New Motorcraft IAC, new issues - Ford Mustang Forum
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post #1 of 16 (permalink) Old 08-19-2017 Thread Starter
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New Motorcraft IAC, new issues

My 93 occasionally would hunt for idle while stopped so I figured it was the IAC, ordered a new Motorcraft & installed it, reset the computer let it idle 2 min, off 2 min, on with all accessories on which I doubt would make a difference but it was something to do. I drove the car and noticed if I press in the clutch between any gears the idle would hang at 1400, if I come to a stop in neutral the RPMs would drop so I guess that has to do with the VSS, the old IAC would only hang at around 1100 RPM between shifts but would drop without stopping unlike the new one. I drove it a few miles and come home, I revved the motor with the car stopped and the idle would still hang but briefly at 1400 and slowly drop in increments over a course of a few seconds which is pretty annoying. I checked for vacuum leaks, timing is at 12, TPS is at .96v and no codes. Could my idle be set too high? I assume it's around 700 going by the dash tach which I doubt is very accurate. I read several post on different forums about backing off the idle stop screw little by little, resetting and test driving. This car runs fine but this idle stuff id driving me nuts, maybe this new one is bad... I changed out my PCV grommet, valve and screen last year "all Motorcraft" and wasn't a easy task using a mirror and a pic, I did noticed the grommet would pull out with the valve which may be a vacuum leak source I need to double check, would black RTV work on this grommet is there is a leak there? Any suggestions

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post #2 of 16 (permalink) Old 08-19-2017
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Let the engine idle while disconnecting the iac harness.The engine should stall or idle should drop to a low rpm.If it doesnt,the idle set screw is adjusted too far inwards (increases idle),a vacuum leak is present,the purge valve is stuck open or the iac is suspect.Use the base idle reset procedure in the following link.Its imperative to set idle as close as possible to 600rpm to prevent idle surging and high idle.Click the following link then scroll to post #2 for the base idle reset.BTW- a suspect tps will also cause a high idle.


https://www.stangnet.com/mustang-for...cklist.698148/

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1991 Mustang lx
D.S.S. 342ci {10.5:1}
TFS 190cc + track heat
Lunati 61011 .549".565"
Pro-M 76mm maf
Jetronic 30lb inj
QA1 Coilovers
17" Weld ProStar XP's
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post #3 of 16 (permalink) Old 08-19-2017 Thread Starter
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High idle may be my issue, with the engine at temp I disconnected the IAC and saw no change in idle other than the rpm dropping to normal without hanging. I'll make some adjustments late this evening after a drive & when it's cooler. Thanks
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post #4 of 16 (permalink) Old 08-19-2017 Thread Starter
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Did find a vacuum leak at the pcv grommet, I put some RTV on the outer lip which helped some. I ordered a new pcv hose due to some cracking on the bend where it connects under the intake, although not leaking might as well before it does. Making some progress...

Last edited by 93Foxer; 08-20-2017 at 07:00 AM.
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post #5 of 16 (permalink) Old 08-21-2017 Thread Starter
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Well I'm at the point of "almost" giving up on this idle issue and living with it. Did check the purge valve and it had no vacuum at the canister side, could not find any vacuum leaks after the pcv fix. At first I unhooked the new iac and saw no difference in idle. Did the base idle reset with the iac/spout disconnected, idled it down to around 600, did a reset. At this point if I unhooked the iac the idle would drop so it was indeed idled too high, my tach may not be accurate because my throttle blade would initially stick so I went to higher base idle 650 also wiped around inside the throttle body which solved the sticking. TPS voltage is .92 and smooth and steady up to 4.61 also double checked my timing which is at 12. The issue still exist but seems intermittent, at times it hangs other times not, went for a drive this evening and it's no longer hanging at 1500 between shifts but at around 1100 for a second or 2, I thought my old iac was bad due to the occasional idle hunt but it still occurs with this one
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post #6 of 16 (permalink) Old 08-21-2017
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If your charcoal canister is still in use,check the vacuum hoses connected to it as they are known to dryrot and leak.

Remove the check valve from the brake booster and vacuum hose and check it for a leak.Use your mouth and try to blow air through both sides of it.Air should only pass from the booster side towards the vacuum hose side.If this test fails,its bad.

One other thing I forgot to mention is the 10 pin (aka: salt & pepper) connectors.Do the maintenance procedure listed below.Theyre also known for causing idle issues.Your throttle body plate doesnt have a hole drilled in it,does it?? Some do,most dont.The hole can take away some of the iac's ability to control idle.The hole was put there in case sludge/carbon built up on the throttle body.Air would still be allowed to pass through the hole into the intake.The following link is a great source for anything iac related.
Ford Bronco Forum

The following link details the 10 pin maintenance procedure.
Fuel Injection Technical Library 10-pin Connector Fix
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1991 Mustang lx
D.S.S. 342ci {10.5:1}
TFS 190cc + track heat
Lunati 61011 .549".565"
Pro-M 76mm maf
Jetronic 30lb inj
QA1 Coilovers
17" Weld ProStar XP's
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post #7 of 16 (permalink) Old 08-21-2017 Thread Starter
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Thanks, I'll check those as well. Yes my throttle plate does have a hole in it, says do not clean on the TB which I did not notice till afterwards but I saw some grime so I took a small amount of gas on a clean rag and wiped the area clean as well as the plate. It acts as though the EEC is somehow perceiving that the car is in motion although it's not if that makes sense "VSS", sometimes it hangs around 1500 other times a slight hang around 1000 at idle and as with most threads across the internet it does fine with the iac disconnected, I do wonder if the new one might be bad because it does seem to run worse & hangs at a higher rpm. I'll check out the link as well & check the other things you mentioned. Thanks again
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post #8 of 16 (permalink) Old 08-21-2017 Thread Starter
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Maybe I did not damage the coating inside the TB I just read about, I should had paid more attention...
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post #9 of 16 (permalink) Old 08-21-2017 Thread Starter
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My tach is not very accurate, I found a procedure for base idle backing off the stop screw and going back in till it lightly touches the linkage & using a .010" feeler gauge, go 1 turn. Does this sound about right? Just food for thought... What years had the hole in the throttle plate and how does it effect the IAC? I do appreciate all the help because I'm lost! The 10 pin article is very helpful, we had a 88 many years ago and instead of pressing the tabs inward we tried to pry apart, I recall it being a hassle and took forever but I never knew till I read to push in to disconnect
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Beginning in 1991, Ford began using a Sludge Tolerant Throttle Body design which includes a special slick Teflon coating inside the throttle bore.This coating minimizes deposit formation and does not require cleaning or the service kit.Ford specifically warns against cleaning these throttle bodies,which can remove the sensitive Teflon coating and can allow additional minimum airflow.These Throttle Bodies can be identified by a black/yellow sticker on the Throttle Body warning against cleaning or adjusting.A component of the Sludge Tolerant Throttle Body is a hole in the throttle plate which allows EEC programmed minimum airflow through the throttle plate.The reason the hole is required is in the Teflon Coating process.The Teflon is applied to the throttle bore and plate.After drying,the Teflon seal is broken to allow throttle plate movement,but it still almost completely seals any air movement around the throttle plate when closed. Hence the hole in the throttle plate,to provide minimum programmed airflow at closed throttle.However,when more airflow has been added due to modifications,it can cause idle issues.If the Teflon coating was removed when you cleaned the throttle body,that will also allow extra air to pass around the throttle plate,which increases the minimum idle airflow too.If it were me,I would try one of the following things,to see if it gets rid of the high idle.
1) make a trip to Pull-A-Part and locate a Ford model vehicle with the same size throttle body as yours (87-93 Mustang tb=60mm),but pick one that does NOT have a hole in the throttle plate.Remove the plate and replace your current plate with it.
2) remove the plate from your throttle body,dab some rtv on a pop rivet & pop it in the throttle plate hole then reinstall.

After installation of choice #1 or #2,do a base idle reset again and post your results.
The main issue I see with using the feeler gauge is,its not a one size fits all solution.A non sludge tolerant tb vs sludge tolerant tb vs a stock engine vs a modified engine are gonna require different minimum airflow values,to idle at the same rpm level.


The purchase of a new/remanufactured iac valve,or any sensor/actuator/solenoid for that matter,is no guarantee of proper function,so its best to test the component before leaving the parts store or after installation,if the part didnt solve your issue.
There are several tests for the iac valve & harness.
1) turn the key on/engine off.
2) backprobe the red iac harness wire with a red multimeter lead & the engine block with the black lead.
3) the meter should read 10.5-12 volts.
4) start the car and let it idle
5) backprobe the white/yellow iac harness wire with the red lead and backprobe the red iac harness wire with the black lead.
6) the meter should read more than .5 volts but less than 12 volts.
7) disconnect the harness from the iac valve & test the terminals at the iac valve itself.
8) touch the red meter lead to the iac red wire terminal and touch the black lead to the other iac terminal.
9) the meter should read 7-13 ohms.
10) now reverse the leads on the terminals.
11) the meter should read OL or a very high resistance value(10k+ ohms)
12) now hold the black meter lead against the iac body while touching the red lead to the iac red wire terminal then the iac white/yellow terminal.
13) the meter should read OL or 10k+ ohms for each terminal.
If steps 1-13 pass,the iac & wiring are good electrically.
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1991 Mustang lx
D.S.S. 342ci {10.5:1}
TFS 190cc + track heat
Lunati 61011 .549".565"
Pro-M 76mm maf
Jetronic 30lb inj
QA1 Coilovers
17" Weld ProStar XP's

Last edited by wbrockstar; 08-22-2017 at 12:48 AM.
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Rent a vacuum gauge from the part store and verify your vacuum. It will let you know if you have a vac leak or not.
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post #12 of 16 (permalink) Old 08-22-2017 Thread Starter
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Thanks, good info. My issue was there prior to wiping out the TB with a gas rag not saying it removed any teflon or not but I think I may have a complete upper intake at my Wife's Father's shop if it hasn't been tossed, been years since I've seen it, will have to check. I put my old iac back on today and it's the same perhaps better, still hunts occasionally. I did however cap off the brake booster to insure there was no vacuum leak as well as the vacuum ball on the pass side and cleaned the 10 pin connectors. Since the 7 years I've owned this car the spark plugs stay snow white but I've read the EFI on these cars are a bit on the lean side especially at idle not sure how true that is. My FP is 37 running with vacuum connected and 41 disconnected as well as with KOEO
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post #13 of 16 (permalink) Old 08-22-2017 Thread Starter
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Did some brief testing 3) 12.36v KOEO , 6) 7-8v running 9) New iac I took off 11.2 ohms/OL between iac connector and body so it test ok, the other is hot and cannot pull test it yet, tried to get my test meter leads, bad angle. I can say I know a great deal more about a 5.0, quite a learning experience
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post #14 of 16 (permalink) Old 08-22-2017 Thread Starter
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I think I may have found a issue possibly related, today I stopped by Napa and got MAF cleaner come home and sprayed the screen as clean as I could and let it dry the reinstalled, reconnected the battery went for a short drive but saw no difference, maybe a tad more throttle response. The car was setting at idle and it raised up 200 rpm and heard a fast paced WO, WO noise under the hood as well as the exhaust note changed, it idled down on it's own and went away. I revved the car and it did it again and the noise appeared to come from the air/smog pump so as it was doing it I disconnected the vacuum line behind the smog pump and the idle went down and the sound went away, there are two vacuum valves on these bypass hoses. Can I plug these with no ill effects till I can get a shorter belt? Oh I have no cats on the car
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Quote:
Originally Posted by 93Foxer View Post
I think I may have found a issue possibly related, today I stopped by Napa and got MAF cleaner come home and sprayed the screen as clean as I could and let it dry the reinstalled, reconnected the battery went for a short drive but saw no difference, maybe a tad more throttle response. The car was setting at idle and it raised up 200 rpm and heard a fast paced WO, WO noise under the hood as well as the exhaust note changed, it idled down on it's own and went away. I revved the car and it did it again and the noise appeared to come from the air/smog pump so as it was doing it I disconnected the vacuum line behind the smog pump and the idle went down and the sound went away, there are two vacuum valves on these bypass hoses. Can I plug these with no ill effects till I can get a shorter belt? Oh I have no cats on the car
Keep us updated! I had a vacuum leak that was pretty hard to find. One of the vacuum caps under the upper intake had started to fall apart. I used a whole can of carb cleaner trying to find it without any luck. I had to pull the upper intake off to find it... It really really sounds like a vacuum leak to me.

You can bypass the smog pump or you can get a pulley to replace it. It is a pain trying to find the right size belt if you bypass it, especially if you have any kind of under drive pulleys.

Did you do anything with the tubes that come from the back of your heads? There are screw in plugs that you can use if you want to delete it.

https://lmr.com/item/LRS-6086AK/1979...gs-for-50L-58L

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