Renix idle speed anamolies '89, 4.0 L

[Ecomike]

My daughter's DD. Thought I knew what the problem was, guess again. Idle speed out of whack, but before you jump to conclusions, here is the story.

About 6 weeks ago it was getting hard to start. The symptoms, overall, had me thinking a sticking IAC (Idle Air Control valve), it was used one I installed 2 years ago after doing the head gasket replacement. I found a little, possible problem with a loose battery, tied things down better....problem went away.


Then 2 weeks ago (apx) I replaced the AC drier, HP hose, and expansion valve, pulled vacuum, added 134a, got AC working (finally.....). Every thing included AC worked perfectly for about 10-12 days.

Friday it was idling way to slow in 5 pm traffic, shaking at idle, and the gauges said it might be over heating (I have since determined it was not overheating (or is not overheating), entire cooling system is working very well).

So I cleaned the throttle body, IAC inlet on top with carburator cleaner aerosol with engine running. The idle speed was 1200 rpm after the cleaning.

So I bought a brand new IAC, installed it, but no joy. I have stopped and started it about 6 times over a 24 hour period, and no change.

I have checked the grounds (all of them, battery cable, both engine grounds, all 3 body grounds, the TPS ground, and the ITS/MATS [Intake Manifold Air Temp Sensor] grounds, and every one is reading 0.78 ohms, and about .03V with engine running, so all the grounds are excellent. TPS is reading .78 Volts at idle, and the TPS reads normal on all ohm tests. The MATS ohmed out normal at ambient temp.

I disconnected the TPS only, MAT only, then both, several times. Started the jeep with the TPS not connected, then later connected the TPS with the engine running, non of these tests made any difference, idle speed stayed at 1000 cold, 1100 hot.

Turning on headlights drops RPMs about 50, turning on AC drops RPMs another 100, then shifting into R or D drops RPS another 200 (apx) RPM. In drive RPM is about 150 RPM above normal, and at idle with no loads it is about 350 RPM above normal.

I do not think the IAC is working at all. I am trying to remember if there is a relay in the IAC circuit, and if there is, which one it is.

In the morning I may pull the new IAC for inspection of the IAC and the throttle body port area. I also need to determine if the IAC is getting any power at all next (relay????, B+ relay comes to mind from memory too).

I am almost 100% positive this is not a vacuum leak issue, as I have also tested for and inspected for that.

At this point I suspect no power to the IAC, or trash in the IAC throttle body port/seal area that is limiting the IAC pintle movement.

 

[xjtrailrider]

There is the latch relay, it resets the IAC on shut down for restart but I don't think that is your issue.

My 90 idles low no matter what i do to it, even if I jack around with the idle screw it readjusts to 450 RPM. With headlights/AC/stereo on it chugs like a John Deere tractor. If someone can figure out what makes a RENIX idle high or low please let me know, its one of the mysteries of life.

You have covered all of the major issues, I really have no suggestions for you.

 

[Ecomike]

There is the latch relay, it resets the IAC on shut down for restart but I don't think that is your issue.

My 90 idles low no matter what i do to it, even if I jack around with the idle screw it readjusts to 450 RPM. With headlights/AC/stereo on it chugs like a John Deere tractor. If someone can figure out what makes a RENIX idle high or low please let me know, its one of the mysteries of life.

You have covered all of the major issues, I really have no suggestions for you.

Thanks. The "B+ latch" relay is one of the ones I need to check, but I don't recall if it powers the IAC while the engine is running, or just after shut down.

Have you read my old thread "the RenX Files" here, it covers a number of causes of low idle on Renix, and high idle issues.

http://www.naxja.org/forum/showthread.php?t=905849&highlight=Renx+files

 

[5-90]

Let's see...

You said you'd serviced all of the grounds. Have you added a ground between the battery and the chassis? I don't recall as I'd read you did - and it can fix a broad assortment of problems (it's one of the few things that ChryCo actually managed to improve over AMC - I've never been impressed with anyone's implementation of OBD-I...)

You've changed out the IAC. Clean the seat for the IAC pintle as well? While you're about it, consider just dismounting the throttle body and cleaning it as thoroughly as possible - pipe cleaners and carburettor cleaner come in handy for detail cleaning, and an old toothbrush and carburettor cleaner for the gross work. The EGR craps everything between the underside of the throttle blade and the intake valve heads with carbon. I've never liked the EGR concept, and there are much better ways to take care of high NOx (that don't cause additional problems in exchange.)

What's the actual reference voltage to the TPS? Recall that the TPS isn't set to idle at any particular set value, but it's relative to the reference voltage supplied (17% of reference,) and therefore varies as the reference voltage supply circuit wanders. I've corrected a number of idle issues that resulted after the IAC was replaced, due to improper adjustment (it was set to the absolute value of 0.83VDC, and not to the actual 17% of reference value. Correct the adjustment, and you usually correct the problem.)

@xjtrailrider - most of the issues you're likely to run across are covered in "RenX Files". Only a few things I can add offhand:
- Clean up the grounds! Amazing how many automotive electrical problems come back to failed/failing/contaminated grounds. This is particularly a problem on RENIX, since the ECU is grounded to the chassis. The engine is grounded to the chassis via an uninsulated braided strap (silly idea) and through a painted surface and screw hole (downright stupid idea!) Replace the ground strap if it fails inspection (it's at least twenty years old - I'd say just replace the thing outright.) Make a new one using wire of at least 10AWG size, and clean the contact patch on the body down to bare metal. Replace the screw if indicated, and you can run the engine end to a more convenient location (the rearmost fuel rail mounting screw works neatly.) The fuel rail screw is 5/16"-18x3/4", and the chassis screw (I believe) is M8-1.0x20m/m.

- Check the vacuum line from the MAP sensor to the throttle body (pax side.) This can often come loose, and need to be pushed back in. If it doesn't retain on its own, put some RTV on the upper (blind hole) bung and push it in. Problem solved! MAP sensor reference voltage (it's a voltage divider, similar to the TPS) should be within a few percent of the TPS supply voltage.

- Check the HEGO supply voltage as well - as I recall, the RENIX HEGO is a variable resistance that is also used as a voltage divider (and the HEGO and TPS, by the by, are the only sensors on the RENIX rig that aren't standard GM parts of the vintage.)

- As mentioned, the rest of the sensors are standard GM goodies. They follow the thermistor response curve (check your service manual - I know the FSMs and TSMs will have a chart of values for them, I believe Chilton's does as well. I don't count on Haynes...) The IAT and CTS are particularly useful values for fuel metering - and fuel metering signals are considerably more important at idle than at part throttle or WOT.

- You don't mention - does the idle problem go away at any particular time? Will warming up the engine settle it down? Is there anything else? Anything that exacerbates or improves the problem condition would be instructive.

 

[joe_peters]

Oops.

Auto trans TPS output should be 83 percent of reference voltage, manual trans TPS output should be 17 percent of reference voltage.

I'm not convinced about the vacuum/EGR system. Start plugging things off and re-checking.

 

[Ecomike]

Oops.

Auto trans TPS output should be 83 percent of reference voltage, manual trans TPS output should be 17 percent of reference voltage.

I'm not convinced about the vacuum/EGR system. Start plugging things off and re-checking.

Joe, the 83% is for the auto, but it requires setting the tranny side of the TPS (4 wire side). The older FSMs 87 & 88, IIRC, used .79 Volts, and .82 volts respectively as the setting for the TPS (auto) using the three wire engine side of the TPS. In 89 they the FSM changed to calibration on the Tranny side of the TPS using the 83% for the Auto transmission.

One of the things I learned is you can calibrate either side on the Auto tranny TPS, BUT MUST VERIFY the numbers, the high, low, range and any noisy spots on both sides of the TPS, as any one, and only one of those areas can be OFF, and give you bizzare results!!! Also, the input voltage (like 5-90 mentioned) from the ECU and TCU, must both be checked, and the ground to both sides of the TPS must be checked (they use 2 different grounds!!!!!!).

Vacuum leaks tend to cause variable idle speeds, as the engine warms up, for instance the leak size can change (intake manifold, throttle body). That is not the case here. The fact that the idle increased from about 300 to 1200 after cleaning the throttle body IAC air inlet port(s) is one big clue. The fact that the idle is only changing now as loads are added (AC), (falling as loads are added, and note that turning on AC on Renix normally raises the idle 100 rpm, in this case it is dropping 100 rpm when the AC is turned on) is the other big clue in my case, that the IAC is not functioning at all!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!. Since the new IAC did not fix the problem, that leaves no power to the IAC as probable issue with mine. Does anyone have the IAC signal wiring diagram handy, a jpg or gif?

5-90, in my case I have the battery to body ground added, I have grounds, grounding the grounds to the grounds on mine, LOL.

 

[joe_peters]

I've been schooled! :worship:

 

[5-90]

Oops.

Auto trans TPS output should be 83 percent of reference voltage, manual trans TPS output should be 17 percent of reference voltage.

I'm not convinced about the vacuum/EGR system. Start plugging things off and re-checking.

As Mike mentioned - it's 83% for the transmission side of the A/T TPS.

Since I invariably set my TPS replacements using the ECU reference voltage, it's 17% for me (and I just check the engine side of the A/T TPS.) Since the ECU reference voltage needs to be checked anyhow, I found it easiest to just develop a "standard routine" and always check the three-pole plug as well.

I sometimes verify adjustment on the TCU side of the A/T part, but I will always set adjustment on the engine side. The manual doesn't say to do it that way, but I've got more than a few other things that I do, that the manual doesn't tell me to do, but I think work better when they're done.

 

[cruiser54]

5-90's advice on the grounds is super important to follow. I've seen the results of cleaning and adding grounds. Another thought, in addition to performing the ground refreshing which I consider maintenance on an XJ, especially Renix, is to check if your intake manifold bolts may have loosened up. Wouldn't be a bad idea to fill a spray bottle with water, set on STREAM, and spritz around the intake and vac hose areas. The engine will let you know when you have sprayed the leaking area by changing idle speed.

 

[5-90]

5-90's advice on the grounds is super important to follow. I've seen the results of cleaning and adding grounds. Another thought, in addition to performing the ground refreshing which I consider maintenance on an XJ, especially Renix, is to check if your intake manifold bolts may have loosened up. Wouldn't be a bad idea to fill a spray bottle with water, set on STREAM, and spritz around the intake and vac hose areas. The engine will let you know when you have sprayed the leaking area by changing idle speed.

Carburettor cleaner, in a fog around suspect areas, will be considerably more obvious than a water spray or stream ever will (and it's better for finding vacuum leaks farther away from the intake as well - water has a "range of utility" that gets in the way.)

Manifold screws will usually (not always...) present as an idle issue that goes away when the engine warms up. If tightening them improves the problem (but doesn't make it go away,) it's gone on long enough to burn through the gasket.

For this reason, I will coat both sides of the manifold gasket with a thin, even layer of RTV copper upon installation. I also check torque on manifold screws annually (I plan to experiment with "true" Belleville spring washers one of these days, to see if the annual torque check can be eliminated safely.)

 

[Ecomike]

In the past, in probably over 100 tests, with a working IAC,

disconnecting only, disconnecting and reconnecting, with engine running, starting the engine with TPS disconnected, then reconnecting, and so on ALWAYS affected, and showed a change in the idle speed at some point. I am not getting any changes in idle speed this time, so once again I am convinced this is a no signal to the IAC incident, as the TPS is working and all voltages to the TPS are correct.

So other than a rock in the throttle body lodged in the IAC port area, what and where could the power be lost to the IAC, other than the ECU? In my case all the grounds, included the dozen or so I added, are all perfect. This is an 89, so there is no C-101 bulkhead connector.

Could shorting out the underhood trouble light blow a fuse that also powers the IAC circuit (relay?)?

 

[Ecomike]

I just reread the Renix FI, FSM manual I have, and I see nothing between the IAC and the ECU, but 4 wires , no relay shown, so I am going to check the IAC wires for pulse signals shortly, then maybe pull the IAC and inspect the TB IAC ports next.

I am also thinking of disconnecting the O2 sensor, to see if I have leaky fuel injector(s), that the O2 sensor/ECU feed back control may be compensating for (but I doubt it since the idle does not seem to change speeds at all except when a load is added, but I will try it just out of sheer curiosity).

 

[PaulJ]

An easy way to figure out if you have a vacuum leak or not is to cover the throttle body with your hand. If it still runs fast, you have a leak that you'll need to find. You can put your thumb over the IAC port on top of the throttle body and see if the IAC is simply passing too much air. If it is, it's broken or not be controlled properly by the ECU. Also put a thumb over the idle port on top of the throttle body on the driver's side and see what happens.

 

[Ecomike]

An easy way to figure out if you have a vacuum leak or not is to cover the throttle body with your hand. If it still runs fast, you have a leak that you'll need to find. You can put your thumb over the IAC port on top of the throttle body and see if the IAC is simply passing too much air. If it is, it's broken or not be controlled properly by the ECU. Also put a thumb over the idle port on top of the throttle body on the driver's side and see what happens.

I sort of did that with carburator cleaner last night spraying into the ports. With the old IAC still installed, the carb cleaner flooded the IAC inlet port, with no change in idle (so I replaced the IAC with a new one).

But, it tried to stall the engine spraying carb cleaner into the open idle air inlet port on the far drivers side of the throttle body!

I wonder if I may have jammed the new IAC up against some junk in the throttle body, and now it can not turn?????

 

[joe_peters]

Did you power the IAC when it wasn't mounted? There is no limiter built in and they can over-extend.

 

[Ecomike]

It is fixed!!!:sunshine:

The new IAC was no good. Called to find another one, no joy:shiver:. Fiddled with the new one...., I pulled back the cap under the pintle and the spring under the cap so I could un-thread the pintle end, and discovered that the coil spring was mangled. I fixed (unmangled) the spring, re-assembled it, and it worked.

If you have 2 IAC's, you can install one in the TB, and then attach the electricals to the loose IAC while the engine is running, and see if the IAC rattles, and if the pintle moves as it rattles (poor boys test rig), it is working. If it rattles, but does not move, it is getting the power and signal, but is stuck. The new one rattled, but the pintle did not move, until I fixed the bound up spring. The old one was just stuck, needed cleaning and lube for the first time in 2 years and 36,000 miles (LPS 1 works real well).

Even after all this I still had a 1000 rpm high idle (always had a high idle on this one after doing the head job (had just bought the jeep, and started with a head job, so prior TB/IAC history was unknown), but the fixed-new IAC got it back to where it had been before, down from 1200 peak to 1000), and I decided the new IAC was at the end of its stroke, which left just the throttle linkage/butterfly valve assy, and the hidden idle air adj screw on the lower drivers side of the TB as the culprits, since I was 100% Utah sure there ARE no vacuum leaks! (READ MY LIPS, NO VACUUM LEAKS HERE, LOL) The idle screw was no help, it was at its lower limit ( I could adjust the other way and raise the idle a little), so I bent the ear that rests on the screw stop of the throttle body, and it now idles perfectly. It also adjusts for Park, Reverse, Drive, and AC on or off. Now I have a steady 1000 rpm start up idle (for first 1-2 minutes), warmed up it is 800 rpm apx in Park, 600 apx in drive (if the electronic RPM gauge is reading right?), and the AC on/off operation only moves the idle 100 rpm for 1 second before the IAC corrects the idle back to normal.:loveu:So I can see and hear the IAC working when I switch the AC on and off!!!!

The engine/tranny/coolant system is also running cooler now, even with the AC on!

Oh, and do not try the remove the pintle by pulling, it will break/strip the plastic threads, you must release the lower spring clip, and then unthread the pintle if you want to disassemble it. I have done this several times (learned by busting the first one I tried to take apart years ago, LOL).

One more Renix good to go again!:loveu:

And I have one Haaaaapy daughter again! Thanks for helping me think this through!!!

 

[Ecomike]

Did you power the IAC when it wasn't mounted? There is no limiter built in and they can over-extend.

Nope, not this time, LOL!

 

[1987Comanche]

A very interesting thread. I've gone through a lot of this troubleshooting with my '90 4.0 AX-15 and have made it drivable. It does, however, have 2 distinct idle speeds, 650 RPM and 1000RPM. Temp, drive time, etc. seem to have no effect on which one I'll get. For example I'll drive it to work (highway driving, the engine gets good and hot) and sitting at the light I'll have 650 RPM one day and 1000 RPM the next. I ultimately gave up *(&*&(_ around with it but if anyone else has ideas I'm open.

 

[5-90]

A very interesting thread. I've gone through a lot of this troubleshooting with my '90 4.0 AX-15 and have made it drivable. It does, however, have 2 distinct idle speeds, 650 RPM and 1000RPM. Temp, drive time, etc. seem to have no effect on which one I'll get. For example I'll drive it to work (highway driving, the engine gets good and hot) and sitting at the light I'll have 650 RPM one day and 1000 RPM the next. I ultimately gave up *(&*&(_ around with it but if anyone else has ideas I'm open.

Sounds like "fast idle" and "slow idle" - with the decision circuit between the two being confused.

IIRC, 650rpm is the "warm idle" for the 242ci with the gearbox behind it, and 1000rpm is the "cold idle" for pretty much any 242ci while it warms up. The AC/REQ signal to the ECU (to bump idle) only brings it up to the level where it should be, as I recall (it just allows more idle air/fuel.) And, there's no pressure switch on the power steering system to kick that idle up - like there is on the 4-150.

Perhaps the AC/REQ signal to the ECU is getting confused? AC/REQ is a signal that is generated by the climate controls (I believe) when the aircon is switched on to a setting that engages the compressor clutch (it also turns the aux fan ON.) If you're getting an AC/REQ signal without the aircon compressor clutch being engaged and having the aircon compressor loading the engine, a 350rpm hike sounds about right. Are you able to identify any particular circumstances that consistently occur when the idle bumps up on you?

 

[1987Comanche]

I haven't been able to identify any particular circumstances (outside temp, humidity, drive time, etc.) but the truck has no A/C. I added the aux cooling fan circuit but the wire from that would go from A/C to diode is not connected and capped off. The aux cooling fan works properly and it doesn't really affect idle if you jump the temp switch to turn it on. It's also worth nothing the problem existed before I added the fan circuit.
Also the available connection for dealer installed A/C (4 pin out plug by the washer tank) has only been used for the f/p 12V+ feed. That pin is the keyed 12V+ power for my aux fan relay. I checked the IAT sensor and the CTS and, whenever I've probed them, the resistance is fairly close on both. It roughly corresponds to the resistance for the given temp in the FSM. It is possible one is acting up and throwing the truck into fast idle...I've just never caught it.
Also of note my truck has the cold idle/driveway dieout issue. I haven't found the 53007843 ECU to resolve that yet.