How do i i know if my iac is bad? - 1995 Volvo 850

not told RPM of surge. (full range of surge)
hot engine surge, or cold or both?
it's old most do this old.
lots of causes.
but why tells the reasons
the IAC is bad or just lost control of engine for 50 other caused.
let say engine as huge vacuum leak
the PCM closes the IAC and can not correct the massive vacuum leak.
or engine goes lean?
or spark misfires so bad IAC can not correcthis.
bad engine, bad spark bad fueling can and will make IAC go dead.
in fact fixing IAC is last, in my book
if all is good just cleaning IAC fixes it. some times.
or replace it.
on old cars the ECU surges the IAC if spec idle speed can not be reached, 800 hot.

John, IAC normally connected on throttle body, could be on under side. Check the attached links,instruction and guides, Good luck
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Car Parts Diagram
99 4Runner IAC Cleaning
Symptoms of bad IAC valve

no year, wow.!
sounds like gross loss of engine power
or
you started car and didnt notice RPM was 400 rpm.
the engine has almost no power at that rpm.
the normal cold start (winter) is about 1500 RPM.
if not at fast idle fix bad fast idle (Bad IAC)

when i say bad IAC that means the system IAC is bad,not just a part.
the IAC /ISC controls idle at all times, via ECU commands.

P1504 Idle Air Control Circuit Malfunction, take off the idle air control and inspect the inside. the pintle inside can get dirty and cause it to get stuck. try cleaning it out with some brake clean and see if it helps. dont spray it ont he electrical connection though.

JUST to be safe, pull the 3 pin plug on the IAC while the engine is idling. Does the idle change (speed should increase or start to fluctuate)? If not, you've got another bad IAC or there's a problem with the wiring/PCM that's not permitting it to work properly.

The code indicates there is an problem with your Idle Air Control System. Most likely failure is that the IAC system failed. There is an 3 pin from the IAC. Remove it from IAC when your engine is idling.Check whether there is an idle speed change? IF it does not fluctuate then your IAC has gone bad,

yes. If you put the pedal down about 5 to 10 percent and the car starts then the IAC is most likely bad. This is providing you have no codes indicating another issue.

The engine will idle very rough or it will die at idle.

A simple way to test, try removing the IAC valve completely from the intake manifold, leaving an exposed opening to let air in the manifold. If the engine idles ok with the IAC removed, you'll know it's a problem related to the IAC. You will need further testing to determine if the problem is with the IAC valve itself or with its circuit.

its located on the throttle body connected to your air intake manifold where your throttle cable connects on the throttle body there is a adjustment screw connected there

A bad IAC unit will cause poor driveablility and poor/high idle, and also stalling and starting issues. There is really no harm by being bad but during rapid deceleration if the IAC does not catch the RPM's the engine could stall out; not a good thing if you were going 60 mph. Remove the IAC motor and clean the valve and also the chamber that it goes into with a good throttle body cleaner and that should really help. Carbon buildup is a big factor in IAC failure. Most CEL will not show a malfunction code with a bad IAC motor.

Sounds like it could be your Idle Air Control valve (IAC). The IAC does as it sounds and controls the idle in your vehicle. When these go bad, you can either clean them with a throttle body cleaner or replace them. Most people replace them as they are normally not very expensive and relatively easy to replace. The IAC is located on the throttle body which is where the air duct hose ends as it comes out of the air filter box. It is normally held on with a couple screws and just needs to be unplugged and plug the new one in. Please, please let me know if you have any further questions. If you can't find the location of the part let me know. Also if you don't think the IAC is your problem and have addition information that could point to something else, let me know. Here is a picture of what you are looking for:
This is the IAC for the 6 cylinder engine but it is different and more expensive on the 4. Let me know if you have the 4 and need a picture of it. Thanks for rating my response and for using FixYa!