Hi folks, low compression on a 2005 Ford Ranger 3.0L V6 RWD with 285,000 km / 178,000 miles on it. I can rustle up some photos if they're helpful. First suggestions from the mechanics (swap engines for $4-5k) was too expensive. I left Calgary, Alberta (Canada) Sept 26th on a US National Park road trip (WY, UT, CA, NM, etc.), and Sept 30th check engine light came on in Jackson WY. Got to Salt Lake City and got O'Reilly's OBD-II scanner to read a P0316 error code (misfire detected on startup - first 1,000 revolutions), with the following codes stored: 1) P3000 random misfire detected, 2) P304 misfire in cylinder #4, 3) P0306 misfire in cylinder #6. Most likely solution identified was "Replace Cylinder Head Assembly." Fellow at O'Reilly's suggested ignition coil given the background, but we decided to do a compression test as a sanity check and we got:

#1 - 90psi #2 - 98psi #3 - 80psi #4 - 65psi #5 - 75psi #6 - 80psi

poured some oil into 4, 5, 6 and got the following:

#4 - 100psi #5 - 120psi #6 - 100psi

Some background: we were driving up to the artic this summer (Hay River, Northwest Territories, Canada), and had a P304. Checked the spark plugs, and replaced spark plugs 4, 5, and 6 as they were eroded damn near down to the porcelain (1,2,3 were fine). This was after only 25,000km when all six were replaced (NKG). Found a scoring in the #4 wire, taped it with electrical tape, and the check engine light went away; realized something was causing premature corroding of the spark plug material, but didn't have time to dig in. Replaced the wires when I got home (Calgary, AB), and hit the road again (quit work to travel for a year, did a bunch of preventative maintenance assuming engine was solid past 300,000km / 200,000miles). Maybe the wire short accelerated this (exhaust valve) issue? It does look like there's a TSB for exhaust valves.

Overall, the engine runs fine - plenty of power. It does seem to idle a little rough. Oil level/colour looks fine, although it looks like it dropped from mid to low in the last 3,000km (hard to know for certain). Coolant level holds (it was leaking via timing cover to the point of dripping on the ground late 2024, which I sealed up along with replacing the timing chain last winter). Right now the spark plugs look fine gap wise, and show nothing that indicates concern.

After the sticker shock of swapping engines, we decided to keep limping this baby along on our road trip as far as she goes, sticking to the main roads more than initially planned. Posting in case someone has some diagnosis suggestions - in a couple weeks / 1,500 miles I can get back to a buddy's in Salt Lake City (is it dumb to keep driving?). If I can confirm exhaust valves, I'd like to establish either a parts / tools list, or a machine shop that doesn't have a crazy lead time, or both. I've got it setup to sleep in the back, and it has a kitchen/gear storage setup drawer system so it has a greater level of utility than just a vehicle for me for the next few months. For now plan to keep the RPMs low, drive easy, and see how far it gets me.

This video

shows a fellow replacing the head and head gaskets without touching the fan clutch or AC compressor - I can take a closer look, removing the exhaust manifold looks like the worst part (aside from the overall number of pieces). If it is exhaust valves, I assume best to do both heads with new valves and whatever else is required. I've never touched an engine before, so doing this myself (and away from home / without my regular tool set) would be bit intimidating, but I have the time.

How do I best figure what the issue is? What do I need to 'get er done'?

 

Since adding oil to the low compression cylinder restored the compression, the low compression appears to be due to worn rings and not a leaking valve. I would just drive it because doing anything to the rings is cost prohibitive. Your spark plug problem might be caused by the type plug used. Your truck requires double platinum plugs which should have a PP in the plug number. If you use regular plugs 3 of them will wear out very quickly.

 

you will need to have your heads machined, valve seats , and new valves. most likely. you might get away with doing it yourself if it was just one valve. dont use oil right now to test compression, that is to test tor rings. i would test again. pull all plugs and cold motor. im not too sure but you want to see about 160lbs in all cylinders. so driving it 1500mi humm dont know, just dont push it hard. im sure the truck i have had some burnt vales for some time.before i got it and i had it a year before i fixed it.

 

There are a number of variables in compression test readings like starter cranking speed, throttle position, engine temperature, and altitude where the test is being run. I generally assume that the highest number is a good cylinder In your case the added oil ran your bad cylinder higher than the "good cylinder" which suggests that all rings are well worn. While I said I would just run it, there are several things that might help. A higher viscosity oil might seal worn rings a bit better. Valvoline has recently come out with an oil that is designed to remove carbon from rings and pistons which might help if your rings are sticking in the pistons. I think it is called "Restore". I have only seen it in 5W30 which is what my Ranger is supposed to run. I think I will try it even though mine has no known problems.

 

ok here this is the way i think, cold motor, all plugs out. test take a reading... then put the same amount of oil in each cylinder, test again, see if there is a difference, or much. oil will test rings. no oil will test the valves.

 

There's a chance the compression test was done incorrectly, the spark plug channels in the block require a very narrow fitting that some test kits don't have. The fitting might only seal on the threads. I'm not sure why, and this is totally anecdotal, but I had a harder time getting a good seal on cylinders 4-6 on my 2003. Just something to consider.

the difference in spark plug wear that you previously noticed is a good clue, though, that does suggest there's something happening there.

have you tried replacing the coil pack? If you can spare the money, it's a good simple place to start. I had a misfire caused by a very hairline crack.

if it is valve recession, my understanding (which could be wrong as I'm a total novice) is that you could get away with limping it for quite a while, unlike a head gasket. Since it sounds like you're not burning coolant or oil, hopefully nothing catastrophic is on the way. Good luck!

 

Hey folks. Thanks for the interest and replies. Few hundred miles down on the road trip. CEL has gone on and off a few times. Anecdotal, but it seems to be on more on hot days / long distance runs, and when we're doing small in city / just out of city runs it has gone away. I'm under no illusions I'm out of the woods. Nothing of note on the dipstick or fluid levels so far. It does seem to misfire at idle / lower RPM, but I don't think that's helpful diagnostically.

I wouldn't read too much into the "with oil" compression readings, I got a loaner kit from O Reilley's, though it didn't look beaten up. We were far from scientific when adding oil - on #5 we thought we spilled some so added more to be sure. Liquid being incompressible will simply result in higher numbers alone. I think a leak down compression test is the only real way to know where the loss of compression is going, and it wouldn't hurt to run the compression test a second time.

Knowing I had a spark plug wire issue (the insulation of wire #4 was very clearly scored where it was rubbing against a valve cover stud), I really should just replace the ignition coil pack and go from there. However, assuming there's an underlying compression issue, I should run (or pay someone to properly do when we get to Las Vegas area) a leak down compression test to have a much better idea of where the loss of compression is going. When it misfires, you can hear it at the tailpipe (I'll rerun the "dollar bill test" another time - trucks been running good today...); hard to say if there's reversion (air sucked in the exhaust) or not.

Pricing out replacing the heads it looks like a $1k parts job (heads, valves, head bolts, gasket kit, valve spring compressor tool, exhaust manifold bolts), and it looks like more than I want to take on in my buddy's driveway if I can avoid it. He's lacking in tools, and there's a few things I have that would make this go easier that I would rather not buy (in-lb torque wrench, ft-lb torque wrench, locking chain wrench to remove the fan, torx socket set, heat, serpentine belt tool, etc). Buuuut I've got like 10,000 miles before I'm home (I'll have places I can borrow tools / a workspace in under 5,000 miles). Thinking to do the ignition coil knowing the muddy history, monitor the fluids, and if it starts acting up past that do another compression test with a loaner tool and get a leak down compression test done.. at some point so I'm traveling forward with my eyes open.

I've had 'Old *****" for 15 years and 200,000 kms now, so I think I would (try) to do the head job (at some point) if it proves to be an exhaust valve issue - partly cuz I love the little truck, and partly cuz it's a great opportunity to learn.

 

Made it back to Canada. CEL continues to be on and off. Alternator / voltage regulator gave me trouble, and it definitely misfires on rare occasions, but otherwise its running reliably so far. Swapped a remanufactured alternator in when I was in Dripping Springs, Texas in O'Reilly's parking lot and took care of the ignition coil at the same time, although I don't think the ignition coil made a difference - but given the damaged spark plug wire insulation that went unnoticed for who knows how long, is wise to do.

Looking to get a leak down compression tester and do that to confirm, and putting a parts list together assuming its the exhaust valve seats. Rock Auto only has heads (not assembled heads) and if the issue is just the valve seats, it seems worth making a call or two to some engine machine shops when it comes time to see their turnaround time.

I'll be parking the truck in a month, and I think I'll deal with it when I'm back in the spring. I've got another 10,000 km or 5-6,000 miles before I'm "home" but I've got a few places I could hole up and deal with it if it acts up and needs dealing with before then along my route.

 

It would be interesting to see what your compression test results are now. Doesn't the fact that #4, 5,and 6 went >= 100psi with oil suggest that your problem is rings rather than valves, or are those numbers too low? With the air pressure fitting for the leak down tester you should be able to hear if pressure is leaking into the crankcase (oil fill cap hole) or out the exhaust (tailpipe) or intake (throttle body) valve.