Hi guys, looking at a 97 Ram 2500 to use as a beater plow truck. Only has 110k, but as soon as I test drove it I could tell it wasn't banging on all 8 cylinders. Pulled plug wires one at a time and found the bad cylinder. Did a compression check on 4 cylinders and ranged from 160~200, but the bad one was only at 120.
Valves? Piston chunks busted from top of ring land (I've already seen that)?....Heads wouldn't be too bad I guess, but I don't feel like diving into the bottom end of a $500 dollar truck....your thoughts on common causes for low compression on these engines?

Does the compression come up if you add a couple squirts of oil into the low cylinder?

An old plow truck could have been overheated, or over revved. If the compression does not come up with oil added, I would pull the head and have a look. The worst that can happen is you replace the head gasket & intake gaskets, and drive on 7 1/2 cylinders. Gene

Those engines are common for cracked cylinder heads also.

even at 120 lbs I'd think it still would be firing. I'd do some more digging.

would the cracked heads have an effect on compression?
obviously if crack was big enough, but by then I would think you'd have a geyser shooting from the radiator....
are these engines capable of coil bind and bending pushrods from over revving?
it does run on the weak cylinder, it's just that the 120 pound cylinder had least effect on the tachometer when pulling the plug wire...
did not squirt oil into the offending cylinder, I guess that might tell me whether it's piston/rings or valves....
in any case it sounds like low compression isn't a common issue with these engines? thanks for the info so far!

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.

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

Cyl #1 has the low compression (120), cyl #2 has 160 psi, #3 and #4 pegged my gauge at 190~200.......

How are you numbering the cylinders?

To you what number is the frontmost driver side cylinder?

To you what number is the rearmost passenger side cylinder?

Did you fully open the throttle butterflys and use a block of wood at the cable lever to hold then open while doing the compression tests?

If #1 is 40 psi low,
and does not improve
with a shot or two of SAE 50 wt motor oil into that cylinder prior to another compression check,
and does not improve
with sprays of Chrysler Combustion Chamber Cleaner-conditioner to remove valve deposits,
I would remove the valve cover and check
rocker arm, pushrod and spring
since those are quickly do-able and less expensive repairs.

One of the inexpensive Harbor Freight video borescopes with the smaller 9 mm camera head would be able to look in through the sparkplug hole, and in through the valve cover oil fill cap without disassembly.

I would want to know why couple cylinders have 200. That seems too high to me.

ya, I too was shocked at the 200 numbers.....I guess I will take those with a grain of salt - as far as I'm concerned, the variation between cylinders is what's most important to me....
I call cyl #1 frontmost drivers side and cyl #8 rearmost passenger side, I held throttle at WOT while wifey was cranking......
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...

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.

For $500 you can find another engine from the junk yards, I bought a core engine in 04 for $125. It had a engine fire and other than the outside componites burnt off, it was perfect inside. Too many in the yards to go dicking with a re-build. If you find one use an air tank and leak down tester, don't need to be spun over. Or take a hot battery to crank it over with comp checker.

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.

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.

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.