Surging Idle. HELP PLEASE

[frankgeno]

Hello. I have a 1992 Ford Ranger 4WD Automatic transmission. 2.9 L V-6 engine. 103,000 miles
I live in southern Alberta and this is a Canada-built truck

I have had this truck for about 18 months and put about 22,000 miles on it.
has been running fine until about a month ago.
It USED TO start-up and the idle goes to about 1500 rpm until it warmed up some and then it would stair step down a couple of steps to about 700 rpm and idle well
But about a month ago it started and the idle did not go to 1500 just about 500 and it died. then start right away and it then ran to about 1500 rpm
however the idle is no longer smooth
it lunges or lurches at idle, the engine up and down up and down a couple of hundred RPMs
if you rev it up some it seems to idle better.
WELL, I am currently on a road trip. Driving up and down hills and going uphill runs fine but going downhill trying to hold the speed limit at first it felt like maybe a wheel is out of balance
a little bit, the engine does not run smoothly when going downhill, if I put the truck in NEUTRAL and coast the out of balance feel goes away so it is not a wheel problem
So I wonder if anyone has any ideas on what I can adjust or replace. I have no scanner or test light with me.
Any help is appreciated.

 

[rusty ol ranger]

Check for vacuum leaks.

 

[JerrySab]

I've read of others who have had this issue, and found a failed MAP Sensor to be the issue, or as ROR mentioned the vacuum line to MAP sensor could be leaking or blocked. Not a difficult part to replace, nor is it terribly expensive.

Secondarily, you may inspect the wiring to the IAC valve, checking that the plug is well attached and there is no damage to harness. It could also be blocked/dirty, but your description of the problem doesnt sound like a blocked idle valve. I also had an instance where the TPS would randomly send high/low voltages to the computer, which (I'm told by mechanic) was causing idle to spike and hold. But again, doesn't sound like the problem youre describing.