Burned Valve??

[84 4Runner]

Does anyone know the specific symptoms of a burned valve?? My 22R has been having inconsistent problems for a few months now. Previously it would hesitate and misfire after warming up any time I tried to accelerate (especially up a hill). I replaced: spark plugs, spark plug wires, battery, coil, distributor cap and rotor, air filter, 02 sensor, and fuel pump.

After learning that it may be a problem with a burned valve, I did a compression test and found that my #4 cylinder had a VERY low compression, even relative to the rest (125, 125, 124, 95)! So I re-adjusted the valves and discovered that the #4 valve was improperly adjusted and was too tight. As far as I could see everything else looked great! This upped the compression in #4 to 112, which seems consitent with the wear #4 had when I rebuilt the head 3 years ago. I know it's not a ring because it doesn't smoke (except a little on starting) and the compression didn't change much when I put a little oil in the cylinder.

Now, however, the 4Runner is running VERY inconsitently. Sometimes it will run great, and other times it lugs like no other! Then on occasion it will missfire and hesitate for a brief second, and then run great again. The hesitation seems mostly to happen when I'm revved to about 3000-3500 rpm, but occasionally in lower rpm as well.

Does this sound like a burned valve??? Or does anyone have other possible ideas or experience? Thanks!!

 

[rustybronco]

You say the valve was too tight? You mean the rocker was too tight? Could be a burnt valve, could be a bent valve stem. Adjusting the valve probably helped compression initially but now I betcha has made the problem worse. Check your compression again. If oil in the cylinder didn't help compression, you're definately in need of a valve job.

 

[Shawn]

Stuck valve,,,,,

Runs good,,,

Then Poor,,,,

 

[Too Far]

My experience with burnt valves was that the cylinder had no or very little compression. If you are getting 112 psi the valve can't be that bad. You are only about 30 psi below what I would expect from a new motor at this altitude, which isnt great, but it should still run ok.

I also noticed that the motor usually runs better at higher rpms when a valve is going, and it runs crappy at lower rpms. I think that this happens because at higher rpms the compression has less time to leak out and this increases the probability of the cylinder firing properly.

I would look into electrical trouble to try and solve the high rmp miss.
A stuck valve, like Shawn said, can make a motor do all sorts of weird things, but I don't know how to test for that, unless you can get it to stick while the motor is not running so you can see it.
Check your plugs. Is one more fouled than the others?
The Mass Air Flow sensor can create lots of trouble as well if it isn't working. They are expensive though so you would hate to replace it just to see if it helps.

 

[84 4Runner]

Thanks for all the input guys!!

Well, if I'm lucky it may not be a valve then. I'm planning on doing a full rebuild next year on the engine, so I guess I'll find out then.