Renix TPS out of spec but seems to work -- should I care?

Short version: with TPS output voltage at spec, idle is crazy high. Below spec, idles and drives fine. Does the voltage out of spec matter if the Jeep works OK?

Had been chasing low/rough idle on my 89 Renix auto. New O2, new EGR which turned out to be bad and I put the old one back on, cleaned IAT, checked sensor grounds. Running pretty well, idling at about 700-800 RPM, drives fine.

Wondering about the remaining slight idle roughness went to check the TPS (using Cruiser's instructions.) Output voltage was about 0.2V against a 4.88V reference voltage. Tried adjusting it up to 0.83V which is what was indicated by the REFERENCE * 0.17 calculation. Start up Jeep, idles at about 3K RPM and doesn't drop.

Backing it off the TPS output until the idle seemed normal landed me at about 0.20V again, with the Jeep seeming to idle and drive fine. Found a new replacement Mopar TPS and put it in. Noting Cruiser's tip that "you may experience a high idle upon starting" I followed the instructions to back it off until the idle dropped to normal then gradually dial it back up to spec. Doing that while it's running worked fine, and the idle stayed sane even at 0.83V output voltage on the TPS. Starting it up again, though, it was back to crazy idle. After multiple adjustments and starts seems I could run the output voltage at about 0.5V throttle closed without getting the crazy high idle at startup. But couldn't get it to spec.

• Identical behavior on the old and new TPSs.
• TPS voltage measured both with a meter tapping the sensor wires and an MT2500 plugged into the ECU. Values were identical on both.
• Sensor ground from the same taps on the TPS harness reads <1 Ohm resistance consistently to negative battery terminal.
• Has a 5-90 grounding kit on it.

So I've confirmed that the Jeep runs OK with the TPS output way off spec, and that I don't know what I'm doing from here. Question is should I care or not?

The check the O2 sensor voltages, per my old thread on O2 sensors here.

Check the CTS and IAT temperature sensors against the factory manual specs. The specs are posted here in old threads. Need to compare resistance of the isolated temp sensors verses their ambient / actual temp. Boiling water and ice water work great for the unmounted sensors. I have O2 sensor issues (sensor and damaged wires) and the IAT sensor give me crazy high idles randomly.

Lastly and most important it really sounds like you have a hidden vacuum leak. They be just about anywhere. I found some of mine just after turning the engine off.

Does Cruiser have a how to find jeep vacuum leaks write up? Very small Vac leaks can cause a 3000 rpm idle. Have you checked the lines all the way to the bumper vac storage bottle and the bottle? The inside dash AC/Heat vac lines? And the fire wall vac lines to the cabin AC accesories is a common place for breaks and leaks. Intake manifold is very possible!!!!! I had to sand and smooth out warpage in the exhaust manifold with out taking off too much metal to get the common clamps on the Intake-exhaust bolts to work right and seal the intake and exhaust. No fun!!!! Throttle body gasket, IAC o'ring, injector intake manifold o'rings.

I am satisfied that the TPS is not your problem, nor its wiring!!!

Try disconnecting the O2 sensor and IAT sensor and CTS while the jeep is running and see what the idle does. Might help eliminate them leaving just a vac leak.

Ecomike said:

Try disconnecting the O2 sensor and IAT sensor and CTS while the jeep is running and see what the idle does. Might help eliminate them leaving just a vac leak.

Click to expand...

I'll give them a look. Already got a spare CTS but haven't figured out a way to actually install it without pulling the manifolds.

Being a bit more obtuse though, are we actually talking about vacuum leaks on the O2 and CTS? I can see the O2 letting gas in/out of the exhaust, but I don't see vac lines on either sensor. Guessing this is part of that me not knowing what I'm doing thing, but I'm trying to get there.

One last question if you know: what are reasonable vacuum readings? The ECU tells me what it's seeing via the MT2500, but I haven't found a useful reference table.

Oh, and the O2 sensor is new. Old one was fluctuating but sticking up near 5V. New one fluctuates in either a low range (~1.x V fluctuating) or a high range (~4.x V fluctuating) as I think it should based on what the MT2500 Troubleshooter is telling me and other threads. Engine also drops neatly into closed loop at idle with the new sensor, didn't with the old one. But still got the TPS set right == crazy idle with that new O2 sensor.

I was not talking about vacuum leaks around the CTS or O2 sensor, but an exhaust leak (not vacuum) can screw with O2 sensor data to the ECU.

The new O2 sensor should show a narrow 2.0 to 3.0 volts or even tighter range at 2000 rpm.

I was talking about intake manifold and vacuum lines attached to the intake manifold that wander all over the place. Intake at the throttle body to manifold gaslet, intake at the injector ports (o'ring seal), the valve cover vacuum lines, lines to the air filter box and cold morning start up solenoid. vac lines at the passenger side front bumper tucked inside the bumper, vac lines at the passenger side firewall that feed the vacuum to the AC/Heat control dampers....Get your self a diagram. The is a factory one at the drivers side fire wall.

Sounds like you CTS and O2 sensor are in good share, leaving a vacuum leak (or a bad IAT intake temp sensor in the intake manifold) which is no longer made but can usually be cleaned and restored with a gentle solvent or soap and water cleaners. There is a substitute IAT available but you need to change the wiring connectors.

Ecomike said:

I was not talking about vacuum leaks around the CTS or O2 sensor, but an exhaust leak (not vacuum) can screw with O2 sensor data to the ECU.

The new O2 sensor should show a narrow 2.0 to 3.0 volts or even tighter range at 2000 rpm.

I was talking about intake manifold and vacuum lines attached to the intake manifold that wander all over the place. Intake at the throttle body to manifold gaslet, intake at the injector ports (o'ring seal), the valve cover vacuum lines, lines to the air filter box and cold morning start up solenoid. vac lines at the passenger side front bumper tucked inside the bumper, vac lines at the passenger side firewall that feed the vacuum to the AC/Heat control dampers....Get your self a diagram. The is a factory one at the drivers side fire wall.

Click to expand...

Yeah, looks like I'll need to give them a proper going over rather than just a bit of spot-the-leak. I have a vac diagram in the FSM. Reservoir and lines from behind the bumper are already relocated up near the relays from when I put the JCR bumper on.

spaceharrier said:

Yeah, looks like I'll need to give them a proper going over rather than just a bit of spot-the-leak. I have a vac diagram in the FSM. Reservoir and lines from behind the bumper are already relocated up near the relays from when I put the JCR bumper on.

Click to expand...

... and curiouser. Idle still weird with TPS at spec.

Replaced 'front' and 'rear' vacuum harnesses (which are actually two halves of the same harness) and the rear vac line from the valve cover with its associated grommet. MT-2500 says the ECU is reading a steady 18-19" Hg vacuum at idle.

Also replaced the EGR valve with a Mopar part, suspecting that it might not have been closing fully when ECU commanded closed.

So everything reads sensible on the MT-2500 at idle: 18"+ of vacuum, closed loop, short term fuel trim around 128. But put the (new, Mopar) TPS up to spec output voltage (0.83V) and start idle is crazy high (>2.8KRPM) and stays there. Back it off to 0.51V and idle seems fine.

Jeep still seems to run very well, with good power, good shifts and without serious idle issues (still a little fluctuation at idle, but not enough to be alarming.)

One of the first things I do when suspecting a vacuum leak is to cut some short pieces of tubing to fit over most of the vacuum inlets on the intake manifold. I Plug the ends of the tubing with an appropriate screw or bolt. You can plug off everything but the fuel regulator and the motor will run fine. I save the tubing pieces in my tool box for next time.

Just for info, put your meter on a low voltage setting and check for voltage between the TPS ground and the battery negative, with the engine running. Only takes minutes and can be an indicator of an issue. Two threads of a 12-14 gauge wire will pass an ohm, but very few amps.

Another oddity that can/may be an issue is alternating or high frequency voltage in your DC system. Don't really know if it is an issue or not, but I have measured it when alternator diodes are failing etc. Some meters will pic it up on the AC voltage mode.

Good luck with your quest. Been there, done that

You've covered most of the common stuff, time to grasp at straws.

I had an erratic idle, sometimes it was high, sometimes it was low and I had trouble repeating my TPS adjustment after a few days. Turned out to be the ground for the TPS (which the TPS shares with lot of stuff on the fuel rail etc. and sensors). The ground always ohm tested within spec. but part of the that ground tree was just sitting in an uncrimped splice and not making consistent contact.

8mud, don't forget one vacuum line he needs to keep (do not plug) in the test, and that is the air flow into, and through the engine valve cover and out to the intake manifold, through his new PCV hose set up he just bought and installed.

I am convinced he has a hard to find vacuum leak from reading all his posts.

Still haven't managed to track down a vacuum leak. While spraying lines, though, I did manage to brush a spark plug wire and get a nice high voltage twang. So replaced the plug wires, having turned it off and noticed a couple of others didn't seem as tightly seated as I'd like.

Seems to idle smoother, still seems a bit like witchcraft.

Alot of those sensors share the same ground and it's many times the problem.

From www.cruiser54.com:

http://cruiser54.com/?p=19

http://cruiser54.com/?p=44

http://cruiser54.com/?p=54