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Idle flare, 87xj

Ron Hyslop

NAXJA Forum User
Madison, WI
87xj, 4.0l, aw-4, every once and a while I get a short duration idle flare, up to about 2000rpm that comes back to normal idle as soon as it peaks. As per Satan's post back March 3, I've measured voltage drop across the flat TPS connector pins as follows: C to engine ground strap=.77v, C to battery ground=.77v, B to engine ground=.03v, B to battery ground=.03v, A to engine ground strap=4.68v, A to battery ground=4.67v. I think that B to both grounds being .03v means a good ground path through the c101 connector to the ECU and that my idle flare is probably caused by a vacuum problem or IAC/thottle body problem. Any comments, suggestions or smart remarks are welcome. By the way, anybody know the part number for the Renix TPS patch kit?
TPS is of course the prime suspect. you've probably noticed that adjusting and diagnosing this is a PIA.

Before I got into that, I took off my throttle body and thouroly cleaned it. pay special attention to the IAC hole, and other little nooks and crannys.

Another thing to check is the MAP sensor. it sences the pressure of the intake. there's a little vaccum hose going from the TB to the sensor on the firewall. clean the pins in that sensor, and make sure the tube isn't clogged or cracked.

Lots of people have this problem, and few have solved it, good luck
Ron - is your flare during "normal operation" (like just driving around), or is it happening right after you start it ...?

Numbers all look good for the conditions that you measured em in, but it still could be a situation with an intermittant dirty ground in C101 (or elsewhere along that path).

Assuming that the electrical stuff is all happy and joyful, looking at the idle air passages for cleanliness could be a good next step - especially if your XJ is experiencing a little blow-by. The stepper motor can stick (sometimes causing unexplained idle speeds) and often the passage will partially block or contaminate with blow-by gasses and their precipitates; the idle will compensate for the blockage, then at some point, the crud will suck past the idle pintle and leave a much larger air path (though these types of fluctuations are not usually on par with the 2Krpm that you describe.

While your at the idle passages, cleaning or replacing the Idle stepper motor itself may help (could just be sticky - the threads in the pintle do get a lot of wear in the same place over and over...)

Have you ruled-out a mechanical issues (the linkage on the RenixII throttles is not exactly simple and straight forward; binding is common enough to warrant another check as well. You idle return springs are "gettin' up there" in age...)?
Idle flare only occurs on startup. I replaced the throttle cable and bellcrank assembly a year ago because of a sticking throttle. New cable and bellcrank made a big difference in performance, I suspect the slop in the bellcrank was preventing the TPS from going through it's full range of motion. If somebody knows the part number for that TPS patch kit, I think I'll try that. I cleaned the thottlebody about ayear ago, probably should do that again. In general the car runs great, 148,000miles.The trouble is I'm a perfectionist. I've got a spare IAC motor, I'll try that too. Thanks for the advise!
Before installing the IAC - try soaking it in your favorite synthetic gun oil (NO SILICONES!!!) for a day or so first (point the pintle up and fill around it's base - I even bake mine in the oven to encourage oil-creeping!) -- not only with that loosen things up (be sure to drain-off the excess before installation) but it'll make it far easier to clean in the future...

A little flare is not uncommon; especially when the engine was shut off under load (a/c, fans, lights, auto-in-gear, etc) or not fully warmed, and subsequently restarted in better conditions (warmer or no lights or ...?)...

... The RenixII uses a b+ latch relay to hold power to the ECU after shut-down so that the ECU can open the IAC for the next restart (you gotta get SOME air in the intake so the fuel will get into the cylinders with something to burn). The ECU has no feed back on the IAC's position (in the "run mode" it's all about "more" or "less" - and there's no feedback without the engine running)... On top of that, the ECU also "opens" the IAC while the engine is cranking - so the longer you have to crank, the more the IAC will open when the engine finally fires-up (remember here that the ECU won't fire the injectors or ignition until it receives qty=2 CPS pulses after a valid transitions from the distributor's sync generator, or until the engine tries to fire in a limp-home mode in the absence of that sync signal - may take more than 2 full revolution of the engine, allows a lot of time for that stepper to open!)-- Of course, then, with a good TPS value (less than 0.8v) and solid data-clocking from the CPS the ECU will begin to control the idle by closing the IAC.

There are a few tricks to minimize the flair, but many will impact ultra-cold or low-density-air starting (up there in WI, you know all about them artic Lows that sneak outta Canada, eh??). We could banter those around here, or work em PM if y'wanna play around with a few of em!
Don't want to hijack this thread-hope any answers helpful to Ron as well. I also have this idle flare returning. Thought I had beat it with a throttle body cleanout. I removed the distributor to replace but my Autozone replacement would not fit so I reinstalled original. The two rebuilt ones I handled are looser than mine. I have a long crank time.
1. So maybe the long crank time is causing my IAC to crank open more?
2. How can you tell if the thing is starting in limp-home mode?
3. I don't understand how a sync sensor fails, there's nothing to wear. If the sync sensor is totally gone, will limp home mode run the thing perpetually? If the sync sensor fails, will it be gradual or sudden? I have good power and throttle response-just long crank times-the crank time is consistent-its not the CPS.
4. Can the fuel injectors leak droplets into the combustion chamber during off times and result in flare up upon ignition? I never have idle flare except upon startup-assume this means it is not the TPS.
Yo XJTRIPP, feel free to hijack! All the subjects you brought up are of interest to me. Satan, I'm game to try some of that other stuff as well. In one of your other posts, you referenced removing the IAC, and plugging the whole with your finger and setting min airflow. I assume this means setting the gap between the throttle plate and throttle bore, and where does adjustment of the little screw behind the welch plug enter in?
When you block off the IAC port, you want to adjust the screw on the right side of the TB that covered by the plug you mentioned. If everything is working right you should need to use the throttle plate stop to adjust idle. I've read of "adjusting" the CPS with a crow bar to reduce the gap between it's business end and the flywheel. This reportedly will help reduce the crank time when starting. I haven't yet tried this myself but if you "adjust" it too much then it hit the flywheel and is ruined.
The part numbers for the CPS kits (includes patch harness and CPS) are 83100066 (automatic) and 83100063 (manual). As mentioned the 87-88 models have problems at the C101 connector. I cut mine out soldered all the wires one at a time. There have been a few bulletins over the years for connection problems from the TPS to the ECU. There was even one that involved replacing the ECU.

Yes the CPS can be "adjusted" by using a long pry bar and hammer very carefully (or damage may occur). The optimum voltage for the CPS is .8V AC during cranking.
A few days ago I attempted to solve a fuel leak down by installing a BMW in-line fuel check valve (cost $3.83). Crank times are often shorter than before and I think I'm noticing less frequent idle flare-ups upon starting. Maybe during the preovious long cranks the IAC was opening more and more. I also recleaned the IAC hole after removing the IAC and sprayed the IAC pintle down good. If you soak the IAC, is it ok to submerge the IAC motor?