compression test questions

[bansheenut420]

I haven't been driving my RT lately due to a knocking nosie in the motor. I think its rod knock, but I wanted to do a comp. test just to check things anyway. My first question is, when I did it the motor was cold, is that right? will it affect my readings? I figure it will but how much? Here are the numbers I got: #1=100 #2=85:yuck: #3=90 #4=100. They all seem kinda low to me, but I'm not sure what they are suposed to be. I think it may be time for a complete rebuild. what do you guys think? Thanks in advance :thumb:

[cordes]

I haven't been driving my RT lately due to a knocking nosie in the motor. I think its rod knock, but I wanted to do a comp. test just to check things anyway. My first question is, when I did it the motor was cold, is that right? will it affect my readings? I figure it will but how much? Here are the numbers I got: #1=100 #2=85:yuck: #3=90 #4=100. They all seem kinda low to me, but I'm not sure what they are suposed to be. I think it may be time for a complete rebuild. what do you guys think? Thanks in advance :thumb:

Did you have the TB held open, and the air filter etc. disconected? It should be way higher than that. #2 and #3 are looking pretty rough for sure. I think that 100PSI is the lower servicable limit according to the FSM, but I could be wrong.

[contraption22]

My last compression test was 135-155-155-155 for reference.

[bansheenut420]

Did you have the TB held open, and the air filter etc. disconected? It should be way higher than that. #2 and #3 are looking pretty rough for sure. I think that 100PSI is the lower servicable limit according to the FSM, but I could be wrong.
Yeah I thought those were very low also. I did have the tb open, but didn't have the air cleaner off. Should I unhook the hose from the tb? I also havent run the motor in about two weeks, so maybe try squirting some oil in there? Thanks for the help guys!.:thumb:

[GLHSKEN]

Run the engine to temp... Then do the test. R/t's have forged slugs (worth a shot). Even cold, it should be higher than that...

[mo' parts]

just for reference, of the many tests i have done(prob a couple hundred on 4 motors, lol), cold, warm, hot, stock pistons, forged slugs ect. readings were never more than 5-10psi+/- different. test were done with all plugs out, hep disconnected, closed throttle and wot. i have them all on my garage wall with mileage, temp, ect. just a bit over cautious on engines i build, course i run'em hard:)

sounds like your def in for a full rebuild. could be a gasket at 2&3 or cracked pistons. did it have any blo-by? i had very similar readings once (125,90,90,125) both mahles had hairline cracks, the comp. didnt come up with a shot of oil either.
hope it helps

[bansheenut420]

just for reference, of the many tests i have done(prob a couple hundred on 4 motors, lol), cold, warm, hot, stock pistons, forged slugs ect. readings were never more than 5-10psi+/- different. test were done with all plugs out, hep disconnected, closed throttle and wot. i have them all on my garage wall with mileage, temp, ect. just a bit over cautious on engines i build, course i run'em hard:)

sounds like your def in for a full rebuild. could be a gasket at 2&3 or cracked pistons. did it have any blo-by? i had very similar readings once (125,90,90,125) both mahles had hairline cracks, the comp. didnt come up with a shot of oil either.
hope it helps
There was some blow by, my oil dipstick would lift. I just did the headgasket about three months ago. I knew I needed to rebuild the bottom end, now I guess I will do the whole motor :eyebrows: She was runnin so good before the rod knock, I can't wait to see what she will do with some fresh compression! (and the Lonewolf head, cams and a spearco intercooler:evil:)
Thanks for everyones help and input!:D I am sure I will be needing some more!

[turbovanman²]

100 is low, also, you should disable the fuel system so the other cylinders don't get washed down with fuel as your checking each one. Oil test would be good, if the readings come up alot, then your rings are tired, but you know that by the dipstick tube lifting, :thumb: