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89XT6

XT6 high idle after hot start

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Spring is here and I'm tinkering with my 1989 XT6.  It sat in the garage for the winter, with a trickle charger and a weekly startup.  I recently did a complete reseal of the H6, timing belts, water and oil pump, etc.  It runs like a top, starting right up and idling  perfectly after a cold start.  I let it warm up to the point the fan kicks in, cycling the cooling system.  When I shut it down and restart it hot, the idle RPMs shoot up to 1,500 or so and stays there, something is convincing the ECU to do this I guess.  I cleaned the IACV, MAF, throttle linkage and all the wiring to them.  I get 3.2 volts at the CTS wiring plug (I replaced the original with an injector harness).  The CTS seems bad, it's 30 years old!  I read an open circuit between the two conductors, tested when warm and then hot.  I will check again when cold and hot, but an open is bad when it's hot and should be talking to the ECU.  I'll try a new CTS first, they're not expensive.

Questions...

Should I get a $15 aftermarket CTS, or the good old NOS Subaru CTS for $40?  The original lasted 30 years, so I'm leaning towards NOS Subaru.

Any other suggestions on this issue?  

I can drive the car all day after a cold start, idling at a stop around 750 RPMs but if I shut it down and restart it hot after a few minutes, the idle zips up to 1,500 and is reluctant to settle down.

I could check for vacuum leaks, but figured if this caused the high idle, it would occur even with a cold start.

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When one of mine went bad at about 30yrs, I got a new Subaru one.   At $3.00 per year, an OEM one is not a bad deal.  The only time I go cheap on those kind of parts is if It were a beater I planned on running until it broke from rust, and had no use for the engine as a spare.

 

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I would not worry about CTS branding.Subaru does not manufacture them.

Are you sure it is  open? An open one should have triggered a trouble code and dash light.

They can be up to around 20,000 ohms,If you do not have your meter on the right scale,they can look open.

3.2 volts at the plug w/the CTS unplugged sounds flakey. Should be more like 5.0.

I would be suspicous of the ECU if the 3.2 volts was measured  with the CTS disconnected and a new CTS has no effect.

Disprove vacuum leaks by unplugging the IACV for a hot start,If the idle lowers now,it is not a vacuum leak.

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2 hours ago, naru2 said:

I would not worry about CTS branding.Subaru does not manufacture them.

Are you sure it is  open? An open one should have triggered a trouble code and dash light.

They can be up to around 20,000 ohms,If you do not have your meter on the right scale,they can look open.

3.2 volts at the plug w/the CTS unplugged sounds flakey. Should be more like 5.0.

I would be suspicous of the ECU if the 3.2 volts was measured  with the CTS disconnected and a new CTS has no effect.

Disprove vacuum leaks by unplugging the IACV for a hot start,If the idle lowers now,it is not a vacuum leak.

Thanks for the info, this is what I found this morning...

Key on, 3.2v at the CTS pigtail, 3.2k ohms at the CTS.  Starts up with normal idle, warms up and maintains 750 RPMs.

After letting it get hot, the fan cycles and the engine stays at perfect temperature. 

Now I see 0.23k ohms at the CTS when it's hot.  I notice a hum coming from the IACV with the key on, engine not started.  I disconnected the pigtail at the IACV and gave it a hot restart.

The idle goes up to 1,250 RPMs when in park, down to 750 RPMs in gear, but it runs rough with the check engine light on, can this be a vacuum leak? 

I noticed that the IACV pigtail is a bit of a mess, I'm going to order a new one along with a new CTS and see what happens after cleaning the IACV again.  

Would a little bit of WD40 help the IACV work better?  If this doesn't work, I'll have to splurge for a new IACV.  Rockauto, or Subaru?

I know the harnesses on these cars cause problems, I have solder sleeves and tape ready to go.  I found a few testing procedures to follow.

Any more advise is appreciated.

Edited by 89XT6

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Trouble code to be expected upon unplugging the IACV.

Rough warm  idle at the same rpm is a little unexpected.

The high idle appears to be due to the IACV in some way and not a vacuum leak.Probably the bad pigtail.

You might try again by leaving the IACV plugged in and disabling it by pinching one of the hoses closed.

That 3.2V is still troubling.Does it remain constant cold/hot?

I would use an intake cleaner on the IACV.

The CTS does not control the fan.

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3 hours ago, 89XT6 said:

The idle goes up to 1,250 RPMs when in park, down to 750 RPMs in gear, but it runs rough with the check engine light on, can this be a vacuum leak? 

 

yup.

Idle should not change more than 100~200 Rpms between P and D

 

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1 hour ago, naru2 said:

Trouble code to be expected upon unplugging the IACV.

Rough warm  idle at the same rpm is a little unexpected.

The high idle appears to be due to the IACV in some way and not a vacuum leak.Probably the bad pigtail.

You might try again by leaving the IACV plugged in and disabling it by pinching one of the hoses closed.

That 3.2V is still troubling.Does it remain constant cold/hot?

I would use an intake cleaner on the IACV.

The CTS does not control the fan.

Lol, I'd have to learn Morse Code to read the trouble codes.  I have original repair manuals but I don't want to fiddle with the main harness at the ECU yet.   The IACV pigtail is definitely part of the problem,  12.2V at pin 2, but no voltage or OHM readings on pins 1 or 3.  I didn't check voltage at the CTS pigtail when hot but will do.  The CTS pigtail is new but I'll try to get 5V out of it.  I'll check an auto parts or electrical supply store for new pin connectors for the IACV, they look unique but nothing I can't fix there.  The plastic housing is also unique, hopefully I can find a suitable replacement or just reuse the original.  A pinched hose is possible, the input hose is new but it's just a straight generic hose making a pretty tight turn,  Subaru didn't have one.  I'll keep this in mind when everything else is done.  I'll clean the IACV good, with  intake cleaner when I put everything together again.  I should have results by next weekend, thanks for your help.

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1 hour ago, FerGloyale said:

yup.

Idle should not change more than 100~200 Rpms between P and D

 

A vacuum leak is possible, after I repair the IACV pigtail, clean the valve,  try to get 5V instead of the 3.2V I read at the CTS pigtail and replace the CTS, I'll check for a vacuum leak.  With a cold start, the motor runs fine, idling at 750 in park and drive, drivability is fine, so I'm pretty sure vacuum is okay but you never know.  It's when I do a hot restart that RPMs zip up to 1,500 and don't settle down.  We all know corroded wiring is the main suspect on these early ECU Subbies, yet we all throw money at everything else:angry:

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Quite likely there is a vacuum leak as well as an IACV related issue.Would explain the rough idle and rpm variation.

Corroded connections won`t affect the 3.2 volt reading w/the CTS  unplugged as there is no current flowing.

AFAIK,the only fault that could make the voltage too low here is a failed voltage regulator in the ECU.

Pretty sure the same voltage regulator also supplies 5 volts to the TPS.You might want to check voltages there.

I did not mean to suggest you had a pinched hose,I meant you could disable the IACV by pinching a hose instaed of unplugging it and possibly sending the ECU into a limp mode.

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5 hours ago, naru2 said:

Quite likely there is a vacuum leak as well as an IACV related issue.Would explain the rough idle and rpm variation.

Corroded connections won`t affect the 3.2 volt reading w/the CTS  unplugged as there is no current flowing.

AFAIK,the only fault that could make the voltage too low here is a failed voltage regulator in the ECU.

Pretty sure the same voltage regulator also supplies 5 volts to the TPS.You might want to check voltages there.

I did not mean to suggest you had a pinched hose,I meant you could disable the IACV by pinching a hose instaed of unplugging it and possibly sending the ECU into a limp mode.

I recently replaced the original CTS pigtail with an injector pigtail, but I should take a voltage reading further back.  I can't do any testing until I get the IACV pigtail back in order and running. Right now the main culprit is the IACV pigtail, the connectors were bent and corroded.  Do you have any idea where I can get an original replacement pigtail?  Otherwise I'll just re-pin the old one.  A vacuum leak is possible but wouldn't that affect the idle from a cold start?  It runs fine from a cold start, hot restarts are where the high idle shows up.  I picked up some carb cleaner for the IACV, maybe its getting stuck with hot starts.  Rewiring the pigtail is my best bet here and trouble shooting the 3.2 V issue at the CTS and TPS would help also.  I contacted a few auto electric supply places and sent them photos of the IACV pigtail, they may be able to find something that works, preferably a complete pigtail, ready to splice.

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Find an XT6 here http://car-part.com/cgi-bin/search.cgi

Search for a different part.I used a camshaft.

Phone one of the wreckers and have them cut off the pigtail.

Those pins will be available somewhere.I bought subaru ECU pins from a different dealer-Honda maybe?

Or ask these guys https://www.iwireservices.com/3-pin-plugs

You don`t need to run the car to troubleshoot the 3.2 volts.Just have the key on.

You should  read the codes.Easy and possibly informative.

Check the 3.2 at the cts plug while both the TPS and MAF are unplugged.

It is possible that a fault there could load the 5 volt supply enough to drop the voltage.I would expect a trouble code.

I would get a backup ECU while they are still around.

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2 hours ago, naru2 said:

Find an XT6 here http://car-part.com/cgi-bin/search.cgi

Search for a different part.I used a camshaft.

Phone one of the wreckers and have them cut off the pigtail.

Those pins will be available somewhere.I bought subaru ECU pins from a different dealer-Honda maybe?

Or ask these guys https://www.iwireservices.com/3-pin-plugs

You don`t need to run the car to troubleshoot the 3.2 volts.Just have the key on.

You should  read the codes.Easy and possibly informative.

Check the 3.2 at the cts plug while both the TPS and MAF are unplugged.

It is possible that a fault there could load the 5 volt supply enough to drop the voltage.I would expect a trouble code.

I would get a backup ECU while they are still around.

Great info, thanks!  I see a backup ECU and engine harness in my near future, just have to get the exact part number off the ECU in the car. 

I see some Grade A parts at Gerry's Foreign Auto Parts http://www.gerrysap.com/

I found a dealer that sells all types of connectors and terminals https://connectorexperts.com/

Their prices are high, but you can do a specific Subaru connector search.  I see connectors and terminals that might work.

Tomorrow I should have time to wire up the temporary pigtail for the IACV and test the CTS with the TPS and MAF unplugged.  I'll test and clean the IACV, TPS and MAF also.

Hopefully the hot start issue will be corrected with this simple fix.

 

 

 

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In some cases I have cut a notch through the side of the sensor-side connector in order to facilitate soldering a wire to each metal pin on the connector. Then end each wire with an individual connector and it’s appropriate matching half on the engine side. 

Its easy, cheap, repeatable and a good solution in some cases but I can see that you probably won’t need to nor want to here. Someone else may stumble onto this later or be following along. 

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1 hour ago, idosubaru said:

In some cases I have cut a notch through the side of the sensor-side connector in order to facilitate soldering a wire to each metal pin on the connector. Then end each wire with an individual connector and it’s appropriate matching half on the engine side. 

Its easy, cheap, repeatable and a good solution in some cases but I can see that you probably won’t need to nor want to here. Someone else may stumble onto this later or be following along. 

I have a thin soldering iron ready, but I want to try a few prewired IACV tails first.  This tail is a 3 pin Bosch type, other makes use these, Hyundai uses one that looks close enough to gamble on for $15.  

Today I did a temporary  IACV pigtail, cleaned the MAF, TPS, throttle body/linkage.  The car cold started perfectly as usual, 750 RPM after warmup, even smoother after cleaning stuff, I even put a new air filter in.  Two hot restarts were good, 750 RPM after the usual startup surge.  On the third hot start the RPMs went up to almost 2,000 RPM, but this time I was able to get it down to 750 RPM after goosing the throttle a few times, like knocking the choke off of a carburetor.  Something might be sticking, I'll take the IACV off again and use carb cleaner this time.

I have a new CTS to replace the old one.  After that, some further wiring cleanup might help.  I have repair manuals and could read the codes on the dash, I just have to be careful with the 30 year old ECU and it's harnesses.

 

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Good job.  I assumed you wouldn’t need that solder option, I just put it out there for others.  

If the check engine light is on read the codes, don’t just keep guessing. It takes 2 minutes. Ive daily driven XT6s for 25 years and owned a ton of them, there is zero worry about the ECU or wiring. You pop the trunk and count a blinking light. 

It seems like the failed component should test differently between  cold start and hot restart.   I would write down readings when cold and test again during a *hot restart with symptoms presenting*. Don’t test unless the symptoms present. 

I would wonder if you pulled the hot IAC if the visible valve would be partially stuck and physically appear in a different orientation than when cold.

The hot-only restart issue does seem like an electro mechanical device is physically sticking, either the CTS or IAC.

Electrical properties frequently present at higher temps too but this seems to have an electrical-mechanical feel to it and not just wiring.

 

 

 

 

 

 

 

 

 

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17 hours ago, idosubaru said:

Good job.  I assumed you wouldn’t need that solder option, I just put it out there for others.  

If the check engine light is on read the codes, don’t just keep guessing. It takes 2 minutes. Ive daily driven XT6s for 25 years and owned a ton of them, there is zero worry about the ECU or wiring. You pop the trunk and count a blinking light. 

It seems like the failed component should test differently between  cold start and hot restart.   I would write down readings when cold and test again during a *hot restart with symptoms presenting*. Don’t test unless the symptoms present. 

I would wonder if you pulled the hot IAC if the visible valve would be partially stuck and physically appear in a different orientation than when cold.

The hot-only restart issue does seem like an electro mechanical device is physically sticking, either the CTS or IAC.

Electrical properties frequently present at higher temps too but this seems to have an electrical-mechanical feel to it and not just wiring.

 

 

 

 

 

 

 

 

 

I'm not getting a check engine light, but if I allow 2,000 RPM for too long, I'll get a check engine light that's much worse than a hot start issue! :mad:.  Something is just tricking the ECU into cold start mode I guess.  I only got a check engine light when I disconnected the IACV, to see if it made any difference, the car ran rough of course.  I'll replace the CTS, remove the IACV, clean it good with carb cleaner and maybe a hint of WD40 to really loosen it up.  I notice a slight hum coming from the IACV when the key is on, engine not started.  This is an electromechanical part, so I assume it's normal.  I'll read the repair manuals and study up on reading the codes.  The ECU and wiring in the trunk look perfect, I don't want to mess with it.  There could be a bad ground or other connection under the carpet.  Is 3.2V on a cold CTS pigtail ok?  I've read that 5V is required.

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I put a new CTS in today, cleaned the IACV also, its spotless and free.  The old CTS was tired, the connector snapped off when I removed it, it looked corroded.  

Same thing though, only a smoother initial warmup from a cold start, the new CTS made a noticeable difference.  Perfect cold start and two hot starts where the RPMs shot up to 1,500 RPMs and settled to 750 after I revved the throttle a couple times.  However, on the third hot start, the idle ran up to 2,000 RPMs and refused to settle down to normal.

The Hyundai IACV pigtail will fit with a little measuring and file work on the plastic connector housings.  I ran out of time today, once I have the pigtail done, I'll give it another chance.

I may have to dig into my wallet for a new IACV, maybe even a new TPS,  MAF...$$$ :unsure:.  Maybe if I have a full day available, I'll take a look at the wiring under the carpet.  The carpet is original and dry, the car has zero rust, maybe I'll find that million dollar penny under the seats!

I'm not getting any check engine light and the car runs great from a cold start, something may be getting heat soaked?

Edited by 89XT6

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On 4/5/2019 at 11:40 AM, 89XT6 said:

I'm not getting a check engine light, but if I allow 2,000 RPM for too long, I'll get a check engine light that's much worse than a hot start issue! :mad:.  Something is just tricking the ECU into cold start mode I guess.  I only got a check engine light when I disconnected the IACV, to see if it made any difference, the car ran rough of course.    Is 3.2V on a cold CTS pigtail ok?  

READ THE CODE

There is no good reason for a rough hot idle w/the IACV  inoperative.I would try to isolate and eliminate IACV symptoms by  pinching a hose closed  and leaving it electricly connected.

Voltage at the CTS pigtail is not dependent on temperature when the sensor is disconnected.

Sounds to me like the IACV sticks once hot.I would try cooling it with one of those aerosol sprays while it is doing the hot fast idle thing.

Edited by naru2

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7 hours ago, naru2 said:

READ THE CODE

There is no good reason for a rough hot idle w/the IACV  inoperative.I would try to isolate and eliminate IACV symptoms by  pinching a hose closed  and leaving it electricly connected.

Voltage at the CTS pigtail is not dependent on temperature when the sensor is disconnected.

Sounds to me like the IACV sticks once hot.I would try cooling it with one of those aerosol sprays while it is doing the hot fast idle thing.

Great idea with cooling the IACV, I'll try this tomorrow.  There were no codes and the check engine light doesn't come on, yet there is definitely an issue that will overheat the motor if I don't take care of it.  I used a Hyundai IACV pigtail, it fit perfectly with a little filing of the connector notches on the IACV.

I'm also thinking that the IACV becomes heat soaked and malfunctions when hot, but $320 for a new one. :eek:

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Well, we are making some progress...

Installed new MAF, IACV, 90 deg. inlet hose and pigtail, CTS and pigtail.  

Cold starts are even better than before, I notice about five steps of idle dropping to 750 RPM, perfect.

Two hot starts with idle up, then down to 750, without the help of my right foot.

Third hot start is still not coming down, stays around 1,750 RPM, down to 1,250 in drive.

The new IACV definitely makes a difference, but it isn't the cure.

What's next...

I have a new TPS and pigtail coming, the original is beat up.  Before I install it, I'll clean the throttle area that it connects to, it may be sticking.

I'll also look into a new ECU, even though the one I have looks like new, besides the half rump roast radio splices done by a previous owner.  Not an expensive NOS, if they even exist they would cost well over $500, no thanks!  I've seen Grade A used ones for about $125.

There are no codes coming from the ECU, no check engine light, no vacuum leaks, just one stubborn component somewhere.

Another piece could be the voltage regulator, I'll look into that.

Who knows, maybe the 4EAT has too much fluid in it, causing idle issues when it gets hot?

If all else fails, I'll just live with it, being careful not to let it idle so high for a long time, the cars has never overheated and we know not to ever let that happen.

 

 

MAF.jpg

IACV.jpg

CTS.jpg

ECU.jpg

Edited by 89XT6

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FYI...

Mitsubishi TPS pigtail part # MD614772 fits Subaru TPS Intermotor part # TH377 perfectly.

I'll be installing this over the weekend.  I'm waiting for an ECU and a Nissan MAF pigtail which should also fit.

TPS with pigtail.jpg

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