. I think my numbers look great, but that's only b/c they're so close to each other. If it's suppose to be 190 or something, than I guess I'm f*ed....
 
gone postal said:
Okay, just did the test. It is a '91 HO w/ 166k miles. It has a knock to it that I haven't been able to diagnose - even w/ multiple videos posted on this site. It has solid oil pressure, but figured I'd do this too since it's relatively easy.

I just read online that the vehicle should be at normal operating temp when the test is performed - ooops!! It last ran a few hours ago, but has been sitting in the sun in low 90* weather. I guess the colder thicker oil will skew my results some?!?

Anyway, here are the results: (two tests - no oil squirted for either)

Cyl 1 - 145 / 137
Cyl 2 - 139 / 139
Cyl 3 - 145 / 146
Cyl 4 - 144 / 144
Cyl 5 - 136 / 135
Cyl 6 - 139 / 144

What is compression suppose to be? I looked in my Chilton's, but could only find directions on how to test (duh, should've checked that earlier), but not what it should be - just that it should be within 20%. I think my numbers look great, but that's only b/c they're so close to each other. If it's suppose to be 190 or something, than I guess I'm f*ed....
Click to expand...
Your not f*ed. The numbers look lower than new, but look fine for 166k.
 
I dont think you'll ever find an actual compression test figure to compare against. It really depends on a lot of different things and its different for every engine. Even if you know the compression ratio which corresponds to a cylinder pressure for one engine, it could be different on another engine with a similar compression ratio. The only thing you need to worry about is if its within 20% of each other. Sometimes the front cylinder will wear more than the rest due to the colder coolant that comes in around that cylinder from the thermostat. So what the numbers should be is within 20% of each other.
 
I don't have a 91 FSM but the 88 FSM (RENIX) says 120-150 PSI with a variation of no more than 30PSI (hmmm.. 20% of max). You look fine to me.
 
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