I purchased an 88 4runner in January 2011 with 245K on the odometer. It runs as good as I would expect a truck this old to run. But the other day I came to a stoplight and the idle started to go up and down. Nothing major or high revs but just a pulsating short interval up and down. I don't have a tachometer but its probably 2,000 rpm or so at the height of the noise. Is this a big deal at all? Its a daily driver for me and my only vehicle so I'm wondering if this is the start of a bigger problem.

Also, the guy who sold me the truck changed the timing belt and did some cylinder head work to it. It runs fine for me but does sound kind of raggedy compared to my old 1984 22r truck.

 

If the base idle is set too high, when you press the brake the idle will fluctuate. So step 1 would be to check the idle, supposed to be 750 rpms I believe, although I usually set mine a little higher.

If that doesn't fix it report back on here.

 

If the idle goes up past 2k, the ECU will cut fuel, causing it to drop down, and repeat the whole procedure. That's typically where the hunting idles comes from. The ECU knows the engine is in an idle state when the TPS tells it that the throttle is fully closed.

The problem is likely air getting in to the intake manifold past the throttle body. Is the idle adjustment screw too far out? Do you have a cracked vacuum line running to your valve cover?

Another issue it mighe be, however really unlikely, is that your tps is mis-adjusted.

 

Thanks for the replies. I'll have to look at it closer when I get a chance. It has been idling perfectly since January so I don't think the TPS is mis-adjusted. But maybe something came loose. I looked briefly when I stopped for gas today but didn't see anything strange. Although I don't really know what to look for. When I first start the engine it idles fine for about 5 minutes or so and then it starts fluctuating. I'll keep listening and looking. Thanks for the help.