I am in the middle of replacing the lower intake gasket on our 2000 ranger 4.0l and I'd appreciate some feedback. At 270k miles:

We removed the plugs and ran a compression test. The results were
Dry=
1=120
2=118
3=120
4=105
5=103
6=105

We ran out of time to do a wet test. It's on the list.

We scoped the cylinders with an endoscope, the piston faces are normal, brown w/the expected light carbon scale. the piston edges are fine, the cylinder walls have good crosshatching left, no scoring scratches or discoloration. The valve heads appear to all be in good condition. PLugs are all burning the same normal tan color.

There is a good deal of carbon buildup on the intake ports but those will be cleaned. There is no oil sludge or residue visible anywhere in the upper levels of the engine we could see or stick the scope down. The engine has has religious 3k oil changes since its 60k or so when we got it.

It runs fine, no smoke no noise. It gets around 18 MPG on a good day mixed driving. The engine seems to pull the truck along quite well with no struggle. We are going to finish the intake gaskets and put it back together.

Referring to the above data, do you think this engine has a couple good years left? The compression levels have me a little worried, why they are 15# lower on one side is odd. We can find no obvious reasons for that.

I can't find anywhere what the lower limit to the compression # can be before it causes issues. If the wet test boosts the # then running lucas or something in the oil should keep the compression up a bit.... in theory.

It runs so nice, I was a little stunned the compression was so low.

Any general feedback or advice is appreciated. I'd like to extend this things life as long as possible. It will be for extra truck/light duty mostly. We'll probably put less than 3k a yr on it.

What could be killing the compression on one head like that? There is no sign of a leak. A wet test may show leaky valves..... I'll get back to this thread when I run that. I probably should do a leak down on it too.

Or not... I dunno. Whatcha think?

 

Hate to be thrower of bombs, but question the checking process. Was the throttle plate propped wide open? Was the check done with cranking on each cylinder, continuing until the reading did not increase any more (likely 3-4 cycles for each cylinder)?
If you are not at an altitude such as Denver, your numbers should be a bit higher. The drivers side being down 15% relative to the other side is interesting, but could be explained by technique.
Did you use a press-on gauge or a screw-in gauge? The former is harder to hold and get a good consistent seal, and can lead to bogus numbers. The latter is easier to use, but requires that the flex tube be screwed into the plug hole and have a good seal, then the actual gauge is snapped onto the connector. It will take a bit longer to develop peak pressure as the air inside the hose & gauge must be compressed to get a true reading of pressure. That is the 'general' reason it takes more than one pump to get a good reading, FWIW.
tom

 

Throttle blocked open, all the plugs removed. Screw in type tester, each cyl was tested 3 times cranked until the needle stopped climbing.

The one side 15# lower than the other was kinda strange. There was no signs of head damage, no "steam cleaned" pistons, no oil burning signs, all the pistons and sleeves looked pretty much identical condition wise. I don't think the readings were in error as each cyl was tested 3 times before moving to the next. We started with the drivers side the engine was cold. The tester was new, the screw in fitting O ring was oiled and snugged down with a plug socket.

When its all back together I will re run the test on all 6 with a shot of oil. Maybe they will come up different.

The only thing I can add is the lower intake bolts were barely even finger tight when we removed them. We thought that was pretty strange.

 

This probably should have been put in the 2.6, 2.8, 2.9, 4.0 & SOHC 4.0 V6 section...

Maybe a mod can move it.

 

A leakdown will tell the tale. It may be just tired. The rings may have lost tension, and the valves not be seating as well, and the combination leads to lower compression.
If it runs smooth on all cylinders, and you will have limited miles per year, I don't see why it won't keep going for another 10 years at just about the same rate.
I have experience only with two high mileage engines, my Dad's Monarch with a 302 that was just tired at about 170k and my Vulcan 3.0 with 248k, which seems a bit tired, but I don't drive a lot. A critter chewed through the SPOUT, so timing has been 'fixed' at 10BTDC recently until I discovered the dining, and installed a connector. Neither engine burned oil, would go down the highway at traffic speeds, and was reasonable on fuel, maybe a little down on mpg. We replaced the 302 with a 255 after he was gone, and my Mom drove that until she got sick. That was a bit lighter, and seemed to have more power. The Vulcan trucks on as needed.
You might take a look at your cam timing, though I can't think of how to check w/o a distributor. If there's too much slack, the valves will be opening late, perhaps not allowing a full 'fill' to compress, or the valve lift may be down, doing the same.
If you don't eat antifreeze, nor oil, there's not much left to consider that I can think of beyond just 'tired'. I do note that old, old engines would keep turning after spark was removed, and the Vulcan which I still have will stop turning immediately after ignition is removed, so there's still compression. Turning the crank damper by hand the engine seems to have low friction. All the fricting stuff has worn away???
tom

 

Now if I can get this pesky wire harness out of my way good enough to clean and replace the valve covers.... and not break anything.

Can you have a slight timing problem on one side?

 

One other point: compression tests should be done on a hot engine. Cold tests would result in lower readings.

 

I've mostly read/been told cold but.... I'll run it again with a hot engine when we are done.

 

These are several pages from my 2004 Ford manual. This is from their "Diagnosis & Testing" section, which is generic to all their manuals. Notice that step one of the compression test is to bring the engine up to operating temperature.

 

Thanks! I'll do it that way.

 

Warmed up all 6 tested around 122#.
I have no idea why bank 1 was low on all 3 before. Go figure.

 

You parked on a hill and all the compression leaked out the low side...

Or you thought you had the adapter screwed in tight, but it were not, and leaked past the seal soundlessly as you cranked.
Murphy was an optimist.
tom