Renix Low idle issues (not your average bear)

So i have a 90 RENIX. Im pretty versed in RENIX-ese and i have FSM;s etc.

I have been chasing a low idle issue on my RENIX, and its bugging the crap out of me.

Ive tested every sensor and every ground both at the ECU, and the sensor directly. The only ground that doesnt have continuity to negative is the B+ latch relay, but this is by design. From studying the FSM, this is what the ECU uses to energize the +B latch, so it wouldnt have continuity to ground. The wire itself tests at .002 OHMS.

Ive narrowed the problem down to my IAC. What happens is that when the engine starts, it immediately pushes the pintle completly closed thus causing the low idle. When you start it, it doesnt rev high and come down as usual; instead it sticks at ~500RPMs and stays there.. thats where it idles all the time.

Obviously ive tried a new and known good IAC, and i get the same problem. When the throttle is plugged with an IAC, and i plug in another IAC to the harness, it pushes that pintle out all the way, and retracts it when you turn the engine off. That tells me the IAC is working and so is the B+ latched relay.

Interestingly enough, if i manually rev it with the accelerator, it will come down fast till about 1200 RPMS (which is normal) and then go slower down to the base idle.. however, instead of stopping at ~750 it then again drops down to ~450-500. Cycle is easily repeatable. This also tells me that the computer is using the IAC.. but it closes the air passage too much.

What would cause the pintle to close in the throttle completely killing the Idle? Would the Engine Speed Sensor do this? (aka Crankshaft Position Sensor). Mine is brand new, but im grasping at straws here. Yes TPS has been tested and adjusted correctly.

Not only does it bug me, but on a cold start it makes it hard to start. Additionally, since the engine isnt getting enough air it runs rich at idle.

Does anyone have any suggestions on this one? Im at my last leg. ECU is my only suspect at this point.

Plugged cat? How is the power at higher rpms?

When did this start? Sudden or slow?

My cat lost all its hair so im currently running hairless.

Engine runs fine otherwise (besides a lean tip in issue but thats unrelated)

IIRC it happened suddenly.

I'm old school. With an engine that is running wierd, the first thing I do is pull and look at the plugs and while I am at it, run a compression test, and a vacuum test. That rules out a ton of things.

I too am suspicious of the ECU. I know you've been probing around there in your testing procedures, but have you ever done the following?

Remove the ECU harness plugs from the ECU. Spray out both sides with contact cleaner. Using a small pick, tweak the female receptacles so that they will grip the pins more tightly.

Thanks for that suggestion, although i have tried that already. Ive also back probed the ECU connectors with the engine running just to verify my findings at the sensors themselves.

I dont know what the heck would cause the ECU to think it needs to push the IAC all the way into the TB.

How are you telling that the pintle is being pushed all the way in? You should never run the IAC with it not in the TB. The stepper motor is normally run until it stalls out. If done outside the TB it can break the plastic gears inside.

Im using my own little test rig that doesnt allow the pintle to push all the way out past the gears. If i was that silly, then im sure my post would have been something like "I tried to test the IAC, but it shot across the garage".

Another test is to make sure the IAC is in its normal 1-1/8" position and leave it unplugged. Start the engine. Observe high idle. Plug in and get instant low idle.

I figured you knew what you were doing, but you never know.

Maybe you should try another ECU then. Auto trans?

Yep Auto Trans.. need to get my hands on a ECU for a 90... from the wiring diagrams ive seen the 90 MY has alot of differences compared to 87-89. One prime example is the Orange Oxygen Sensor Heater +12v. On a87-90, its hot all the time with key on.. but on the 90 MY its only hot with engine running (besides 3 second prime).

Have you tried "Uncle Bobs" approach, adjust the idle screw in the side of the throttle body, and or the idle stop screw, to force a minimum idle on the beast?

Mine ran at 700 to 750 for the last few years once I got all the random idle variation gremlins exorcised, but it still had a slight miss, and when I finally found the source of the miss and fixed it (Poor injector connection, #3), my 87 now refuses to run above 500 rpm at hot or warm idle now, but it also runs as smooth as glass now, no knock, no ping, no tick, just smooooooth, so I have left it alone the last 5-6 months and just drove it. It does speed up to 700 rpm when I turn the AC on, and stays there which I find interesting!!!!

How are you determining idle speed?

I am going by the dash Tach, which I am not sure is accruate, nor I am sure what the two inside lines really are, but I always assumed they were 500 and 750 rpm, between 0 and 1000.


Oh, and I got a good laugh out of ""I tried to test the IAC, but it shot across the garage" LOL! But Coil springs are much more entertaining!!!!:cheers:

lol!

I havent tried to adjust the TB, but it shouldnt need it. First and foremost, it was running just fine then all of the sudden the low idle. Second, the Throttle plate should only be adjusted to close enough as to where it doesnt contact the inside of the Throttle Body. Additionally, i dont use a RENIX or HO Throttle Body. Im using a 68mm from a Dodge Dakota. Dont let that be the focus though because it has been running and working flawlessly for almost a year now.

I am going off of the Tach, but its obvious that the RPM's are not right. When you start the Engine, the RPMS should jump to maybe 2500 and then drop down to about 1000 on cold start. Currently on mine, when you start it the RPMS go to 750 max and usually more like 500 which is where it sits at idle. That darn IAC is buggering me up! Im pretty confident that the notches go 0, 500, 750 and then 1000.

How smooth is the idle?

I dont know what the heck would cause the ECU to think it needs to push the IAC all the way into the TB.

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Not sure how important it is, but the first IAC I ever installed had some special instructions, and I seem to recall that pintle needed to be back off a good bit so the ECU could cycle the IAC fully closed, and then reset the damn fully closed to figure out what the tick count over a certain length was, or some such crap? I have not done it on an IAC replacement since that one (about 6 years ago), and have not seen those instructions since then, but I recall that instruction sheet made a big deal that the pintle needed to be in the right spot when installed, and then the ECU needed to cycle it closed once.

Ecomike said:

How smooth is the idle?

Not sure how important it is, but the first IAC I ever installed had some special instructions, and I seem to recall that pintle needed to be back off a good bit so the ECU could cycle the IAC fully closed, and then reset the damn fully closed to figure out what the tick count over a certain length was, or some such crap? I have not done it on an IAC replacement since that one (about 6 years ago), and have not seen those instructions since then, but I recall that instruction sheet made a big deal that the pintle needed to be in the right spot when installed, and then the ECU needed to cycle it closed once.

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For the most part the idle is smooth... a little rough but not bad. Not bucking and surging but its not getting enough air. In Closed loop it will run rich (12.5ish) at idle. As soon as you accelerate just a tad it will pop back up to stoich and stay there until you idle again.


I know what you mean about the IAC instructions. When i was doing preliminary testing, i purchased an IAC from NAPA and it had those instructions.

Muad'Dib said:

lol!

I havent tried to adjust the TB, but it shouldnt need it. First and foremost, it was running just fine then all of the sudden the low idle. Second, the Throttle plate should only be adjusted to close enough as to where it doesnt contact the inside of the Throttle Body. Additionally, i dont use a RENIX or HO Throttle Body. Im using a 68mm from a Dodge Dakota. Dont let that be the focus though because it has been running and working flawlessly for almost a year now.

I am going off of the Tach, but its obvious that the RPM's are not right. When you start the Engine, the RPMS should jump to maybe 2500 and then drop down to about 1000 on cold start. Currently on mine, when you start it the RPMS go to 750 max and usually more like 500 which is where it sits at idle. That darn IAC is buggering me up! Im pretty confident that the notches go 0, 500, 750 and then 1000.

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When my renix rigs are working right, and start up quickly, they never rev past 1000 to 1200 now that have most of the old skeletons fixed and exorcized. They never get past 1300 rpm at start up now.

Not sure if that is OEM typical?

Perhaps the throttle body stop wore and made a sudden permanent change in the idle air throttle stop setting? Or the old IAC was not fully closing (for what ever reason, dirt...) while the throttle stop metal wore?

I have never found anyone that was 100% sure that Renix truly controls the low idle speed 100% with the IAC, versus an idle stop and idle screw setting being the OEM low idle control, where the IAC closes 100% to set the minimum idle, or ?????, which is probably a good topic for the 4 of us right now!!!!

Muad'Dib said:

For the most part the idle is smooth... a little rough but not bad. Not bucking and surging but its not getting enough air. In Closed loop it will run rich (12.5ish) at idle. As soon as you accelerate just a tad it will pop back up to stoich and stay there until you idle again.


I know what you mean about the IAC instructions. When i was doing preliminary testing, i purchased an IAC from NAPA and it had those instructions.

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Yep, I think it was NAPA on the one I had too.

OK, how about this. Mine now runs smooth as glass dead on at 500 rpm, purrs like a kitten, but at 700 it wandered up and down about 25 rpm before I found the one poor injector connection that was making one injector fire 2 out of 3 (est) times at idle, but it ran fine under load. Perhaps you have a slight blockage in an injector, or a slight leaky injector, or one of each, at idle, screwing with the O2 sensor ECU stoich calcs?

Also, since the throttle body is not OEM, how do you know it had set right to begin with? Perhaps, like I said, something was still off a little when you installed and set it (throttle body stop and idle screw), and now it has gone away, with new parts added over time, like new TPS, new IAC, new tune up parts?????

At some point, when you have eliminated the logical, look at the illogical, or start over at the beginning and look for something that changed since you last tested it. When I run into brick walls I usually find it is not one item causing the problem, but multiple issues competing with each other, thus the rich and lean injector idea, causing a not perfect idle yet (tiny slight miss?).

I am still shocked that after 8 years of tinkering and troubleshooting I finally got the 87 to idle like a brand new car, at 275,000 miles!!!! But the idle is at 500 rpm, LOL!!!!!

Muad'Dib said:

Yep Auto Trans.. need to get my hands on a ECU for a 90... from the wiring diagrams ive seen the 90 MY has alot of differences compared to 87-89. One prime example is the Orange Oxygen Sensor Heater +12v. On a87-90, its hot all the time with key on.. but on the 90 MY its only hot with engine running (besides 3 second prime).

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I've used different ECUs across the years. I have a 90 ECU in my wife's 88 right now. You want the one ending in 428 for your 90. Same as an 89 BTW. For a test ECU, any of them would be fine for troubleshooting purposes.

Did the final check on the CrankShaft position sensor tonight thinking that there was no way the CPS could have anything to do with a low idle.

Test #1 Ohm Check = To Spec and OK!

Test #2 AC Cranking Voltage. = Not Sure?

From what i read it is suppose to be .5 volts and above... however mine tested .33 to .34 volts while cranking (I used a Digital Multimeter and i have the High Altitude CPS)

Could a faulty CPS really cause a low idle like this??? Im starting to wonder because it does give the computer engine speed.. but then again if the computer was seeing lower voltage, wouldnt it raise and not lower the idle?? Im confused about this one... or if its even a problem.

No it would not cause a low idle. Besides the running voltage is a whole lot higher than the cranking voltage on the CPS. Back probe and test the running voltage at idle.

I had one as low as .17 V on my 87 for years that started fine. But another new one at .45 V that had intermittent no starts due to a cold, loose electrical connection inside the CPS that would come and go.

If you want the cranking voltage higher you need drill out the CPS mounting holes one drill bit size larger than stock OEM in the CPS, and bolt the CPS a little closer to the flexplate.

The CPS signal is used for timing the fuel injection pulse and spark timing. If the CPS was a problem you have a no start, or bad miss, and dying problems. It is not the droid you are looking for! LOL

Muad'Dib said:

Did the final check on the CrankShaft position sensor tonight thinking that there was no way the CPS could have anything to do with a low idle.

Test #1 Ohm Check = To Spec and OK!

Test #2 AC Cranking Voltage. = Not Sure?

From what i read it is suppose to be .5 volts and above... however mine tested .33 to .34 volts while cranking (I used a Digital Multimeter and i have the High Altitude CPS)

Could a faulty CPS really cause a low idle like this??? Im starting to wonder because it does give the computer engine speed.. but then again if the computer was seeing lower voltage, wouldnt it raise and not lower the idle?? Im confused about this one... or if its even a problem.

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