Bosch Idle Air Control (IAC) Valve - Rough Idle/Stalling Problem Cured!
04-20-2007, 12:07 AM
#1
Instructor
Thread Starter
Bosch Idle Air Control (IAC) Valve - Rough Idle/Stalling Problem Cured!
Folks,
After months of despair I'm celebrating to no end tonight!!!! :-) :-) :-)
About 7 months ago my 1986 3.2L Motronic 911 Carrera slowly worked its way (over a period of about 4 months) to a point where it got harder and harder to start cold and would not idle until it got good & warm. Even after warming the idle was VERY iffy.
When approaching a stop sign/light, if I got off the throttle early and let the RPMs slowly stabilize, it might not stall when I came to a stop. If I got off the gas quickly it would stall for sure.
For fuel efficiency/emissions reasons these cars have a fuel 'cut-off' feature built into the Motronic system where all fuel delivery from the injectors is stopped (under certain RPM & engine temperature conditions) when you're completely off the throttle and decelerating. At the appropriate time (determined by the Motronic control unit via various sensor inputs) fuel delivery is resumed so a proper idle will result as you roll to a closed throttle stop.
After fuel 'cut-off' my car would just not recover in time to maintain idle unless I nursed the RPMs down first. Cold starts & exiting a parking lot cold were worse, with the engine stalling everytime I got off the gas.
I like to think I'm pretty smart (?) so out came the testing equipment and into the books & engine compartment I jumped. All to no avail.
With the Bentley Carrera manual and Charles O. Probst's excellent Bosch Fuel Injection & Engine Management book I came up stymied and feeling dumber than a bag of hammers.
I won't go into the the gory details (and there are a LOT) but all I found out was that I had a lot of things GOOD in the Fuel Injection (FI) department and NOTHING bad... This can't be... (although I'm VERY pleased to know that I DO have many good FI components!!!)
Then last week I got MAD and set out to solve the problem. Since the IAC Valve (Bosch part# 0 280 140 501) is controlled via pulse/duty cycle modulation on either of its two grounds at any given instant (one series of signals to open the valve and one series to close it) and knowing that many 'home-fixit' folks can't perform such a 'full operation' test on the unit, I got VERY interested in if it was REALLY working.
All of the test procedures I've seen have you perform a few basic resistance checks between the hot (12V) center pin on the unit and the 'open valve' and 'close valve' grounds. If the values are correct and the 'shake' test (to detect free piston movement) succeeds (mine did) then you assume the valve's OK. Maybe...Maybe NOT!
My IAC Valve passed these test but was still the problem!
At this point I have to defer to the more electrically knowledgeable folks (and there are MANY on this board) as to the safety of my improvised test procedure so don't try this at home unless you're willing to fry a potentially working valve, but here's how I found the valve to be faulty.
The valve operates on 12V DC and in desperation I wanted to 'see it work'. I felt that a lower voltage might keep me from smoking it if I was going down the wrong path so I used a partially discharged 9V radio battery (7.2V on the Volt Meter) to test it.
After manually moving the valve to about its halfway position with a small probe, I connected the battery's positive terminal to the center pin. I then used a jumper from the battery's negative terminal to give the 'close valve' ground pin on the valve a quick 'tap' to simulate a 'pulse'. Just a quick 'tap' here mind you, NOT a constant connection.
SNAP - It closed. So far so good. Next a quick tap to the 'open valve' ground pin. NOTHING. At this point, with the valve fully closed, I repeated the Ohms test published in the manual(s) between the center pin and the 'open valve' pin. NOTHING - I.E. Infinite ohms. WOW! It DOES fail the Ohms test - BUT only when fully closed.
I then repositioned the valve to where it was almost (but not quite) closed and the Ohms test was OK! A quick tap of ground on the 'open valve' pin indeed resulted in another SNAP - this time in the open position, just as it should be.
Repeated and varied cycles of this testing all showed the same results - the valve would open & close as expected (and PASS the Ohms test published in many places) UNTIL it reached the fully closed position.
Once the valve was fully closed it would NEVER reopen and, subsequently, would not allow for a needed increase in bypass air for 'idle adjustment' (all of which should be metered as the IAC valve is downstream of the airflow sensor itself) thus having a devastating impact on idling.
Just keep in mind that this failure only goes to show that what works (and PASSES tests) in 'one position' may NOT work or PASS tests in 'another position'.
I don't know the exact layout of the internal electrical design of the valve but it might lend itself to failure in ANY position with VERY different symptoms (I.E. a condition where the valve always opens but won't close would lead to too much bypass air and possibly an INCREASED idle...)
I'm just so damn glad to have an idling car again. A loaner valve from a GOOD friend passed all of my 'improvised' tests and, when installed on the car, immediately cured the problem.
After 7 months of stalling and grief, perseverance, study and a bit of 'questionable improvisation' it ALL finally paid off!
Barry
Who will have BOTH the Lobster & Cracked Crab tonight! ;-)
After months of despair I'm celebrating to no end tonight!!!! :-) :-) :-)
About 7 months ago my 1986 3.2L Motronic 911 Carrera slowly worked its way (over a period of about 4 months) to a point where it got harder and harder to start cold and would not idle until it got good & warm. Even after warming the idle was VERY iffy.
When approaching a stop sign/light, if I got off the throttle early and let the RPMs slowly stabilize, it might not stall when I came to a stop. If I got off the gas quickly it would stall for sure.
For fuel efficiency/emissions reasons these cars have a fuel 'cut-off' feature built into the Motronic system where all fuel delivery from the injectors is stopped (under certain RPM & engine temperature conditions) when you're completely off the throttle and decelerating. At the appropriate time (determined by the Motronic control unit via various sensor inputs) fuel delivery is resumed so a proper idle will result as you roll to a closed throttle stop.
After fuel 'cut-off' my car would just not recover in time to maintain idle unless I nursed the RPMs down first. Cold starts & exiting a parking lot cold were worse, with the engine stalling everytime I got off the gas.
I like to think I'm pretty smart (?) so out came the testing equipment and into the books & engine compartment I jumped. All to no avail.
With the Bentley Carrera manual and Charles O. Probst's excellent Bosch Fuel Injection & Engine Management book I came up stymied and feeling dumber than a bag of hammers.
I won't go into the the gory details (and there are a LOT) but all I found out was that I had a lot of things GOOD in the Fuel Injection (FI) department and NOTHING bad... This can't be... (although I'm VERY pleased to know that I DO have many good FI components!!!)
Then last week I got MAD and set out to solve the problem. Since the IAC Valve (Bosch part# 0 280 140 501) is controlled via pulse/duty cycle modulation on either of its two grounds at any given instant (one series of signals to open the valve and one series to close it) and knowing that many 'home-fixit' folks can't perform such a 'full operation' test on the unit, I got VERY interested in if it was REALLY working.
All of the test procedures I've seen have you perform a few basic resistance checks between the hot (12V) center pin on the unit and the 'open valve' and 'close valve' grounds. If the values are correct and the 'shake' test (to detect free piston movement) succeeds (mine did) then you assume the valve's OK. Maybe...Maybe NOT!
My IAC Valve passed these test but was still the problem!
At this point I have to defer to the more electrically knowledgeable folks (and there are MANY on this board) as to the safety of my improvised test procedure so don't try this at home unless you're willing to fry a potentially working valve, but here's how I found the valve to be faulty.
The valve operates on 12V DC and in desperation I wanted to 'see it work'. I felt that a lower voltage might keep me from smoking it if I was going down the wrong path so I used a partially discharged 9V radio battery (7.2V on the Volt Meter) to test it.
After manually moving the valve to about its halfway position with a small probe, I connected the battery's positive terminal to the center pin. I then used a jumper from the battery's negative terminal to give the 'close valve' ground pin on the valve a quick 'tap' to simulate a 'pulse'. Just a quick 'tap' here mind you, NOT a constant connection.
SNAP - It closed. So far so good. Next a quick tap to the 'open valve' ground pin. NOTHING. At this point, with the valve fully closed, I repeated the Ohms test published in the manual(s) between the center pin and the 'open valve' pin. NOTHING - I.E. Infinite ohms. WOW! It DOES fail the Ohms test - BUT only when fully closed.
I then repositioned the valve to where it was almost (but not quite) closed and the Ohms test was OK! A quick tap of ground on the 'open valve' pin indeed resulted in another SNAP - this time in the open position, just as it should be.
Repeated and varied cycles of this testing all showed the same results - the valve would open & close as expected (and PASS the Ohms test published in many places) UNTIL it reached the fully closed position.
Once the valve was fully closed it would NEVER reopen and, subsequently, would not allow for a needed increase in bypass air for 'idle adjustment' (all of which should be metered as the IAC valve is downstream of the airflow sensor itself) thus having a devastating impact on idling.
Just keep in mind that this failure only goes to show that what works (and PASSES tests) in 'one position' may NOT work or PASS tests in 'another position'.
I don't know the exact layout of the internal electrical design of the valve but it might lend itself to failure in ANY position with VERY different symptoms (I.E. a condition where the valve always opens but won't close would lead to too much bypass air and possibly an INCREASED idle...)
I'm just so damn glad to have an idling car again. A loaner valve from a GOOD friend passed all of my 'improvised' tests and, when installed on the car, immediately cured the problem.
After 7 months of stalling and grief, perseverance, study and a bit of 'questionable improvisation' it ALL finally paid off!
Barry
Who will have BOTH the Lobster & Cracked Crab tonight! ;-)
04-20-2007, 01:40 AM
#2
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Looking good Billy Ray! I've found IAC's to last 4-5 yrs, then start to deteriorate on the Motronic cars I've owned. Strange idle? Car wants to, or does, stall when coming to a stop (learn to heal toe to keep it from dying) or using a/c? Smack the IAC with a hammer and it usually changes. You've gone the furthest I've seen in diagnosing a bad valve. Congrats! Next time, just replace it and be merry.
David
David
04-20-2007, 02:09 AM
#3
Nice work, and thanks for the very detailed info. Great stuff.
I've been there with a fast idle on my 78 CIS 911. I read the books and all the forum posts on fast idle on multiple forums. I bought the tools and diagnosed every component just to find that it all was working within spec, which was satisfying and disappointing at the same time. After a few months of scratching my head, my father-in-law and I decided to have a crack at it one day. Being the "old fashioned" mechanic, his method was just to begin wiggling things. When he came to the cruise control cable, the idle magically dropped back to normal. That was it. A slight loosening of the tension on the cable via its adjustment nut solved the problem for good. I had to smack my forehead more than once over that one.
Brett
I've been there with a fast idle on my 78 CIS 911. I read the books and all the forum posts on fast idle on multiple forums. I bought the tools and diagnosed every component just to find that it all was working within spec, which was satisfying and disappointing at the same time. After a few months of scratching my head, my father-in-law and I decided to have a crack at it one day. Being the "old fashioned" mechanic, his method was just to begin wiggling things. When he came to the cruise control cable, the idle magically dropped back to normal. That was it. A slight loosening of the tension on the cable via its adjustment nut solved the problem for good. I had to smack my forehead more than once over that one.
Brett
04-20-2007, 06:16 PM
#4
Rennlist Member
Barry, is the IAC valve you are talking about the same one as the Idle Control Valve (ICV)? Do you have a picture of the IAC valve? The ICV is in the middle of the intake and just back of the top of the fan shroud housing. My ICV valve is goldish in color and has an arrow pointing in the direction of air flow. A picture is worth a thousand words.
04-20-2007, 07:29 PM
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Originally Posted by Spider911
Barry, is the IAC valve you are talking about the same one as the Idle Control Valve (ICV)? Do you have a picture of the IAC valve? The ICV is in the middle of the intake and just back of the top of the fan shroud housing. My ICV valve is goldish in color and has an arrow pointing in the direction of air flow. A picture is worth a thousand words.
AKA
Air Idle Control (AIC)
Idle Air Control (IAC)
Idle Control Valve (ICV)
04-22-2007, 11:16 PM
#6
Instructor
Thread Starter
Spider911,
Yes - as dsmith stated, you've found the Bosch IAC Valve. A photo is attached. The valve is the horizontally mounted 'can' with an upper hose to the intake rubber boot and a lower hose to the left-side intake manifold itself. There is also an electical connection (three pin) to the leftmost 'bottom' (horizontally speaking) of the 'can'.
And yes - there is a vertical airflow direction 'arrow' (pointing downward) moulded into the section between the two hoses .
Hope this helps!
Barry
Yes - as dsmith stated, you've found the Bosch IAC Valve. A photo is attached. The valve is the horizontally mounted 'can' with an upper hose to the intake rubber boot and a lower hose to the left-side intake manifold itself. There is also an electical connection (three pin) to the leftmost 'bottom' (horizontally speaking) of the 'can'.
And yes - there is a vertical airflow direction 'arrow' (pointing downward) moulded into the section between the two hoses .
Hope this helps!
Barry
