low idle

'89 s15 jimmy 4.3, 4wd 114,000 miles.
it keeps dropping to a low idle. If I put it in neutral or park it's fine but if it's in gear and I stop, then it drops and almost shuts off. I was thinking it's something to do with the TCC. any ideas?

Quite possible the TCC is causing the problem. Is the check engine light on?

I agree with oldmaster, itcould be a possible TCC problem, but I'll give a couple of other ideas to think about too.

Being in park and neutral, it may just that because there is no load on the engine (TCC), or, it may be the EGR valve solonoid is not working properly. The solonoid is energized only when in gear, in neutral and park the EGR valve solonoid is de-energized and not used. It could also be that your IAC valve may not be working properly too. For EGR valve, pull the vacuum line off from the solonoid and see if that helps. If it does, then your EGR valve solonid needs to be replaced. If it doesn't change anything, then check your IAC valve and make sure it is not clogged and make sure it is working. Hope this gives you something to look at.

I have changed the EGR vacuum solenoid but not the valve. I will check that and the IAC this weekend after I do my brakes.