View Full Version : High idle with AC on

07-07-2007, 03:03 PM
Hi All!

I just searched the forums for high idle problems and did not find one that is close to my problem, so here goes...

On my '96 it started idling high (1700-2000) with AC on. If you turn the AC off, it would fall to normal and idle fine. Turn on the AC and you would see a little bump in RPM (+200?) but if you bumped the throttle, it would return to 1700-2000.

Any ideas? It seems to run fine with the AC off.

Also, what is the IAC? Idle Air Control? Where is it located?


07-07-2007, 11:36 PM
That was yesterday. Now today it runs fine, AC on or off. Any ideas?

07-08-2007, 10:58 AM
Have you had the battery disconnected recently? If so, the PCM is learning to recontrol idle speed by adjusting the Idle Air Control valve. This part is located close to the throttle body on the intake plenum which is basically an air bypass to the throttle body to regulate idle speed. The idle will rise up slightly when the load on the engine changes such as turning on the AC. Also, change in vacuum draw will also cause the IAC to increase idle speed when you push hard on the brakes.

If you are having issues with the IAC, sooner or later the check engine light will come on if the idle speed cannot be maintained. Check the wire harnes connection to the IAC (has a diamond shaped base that mounts to the intake plenum, and a small cylindar solenoid motor attached to it).

07-08-2007, 12:05 PM

Thanks for the info. No, the battery has not been disconnected lately. It has been driven every 3 days or so.

From the posts in this forum and other research on the internet, it is sounding like classic IAC. The car has 67k on it and may be time for a new one.

Thanks again for the help.

07-11-2007, 11:53 PM

I think I found the IAC. On my 96, it appears to be on the drivers side of the plenum, towards the front. My throttle body is on the passenger side. It has a square base with two holes in it and a solenoid mounted in alignment with one of the corners of the square base making it look like a diamond.

Instead of cleaning is a guy just better off getting a new one? They dont seem that expensive.


07-12-2007, 08:44 AM
Im a firm believer in replacing, b/c it just seems like you can never get them as clean as if they were new...I think an IAC is going to run you about $50...

07-12-2007, 11:32 AM
Replacement of the part does not mean that the issue will go away. In some cases it will correct the problem but since you mentioned that the idle is fine when the AC is off. One question: Do you get a hanging idle speed when you are driving (shifting gears) or at idle and tap the throttle a bit similar to what you have noticed when the AC is on. IF you are not experiencing any of the issues without the AC on, more than likely there is something else causing the issue. When I had the 01, it developed a simialr problem but got worse over time. Throttle would hang every now and then and the idle speed would not recover properly (sometimes the IAC would over compensate and cause a stall). The same thing occured after replacing the IAC. Cleaning the throttle body did not correct the issue either but it did make improvements. There was one thing I did notice that was different, I no longer heard the click of the EGR valve which I used to hear 5 to 10 seconds after starting the car. After replacing the EGR valve, the car ran better than it did when I bought it. With the stock tune, EGR valve should cycle every minute and open for about 5 seconds. As the engine heats up the cycle time will increase. I am not saying that the EGR is the problem in your case.

IF you do replace the IAC, make sure it comes with a new gasket (note: do not attempt to port match the gasket to the opening in the plenum as this will cause high idle speed and possible hanging idle when shifting gears, I did this once and was a big mistake). Also you may want to inspect the throttle body blades and clean if needed. Also inspect and verify that the throttle body blades do not have interference on closing. Make sure the return springs are in good shape too.

07-12-2007, 10:29 PM

Very interesting comments. I have noticed the idle does not drop like it should with the AC off. Sometimes a little hang and every once in a while you get a stumble when the idle is coming down. What happens is you blip the throttle, it slowly comes down and then the idle gets real low and the engine stumbles and fumbles a bit (real technical terms) and then the idle jumps a couple hundred and finally settles down.

67k on the car and nothing has every been replaced, to my knowledge. Ausie, you could be correct in that the IAC may/may not be dirty, but is having trouble compensating for another issue.

What should the EGR sound like? Just and on/off click?


07-12-2007, 11:40 PM
Hi again...

I did a little more diagnosis this evening. I cannot hear the EGR cycle, ever. It didn't make a noise during startup/warmup (about 10 minutes). The valve and piping were cold even though the car was coming up to temp. I took it out and drove it for about 30 minutes. When I got back, everything under the hood was warm. I put a screwdriver handle on my ear and put the tip against the EGR valve in different places. I never heard and kind of different noise.

I unplugged the IAC and the car instantly dies.

Any further ideas or does this confirm EGR is bad?


07-13-2007, 07:48 AM
Chris, Let me start with the EGR. The EGR will make a click sound when it is activated as loud and similar to the AC compressor clutch (but without the compressor noise). A screw driver will not be needed to hear it. There may be a different PCM control of the EGR given your model year so it may not perform any duty cycles at warm up. I do know that the 01 and 04 Cobras do this since I have/had both vehicles. There are two components that are used by the PCM that control the EGR valve, EGR vacuum regulator and the DPFE (differential feedback pressure sensor). If the EGR valve was stuck open chances are that the Check engine light would come on indicating excessive EGR. Actually the EGR valve is not supposed to be active at idle, when the TPS is below a specified value or at WOT. However, this is not the case in the 01 and 04 model years and it will cycle on and off during the warm up period. Actually I never heard it in the 01 until after replacing the EGR valve. Symptoms of excessive EGR: engine could stall after a cold start or at idle after deceleration, engine can surge at cruising speeds or idle may be rough. IF there is no EGR: this would cause combustion temperatures to rise too high resulting in detonation, or engine overheating. This was the issue I had with the 01, excessive ping and detonation after engine reached operating temperature. Replacing the EGR fixed the problem as well as the idle issues I had. If you have noticed any pinging when accelerating, chances are that the EGR valve is not opening. Another issue is if the EGR tube is damaged or rusted through this would allow for unmetered air to enter the intake plenum. You would know if it was since the exaust would sound different and the engine compartment would be considerably dirty from carbon suit if there was a breach in the EGR tube. You would also have far more issues if this was the case.

IAC valve: there may be different designs depending on model year. 99-04 have the IAC mounted next to the throttle body. They function by applying voltage to the solenoid to close the IAC valve, under no voltage (unplugged) the engine vacuum will cause the IAC valve to open which will result in high idle 1200-1500 RPM. I am not sure if the IAC valve functions the same with the 96-98 model years. Consider this.... There are 4 sources of air for the intake plenum, three of which are metered air sources and the fourth is recirculated by the EGR valve. The three metered air sources are as follows: throttle body, IAC, and PCV system. At idle, there are only three contributors, IAC, PCV and EGR (may depend on operating conditions). IF the EGR and or PCV sources are excessive, the IAC valve cannot be regulated properly resulting in a stall at idle or during deceleration.

I would probably start off by cleaning or replacing the PCV since that typically gets gummed up easily and is the least expesive component to replace.

The DPFE may be malfunctioning which is the common issue with problems regarding the EGR valve since it has one connection to the EGR tube. DPFE component is typically rectangular in shape, has two vacuum lines connected to it along with a wire harness connector. If any EGR codes are generated by the PCM, this would be the first component to suspect or replace.

The EGR vacuum regulator may also be at fault but not a common failure. The EGR vacuum regulator (or actuator in other words) also has two vacuum lines connected to it but one is connected directly onto the EGR valve. This also has an electrical connector.

The EGR valve is not an easy task to replace especially if it is corroded or rusted. When replacing the EGR valve, you will need a new metal flake gasket, use antisize compound (same as what you use on spark plugs) on the cast iron threads of the new EGR (try not to get any of that on the inside area of the EGR valve). You may also need some rust loostener to remove the EGR coupling from the tube if it is corroded enough. To remove the old EGR, loosen the coupling first and do not pull the tube too far as this may cause damage to the tube if it has become weak over time. It is possible to clean the pintle valve of the old EGR, but not worth the effort especially if the diaphram of the EGR is not functional. There are ways of testing the EGR valve diaphram. The easiest way to verify if it is functional is to disconnect the vacuum lines from the EGR vacuum regulator and use a small tube or vacuum tree to connect the two lines, or if the vacuum lines are long enough connect the feed (usually red) to the EGR valve. When the engine is at idle, you will hear the click of the EGR valve diaphram.

07-13-2007, 03:31 PM

Thanks for the info. You have been most helpful.

When unplugging the IAC, the car dies right away. Any thoughts there?

07-13-2007, 08:12 PM
the stall could be related to a too much air or not enough air. The issues may be related to the IAC valve since I would not rule that out. If you had something to block off the pvc line or pvc valve and try the idle test with the iac again and see if there is any change. I believe your model year feeds the IAC through a rubber hose connected to the intake. If so, there may be a poor seal of the hose or a leak in the hose that is allowing unmetered air into the intake throught the IAC. What is supposed to happen when the IAC is disconnected is that the valve will be drawn open and the rpm will rise to a maximum within the range of 1500 to 2200 RPM. IF you disconnect the IAC and let the engine run too long the Check engine light will come on and the codes if read would indicate idle speed out of range. IF you can rule out any vacuum leaks or issues with the pcv system, then the IAC may be sluggish to compensate for changes in operating conditions. The problem is after you replace the iac and it still continues to do the same thing..... that would definately point towards the EGR valve or one of its controlling components.

07-14-2007, 12:03 AM
Ausie, hi again.

OK, I will do some more checking. My IAC is attached right to the plenum. It does not look like any external hoses other than the wire connector.

I checked with the Ford dealer today, and the IAC is $103 and the EGR valve is a little over $100 also.

I will also check the PVC that is easy.

Thanks again for all the advice.

07-14-2007, 07:17 AM
oops my bad, was thinking of the 2v GT motor. That one has an external air bypass hose. I had too look in the picture and video area to look at engine pictures, SuperG has one that shows the IAC. Must have internal plumbing for the air bypass.

It appears as if you have an intermittant issue. Which component is the problem.... EGR valves typically are reliable but are susceptable to diaphram failure or clogging of the pintle valve which could prevent them from closing completely. If the car had a surge issue (idle would oscillate low to high repeatedly) I would suspect the EGR valve or vacuum leak of some sort. If you were getting any pinging after warm up that would also point towards the EGR but could also be related to head temperatures or carbon in the combustion chamber. At this point, I would probably order the IAC valve and replace it. They can be cleaned as long as you hold the solenoid motor to prevent any solvents from dripping in throught the shaft. Age, corrosion, heat, etc will cause the solenoid motor to degrade over time and it is possible that the coil may not pull in properly to manage the idle speed. What comes to mind is that it could be probable that the voltage is drifting when the electrical load increases (this could be caused by old battery, or underdrive pullies and too slow a speed on the alternator).

If I think of the issue in a different manner, such as there is nothing wrong with the IAC since the problem only occurs when the AC is on. There may have been some data logged into the EPROM that should have not been stored for certain load conditions. I cannot recall the extact term but in some instances, no load high rpm (banging the rev limiter) can cause some data to get remapped in areas it should not be in. The hanging idle or dashpot setting is used to allow the engine to slowly lower the RPM at a different rate depending on the given engine load. All of this information is stored as corrections to Air/fuel ratio and load table corrections. There is a way to clear all the data stored in the EPROM memory which is disconnecting the battery for 1/2 hour. After reconnecting the Battery, the PCM will relearn all of the data it needs to manage engine operations, all that is needed is to drive.

If you decide to dump the EPROM, at this point would be a good time to inspect spark plugs and replace if necessary, check throttle body for dirt and clean if needed, clean the MAF, inspect and replace the air filter (or clean and reoil if it is of the cotton variety K&N type B&C or other reusable filters). After reconnecting the battery, turn the igition key to on but do not start the engine (wait about 15 seconds, the guages may do some funky things like max out then drop to normal) then start the engine and let idle for a few minutes, once the idle stabilizes, turn the engine off and restart. Take the car out for a ride but do not punch the throttle take it easy since the PCM will be recalculating the A/F corrections (starts off rich then slowly leans it out, then returns it to a nominal A/F ratio). Drive for about 1/2 hour in various speeds, also turn on the AC for a bit during the latter part of the test drive. Note that the idle speed may be a bit higher than usual during the test drive and expect it to hang a bit during shifting. On some occasions it may even stall since the PCM is recalculating the corrections needed. If the iac is indeed bad, you may experience surging of idle, and possible the check engine light may come on. If that happens, take the car to the nearest auto parts shop that can read the codes for you for no charge or if you have a scan tool you can read the codes yourself.

Although, replacing the parts will not hurt the performance of the vehicle it will hurt the bank account. Besides that if you do replace both the IAC and EGR you should purge the EPROM since the parts have changed and the PCM should relearn the load tables with the new parts in place. The same applies for replacing spark plugs. After 60k miles there are some parts that need replacement, namely the spark plugs, in some cases the O2 sensors (expensive and not an easy task to replace), plug wires where applicable. Fuel filter should be changed every 16k miles, thermostat every 24k miles.
At a minimum, replace PCV valve it you still have the original, and if the plugs have not been replaced in a long time, changing them will improve performance. The other items are not necessary or relavant to have replaced until after you correct the idle issue.

07-14-2007, 02:13 PM
Awesome information. Thank you!!!!

I thought I heard somewhere that when you replace the spark plugs, you need to put anti-seize on them (steel on aluminum?). Is that correct?


07-15-2007, 08:17 AM
Yes, anti-sieze is alway a good idea. Permatex makes a product that has aluminum powder in it. It is basically a silver high temperature lubricant. It is conductive once you place torque on the plug and very messy to work with.

Before using anti-sieze, gap the plug first, then insert the plug into the sparkplug socket and then apply the anti-sieze. Use only a small amount, a few dabs around the threads closer to the seat (you want to keep this away from the lower part of the plug and away from the electrod and ground strap).

When working on the spark plugs, wait for the car to cool down if hot, best to let sit overnight and do it with the engine cold. Do not worry about the small glaze of oil on the seat and thread of the plugs, this is normal since the O rings will expand and contract as the engine heat up and cools down. Take your time in removing the spark plugs. Always threat the new plugs in by hand. Also best to have a spark plug socket that has a 12 inch extension swivel or non-swivel that is permanently attached to the socket otherwise the socket will stay on the plug once you attempt to remove it if you are just using a standard plug socket and snap in extension. If your engine is not enhanced with power adders (nitrous, turbo, supercharger) just stay with the stock plugs or replace with same plug that comes out. Gap should be listed on the Catylist sticker in the engine bay area. I am not sure what the toruque should be for your model year. Torque wrench is a good thing to use to prevent over tightening the plugs.

A new set of wires would not hurt either. If you had a 99-04 you can opt out on the wires since they use coil on plug set up. Since your coil pack are not on the plugs, the easy way would be to remove one wire and plug at a time and replace with new before tackling the next plug.

Note: After attending a Mustang show in Harrisburg (CJ pony parts customer appreciation day) yesterday, I was able to look at a few 96-98 Cobras. Even thought the IAC looks like it sits by itself, below the solenoid at the bottom of the intake plenum there is a one inch diameter hose that bends out in front of the plenum, joins into what looks like a plastic breather box (probably has a filter in there) and continues on to the intake in front of the throttle body. However, if you did have a leak in the plumbing it would be evident in all modes of operation and not just when the AC is on. I spoke with two owners that have 89 Cobras about the IAC. I asked them if they were to unplug the IAC when the engine is at idle, the charactersictic is a stall. So it sounds like your IAC is operating fine.

I am going to another show today, I will ask around and see what suggestions I get for the issues you are having with RPM surge when AC is on. It will surge a bit and run slightly over 1000 RPM when the AC is running which is normal due to the additional load on the engine, AC compressors consume approximately 8 to 15 hp to operate ( I am only guessing at this figure but I do know it is much higher than an electrical AC condensor unit used in homes and buildings which usually use 1/2Hp to 4Hp depending on size and can even be much larger than that. I have seen some screw compressor components that are gigantic in size where one of the rotors was bigger than the size of the 4v head on the Cobra (8 inch diameter and 1.5feet long).

07-20-2007, 06:09 PM
OK, here is the follow up on this thread.

I put in new spark plugs, pcv valve and dumped the computer. After the computer re-learned everything it seems to be much better. RPMs coming down to idle are different with the AC on than off, but it does not hang around 1400 any more. Also the RPMS drop to idle much quicker than before.

So, not sure what all was wrong, but perhaps Ausie was right about the computer being messed up.

At any rate, the Cobra is running good again!

Thanks for all the info, it was of great help!

07-21-2007, 06:30 AM
Good to see the issue is resolved. Hopefully it will not return....but if it does the next part to change may be the IAC unless you changed it already.