What are the symptoms of a failing IAC valve?
Can it be tested?
Dans

Possible symptoms of a Stuck or Failing Idle Air Control Valve (also called a "Constant Idle Speed Motor" in many Volvo service manuals):

Fast idle.
Slow idle.
Slow idle and rich mixture.
Erratic idle.
No fast warmup idle.
Hard starting.
Engine surging.
Idle problems that seem to come and go at random.

Here's the big picture and how the IAC Valve works: When the engine is at idle and your foot is off the pedal, the throttle plate (butterfly valve) is closed and prevents air from entering the engine. The black thumbwheel screw (on the throttle body) is used to allow a constant amount of airflow around the throttle plate and thus provide the air required for maintaining a 'base' idle speed. However, additional air is required when various factors are present, such as cold idle warmup, high altitude, etc. Basically, the computer is watching the oxygen senor and trying to keep a stable 14.7 to 1 Air-to-Fuel ratio when you're idling, and the IAC Valve is used as needed to keep this in check. The later vehicles that use the Bosch 516 IAC Valve have completely eliminated the black thumbwheel screw on the throttle body, and now all air entering the engine at idle speed must pass through the 516 IAC Valve.
I've also written a bit more info in a previous post:
http://www.brickboard.com/RWD/index.htm?id=777199

Since the engine purges itself of it's crankcase vapors into the air intake (via the PCV system), some oil usually accompanies the vapors and can varnish or obstruct the IAC Valve and the thumbwheel passegeway. Due to the importance of this little device, I remove my IAC Valve once a year and clean it thoroughly. I also clean the Throttle Body at the same time and make sure the passageway around the throttle plate is clean by removing the black thumbwheel and giving it a good spraying (if you adjust the thumbwheel, you will need to reset you idle, see the Haynes manual). If you have a Bosch 501 or a 520, you should be able to twist the unit along it's motor axis and the valve should move freely (open and close). As you're holding it and twisting it, you should hear it open and close with a soft 'tick' sound. If you don't hear a soft tick, or it takes a firm jerk to make it open/close, then you need to keep cleaning it a bit more. I usually go through about 15 Q-Tips and a good amount of Throttle Body cleaner before I'm satisfied. If you're cleaning a 516 IAC Valve, just try to make everything look nice and shiny inside (it's spring loaded so there's no easy "twist-test" for this guy). Use a Q-Tip or a pen and open and close him several times while cleaning.

A good carb cleaner is required to clean the IAC Valve. Valvoline, Pyroil, and STP Throttle Body (carb) cleaners come highly recommended. Avoid the cheap stuff like Gumout, etc, or you'll be spending a lot of unnecessary time trying to clean the parts. Spend the extra $1 or so and buy good cleaner, it's worth the time you'll save.

God bless,
Fitz Fitzgerald.
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'87 Blue 245, NA 236K

Very good info. I think my IAC is fine (the engine never dies), but how _quickly_ is the system supposed to react to changing loads? For example, when I put it in gear (auto), the idle falls and then catches up; and vice-versa. It's about a 3-5 second process (load change, idle changes, computer corrects). Not too bad, but I would expect a closed-loop control to be more aggressive than that.

Quote: I would expect a closed-loop control to be more aggressive than that.

I agree. It may be a good time to remove and clean the IAC Valve, the tubing for it, and the throttle body. Also, seriously consider replacing your O2 sensor if it's older than 5 years.

how _quickly_ is the system supposed to react to changing loads?

Just about seamlessly. I do not notice the problems you mentioned in my vehicles. When shifting from "Park" to "Drive" and holding my foot on the brake, the engine idle drops maybe 100 or 150 RPM or so. I can't remember the actual numbers off hand, but I'm certain that it doesn't drop too low and then rise back to where it should be. Of note, my IAC Valve is cleaned once a year (and so is the throttle body, with it still on the vehicle), and the O2 sensor is only a year old. I also use full synthetic oil in my engine. This reduces (and almost eliminates) the amount of future deposits and varnish that would be formed from oil droplets blowing through the PCV system (since the synthetic oil stayes liquid and doesn't harden up like dino oil).

God bless,
Fitz Fitzgerald.
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'87 Blue 245, NA 236K

Fitz,

I just want to compliment you on your great explanation of how the IAC works. I am sure that you have educated many with this excellent thread. I personally can't remember facts very well, but I find that if I have an understanding of how something works it will stick with me. I greatly appreciate postings like this. Thanks for sharing.

This posting would be in good company if incuded in the FAQs.

Dan

You're welcome. I'm glad I could help.

I'm wondering what would be required to get additions made to the FAQ. I'm also seriously considering putting together a DIY page with detailed procedures and pictures for some of the more common preventative maintance items (such as this). In turn, this board and the FAQs have been a huge help for me and I've solved many Volvo issues thanks to the great people who contribute here. Hearing of part failures (and solutions) on other people's cars encourages me to inspect them on my own vehicles and those that I happen to find myself working on.

God bless,
Fitz Fitzgerald.
--
'87 Blue 245, NA 236K

Let STeve Ringlee know - he manages the FAQ
--
Darius in Albany NY '86 744 GLE (175K, given to my mother-in-law); '91 745 Turbo (167K-my 'Little Red Wagon')

Fitz,
I have a 501 and I can't figure out what you mean by twisting along the motor axis to get the valve to open. Externally, there is nothing on mine that moves. I can look in the tubing, and move it with a screwdriver.

I have a 501 and I can't figure out what you mean by twisting along the motor axis to get the valve to open.

Hold the IAC Valve up in front of you so that it looks like a big letter "T" with the electrical connector pointing down. The thick vertical body that houses the electrical motor is the axis that we're working with. Grasp the motor in such a way that you can twist it about 90 degrees back and forth. -almost like you're going to spin it like a Top (child's toy). When I'm grasping it in my hand, the thick motor body is in line with the axis of my arm (the electrial connector is pointing through my hand, down my arm, and at my elbow). Twist it back and forth in firm, swift, deliberate motions. It's designed to work in just about any position, so as long as you're twisting on the right axis, the moment of inertia should do the job of opening and closing the valve. The twist test can only be performed on 501 and 520 IAC Valves, and not on a 516 since the 516 is spring loaded.

Externally, there is nothing on mine that moves.

You're right, we're listening for the internal valve to open and close. If it's spinning freely (501 and 520), it should hit a mechanical stop at each end of travel, -thus producing a soft 'tick' sound.

I can look in the tubing, and move it with a screwdriver.

Keep cleaning, if things are looking shiny but it still requires a screwdriver to open, -you're almost there. You may need to gently force the valve open and closed with the screwdriver a few times while you're cleaning in order to break up some serious varnish and deposits that are grabbing the valve gate. I've had to do that on several occasions and once I had to fill the valve area half way up with carb cleaner and work the valve back and forth until it moved smoothly. When you're done, a quick 90 degree twist of your wrist should be all that's required to open the valve from a closed position. It should go back to a closed position just as easily, -with a quick twist in the opposite direction.

Lastly, if you've worked on an IAC Valve for an unusually long amount of time with no improvement, you may have damage inside the electrical motor assembly. If this has flooded with dino oil (unlikely since it's decently sealed) or the motor is damaged due to worn brushes or a commutator, I would lean towards replacing the entire IAC Valve with a used one from a scrap yard. I've had less-than-acceptable results from dismantling the electrical motor and reassembling it on the two occasions that I've tried. While I did clean it out and get everything to go back together, it didn't perform as well as I had hoped. The wire that is used to wind the coils is about the thickness of a human hair and can be easily broken at the several points where it's soldered to the commutator (if you're not careful when taking it apart and putting it back together).

If you're interested, a working model of a magnetic motor can be found on the following webpage: Click Here. The moving picture of an electric motor, halfway down the page, is very similar to how our IAC Valves work (move your mouse over the highlighted test to see it work). If I remember correctly, our IAC Valves have 3 coils, 3 brushes, a 6-sectioned commutator, and the motor only has about 90 degress of travel. (it changes directions for open/closed positions rather than spinning around in a complete circle)

God bless,
Fitz Fitzgerald.
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'87 Blue 245, NA 236K

Thanks for the clear explanation. I found buried in the FAQs a section the discusses testing the IAC, 12volt to the center pin, and grounded either of the others. It doesn't address if this test covers the 501/520. Do you know?

Great information...just had the TB off and cleaned, it was a mess...
but idle is still rough at times...
Where can you get the IAC's? .. Specifically if I were to get a
520 as I think the 501 must be on my 1991 vintage wagon what cars use the
520?..
I've had it off for cleaning and it seems ok , but given that the problems like idle hunting etc...I may just replace it...
Regards & thanks
Dans in Toronto

Where can you get the IAC's?

I get mine from a local U-Wrench-It scrap yard for about $4 each. I clean them up and keep a couple on the shelf in ready-to-go condition so I don't have to spend a lot of time cleaning one during a tuneup of a friend's vehicle. If you insist on buying one new or factory rebuilt, prices range from about $100 to $300 depending on the supplier.

Specifically if I were to get a 520 as I think the 501 must be on my 1991 vintage wagon what cars use the 520?..

The switch from the 501 to the 520 was in May, 1987. However, I've pulled them apart and compared a 501 and 520 side by side and as best as I can tell, they are identical parts. There might have been some sort of sealing improvement or some undetectable change that I can't find, but otherwise they are the same. If you go to your local Volvo dealer and attempt to buy a replacement 501, the parts guy will sell you a 520 instead. Even if you manually type in the Volvo part number of 1317957-7 (the old 501) into the Volvo parts database, it will automatically change the number to 3517067 (the new 520) and advise you that the new part supercedes the old one. For some unknown reason, IPD has not yet figured this out and they still sell both 501 and 520 units.

I've had it off for cleaning and it seems ok, but given that the problems like idle hunting etc...I may just replace it...

Are you still having problems with idle hunting? If you are, consider cleaning it again or replacing it with a scrap-yard-special (after a good cleaning). Also check for vacuum leaks and a bad Oxygen Sensor. If your Oxygen sensor is older than 5 years, I would replace it even if it seems to be working. The chemical reactions slow down and it looses it's voltage calibration (starts to produce a lower output and thus cause the engine to run rich). If you have a 3-wire O2 sensor, you can get a replacement for under $40 if you're willing to splice a couple wires. Check out the Bosch 13913 (cheapest) and 13953 (has longer wires) for a cheap and effective replacement of your 3-wire Oxygen sensor. More info can be found here: Click Here. Avoid non-Bosch O2 sensors as if they were the plague...

If that doesn't solve your problem, double check your engine temp sensor (the one that sends the signal to the computer) for acceptable output and then check your fuel injection computer. If you're driving with either of the 511 or 544 ECUs, these had cold-idle-mixture issues and are superceded by the 554. However, I wouldn't expect to find a 511 or 544 in your '91 Volvo so you should be ok.

God bless,
Fitz Fitzgerald.
--
'87 Blue 245, NA 236K

>If you're driving with either of the 511 or 544 ECUs, these had cold-idle-mixture issues and are superceded by the 554.<

That's a new one to me -- makes me want to check what's in my '88 745T. Were the 511 and 544 ECUs N.A. only?

You said that you have an '88 740 Turbo? -That should be a good unit. I'm not certain, but I beleive that Volvo stopped using the 511 & 544 units by the end of the production year in 1987 (thus all the '88s should be good). However, this is off the top of my head and I can't say for certain (at least regarding the 700 series vehicles).

However, in the 240 lineup, the 511 was used in the 240s (starting in '85 I think) until '87, and then all the '88s had the 554. However, in '89 they switched from Jetronic 2.2 Fuel Injection to Jetronic 2.4 Fuel Injection, and thus a completely new ECU design/platform was used. If you should come across an '88 240, you can assume that it has both the good 554 ECU and a non-biodegradeable wiring harness (also discontinued at the end of '87). Double bonus for the '88 240 owners!

FYI, today I just bought a 554 ECU from a salvage '88 240 to replace the 511 in my '87 240. Sometime in the next week or so, I'll change them.

God bless,
Fitz Fitzgerald.
--
'87 Blue 245, NA 236K

Alright, I'll bite. What sort of cold idle behavior should I expect to see on my wagon?

--
alex

'89 765T, 170,9xx mi

I think you're supposed to see a cold idle of around 1,000 to 1,100 RPMs or so until the car warms up a bit (determined from the coolant temp sensor). However, there is little info available on the subject of cold-idle speeds for 200/700 Volvos. The service manuals and Haynes books show how to set the base idle speed with the engine warm, but nobody really discusses the cold-idle behaviors. There may also be a difference in cold-idle behavior depending on the computer. Switching from a 511 or 544 to a 554 should change the cold-idle-mixture, and thus might change the cold-idle-speed as well.

Sorry, I don't have any more info on the subject right now. (but I'm curious myself)
Fitz Fitzgerald.

Mine had the problem described and the milage was much lower than expected. After 165,000 miles I owed it a new one. When I took it out I could get it's mechanism to move but not with the electric control. I have some parts for that engine for sale at the best price I can get, since the car was totaled right after I put $1000 of parts in it. See my other posting today at
http://www.brickboard.com/RWD/index.htm?id=778443

bobz

see my last post re the icv...I'm interested. Dans

Ok...you have an almost new IAC?...
Dans

the car doesnt want to idle! But the symptons on mine were very rough idle. The car seemed to want to surge and then drop from about 1500 to 500rpm.

The way I found out it was the IAC was when it was having problems, I tapped on it with an open ended wrench, (19MM I think). And it ran okay for a short period of time.
these can be cleaned and there is a definite test procedure, Ohming it out, checking voltage, etc, but I don't have my manual handy.

good luck,

jc
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1985 740gle na, 1990 740gle 16 valve

Hmmm, I'll try tapping it...I'm having some funny idle as well...
and stalling in very wet weather...not sure if they are related or not.
Probably worth getting a new one.
Dans