Yet another Renix high idle thread...

'89 XJ 4.0, automagic, 4x4, no engine mods. Pulled the throttle body off today to clean it (trying to narrow down a rough idle issue, it needed cleaning anyways.) Pulled it all apart, IAC, TPS sensor, throttle butterfly, shaft, everything, and cleaned it all out with carb and choke cleaner. Put everything back together, cleaned manifold and throttle body surfaces and used a new throttle body gasket. Admittedly, did not use new O-rings on anything. Put new vacuum caps on EGR solenoid and valve. Also put Teflon tape on the threads of the brake booster hose nipple and a screw in the intake manifold (more on that one later.)

Measured 4.88 volts TPS reference voltage and 1 ohm on the sensor ground between it's wire and battery ground. (Kinda had to find the sweet spot with my leads to get that ohm, but I got.) Didn't notice a change in resistance when wiggling/shaking the wiring harness near the valve cover. Adjusted TPS until I got 0.83 volts on the output wire.

Starts without a problem and idles smoother, but it sounds like it's idling high. I don't have a tach I can check the RPM with, but if I had to guess, I'd say at least 1,000 RPM, maybe as high as 1,500 RPM. Checked output voltage, it's still at 0.82. Loosened the screw and moved the TPS around just for the Hell of it, still idles high. I should mention that I broke the bottom TPS screw off while taking everything apart, but for now I'm just running with it. I can get the top screw tight enough that it doesn't affect my voltage readings.

Checked the idle stop screw, it has the tiniest gap between it and the throttle arm, so no problems there. I did notice while cleaning up the throttle body that the throttle plate has slotted holes in it and the tiniest of gaps between it and the body. Should I be worried? Should I try to minimize those gaps by wiggling stuff around? What's the point of the screw below the throttle plate? Should I take the Teflon tape off it and/or replace the O-ring? Sprayed carb and choke cleaner around possible vacuum leak locations while listening to the idle, didn't notice any changes. Anybody have other ideas for finding small vacuum leaks? I'll put a vacuum gauge on it tomorrow and adjust the transmission cable to see if that helps anything. Will probably also do cruiser's sensor ground mix. What else could I be missing?

TPS is likely no good.

Did this just happen?

The rough idle had been going on for a while, the high idle started after removing the throttle body, disassembling and cleaning everything, putting it back together and re-installing. Just adjusted the TV cable and took some preliminary vacuum readings. I think I'll make my high speed bombing run into work, check vacuum readings with the engine warm, and report back.

When I went to start it just now, it seems to be back to a normal-ish, idle speed, albiet still hunt-y, like it can't decide what speed it wants to idle at. My vacuum readings did improve the one time the idle speed picked up. Weird man... :wow:

More than likely you have a vacuum leak. Use some carb cleaner on all the vacuum lines and around the base of the throttle body, while listening for a RPM fluctuation.

The TPS has virtually nothing to do with idle. That is controlled by the ECU from the engine speed.

How did you clean the IAC motor. If you forced the pintle, you most likely need to replace it. If you turned on the key with it removed, you most likely need to replace it.

You might want to check the relay that keeps the ECU energized for a couple of seconds after the key is turned off. It allows the ECU time to reset the IAC for starting conditions. For the moment I can't remember the name/initials for it. It is located on the passenger fender well up by the battery in a row with the other relays.

Update: Got it warmed up, no problems driving, transmission shifts through all the gears fine, downshifts seem a little late/harsh, but nothing unbearable. Connected a vacuum gauge to the rear-most vacuum port that runs to the axle disconnect and HVAC stuff, transmission in park.

Got a consistent-ish reading of 18 in/Hg at 1,000 ASL, needle was quivering around a bit, but less than +/- 1 in/Hg. Not exactly great, but not horrible either. Idle speed sounded like it came down a bit as well.Twice I noticed the idle speed increased for 5 seconds or so, and then decreased, like the IAC was being commanded open and then shut, and when it did, the vacuum reading increased to 20+ in/Hg. Again, weird, and will make finding vacuum leaks that much more fun...

FWIW, I removed the IAC from the throttle body, cleaned it wiith carb cleaner and a wire brush and re-installed it. At no point was it energized while not firmly affixed to the throttle body.

Check that your intake manifold bolts are snug, and use carb cleaner to check for vacuum leaks.