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Ok, I’m working on a 97 thunderbird with a 4.6L engine. Here is the problem. When I turn on the heater or the A/C it makes my idle fluctuate wildly. The idle changes along with the compressor turning on and shutting off, (which happens with the A/C and Heat.) I’m not sure if it’s supposed to be running when it’s set
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I would check for carboned up iac passages or heavy carbon build up in the throttle body. The next thing i would possibly pursue is a carboned up or failing iac motor. To clean it i would remove it and hose it down good with some carb cleaner. When you re install it remove the positive battery cable let it sit for like fifteen minutes re attach it and start the vehicle. let it try to find its idle for a few mins then attempt to see if it will replicate the same symptoms
The compressor cycling could also be caused by low refrigarent charge but i would check the tb iac for the erratic idle symptoms.
As a definite non-expert on this forum, I am seeing this from a different angle. It sounds to me, whenever the A/C clutch kicks in, your motor slows. Have you looked at the compressor to see if the bearings are bad? Extra load from a bad compressor might give a transient drop in idle before the computer can adjust. Incidentally, on my truck the compressor clutch activates even when heat is on — it seems counterintuitive but that’s how it happens on my vehicle, normally.
Is the idle just looping or can you explain what RPM’s the needle bounces to and from? Does it do this in all gears?
The idle on vehicles will always change when there is an extra load on the engine, such as the A/C compressor engaging.
Out of curiosity, how do your battery connections look? Any susbstancial change is operating voltage at the battery when this issue occurs?
I cleaned the IAC a few weeks ago, and that tackled a larger fluctuation problem that I had. Now with the AC and Heat off it runs nice and steady. The problem only occurs when the Heating or cooling is on. It seems counter-intuitive to me but the idle jumps up around 300 rpm (600 to 900 in park, and 800 to 1100 in Drive gears) when the compressor kicks on. The compressor is cycling around every 5-10 seconds.
I have not ruled out a problem with the IAC since it was very dirty before I cleaned it. I tried smacking it with the screwdriver too with no change. I haven’t ruled out the possibility that I may have bad bearings in the compressor either. This car sat for a very long time and was ill maintained before I bought it (come to find out after the fact.) So it could be anything.
The battery voltage holds steady at 14 volts dc. The headlights dim when the compressor kicks on though. (could this be caused by a failing alternator?)
By the way…. thanks for the quick replies…. this forum is awesome.
If the compressor is cycling on and off in that time span, i would hook up some A/C manifold gauges and check for refrigerant issues seeing as this problem is only evident when the compressor is engaging.
Thats all that comes to mind right now, sorry, im exhausted from work.
Keep us updated though!
Yeah thats the next avenue i would pursue is make sure the system is charged. I wouldnt worry about the alternator with the voltage you stated, a compressor is a big amp draw so i wouldnt be to concerned about a little dimming. But i would pursue system pressure first then i would check out the compressor/compressor clutch assembly also like spelunkered said because your iac might be working decent now but the compressor is that much of a drag where the pcm is demanding the added rpm input and not letting the iac bring the idle back down plus it coincides with excessive headlight dimming and the erratic idle only when compressor is cycling. Could be a combo of low system pressure and a whooped compressor clutch. Poorly maintained cars can be like a big cause and effect game lol. Keep us posted i wanna see this fixed.
Also 2 dumb questions. First did you do a iac relearn after you cleaned it?
And have you checked the thing for vacuum leaks because the are famous for them? because normally a maf car will throw some code for a unmetered air leak but sometimes they dont catch it. If its a big enough leak the iac steps might be maxing out when you have the additional load of the compressor but it can easilly control idle without the extra load. Usually it will throw a cel for that too but sometimes eec v cars are weird lol. Just some more food for thought lolhey, I had some what same problem.. every time i turn on ac idle go down.. after some investigation.. i found out that my temperature sensor was bad.. i replace it and all was good… i also clean out the throttle body and i check the idle valve…
The compressor turning on with the heat on is normal especially when in defrost mode. The system is designed to raise the idle when the AC compressor is engaged to keep the idle steady, it sounds like yours might be ‘overcorrecting’, considering that it was very dirty and you found an issue there you might consider replacing the IAC, I often find that cleaning them isn’t enough and sometimes you just need to replace them. You might start with that just to eliminate that as a cause. Keep in mind that after the replacement you may have to wait some time for the idle to level out as it takes a bit for it to ‘learn’ the new part.
That said you might also want to check for vacuum leaks just to be on the safe side as the computer may be trying to correct for something that is out of range because of an existing mixture problem. Here ia a video that might help you with that.
With the A/C compressor cycling every 5-10 sec, something’s gotta be wrong with your A/C…. I agree with Dreamer2355, check your A/C system pressures, and compressor.
Quoted From spelunkerd:
With the A/C compressor cycling every 5-10 sec, something’s gotta be wrong with your A/C…. I agree with Dreamer2355, check your A/C system pressures, and compressor.
I agree this could be an issue and probably indicates low pressure in the system however I think it might be best to address the idle issue first and then move to the AC. Just my opinion anyway.
I’ve been trying to get out there to get this thing sorted. This car seems to be one thing after another though. Tonight I addressed another issue with the bearings on the idler pulley, and the tentioner pulley. I got a new bottle of propane for my torch, and am going to check for vacuum leaks in the morning. I have decided to attack this thing in financial order. Cheapest possible fix first.
I GREATLY appreciate the responses and suggestions in regards to this problem I’m having. I am quite sure that the solution has been presented in this thread. I just have to find the time to get out there and do the work, and the money to buy the parts if required. Regardless I will keep you all updated. I hope to get it sorted by days end tomorrow (but you know how those things tend to go.)
When considering your ‘approach’ of cheapest first also consider that some things left unchecked can cause other more expensive problems down the road. Keep us posted on what you find along the way and we will do our best to help you. Thanks for posting on the ETCG forum.
okay… it’s been a while but I finally tracked down the problem and it was a weird one. After a while of driving it like this, I developed a squeek from the engine bay. It wasn’t constant like a fan belt but more like a bearing problem. I took the belt off and the bearings in the idler pulley and the tensioner pulley were completely toasted. I replaced them, and a couple of days later the problem just went away….
Not what I expected, but may be something to keep in mind if you have someone else with this problem.
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