[Randy S]

Not sure if this belongs in the troubleshooting section as much as the rant section, but some may find the info useful.

My daily driver pickup (95 Z-71) developed a fast idle a few weeks ago. It would race up around 1800 RPM's when first cranked and then slowly idle down to around 900 RPM's. Slowing to a stop produced similar results, a fast idle as if you were pressing the accelerator, and then a slow decrease to "near" normal idle. I figured IAC, TPS, or vacuum leak. A quick shot around the TBI base with WD-40 did not change the idle and I couldn't find any cracked or broken hoses. I pulled the IAC out and cleaned it with carb cleaner. No change.

It finally set a trouble code. It's OBD-I, so you can pull the codes out by shorting the diagnostic pin to ground. Code was 35. My manual says something to the effect of "can't set idle speed - replace IAC". What I didn't pay attention to was the small asterisk caveat that said "may not fix all idle issues".

Anyway, I replaced the IAC ($30). No change. In a moment of weak-minded parts changing madness, I replaced the TPS without even trying to troubleshoot it ($20). No change. [img]graemlins/sad.gif[/img]

This leaves me staring at the MAP sensor or a vacuum leak. Being $50 in with no improvement encourages me to break out the manual for the MAP sensor test procedure. Test indicates I should get around 5.0 volts with the engine OFF and the key ON. I got 6.48. Hmmmmm. Next step is whether or not it pulls down to between .5 and 2.0 volts when the engine cranks. It passed this test with a reading of 1.52 volts. The fact that it pulled down within spec led me away from the MAP sensor.

Ok, back to a vacuum leak. This time I used starting fluid to ensure the engine would choke if I hit a leak. I started by spraying the front left side of the TBI base - a known problem area. Engine immediately stumbled. Another shot killed the engine.

Here's what I found:


$6 at the local parts house.

[dnult]

Excellent! Bill, what do you think about starting a section on the front page of troubleshooting success stories. There was one here a while back in the electrical forum also. While there may not be that many folks with this exact problem, it is an excellent example of troubleshooting tactics.