1991 Rodeo Rough Idle - Help!

Hi guys, my first post to this group. Did a lot of searching and found a couple of things about rough idle problems on the 2.6 motor. I have a 91 Rodeo 2.6 that's been having a rough idle problem for
quite some time now. I have taken it to a mechanic a couple of times with no fix.
I have replaced the EGR Valve, PCV Valve, checked timing, replaced spark plugs, replaced O2 sensor, etc etc. Still no fix. What is weird is that i pinched the lines from the thermal valve in both directions as a test with channel locks on both sides. When I did this the idle improved about 90% and seems to be working well, but idles high quite a bit. I imagine there is something wrong with the high idle solenoid as well but i don't see any codes or a check engine light. As a matter of fact, I have NEVER seen a check engine light with this truck, and you would think I would have by now with all the problems I'm having with hit.
I'm wondering if anyone here has replaced a thermal valve to clear this problem. If so, did it work? Are there some other things I can check? The valves seem fine within tolerances as well.
Any help would be appreciated.
Jack
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I have a 98 rodeo that started with rough idle then progressed to high idle (around 3000 rpm) after driving on the highway. Later it would sometimes high idle right from startup so I knew it wasn't tempeture sensitive. The last symptoms included the idle dropping so low that it wouldn't stay running. Who would have thunk it. The Isuzu dealer couldn't find the problem. After a whole lot of reading and research, the clerk at the local AutoZone clued me in on the problem.
Turned out to be........ The vehicle speed sensor located on the transmission. It did take about a week for the computer to quit acting up after changing it. I guess I should have reset it, but now it runs fine.
Be sure before you swap it out tho, it retails for $170.
Keith
On 12 Jan 2004 11:33:10 -0800, snipped-for-privacy@my-deja.com (Jack Thorpe) wrote:

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Cancel that last message. The dumb thing is starting to do it again. Back to the drawing board. It did clear the check engine light though.
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