My XJ is trying to kill me!! More Renix high idle nonsense.

I don't even know if I want to call it a high "idle" problem. At cold start, the engine revs itself up to 3000 rpm, as if the throttle is stuck. Putting the truck in gear brings it down to roughly 1500rpm, but as soon as you take your foot off the brake, it takes off as if you're standing on the pedal. Most of the time this will calm down a bit once the truck warms up, but not always.

Today I almost rear-ended someone in traffic because the truck tried to outdrive the brakes. Sometimes I can get the problem to clear up by turning the key on and off a few times before starting, but not always. The truck usually has to be warmed up a bit for that. En route to work(after my near-wreck experience), I put the truck into neutral while cruising down the highway, and the idle ran itself all the way up to 3500 rpm(!!), then it dropped down to a loping ~1500rpm "idle". At other times, I have been able to get the idle to drop down by jiggling the relays, though that's not a consistent thing. I have also been able to do it by disconnecting the IAT sensor, but the idle fluctuates so much that the truck is undriveable.

Things I have done:

*Cleaned and added grounds. I added two large ground cables, one running from the battery to the firewall, and one running from the firewall to the passenger's side of the engine. Battery ground cable is brand new OEM. Battery terminals have been cleaned and checked.

*Cleaned and dielectric greased sensor connectors. TPS, IAT, o2, IAC, CTS, and other connectors have all been cleaned and greased. The knock sensor is brand new.

*Cleaned and greased relay connections on the passenger's side of the engine bay. Replaced LATCH relay.

*Checked and adjusted TPS to 0.827 ratio. This has been checked numerous times.

*Cleaned, checked, and lubed IAC valve. Twice.

*Checked plugs, wires, cap, and rotor - all are in near-new condition.

*Replaced ignition control unit(under the coil) for an un-related(at least I assume) problem.

Other problems I'm having that may or may not be related:

*Hesitation and near-stall(sometimes it does stall) at throttle tip-in. It's fine if I don't touch the throttle, and it's fine if I gun it, but if I apply very light throttle(think of sitting in stop and go traffic), it stumbles and tries to stall. It's twice as bad going in reverse(backing out of a parking space, for instance).

*Intermittant power loss, usually clears up at about 3000rpm. This has gotten better since I cleaned the engine bay connectors up, but it still does occur.

*Occasionally backfiring, severe power loss. 2 days ago, I drove to work, and the truck was fine(aside from the inital high idle problem of course), but when I got off the highway, the truck tried to die every time i pushed the gas, backfired through the intake, and just generally ran like shit. Cleared up after I stopped, turned it off, and re-started it.


I'm open to suggestions at this point, but I'd really like to get to the bottom of this so I can enjoy my truck again. I don't trust it enough to let my wife drive it out of fear for her safety.I'm open to suggestions at this point, but I'd really like to get to the bottom of this so I can enjoy my truck again. I don't trust it enough to let my wife drive it out of fear for her safety.

Thanks in advance for any suggestions!

On my 90 Renix I had to set my TPS on the ECU side to .44volts from the original setting of .80 volts which is the correct 17% of my input voltage.

Set your TPS to .50 volts and keep going lower if you must. Just make sure when you go WOT that it reads atleast 4.70 volts.

Have you checked the TPS with a meter, to make sure there is a smooth transition across its range?

I would also check the bolts on the exhaust manifold.


I had the same problem. Search for a post called "the renix files" it has everything that could cause it.

For me, it was TPS, and loose bolts on the very back bottom of the manifold.

Mine was loose manifold bolts - and they can be a little hard to reach. To check, spray some carb cleaner around the injector bases - if it revs up, it's loose.

The computer sees this as a slowly developing leaness and compensates the injection until it can't handle it. After tightening, you may want to reset the computer to baseline so it readjusts more quickly - which will cause some driveability roughness until it maps the current ranges again.

My 87 xj had this same problem. All my intake/exhaust bolts where loose. Some of them can be a bear to get to,like the bottom ones . I used a wrench on most of them, It ran great afterwards. Good luck

What year XJ, if you mentioned it I overlooked it. Some general stuff, pay attention to whether the problems are cold problems, hot problems or occur at a specific engine temperature.
The IAC needs a lot of current to function properly, mine messed up a lot worse when the battery was weal or I was having charging problems. Not to mention grounds and connectors. IAC#s can also be temperature sensitve, the sometimes mess u cold and work OK warm.
A marginal CPS can give al sorts of grief, they can also be temperature sensitive. Start but barley run, run like crap as soon as a certain temperature is reached and other oddities.
You have to make sure the die electric grease you are using doesn't induce any resistance in the connectors. Most all of the sensors are pulse or resistance. Excess resistance confuses the ECU.
It sounds like you have more than one problem going on. Best guess is low voltage someplace and/or a marginal CPS or MAP sensor.
Loose manifold bolts were mentioned, possible, this could also affect MAP fiunction (low vacuum). Vacuum leaks at the manifold often cause a hunting idle, the idle will go up, then the ECU will correct, it will go down some and then repeat in a kind of cycle.

Maalox,

I recently went through high idles on my 88XJ. The first thing I discovered was a crack in my manifold. The previous owner had cleverly coated the crack with an aluminum epoxy when it was small and I never noticed it until the crack grew larger. I changed out the cracked manifold and that helped some. Then I ran all new ground cables, plus some extras, and ran separate grounds from my TPS to the negative terminal of my battery. It got steadily better. During all this, it seemed like I had pulled connectors apart a million times to clean them. Then I recalled some connectors on my son's FSJ that were always green, no matter how many times we cleaned them. So finally I decided that I must have a stubborn oxide layer on my connector pins and bought some Deoxit. I applied it to my connector pins and that seemed to help a lot. My XJ is running pretty well now.

Best regards,

CJR

Damn I know the feeling. My 88 use to do that all the time, very scary. It would accelerate from 45-50mph without my foot even touching the gas. I think I tried just about everything. Replaced vacuum lines, tps, cps, etc, but i could never completely solve it. I would start with the header and go from there.

First I would be trying to see if you have any intake manifold leaks (especially on the intake gasket.) Then try checking all of your sensors. Im thinking your IAC is bad.

http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm

Another thing that can mess with you is water (or moisture) in the TPS. Most times it happened to me the idle would go way up at start, but then may eventually settle back down to something nearly acceptable (eventually). It was often kind of a random idle RPM, mostly high.
It may adjust correctly but still mess up during the sweep from .8 to */- 5 volts. The higher the voltage the more likely to have a moisture driven electrical leak or partial short inside the TPS.