TBI High idle

[bobby v]

My truck (1990 SS454) idles too high, varies from 800-1000rpms. So far Ive relaced the plugs, IAC valve, new air cleaner, tried cleaning the TPS but didnt think that would actually work and it didn't. Next is to buy a new TPS I guess, is there anything Im missing after that
Thanks
Bobby V

 

[smdk2500]

have you checked for vacuum leaks

 

[evilunclegrimace]

How about changing the TBI base gasket before you go spending a lot of money.

 

[bobby v]

So I see the 2 ways to check for vacuum leaks are with carb cleaner or propane (unlit of course and on a cool motor) around the areas of a suspected leak, is that about right"??
Thanks for the help
I do have an after market air cleaner and I don't what all has been disconnected but there are some plugins that are not plugged into anything, my knowledge is limited about this

 

[Wheeler]

Brake clean & starting fluid (ether) work great. Spray all around motor.. If it revs up at a certain spot, that’s the leak!!

 

[evilunclegrimace]

Brake clean & starting fluid (ether) work great. Spray all around motor.. If it revs up at a certain spot, that’s the leak!!

Brake clean is a bad idea as it melts some plastic and rubber parts. Carb cleaner or propane is the way to go

 

[Wheeler]

Never had any issues in the last 25 years using brake clean or starter fluid...

 

[bobby v]

One of the YouTubers say soapy water ????? If so that seems a lot safer to me. I'll give it a try 2morrow because Im tired of this, plus the bogging down. I'll report back if I find anything. If I don't I guess the TPS will be replaced, not too expensive.
Thanks
Bobby V

 

[DerekTheGreat]

Have you hooked a scanner up to it to see what the sensors were reporting? My '89 TBI 350 had an erratic idle which turned out to be a defective TPS. The voltage would vary from ~0.6v to ~0.9ish and when it would, the computer would respond by increasing or decreasing the IAC counts depending on if the TPS voltage was increasing or decreasing. Made the idle surge or just higher at times depending on what mood the TPS was in.

Note: I had an '88 C3500 with a 454 & SM465. I believe the factory idle is set to 800 rpm, something to keep in mind. My tach (did the moonie to needle swap) would indicate 1000rpm or so which drove me nuts and I did all kinds of things trying to fix it. I finally hooked my scanner up to it and saw it was actually idling at 800 +/- 50rpm or so.

 

[bobby v]

Thanks Derek, My tach is on the a pillar, one of 3 GlowShift gauges, that's all I have. I tried the propane test this morning to no avail, motor didn't change one bit.
I have a new TPS coming 2morrow so hopefully that solves it because after that I have no clue.
I wish this forum would get up to speed with posting protcol such as the Corvette Forums, Its SO MUCH EASIER, photos are a snap to post, just a much much easier forum to navigate
Thanks for the help, I appreciate it
Bobby V

Have you hooked a scanner up to it to see what the sensors were reporting? My '89 TBI 350 had an erratic idle which turned out to be a defective TPS. The voltage would vary from ~0.6v to ~0.9ish and when it would, the computer would respond by increasing or decreasing the IAC counts depending on if the TPS voltage was increasing or decreasing. Made the idle surge or just higher at times depending on what mood the TPS was in.

Note: I had an '88 C3500 with a 454 & SM465. I believe the factory idle is set to 800 rpm, something to keep in mind. My tach (did the moonie to needle swap) would indicate 1000rpm or so which drove me nuts and I did all kinds of things trying to fix it. I finally hooked my scanner up to it and saw it was actually idling at 800 +/- 50rpm or so.

??