I need some help here please. My 91 with a 302 started idling real high and rough a few weeks ago. It seems like a vacuum leak but I have not been able to locate it if that's the problem. I pulled the vacuum line off of some kind of sensor that is mounted up near the firewall on the passenger side of the engine (it also has an electrical connection) and the idle dropped way down but was surging real bad. What is this sensor and is that the way it should react?
Also, I pulled the vacuum line off of the EGR valve and there was no change. There is no vacuum on the line. Should there be? I tried to trace this line down but it goes behind the engine and I was unable to locate the other end (engine was hot). Anyone know where it goes?
In addition it seems like my fuel milage has dropped WAY down (never was great). Not getting smoke or anything but it sure seems to be sucking some serious juice.
Any help will be very much appreciated!

 

The device between the right-hand cylinder head and the firewall is the TAD (Thermactor Air Diverter) valve. It should have TWO vacuum lines. The first vacuum line runs to the TAD solenoid which controls the opening and closing of the diverter. The second vacuum line is the TAB (Thermactor Air Bypass) valve and runs to the TAB solenoid. Both valves are incorporated into one unit. The TAD and the TAB solenoids are mounted on the intake manifold behind the ignition coil. Vacuum to the EGR and the TAB/TAD solenoids is controlled by the computer and may or may not be present depending upon the operating conditions at the time. The important thing here is to be certain all vacuum lines are intact (which can be a chore with older plastic vacuum lines).

While you are there make certain the EGR fresh air plumbing to and from the smog pump, diverter valve and catalytic converter are intact as well. There are TWO metal check valves in this plumbing that can fail. They tend to make noise and/or leak dangerous exhaust backwards through them when they do. The first of these (and the one that fails most often is the one mounted in the air bypass cross over tube that runs across the top rear of the engine between the cylinder heads. Its about center and points straight upwards with a formed rubber hose extending back to the diverter valve. The other is harder to get to and much harder to replace as it is mounted to the top of the metal air injection tube that extends up from the catalytic converter. Again it has a formed rubber hose back to the diverter valve as well.

If NONE of this yields any kind of failure point or broken lines, have a close look at the vacuum tree extending from the center of the upper intake plenum (@ the midpoint of the plenum front to rear) and check for vacuum leaks or missing caps.

If none of this helps send me a PM and let me know the color of the vacuum line that came disconnected.

 

Wow, lots of info there! Thank you for the reply! I'll try to follow some of your leads asap and see if I can find anything. Again, I really appreciate you taking the time to help out.
Fred

 

The sensor on the firewall with a vacuum line from the tree and wiring should be the MAP sensor. The EGR may or may not have vacuum but is controlled by the electronic valves on the drives side of the engine. Two valves control the air diverter valves for the A.I.R. system with the front being the valve for the EGR. THe supply line comes from the Vacuum resivior canister on the passenger side of the engine compartment. Then there is a line that runs to the HVAC system with AC, not sure about the Non-AC systems. Another line runs to supply the Cruise system and I have had a leak show up on the cruise control system and leaks on the brake booster have been noted. I have had the EGR vacuum line fall to pieces inside the harness but still remain attached to the harness, but the EGR valve is a normal leaking point as this valve operates as a cycle to turn the vacuum on or to vent it to atmosphere as per the needs set out by the computer.

Since this is such a common issue on the fuel injected engines, isnt there a FAQ on the subject somewhere?

 

More great info. Thanks so much! I'm working on it. Will let you know what I find. Thanks again...
Fred