Hmmm where to begin. Truck is running a bit rough under heavy load. I'm suspecting a fuel delivery problem, either at the tank or maybe the fuel pump. But before digging into that I figured I would check the cylinder contribution and compression.

I got Torque Pro and Car Gauge Pro working (thanks all!). No codes. Injector buzz test sounded good. The cylinder contribution test passed with flying colors.

I got the Harbour Freight compression tester and rigged up the grease gun hose adapter (thanks again!), managed to get the valve covers off, and got the following cylinder pressures:

1 365
2 365
3 365
4 360
5 365
6 280
7 360
8 360

So I squirted a tablespoon of Rotella T into the number 6 hole and the compression jumped to 410. Damn! I've read this means bad rings or a scored cylinder wall. And no way to fix that without taking the engine out.

In desperation I checked #6 a few more times and I see it's slowly working down to 360 or so. At this point I'm starting to wonder just how long that tablespoon of oil is supposed to seal the rings.

So I put all the plugs back except for #6, pull the #6 injector connector and watch the #6 compression with the engine running. Steady at 450. I did the same with #4 and it's steady at 460. Went back and did the #6 compression with just the starter and it's now steady at 365, even with the starter working against all of the other glow plugs in!

So did I 'heal' #6 with a tablespoon of oil? Unstick a stuck ring? This is new territory for me... first time I've done a compression test on a diesel.

'97 F-350, 240000 miles, 4th owner, turbo looks real pretty with no play, air box and air supply tube in good shape. I've owned the truck for the last 10 years or so and have had no major issues with the engine until this gradual loss of power over the last year or so with a bit of stutter at WOT.

At this point I have the valve covers back on and I'm planning on checking the fuel pressure when I get a chance in a couple of weeks (busy at work).

I've been lurking here for years, and you guys have been a huge help. Any help on this odd compression 'cure' is much appreciated!

p.s. Don't try the 'engine running compression test', at least not with the Harbor Freight tester, unless you're prepared to disassemble the tester's check valve and recover the swallowed little o-ring under the check ball. Repeatedly. Oh well, $25 what do you expect.

 

Was the engine warm or cold when you did the compression test? Best results come from a warm engine. My WAG is that the cylinder was dry, and that oil helped to "seal" the rings by lubing the cylinder walls and rings. I seem to recall that you should squirt a little oil into the cylinder before doing a compression test on a cold engine.

 

Thanks, the engine was warm. Plugged in overnight, then idled in drive (automatic tranny) for 10 minutes or so with the exhaust partially blocked. Oil temp gauge was up at the normal driving point. Funny how hard it is to warm up these beasts!

 

Maybe the test probe didn't seat well the first time which caused the low pressure reading

 

Maybe you're right. I did the measurement twice, but I don't think I reseated the adapter into the glowplug hole.