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Old 08-04-2019, 07:13 PM   #1
mbiver
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Default Compression difference in a 345?

Just did a compression test on my 345 and was wondering if I have too big of a difference between the cylinders or if any of the numbers indicate serious wear/damage? It was the first time I’ve ever done one so when I did the “dry” test I didn’t crank it long enough to get max pressure. Just for 1 second. The numbers below are for the “wet” test where I did a couple of squirts of 10-30 with an oil can directly into the spark plug hole, then screwed in the compression gauge and cranked it for about 4-5 seconds to get max pressure. Any thoughts appreciated and thank you!

Cylinder#/Pressure

1 - 132
2 - 134
3 - 145
4 - 140
5 - 135
6 - 129
7 - 132
8 - 135
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1978 Scout II with 1977 345 engine
Four wheel drive Dana 44 Axle's, Dana 20 T-Case, Power Steering, Power Brakes (Disc/Drum), no A/C.
Manual trans, Borg Warner 3-speed w/floor shift.
Prestolite distributor with petronix.
Fuel System: New 19Gal. Poly tank/Sending unit and Holley 2300 P/N 07448 kit from IHPartsAmerica, Nylon fuel line, relined original fuel neck filler tube.
Unknown non-oem modifications
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Old 08-04-2019, 09:04 PM   #2
Scoutboy74
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Default Re: Compression difference in a 345?

Were all the spark plugs out? Was the throttle blocked open? Those numbers are consistent. There's only a 16 psi difference between the highest and the lowest.
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Coal Trickle - '99 Dodge Ram SLT 2500 Q-Cab SWB 4x4 - 5.9L 24V CTD/NV4500/8800# GVW/12K# winch
Hooty - '74 SII 4x4 - 392/TF727/D20/3.73 D44's/Spartan FA/Powr-lok RA/33g Fuel
Lemuel - '73 1210 Reg Cab 4x4 CS - 304/TF727/NP205/4.09 FA25 & 4.10 RA16/6800# GVW - "Pull Lemuel, pull!"
Organ Donor - '72 1210 Reg Cab 2WD - seized 392/TF727/7500# GVW - "Use my body to keep you alive!"
Mongo - '71 1210 Reg Cab 2WD - 345/TF727/4.10 RA17/6100# GVW - "Mongo love candy! Duh, huh, huh!"
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Old 08-04-2019, 10:42 PM   #3
mbiver
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Default Re: Compression difference in a 345?

All spark plugs were out for all tests. Was I supposed to put something on the accelerator to open the throttle during the tests? Like I said, it was the first compression test I’ve ever done, so kinda learning as I went. Thanks again.
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1978 Scout II with 1977 345 engine
Four wheel drive Dana 44 Axle's, Dana 20 T-Case, Power Steering, Power Brakes (Disc/Drum), no A/C.
Manual trans, Borg Warner 3-speed w/floor shift.
Prestolite distributor with petronix.
Fuel System: New 19Gal. Poly tank/Sending unit and Holley 2300 P/N 07448 kit from IHPartsAmerica, Nylon fuel line, relined original fuel neck filler tube.
Unknown non-oem modifications
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Old 08-05-2019, 05:17 AM   #4
Scoutboy74
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Default Re: Compression difference in a 345?

Yes, or wedge the linkage wide open at the carb. You want air to be drawn into the engine unobstructed. Is the engine able to be started and ran? If so, it would be e best to warm it up first, then pull all the plugs and do the test to a warm, lubed engine.
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Coal Trickle - '99 Dodge Ram SLT 2500 Q-Cab SWB 4x4 - 5.9L 24V CTD/NV4500/8800# GVW/12K# winch
Hooty - '74 SII 4x4 - 392/TF727/D20/3.73 D44's/Spartan FA/Powr-lok RA/33g Fuel
Lemuel - '73 1210 Reg Cab 4x4 CS - 304/TF727/NP205/4.09 FA25 & 4.10 RA16/6800# GVW - "Pull Lemuel, pull!"
Organ Donor - '72 1210 Reg Cab 2WD - seized 392/TF727/7500# GVW - "Use my body to keep you alive!"
Mongo - '71 1210 Reg Cab 2WD - 345/TF727/4.10 RA17/6100# GVW - "Mongo love candy! Duh, huh, huh!"
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Old 08-05-2019, 05:46 AM   #5
mbiver
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Default Re: Compression difference in a 345?

Still has a slight hiccup that I’m trying to diagnose, but it definitely runs. Will be getting a new 2300 carburetor soon that I hope will help narrow things down. So do I need to disconnect the fuel line when performing the compression test with something holding down on the accelerator? Thanks.
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1978 Scout II with 1977 345 engine
Four wheel drive Dana 44 Axle's, Dana 20 T-Case, Power Steering, Power Brakes (Disc/Drum), no A/C.
Manual trans, Borg Warner 3-speed w/floor shift.
Prestolite distributor with petronix.
Fuel System: New 19Gal. Poly tank/Sending unit and Holley 2300 P/N 07448 kit from IHPartsAmerica, Nylon fuel line, relined original fuel neck filler tube.
Unknown non-oem modifications
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Old 08-05-2019, 05:57 AM   #6
Scoutboy74
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Default Re: Compression difference in a 345?

No, just don't cycle it a bunch of times. One squirt won't hurt.
__________________
Coal Trickle - '99 Dodge Ram SLT 2500 Q-Cab SWB 4x4 - 5.9L 24V CTD/NV4500/8800# GVW/12K# winch
Hooty - '74 SII 4x4 - 392/TF727/D20/3.73 D44's/Spartan FA/Powr-lok RA/33g Fuel
Lemuel - '73 1210 Reg Cab 4x4 CS - 304/TF727/NP205/4.09 FA25 & 4.10 RA16/6800# GVW - "Pull Lemuel, pull!"
Organ Donor - '72 1210 Reg Cab 2WD - seized 392/TF727/7500# GVW - "Use my body to keep you alive!"
Mongo - '71 1210 Reg Cab 2WD - 345/TF727/4.10 RA17/6100# GVW - "Mongo love candy! Duh, huh, huh!"
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Old 08-05-2019, 10:02 AM   #7
Robert Kenney
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Default Re: Compression difference in a 345?

Don't disconnect the fuel line for a compression test.
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