Burned exhaust valve?

[Automan1988]

I have a 92 4.0 and after it idles for a few minutes it will drop down to around 500-600 RPMs. I recall reading that if you stick a dollar bill at the tail pipe and it sucks it in you could have a burned exhaust valve. Could a miss also cause the dollar bill to do this? Also would a burned exhaust valve cause it to start idling rough after a few minutes and have no affect during normal driving. Any ideas?

[Explorin94]

If it was/is a miss then no, the dollar would not suck into the tailpipe. The burnt exhaust valve allows air to be pulled back in through the exhaust side. Sounds like you need a good tune up if you have not already done it.

[Automan1988]

I have replaced spark plugs and wires, new air filter, PCV valve, CTS, O2 sensor, I have cleaned the Mass Air Sensor, IAC and Throttle body. Is there anything that I am missing?

Also is there anything else might cause it to idle fine and then drop down and start running rough?

[Explorin94]

Check for vacuum leaks (IE loose intake, broken hose, power brake booster) check your injector connections. Is your check engine light coming on? If it is does it show a misfire. Could be a bad injector or even one of the new spark plugs or wires.

[Automan1988]

There is no check engine light but a few months ago when I had access to a scan tool, it was giving the codes 172 and 565. The 565 is the canister purge solenoid and we tested it and it was leaking. I unhooked the vacuum line and pluged it and it made no difference. As for the 172 it still came up after I replaced the O2 sensor. We also checked the vacuum and it was about 14-15in. which I know might be a little low, so I may have a small leak.