I bought a 1990 4runner 3.0 about a month ago 160k mi on it. about two weeks later it started idling rough and the power droped. did a compression test and the number 4 cylinder only had 75 psi. ran a bottle of marval mystery oil through the gas tank and it cleared up and the compression went up to 115 psi. two days ago it started idling rough again and the compression dropped. what do you think?

 

burning oil? I'd venture a guess as to a bad piston ring.

 

its not burning oil i have kept a close eye on that. will a cylinder leak down test give me a better idea whats going on?

 

leakdown test would be the way to go yes. you can pinpoint where its leaking from.

every engine burns oil. you dont notice the level drop because of blowby gasses getting into the crankcase. you can actually have a fairly decent leak/oil burning and not notice if you only drive shorter trips.

 

I think you've arleady pinpointed it as your compression rises significantly when you add oil...

 

^if you add oil to the cylinder and the comp rises=rings
if it were a valve, then the comp wouldnt change w/ oil

valve adjustment?

a 'seafoam' treatment might help for the short term

ive been using bardahls 'no smoke/no leak' in a different motor its helped and i know the rings are toast

 

Well, this is usually a burnt valve from too little clearance.

 

Hey guys i found a product better than seafoam. It's called mopar combustion cleaner, you get it from any dodge crysler jeep dealer, it's about 6 bucks a can. It comes in a spray can, and man this stuf blows seafoam away by far. You take the brake hose off the intake and spray the whole can intop it, smoke will plume out of your exhaust and anywhere you have a leak, this stuff cleans the valves and tops of the pistons like i have never seen. I had a turbo talon and looked down the sparkplug hole with a flashlight and saw that the top of the piston was coated with carbon build up. I then used a cab of this stuff and looked after and the top of the piston was clean, and the car ran awesome, i even picked up twop miles per hour in the quarter mile. Try picking up some up and trying it.

 

GM dealers also carry great stuff called "GM Top End Cleaner" also works great. Seems the dealer stuff, regardless of make works good.