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Iac Controller?

pbandj

NAXJA Forum User
Location
AK
who can tell me what exactly these specific symptoms mean:
-1988 4.0L
-New intake manifold (used to leak, not any more!)
-No vacuum leaks and egr is a non-issue (disconnected)
-Starts fine but idles low/rough at first then slowly builds to high rev unless tps is unplugged
-IAC sucks really hard and causes 1500rpm rev only when tps is plugged in and then the throttle opened and closed (but returns to normal idle speed when I plug it with my hand or something)
-TPS unplugged= engine idles normal and IAC doesn't cause high rev even after throttle is opened and closed (I feel no sucking from the IAC port then)
-IAC/throttle body/intake manifold/upholstery was just cleaned and still does problem
Sooo, what causes the IAC to hold higher idle speed when the tps is plugged in? is it a faulty tps? or is the IAC controlled by cumulative sensor variables via the computer?

I am gonna take a quick little beer run while you all chime in on what my quick fix is:) (after this much messing with the motor I've got a high idle problem too)

BTW can anyone tell me why the motor will still (barely) run when you disconnect the vacuum hose to the MAP sensor but will instantly die when you unplug the MAP sensor harness?
 
Just a question where did you disconnect the EGR? I plugged mine next to the EGR valve, didn´t help my rough idle. On a hunch, I disconnected the transducer and plugged that line. Smooth idle. Transducer was leaking vacuum, not the EGR.
I changed my TPS sensor out readings to like 86% or around 3.9 volts. Idled low. But sure had better throttle responce and better shift points.
Was wondering myself, about the relastionship between the TPS and the IAC. Mine IAC also seems to get some imput from the temp. sensor.
I bought my XJ, from the dealer, the previous owner refused to pay his bill and said keep it. One of the first things I noticed, was that someone had bent the tab on the throttle, that sits on the throttle stop, to adjsut the idle. Found out later the IAC was jammed upwith gunk. I´m still trying to get everything back the way it should be or close.
My idle fluctuates from 600-850 depending on motor temp. (above a 1000-1500 momentarily at start up, depending on motor temp.) The idle moves lower, right with the temp gauge going higher. Occasionally on a warm motor, it revs to 2000 or so on a restart. I may have some intake issues, gaskets are going in this weekend.
 
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actually my egr vacuum line is disconnected and plugged at the manifold, should be foolproof there. I have been trying to figure out how to test the tps with my meter but have gotten conflicting accounts of where to probe and how, and I am not wholly familiar with my meter either (is a millivolt .1, .01, .001 volts?) stupid autoranging meter is outsmarting me and I am trying to lock it on one range so I can make some sense of it.....
 
pbandj said:
TTT

c'mon guys what is controlling the IAC? someone must know something.....

All Data say. SOME (leads me to beleive there are additional imputs) of the sensors that imput the ECU to control the IAC are, coolant temp., knock snesor, TPS and the A/C request. I`d guess the 02 sensor also has to have some imput.
 
hmmm, thanks 8mud. btw are you getting kickbacks or a comission from alldata? I've seen you advertise them in a couple threads....
 
You never hear much about TB gaskets anymore, but I've found more than a few that have shrunk and the TB bolts have loosend. Or like mine, that were reused, baked hard and the TB didn´t seat in exactly the same place and caused a vacuum leak.
 
02 doesn't do anything until the system is in closed loop mode. TPS has some influence but a weak/sluggish IAC will stick in the open position on HOT startup. Remember the latch relay holds ECU power for a few seconds after shutdown so the ECU can park the IAC open for the next restart. Notice how the engine RPM always starts high but "usually" returns to normal in a second or so? I once had a 3 month old Kragen IAC that stuck high when HOT. Changing IAC corrected that problem. Keeping those 4 IAC wires in good condition is important too. If just one breaks the circuit the IAC will freeze in position.

But what sensors directly affect IAC? I figure the MAP has some influence but I suppose the temp sensors do their thing in the picture too. TPS has some influence but I haven't been convinced it's a huge part of the IAC/ECU routines.
 
pbandj said:
hmmm, thanks 8mud. btw are you getting kickbacks or a comission from alldata? I've seen you advertise them in a couple threads....
I just like the way they break, the electrical diagrams, down into sub systems and bite sized pieces. Though there is some large holes in there information.
 
Mine 87xj over revved on startup, it turned out to be a bad TPS ground path through the c101 connector. If you have the c101 connector(it's a big junction block above the master cylinder), take it apart and clean all the contacts. I believe they disscontinued the c101 connector in mid 88.
 
I do have the c101 but I already cut and spliced all the tps wires running through it... my c101 is full of what looks like rtv, like someone thought they were being smart and sealing it but just made a big PITA mess

so XJXJ- you are saying that you replaced your IAC and that did the trick for you? and a bad temp sensor could reasonably leave you in cold start (choked) mode all the time as far as the IAC went? if you unplug the manifold or coolant temp sensor would the computer think it was always cold or always hot?
 
The engine (coolant) temp. sensor (lower left of the block) and the MAT have the same values at the same temp. I check them for resistance in the morning, figure the motor and the air temp are close to the same. Should have close to the same resistance. The lower the temp. the higher the ohms. Don´t really know if infinite resitance would default to open loop or what. Have heard the motor operates open loop, till it reaches around 140 coolant temp (though mine seems to change to closed loop at a lower temp than 140). Supposed to be really bad for the mileage to run it in open loop.
I tested my MAT and engine temp. sensor out of the Jeep, with an ohm meter and a digital thermometer.
I had some WD-40 in my TPS connector, that really messed with my values, TPS operates on fractions of a volt (I usually only measure to one decimal point), doesn´t take much to mess up the ECU imputs.
Test the grounds on your TPS, the TCU grounds to the dipdtick holder and believe so does the ECU. There is some slight ground resitance through the ECU and TCU to ground, I checked two TCU´s and ECU´s, they all had some very small ground circuit resistance inside the controllers.
The harness to the knock sensor, motor temp. sensor and the 02 sensor runs down the front of the motor and is prone to frying on the exhaust. Worth a look.
i just got through squirting some penetrating oil down the IAC orifice (while the motor was running) on the wifes 96, IAC problem disappeared (at least for now), Her IAC was sticking closed. Engine would stall at start up.
 
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pbandj said:
so XJXJ- you are saying that you replaced your IAC and that did the trick for you?

For the condition I described - High idle upon HOT restart and/or sluggish to drop RPM? Yes, a new IAC solved that trouble.

pbandj said:
and a bad temp sensor could reasonably leave you in cold start (choked) mode all the time as far as the IAC went?

As somebody correctly mentioned, the resistance goes higher as the temperature goes lower. But, the IAC actually lets more air IN so I don't look at it like a "choked" condition. I doubt it would make 2,000RPM difference in any case but it may influence things a couple hundred RPM.

pbandj said:
so XJXJ- if you unplug the manifold or coolant temp sensor would the computer think it was always cold or always hot?

Go ahead and find out although we now know how the thermistor reacts. But, with this damn Renix ECU I've found unplugging things when it's running good is a legit way to "learn" what's going on. You will not harm anything and the lesson learned may help you later on down the road.

I also believe many things get lost in translation with Renix. It was designed in a foreign country so I don't expect everything in manuals to be correct. I use FSM as a guideline and to help qualify wiring paths. I do wished there was reliable table of operation routines but without that, we are left to figure it out with our best guess and a little experimentation..

Adjustment-wise, what has worked well for me is/was to adjust a HOT idle RPM with the IAC fully closed and then perform a good TPS adjustment. Pulling vac hoses will usually force IAC into a fully closed position. Re-connect hoses, unplug IAC, adjust throttle plate for about 600-750RPM HOT, perform TPS adjustment, done.
 
There is a way to adjust the throttle plate?

I can't get my TPS to show the values it should. I have the same diagnosis as pb, and dunno what it could be, so as part of it, I replaced the TPS, after discovering my current TPS didn't work.

Now, I have one that varies resistance, but I can't get it to return more than 1.2 volts to the computer.
 
over2land said:
There is a way to adjust the throttle plate?

Yes, it's hard to see but the throttle stop is an allen screw. Best to take it off, clean the TB/IAC and find the right size allen wrench. Gently pull IAC pintle out enough to seal-off air, re-install but leave IAC connector off, warm engine HOT, adjust throttle stop for 600-750RPM. Re-connect, adjust TPS.

over2land said:
Now, I have one that varies resistance, but I can't get it to return more than 1.2 volts to the computer.

What's the ECU supply voltage? Should be around 5Vdc. Are you using the ECU sensor ground for reference or engine ground? There is a difference and you should be using the former. Manual transmission or auto?

1.2 Vdc is a LOT of throttle position for an auto transmission TPS. My last adjustment was about 4.40Vdc for a 5.3Vdc supply in idle position on the square connector, of course I'm an auto transmission. Voltage drops as the throttle is opened.
 
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When I tested it my input was 4.77V, and the ground was the TPS ground, like the FSM says.

I've charged the battery since then, so I'm sure my input voltage would be higher now, but still...

It's a stick, and 1.2V was the most I could get out of it, in any orientation.
 
over2land said:
I've charged the battery since then, so I'm sure my input voltage would be higher now, but still...

Maybe, maybe not since the 5Vdc is supplied by the ECU it's likely there's a voltage regulator IC handling the regulation.

over2land said:
It's a stick, and 1.2V was the most I could get out of it, in any orientation.

1.2Vdc is too high for stick. 0.73Vdc would meet the FSM .170 ratio at idle position. If it doesn't change at all make certain you have the TPS installed properly and the actuator arm is engaged with the throttle linkage. If it still doesn't change, you either have a bad TPS, your measurement procedure is faulty or perhaps wiring on the way to ECU is goofy.
 
Sequoia sent me a diagnostic procedure. Dunno where he got it from...

But, it stated, measure the input voltage, and the output voltage. Then divide output by input. The number (coordin to that) should be .8 Although a range of .75 to .85 is acceptable.

this is all with the butterfly closed.

I've replaced the TPS. The one that came with it didn't do anything regardless of position.
 
update on the iac sustained high idle- now it is super low, maybe 4-500rpms and didn't even have a higher rev warm up cycle.... anyone know any good french swear words that I can encourage my renix with? at least I don't have that peugot tranny stuck behind my motor:)
 
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