Bought a 1995 4Runner. 193,000 miles. Runs like crap and has no power. Changed the busted MAF. Replaced cap, rotor, plugs, and wires. Still runs like crap. Checked timing with jumper wire, timing is ok. Checked compression and 5 and 6 are a big fat 0. All others have compression low 120's. Not great but there is compression. So my question is what do you guys think? Bad valves or bad head gaskets?

 

Do a wet and dry compression test. Any mixing of oil and coolant?

 

No mixing of oil/coolant.

 

Just to clarify: cylinders 5 and 6 are on both side closest to the fire wall. Are these the cylinders you are having problems with?
Cylinder numbering:
5 6
3 4
1 2
Passenger Front Driver

 

Yes 5 and 6 are my problems. Did a quick wet test on 6 with no change. Still 0 guage does not even twitch.

 

3vze engine?

 

Yes. Near as I can tell completely stock. All hoses/lines appear to be in good condition. Sticker indicating timing belt was replaced at 108,000 miles, in 2004. I checked it today, all timing marks look like they line up and belt appears to be in great shape.

 

Probably needs new rings or pistons all together for cylinder 5&6

 

Can you do a leak down test on each cylinder? With only one spark plug removed at a time and that cylinder at TDC you may be able to tell where the air is escaping to.

 

I would put my money on either burned exhaust valves or a blown head gasket. Doesn't seem like rings would normally leak bad enough to give zero compression.

You could pull the valve cover off on the driver's side pretty easily and check the clearances on the #6 valves. If the exhaust valve is at zero clearance there's a good chance it could be burnt.

Another possibility is the engine was overheated sometime causing severe detonation which burned a hole in the pistons. Had that happen to an aircraft engine once.

 

Probably going to be a bit before I can do a leak down test. Might pull off the valve cover and check it out.

 

Ok so I skipped the leak down test and pulled the heads. Exhaust valves in 5 and 6 are both burnt. Headed to the machine shop tomorrow. Hopefully it doesnt cost an arm and a leg!