ChattyFiveOh

OK, so I just picked up a '92 GT vert yesterday. It ran great under load, but the idle was wandering between 600rpm and 1300rpm or so. I'm a little inexperienced with 5.0 EFI so I did some Googling. I found a great article about how to fix this problem. I think it was 5 steps:
1. remove and clean IAC valve - did that even though it wasn't dirty at all.
2. clean throttle body plate- did that even though it wasn't dirty at all.
3. clean mass air sensor wire - didn't do that because I don't have a T20 security bit
4. reset idle- didn't do that because the instructions said to start the motor with the IAC wires disconnected, but mine wouldn't run with the harness unplugged. It would crank and run for 1-2 seconds, but would stall out.
5. set TPS voltage to .98-.99 volts with switch on RUN- did that, but I had to use a Dremel to enlarge the holes on the TPS to get that voltage. The most I could get without using the Dremel was .72 volts.
I did everything the article said (except as noted above) including disconnecting the battery for 30 mins to clean the ECU memory.
So, after all this I fired her up, and the idle was MUCH better. Very steady and probablyy ~700rpm. So I drove to the store to get beer for my reward. It was raining so I had the defrost on. When I got home, I turned the defrost off, and the idle soared to a steady 1500rpm. I turned the defrost back on and it dropped to a steady ~700rpm. Defrost off, high idle again. Defrost on, normal idle again. Any ideas??

The car has headers, BBK catted x-pipe, MAC 3" exhaust, e303 cam, Cobra upper, K&N cone filter, underdrive pulley, 5 spd with King Cobra clutch... maybe some other stuff... help?

86 5.0L

run KOEO/KOER test, you'll need a paper clip, and a $1 test light

also check for vacuum leaks

wadesquad

leme ask u: when you turned the defrost on did the air diverter start blowing air @ the windshield? or did it keep blowing out of the vents?

on the vac tree there are three vac lines: big one brake booster and one is for the air diverter and the other is for evap stuff.

if you unhook the smaller ones and cap em off does your issue go away? dont leave em capped off but maybe this could help you to narrow down where your prob is

ChattyFiveOh

i'm going to do the koeo / koer test now and report the results...

ChattyFiveOh

When the defrost is turned on, the air is diverted to the windshield and no longer comes out the vents. Should I try your test anyway?

ChattyFiveOh

Well, I TRIED to do the tests, but nothing flashed. I hooked everything up according to the sbftech article, and the test light was constantly on before I turned the key on. I turned the key on to do the koeo test, and the light went out. The fuel pump primed 2x, but I didn't hear anything else, and the light never came back on. I let it sit like that for a minute or 2, but nothing happened.
Then I tried the koer test. Hooked everything up as before. Light was constantly on before cranking the motor. When it started running, the light went out. The engine did its thing, revved up and down for 2-3 minutes like the instructions said. The idle stabilized, but the light never flashed even once.
What's going on here?
If I take it to Autozone and ask them to use their scanner, will it give the same codes I would've gotten with my light?

ChattyFiveOh

anyone?

wadesquad

autozone scanner is not a diagnostic tool its jus a dumb box that tells you any stored codes... high idle is usually a vac leak have you sprayed your intake and all vac lines with carb cleaner?