The high idle issue is prolly due to a vacuum leak, or a malfunctioning thermal valve
or the fast idle seloniod. This is based on my novice understanding of the vacuum and
emission system ... so bear with me, heh heh.
There are two things that can effect this, the Aux Air Valve, and the Fast Idle Selonoid.
You should be able to test these and find out which is leaking and causing the problem.
There is a good thread here with pics to help find your way
Linky ==> http://forum.planetisuzoo.com/viewtopic ... ght=plenum
So, when you're driving the truck and it is having the high idle problem ... stop and make the following tests:
1. Remove the bottom hose from between the thermal valve and the plenum (plenum is sometimes
called the intake manifold common)- BE SURE TO BLOCK off both ends -the hose and the thermal
valve and not make more leaks. Did the idle change? This could account for the fast idle when warm.
If the idle doesn't change, then its prolly not the thermal valve.
2. Repeat the same test with upper hose on the thermal valve that goes to the Fast Idle Selonoid.
BE SURE TO BLOCK off both ends. Did the idle change? This could account for the fast idle
when warm. If the idle doesn't change, then its prolly not the fast idle selonoid.
3. Check the all of the hoses running to the other vacuum switching valves. If these are leaking they
would cause a fast idle, but prolly would not account for the warm/cold difference you mentioned.
One of these gets told by the ECM when to suck vapors out of the charcoal canister, and the
other gets told to modify the fuel pressure during high RPM.
4. There is also a possibility that its the aux air valve under the intake manifold - but I haven't
figured out an easy way to test that and don't have the FSM.
Report back and lets see what we gots.
MarkB.NV, white 1984 Trooper, 1.9 / 4 speed, 87k mi.