Since the day I bought my 03 gt 5 speed( about 3 months ago) the idle will drop to 500rpms and almost shut the car off every 10-15 seconds. The thing is it only does it with the ac on. The ac off it idles perfectly all day. I cleaned the maf and iac. Replace the plugs, air filter and charged the ac. The ac runs and cools fine. I havent tested the battery but it cranks fine even if sitting for a week. I was gone for 3 weeks last month, came home and she started fine. Any ideas?
Idle Drops To 500rpm
- Thread starter rebus20
- Start date
wmburns
SN Certified Technician
The way that the PCM handles IAC/idle trim values are covered in this recent thread.
https://www.stangnet.com/mustang-fo...e-under-load-or-when-a-c-is-turned-on.899667/
https://www.stangnet.com/mustang-fo...e-under-load-or-when-a-c-is-turned-on.899667/
wmburns
SN Certified Technician
IF the motor almost stalls from the AC compressor load every time the AC engages some consideration should be given to the AC compressor drawing too much load. What could cause this?
- excessive Freon. There's a correct Freon charge. There has to be room in the system to allow the Freon to expand.
- Dirty/clogged AC evap coil.
- cooling fan not operational
- Blockage to the flow of Freon within the system. Kinked line or blocked orifice tube.
- internal compressor fault
What about the compressor slugging gobs of liquid? Stalls intermittently, slows everything down, maybe slips the belt a bit, then clears, next slug shows up, etc. You just can't compress liquid. I thought about this when you suggested over-charge of Freon: most had a relief valve which started dumping it before any chance of significantly stalling engine/damaging compressor, think the Sankyo 10 was set at, like 450 psi or something. They are self-reseating, but mine ('90 F-150) with 120` ambient one day popped off, did not reseat, bled almost all off. Just thinkin' out loud. imp
Wondering why you say so? We have an engine stalling and so far are blaming alternators??? Given a battery is present which STARTED the engine, unless the vehicle was driven boo-coo miles with no alt. output, till the batt. quit, and then it stalled, why would we think alternator?
Or, I'm missing some vital point here (always a possibility). imp
wmburns
SN Certified Technician
The way today's alternators work is very different from yesterday's units. Today's alternators are very high output devices in a very compact package. This design has some "trade offs". Today's alternator's are no longer self exciting. The battery is used as an energy store to "buffer" the step load changes as will as to provide the excitation voltage needed to "kick start" the field winding into work.
In practice for today's cars no battery = no alternator.
But wait. If this is true why doesn't the motor die if the battery is disconnected? The answer lies in how alternators and excitation work. In practice when the motor is running in a steady state it is possible to disconnect the battery and the alternator will continue to put out. The problem comes when there's a large step load power change. Without the battery to act as a damper the voltage could dip as the regulator is attempting to adjust. IF during this delay the voltage dips too low, the alternator field coil will loose strength (excitation) and shut off. Without a ready battery to provide excitation the voltage regulator can not restart the process.
Virtually every system in today's car relies on a stable power source. But consider the high energy ignition system that is supplying spark. It's it hard to see how the ignition system would be affected by low system supply voltage? Would a drop in RPM's be expected IF the spark were weak?
I'm a big believer in starting with the basics. EVERY trouble shooting session should begin with a review of the battery and charging system. Does that mean replace everything. No. It means check and test that it's performing at some minimum level. At the very least perform a visual inspection.
The good professional mechanics know this to be true.
If still uncertain that there even is an alternator problem why guess? Test! If there's is an problem with the charging system it will show up on a tester. For a major voltage problem that bad enough to starve the ignition this has to be an event that is measurable with a Volt-Ohm meter.
IMO If this were my car and testing showed there was a problem with the alternator, I would not take the chance of going on a wild goose chase. I would always stop and fix the alternator/charging system before performing any additional trouble shooting. This goes double if spending $$'s on parts is involved.
Bonus thought for the day. Can you "bump start" a modern car with a TOTALLY dead battery?
Howto perform charging system voltage drop test
https://www.stangnet.com/mustang-fo...perform-charging-system-voltage-drop-test.56/
In practice for today's cars no battery = no alternator.
But wait. If this is true why doesn't the motor die if the battery is disconnected? The answer lies in how alternators and excitation work. In practice when the motor is running in a steady state it is possible to disconnect the battery and the alternator will continue to put out. The problem comes when there's a large step load power change. Without the battery to act as a damper the voltage could dip as the regulator is attempting to adjust. IF during this delay the voltage dips too low, the alternator field coil will loose strength (excitation) and shut off. Without a ready battery to provide excitation the voltage regulator can not restart the process.
Virtually every system in today's car relies on a stable power source. But consider the high energy ignition system that is supplying spark. It's it hard to see how the ignition system would be affected by low system supply voltage? Would a drop in RPM's be expected IF the spark were weak?
I'm a big believer in starting with the basics. EVERY trouble shooting session should begin with a review of the battery and charging system. Does that mean replace everything. No. It means check and test that it's performing at some minimum level. At the very least perform a visual inspection.
The good professional mechanics know this to be true.
If still uncertain that there even is an alternator problem why guess? Test! If there's is an problem with the charging system it will show up on a tester. For a major voltage problem that bad enough to starve the ignition this has to be an event that is measurable with a Volt-Ohm meter.
IMO If this were my car and testing showed there was a problem with the alternator, I would not take the chance of going on a wild goose chase. I would always stop and fix the alternator/charging system before performing any additional trouble shooting. This goes double if spending $$'s on parts is involved.
Bonus thought for the day. Can you "bump start" a modern car with a TOTALLY dead battery?
Howto perform charging system voltage drop test
https://www.stangnet.com/mustang-fo...perform-charging-system-voltage-drop-test.56/
Last edited:
Bonus thought for the day. Can you "bump start" a modern car with a TOTALLY dead battery?
Don't think so.
I have not had much luck jumping a car since they made the change in battery materials, they seem to short out more internally and give less warning before giving out entirely. Or its just me!!
Don't think so.
I have not had much luck jumping a car since they made the change in battery materials, they seem to short out more internally and give less warning before giving out entirely. Or its just me!!
I also changed the belt. No battery light since changing the alternator. The battery would start the car after sitting 2 or 3 weeks so I think the battery is good.
And the rpms drop quicker with the iac replacement. It was slow to idle down before and was making a howling noise from the intake but has stopped since the iac replacement.
And the rpms drop quicker with the iac replacement. It was slow to idle down before and was making a howling noise from the intake but has stopped since the iac replacement.
Super bump... Hopefully this can reach the OP. Did you ever find a resolution to this? My 2000 Saleen S281 is doing the same exact thing. Bogs down at idle to around 400-500 rpm only when the AC is on and then surges every 5-10 seconds. It did completely kill the engine twice in the past 2 months when pulling up to a stop light.
I've had IACV replaced. My mechanic said it improved it somewhat but it seems the same to me. He also evacuated and recharged the AC system; holding pressures fine... no improvement. It's driving me crazy and it's almost summer.
I've had IACV replaced. My mechanic said it improved it somewhat but it seems the same to me. He also evacuated and recharged the AC system; holding pressures fine... no improvement. It's driving me crazy and it's almost summer.
89ripper
Founding Member
Unfortunately OP has not been back in awhile, in the meantime check out this thread
Ive seen this problem before on all types of cars, and it turned out to be different things. You are going to want to make sure there are no codes present, as that will tell you a lot more than just guessing
Similar threads
