Compression test results : 258

Just borrowed my friends compression tester for the hell of it. My 258 runs great, has plenty of power (for me), one hell of a knock. So I don't mean to be a complainer, but isn't this bad?

1. 95
2. 93
3. 95
4. 106
5. 85
6. 90

 

I check mine a while back. All of mine were around 115. The worst was 110, and the best was about 120. Not too sure, but yours does sound a little low to me.

 

Doesn't sound good. Generally don't want them to differ by more than 10 psi or so. #5 is getting weak. But hey, runs good... Does it burn any amount of oil?

 

nope, burns no oil. well maybe less then a half quart every oil change.

 

The jeep Spend-o-meter is on "time to start saving". If you go in now and take care of the low readings, it's a lot cheaper. Did you do the test to determine if it is rings or valves? I've been known to do a re-ring in the driveway. You've got a while, but it's time to start planning.

 

Not terrible, but not great. Add them up, and the average is 94, and the range is -10%/+12%. Marginal. You'd like to see them be even within +/- 10%, and closer is better. Probably the rings are getting worn - valves not sealing usually shows as one cylinder way down. Put a teaspoon of oil in each cylinder and repeat the test. If the pressures come up and are more even, that indicates worn rings.

Like Dave mentioned, you can replace the rings with the engine in the car. However, if it doesn't use oil, you could just drive it.

 

"One hell of a knock" - what? :? if you have a rod knock, you need to worry about that and not the compression. If it's a rod knock, it's rebuild time. No way out of that. What's your oil pressure?

 

I missed the rod knock, good catch, Timgr. Strider, you may be better off going in sooner than later. A rod thru the block is a very depressing experience, ask me how I know.

 

Nah, I think its had that knock for 10 years. at least since I owned it. I thought that was a 258 characteristic, no?? oil pressure is about 50 or 60 at higher rpms, maybe 20 or so when really warm idleing.

i do have a 232 that runs great sitting in the back incase this one blows. i should probably give it a compression test. should i change the oil before i crank it a few times? my experience is the oil dosn't even start pumping until its been running for a few seconds.

yea, i know, i should have changed it before i let it sit....but don't tell me you never did it!

 

Remember, when doing a compression test correctly, all the plugs should be removed, a battery charger be on the battery, and the throttle depressed fully before each cylinder checking. The throttle being closed creates an air restriction thus reducing the cranking compression. A battery wearing down also will affect your test considerably. If your knock has been around that long, probably piston slap or valve train noise. I think your compression is acceptable, but if you missed any of the steps listed above, I would retest. That way you are making an informed decision using accurate information.

 

i have a question, i didnt do anything that mrhp said to do i just pulled 1 plug at a time and cranked over the engine. my 258 was 160-150 high and low. does that mean my test is wrong? its a 73 ,if those readings are correct do you think someone has rebuilt it somewhere down the line?

 

You're supposed to pull all the plugs out and do as MRHP said. Last time I checked mine the readings were between 165 and 175 for 3 compression strokes with each cylinder pumping 120 on the first stroke.

 

I had a bent rod that made my #5 read -0- once. Jeep wasn't running so great because of it (oh really!). Replaced it, and the valve guide seals, and now the jeep runs great. Used to smoke a lot because of the worn seals when starting cold.

 

Probably a bent push rod, not a bent connecting rod. Big difference.

Also, one man's knock is another man's tap. A knock is bad; a tap you can live with. Tap-tap-tap is usually valve train noise. Fuel pumps can also make a knock.