Parsnip

99 motor In a 90 Cherokee. I sprayed looking for vacuum leaks to no avail. Heres the issue-
Start up when cold- idles perfectly. Push the brake, idle goes up a bit as expected. After it runs for a minute or two, idle climbs to 1500 rpm and will not go down...
Iac was replaced, unplugged it while cold this morning, still climbs rpms. I know my booster/ master are bad and plan on replacing next week, but what else effects the idle??

LilWarWagon

I'm sure some more experienced Jeep Techs will chime in, but couple quick checks to rule out, Throttle Position Sensor Voltage, Also that is exactly what my 91 4.0L did when the Crank Position Sensor started to fail. Would run like new cold, but Idle would skyrocket to the point I had to turn it off at a few lights once engine warmed up. Couple things to check into, hope it helps. TPS is easy access to test, would rule it out first.

tjwalker

Is this symptom immediately after motor swap or did things run okay with the swap and this symptom is new?

Brake boosters are notorious for vacuum leaks. Replace that and then see where you are at. Troubleshooting with a known issue makes things even more difficult.

Parsnip

Sympton is fairly new, prior to the swap I had a bad iac and just manually adjusted it untill it idled around 500 and once warm it got to 900 ish, now with a proper iac it starts at 750 and climbs to 1500 if your on the brakes

Kaylabanana

So I am a super novice.... but i had a similar issue... for me is was on my 99 sport and would idle high randomly off and on- even while driving for no apparent reason. It ended up being the o2 sensors. Good luck!

cruiser54

Clamp off the booster hose as a test.

Have you tested/adjusted your TPS?

Parsnip

Will do, and tps is non adjustable, I'm using the late model tps, 're wired to the renix harness

cruiser54

Renix ECU with HO TPS?

Parsnip

Correct, same as with the map, but since the map reads pressure I figure it .ay not be as suspect

cruiser54

Well, an HO TPS works opposite of a Renix TPS.

Parsnip

Interesting, because running wise, it runs absolutely fine?

cruiser54

Unplug it and go for a drive.

Parsnip

Will do. How does it then read throttle position out of curiosity?

cruiser54

Engine side of a Renix TPS reads from .17 at idle increasing in voltage on up to WOT.

Trans side of a Renix TPS, and an HO TPS read from about 4.15 volts and DECREASE with throttle opening.

Parsnip

I still havent unplugged it but I have tomorrow off from work to test, but I guess my question is how does the ecu still read acceleration if the tps isn't telling it how far open the Tb is?