Low compression - thoughts?

I just bought a 1999 Taco 1, v6, limited (for $10K). 250K miles. Good shape. No rust...

Decided to do a tune-up on it this weekend (yay CV19), and replaced the fuel injectors, plugs, o2 sensors, etc, and while I was at it, decided to run a simple compression test. Don't know the cylinder order yet, but all the cylinders got 205-210 psi EXCEPT for the frontmost cylinder on the passenger side, its pressure raised more slowly than the rest, and topped out at 175!

I figured it would have a different spark plug than the others, but they were all about the same color: Nice & tan, with no wetness, no cracking, no heat burns. All looked nice.

Obviously, the bad compression would be rings or valves.. given that the other cylinders are all at 205+, and assuming all the rings were put in at the same time, I'm assuming it's NOT the rings (why would one wear faster, unless it is cracked?) and therefore must be a valve sticky issue. Anybody see this on that particular cylinder? I've actually not run across this in a car (truck!) before!

 

The one that had the low compression is on the right...

 

Hmm, it's not that low, but is is odd to me that it's different. Any other ominous signs like milky oil?

 

no the oil is oddly perfect. no misses, no smoke, runs pretty good. i can tell the main bearings are just a little loose... nothing else!

 

Squirt a couple shots of oil from an oil can in the spark plug hole and retest. If it goes up, it's going past the rings. No change in compression number it's a leaky valve.

 

Or tight valve adjustment.

 

*forehead smack*. I knew about that oil in the cylinder thing. though I never understood it. if I drop oil into the cylinder, wouldn't it simply puddle against the edge of the cylinder that is pointing down? if the ring problem was on the upper side of the piston, wouldn't the oil not get there?

 

Check valve clearances

 

I've always wondered just what exactly is a "sticky" valve?

Stuck closed? Stuck open? Does the valve head "stick"? Does the valve stem "sticks"? And just what does it stick to... and how does it get unstuck? Just don't understand it.

How can a valve stick to something if the camshaft is okay? I've been hearing about "sticky" valves since long before non-interference engines were built.
After 60+ years it'd be nice to know...

 

The valve sticks in the guide and hangs open longer than it should

 

Get a leakdown tester and see if you're losing compression anywhere

 

205-210 sounds kind of high for that engine. there is a chance the 175 is normal and the 210 ones are due to carbon buildup.
have a look with a flash light through the plug hole, does the top of the cylinder look like flaky carbon gunk?
If it is you need a top end clean. you would need a leakdown and more poking around to know what is bad for sure.

 

anybody know what the normal compression range is for this engine? (I don't have a manual yet). I just did a compression test on my other Toyota, a 20111 RAV4, and they were all 210 across the board. By comparison, on my 96 Bronco, they were all 180. The fact they were all 205 though, makes me suspect they're ok. The spark plugs are in such good condition (I was astonished), that I'm thinking I'm going to do "nothing" and just put it back together and run it until something shows up. (no engine codes either). Not going to do a valve clearance test until things get worse, either!

I guess what I was really asking is if there is something that commonly goes wrong w/ that particular piston, like on my Ford Explorer (!), the #6 piston always has bad compression because it's way too close to the exhaust pipe. Something like that. Otherwise, I'm going to simply let this slightly out of spec dog, lie.

What was weird is that I noticed last night, after writing all this, that two of my coil-over-plugs had severely green corroded contacts! What in the heck? I tried to get down in that little hole and scrap it off, to no avail. I got some new used ones on ebay, but I was astonished these were mossy green. Is there some kind of dielectric grease stuff I should put down in the hole to keep that from happening?

 

250k miles? 175 psi sounds pretty good.

I think the min in FSM is like 145 or so.


I had a 3.4 that allegedly had 208k. 125-150 comp numbers and it was noticeably lacking power

 

the problem cylinders on these engines are the ones in the rear towards the firewall, and usually have valve issues that lead to problems.
since you said it was the front, I don't think that is what is going on.

your rav 4 is probably higher compression, these engines had mid 9 compression which means you are more likely to see numbers in the 160-180 range.
now some gauges testers are just off a certain number of psi, but to have one at 175 and the others at 210 means something is up.
maybe the o-ring wasn't sealing on that front cylinder when you did the test and your gauge generally reads high.
maybe you carbon buildup on the other 5 cylinders, or there was on all 6 and previous owner tried to clean the front one which is easiest to access.
maybe there was an issue with the tester.

first thing is to add some oil to that cylinder and redo the test and see if the number shoots up.
Your plugs look clean so the carbon buildup seems a little less likely, but the fact you did the tune up tells me there was some issue and maybe the old plugs would have told the full story.

I'd peek down those spark plug holes and report back.

edit*
I also once had a slightly leaky hose on a leakdown tester that gave me funny readings here and there, so make sure that is good too, especially if that was your last cylinder to test. its not always obvious when it starts to leak and you are always twisting on it to get it on and off, so just putting that out there also they make replacement ones just for that reason.

 

IMO there would have to a shit ton of carbon on the top of the other 5 pistons to raise the compression from the 175 range to up over 200. Doesn't sound likely to me.

As mentioned above i would recheck in case the gauge hose maybe wasn't tight in the 175 cylinder. You could drop a little oil in there and see if its a ring problem.

If you're not worried and it drives/runs good just keep an eye on the coolant level (for possible slow loss through a headgasket) and drive it.

If you're curious about the low number then a leakdown tester is your friend

 

A sticking valve would usually be very loud or at least discernable as more than valve lash click.

A tight valve may give a lower reading and that will burn a valve so it's a good idea to know your #'s there, especially with your mileage.

 

drop oil in from the spark plug hole, it will sit on top of piston and maybe get on the rings a little, but the idea is when it gets pressurized during compression (or leak down test) it will get pushed out wherever there is a gap, thus sealing small imperfections while that is happening during the test.

If you didn't try that there could be nothing wrong and just needs a wet test.
its not uncommon for the some gauges to read high and you don't know if the previous owner decked the heads, etc.. lots of things can slightly raise compression.