Expert: Sean Halliday - 11/10/2009

Question
QUESTION: Hello Sean, i have questions on 2 differnt trucks, both 1994, the first one is a gas that when it is cold the idle is about 2500  when it first starts ,after 15-20 seconds it idles down. i have changed the air valve and it didnt fix the problem. i can unplug the air valve and it will have a very slow idle.on the second truck it is a diesel and when it sets a few days it wont pick up the fuel with out using starter fluid, what would cause the fuel pressure to disappear when setting.would an electric fuel pump on the rail frame help this problem. i have been using start fluid even in warm weather. thanks Don

ANSWER: Diesels i do not work on and the other problem could be a vacuum leak making the idle rise too much.If you unplug the IAC and the idle settles it may be a clue that the valve needs replaced.

---------- FOLLOW-UP ----------

QUESTION: Hello Sean, I changed the tps today and it didnt help will check for vac. leak soon. when i unplug the new air valve the engine will not start with the throttle in idle pos. it would start with the old valve in idle pos. and run about 500  rpm. Do you know where the voltage source comes from that goes to the air valve. Thanks Don

Answer
The voltage source will be from the pcm.I would test the vacuum with a gauge before you buy anymore parts.The high idle could also be a leak at the intake manifold gasket and a test will confirm this.The tps i meant was on the diesel and not the gas truck.