Within the past month I've noticed my truck starting to run a little rougher, especially when under load. Not quite a misfire but one of the cylinders was definitely not like the others. Today I got around to investigating it. Spark plugs all look identical, so I went for a compression test. 165-175 PSI on all cylinders except for #2 where the highest I was able to get was 125 PSI. I sprayed some white lithium grease into the cylinder and tried again which made no significant change. I pulled my valve cover just to make sure there wasn't anything obvious. Valves aren't stuck, springs look good, pushrods aren't bent, rockers are tight, valves are actuating properly when cranking.

Not sure exactly what would cause one cylinder to go out somewhat suddenly. Just looking for suggestions of where to go from here.

 

I'm not familiar with the 3.9 since mine is a 5.2. The checks are similar. I don't know if the heads are crack-prone like the 5.2l and 5.9L mag engines.
I think it's oil, so it will run down around and help the rings seal. Its just another part of the compression check.
Check the coolant level. The engine might have eaten some. Pull the radiator cap off and look for bubbles in the coolant, both when it's cold and when it's warm, careful of the HOT coolant or when it tries toblow out. Sometimes, if the heads are cracked, like mine, the exhaust will pressurize the cooling system and blow all of the coolant out. A blown head gasket could do the same. Sometimes, if you have added water to the radiator, bubbles will sometimes show.
I had a D-8 cat where one of the cylinder liners had popped up and lifted the head. At idle, it would blow all the coolant out, but if you worked it hard, it was fine till you stopped.
Maybe there is a little oil in the coolant? Check for combustion products in the coolant.

 

Along with above suggestions, I would do a leak down test, this should help you hear where the compression is being lost, a blown head gasket, valve may be operating normal but are they sealing? chipped valves/valve seats is not unreasonable or if it comes out the oil fill its rings

 

Yes, Dodgetruck got it, a leak down tester, which usually has a manifold, sometimes 2 pressure gauges, one to read the cylinder pressure, the other to sometimes read incoming pressure. A valve to pressurize the cyl, sometimes a pressure regulator if the incoming pressure is too hi? And a hose to the sparkplug hole. Google it. And Listen at the oil fill.

Dodgetruck, does the hose to the cylinder have a check valve/Schrader type in it? And what else did I miss? How much pressure is allowed into the cylinder ?
And you can use it to hold the valves closed when changing the springs. And gotta be careful, because it can push the piston down. If its in gear, the vehicle might move.

 

Cylinders operate at truly stupid pressures normally, so, I set input pressure at 100PSI, then it's an easy read to see how much leakdown you are getting. Theoretically, up to 25% is considered *acceptable*....... Though I don't like anything over 20.....

It's not hit it with air, and then turn it off though. Pressure is a constant feed, and you read the second gauge to see how much pressure it will actually maintain. If input is 100PSI, and the cylinder holds 80PSI, that's 20% leakdown. Etc.

 

yes, a cylinder is not air tight by any means, just turn it on let it run, mine had a regulator **** to adjust the output pressure and you can read what it maintains, i think it has green yellow red go no go markings, i haven't actually mathed the % they are considering acceptable, I just read the numbers. Personally ive never used it to make master mechanical decisions, i used it to hold valves in for stem seal change and to go oh heavy yea the head gasket is wiped when I'm feeding #7 100psi and pressure is coming out #5 plug hole

 

Yeah, that is just NOT a good sign.

 

OTC 5609 Cylinder Leakage Tester Kit , Black Amazon $109 ish

When I clicked on the OTC tool, there was a video description. It also shows cheaper kits; maybe they don’t work as well as they should? That’s yer decision.

The last time I used one was in 1985ish, when I was mistakenly thinking about doing an A&P license. With an aircraft engine, if you don’t want a prop strike, you really need to be careful; mindfulness is a ‘mindset’?