Re: common cause for low compression 360 magnum??
[Re: kielbasa]
#2355208
08/17/17 12:28 AM
08/17/17 12:28 AM
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Joined: Feb 2003
Posts: 1,306 Laveen, Arizona
GTSDart340
pro stock
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pro stock
Joined: Feb 2003
Posts: 1,306
Laveen, Arizona
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Those engines are common for cracked cylinder heads also.
1949 International KB-2 "Mater" - 302/T5
1968 Dodge Dart GTS "The Drat" - 340/727
2006 Dodge Magnum R/T - Hemi
2016 Dodge Durango Limited - 3.6
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Re: common cause for low compression 360 magnum??
[Re: GTSDart340]
#2355253
08/17/17 02:12 AM
08/17/17 02:12 AM
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Joined: Nov 2003
Posts: 36,040 Lincoln Nebraska
RapidRobert
Circle Track
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Circle Track
Joined: Nov 2003
Posts: 36,040
Lincoln Nebraska
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even at 120 lbs I'd think it still would be firing. I'd do some more digging.
live every 24 hour block of time like it's your last day on earth
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Re: common cause for low compression 360 magnum??
[Re: kielbasa]
#2355316
08/17/17 09:48 AM
08/17/17 09:48 AM
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Joined: Jan 2006
Posts: 8,162 USA
360view
Moparts resident spammer
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Moparts resident spammer
Joined: Jan 2006
Posts: 8,162
USA
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155 psi dry and 170 psi wet after a squirt with 50w oil Were the wide open throttle/ fourth stroke/ average readings at 140,000 miles on my 1995 Magnum 5.9 V8
Was your bad cylinder #7 ?
Even on a low rpm compression test my cylinder 7 read about 10 psi higher than the average.
The Mopar V8 firing order has number 5 cylinder firing just before number 7.
The "Kegger" Magnum intake manifold has the inlets for cylinders 5 and 7 side by side at the rear passenger side corner.
The air flow inside the Magnum manifold's large plenum gets moving toward the inlet for cylinder 5 and then only has to change direction slightly to begin filling cylinder 7 next. This "air monentum" boosting effect on cylinder 7 causes it to get a somewhat better percentage air fill, which for the same fuel injector squirt causes number 7 cylinder's air fuel ratio to run leaner.
The combination of "higher volumetric efficiency" due to air momentum, and leaner air to fuel ratio causes cylinder 7 of a Magnum V8 to be more prone to ping or detonate first. The cylinder will also run hotter.
If by bad luck your driver's side block deck surface was milled at the factory to slope to the rear, the situation is even worse.
If tiny particles get by your fuel filter, or come off the fuel line's inner liner, notice that the fuel line feeds the fuel injector rails from the rear first, which means particles will settle into cylinders 7 and 8's fuel injector "catch screens" first. If an accumulation of such crud restricts the rearmost fuel injectors output, that will make an even leaner and more dangerous air to fuel ratio.
I have heard from other Magnum V8 owners that they have discovered one rocker arm so worn that the pushrod breaks through, causing one cylinder to have poor compression. The root cause may be a bad hydraulic lifter that is no longer pumping oil up to lube the rocker.
Magnum V8 combustion chambers are prone to carbon deposit build up. Official Chrysler Combustion Chamber Cleaner/Conditioner does a good job of cleaning the carbon out and generates an impressive black smoke cloud.
Intake valve upper surface deposits could cause low compression test readings.
High amounts of carbon deposits inside the chamber and deposits filling up the soapdish shaped cavity in the Magnum piston crown could cause 200 psi readings.
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Re: common cause for low compression 360 magnum??
[Re: kielbasa]
#2355329
08/17/17 10:18 AM
08/17/17 10:18 AM
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Joined: Jan 2006
Posts: 8,162 USA
360view
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Moparts resident spammer
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USA
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Make sure your ignition wires are not arranged in the "pretty and neat way" from the factory but are placed according to the TSB that was issued to prevent unintended inductive cross firing of cylinders 5/7 and cylinders 6/8 The improved wire routing is quite odd looking. http://dodgeram.info/tsb/1998/18-48-98/18-48-98-v8.htm
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Re: common cause for low compression 360 magnum??
[Re: kielbasa]
#2355661
08/17/17 08:47 PM
08/17/17 08:47 PM
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Joined: Jan 2005
Posts: 12,419 Kalispell Mt.
HotRodDave
I Live Here
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I Live Here
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Posts: 12,419
Kalispell Mt.
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I have seen a lot of crappy exhaust valve seats showing up on these engines as the years and miles pile up. Not a lot of bottom end issues but a couple. The cracked heads are not going to cause a noticable compression loss unless they are a lot worse than any of the 100s I have seen.
I am not causing global warming, I am just trying to hold off a impending Ice Age!
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Re: common cause for low compression 360 magnum??
[Re: kielbasa]
#2355913
08/18/17 08:56 AM
08/18/17 08:56 AM
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Joined: Jan 2006
Posts: 8,162 USA
360view
Moparts resident spammer
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Moparts resident spammer
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USA
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I'm sure the seller already thinks I'm a kook for doing a compression test on a $500 truck, not sure I feel like testing his patience by doing it again with squirting oil down the bores,much less pulling valve covers...
If it were me, and its a $500 3/4 ton pickup, I would turn my focus to the transmission's condition, in making my final decision. I would like to find a modest price 1994-1995 2500 Ram that has a PTO on either 46RH transmission or NV 4500 manual.
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Re: common cause for low compression 360 magnum??
[Re: HotRodDave]
#2355918
08/18/17 09:37 AM
08/18/17 09:37 AM
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Joined: Jan 2006
Posts: 8,162 USA
360view
Moparts resident spammer
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Moparts resident spammer
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I have seen a lot of crappy exhaust valve seats showing up on these engines as the years and miles pile up. The Iron 8L V10 has pressed in hardened exhaust valve seats according to the press release. Since it shares the same piston and has a similar combustion chamber, I am guessing that means Chrysler engineers knew that Magnum 5.9 V8s needed them too, but for cost reasons they went with the cheaper induction hardened method.
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Re: common cause for low compression 360 magnum??
[Re: kielbasa]
#2356134
08/18/17 04:41 PM
08/18/17 04:41 PM
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Joined: Jan 2003
Posts: 17,844 S.E. Michigan
ZIPPY
I Live Here
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I Live Here
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S.E. Michigan
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Beater...Honestly It should still hit on 120 although that is weak. but should be enough to fire and not worry about it. Something else is going on.
If you want to chase compression you Can always put compressed air to it through the plug hole and listen for leaks. Breather/intake/exhaust air leak will give you a clue whats going on.
Bore scope might reveal something.
But it's usually the trans that is the weak point on those, and for a plow truck it's got to be solid.
Rich H.
Esse Quam Videri
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