need help with IAC symptoms

[dogfriend]

I looked this up

If you look in the Powertrain Manual (mine is 97) under Typical Diagnostic Reference Values there is a chart titled Measured / PID Values (first chart). The last item on that chart is Engine rpm. Under Hot Idle they give the idle speed as 750 - 830 rpm. It is the same for both OHV and SOHC engines.

 

[Robert]

On two scans I recently made on my '97 and a friends '98 both were around 650 RPM. The scans though were made within 5 minutes of starting both vehicles. I pulled out a scan I did on mine on 7/30 and with my IAT reading 136F and coolant temp reading 188F, my idle was 771 RPM. I would say 750-830 with the engine up to operating temperature is probably pretty accurate.

 

[tbomb]

hrmm, unless the factory tach is off, i think i might be a lil low then.

any thoughts or suggestions?

 

[dogfriend]

Originally posted by tbomb
hrmm, unless the factory tach is off, i think i might be a lil low then.

any thoughts or suggestions?

Check with a scan tool or a digital tach. Generally you check with engine fully warmed up, all accessories (A/C, lights, ect.) off. Trans in Park or Neutral.

Its probably not a big deal if it is a little low, but there could be some other issue that could be causing it to be low including the infamous IAC valve.

 

[tbomb]

Originally posted by dogfriend


Check with a scan tool or a digital tach. Generally you check with engine fully warmed up, all accessories (A/C, lights, ect.) off. Trans in Park or Neutral.

Its probably not a big deal if it is a little low, but there could be some other issue that could be causing it to be low including the infamous IAC valve.

ok, thanks. my truck is going to the body shop/mechanic on friday for 2 weeks for accident repairs, so ill tell them to run a scan and check it out for me. ill post the results.

 

[Robert]

If it is running at least 650 or so, I wouldn't be concerned about it. When coming up to stop lights, mine reads the same according to my tach. My tach is about 10% lower than my OBD-II scanner though. As long as the engine isn't stumbling or idleing roughly, it is probably OK. If the idle does get too low, it could be symptoms of a failing IAC valve. Mine has already gone throught he "humming" stage in its life. Now that it is loosing its voice, it just yells out every once in a while.

 

[tbomb]

thanks robert. im gonna have a scan run on it while its in the shop for accident repairs and see what they say. if its low, then the first things im doing is replacing the IAC, and then if its still low, i think ill sit in the corner and worry about it ya know, if its not one thing with these trucks, its another, heh.

 

[X24]

I put the new IAC valve on tonight and the idle came up. It idles warm around 700-750 and around 650 when in gear. It has not stalled yet but that only happened about once a week so hopefully this fixed that problem. The RPMs dont drop to 200 when I shift from R to D anymore.

 

[dogfriend]

When I had that problem, I bought the new IAC but didn't put it on right away. I waited until it started causing a problem again, then took it off in the parking lot and replaced it right there so I would be able to tell if that was the problem (it was).

 

[X24]

Well, I'm going to clean the old one and keep it. Apparently these things go bad quickly so maybe I can make it work again if I need it.

 

[dogfriend]

The original one lasted 3 years, the replacement has 2 years on it.

On my original one, I could see a shiny area on the shaft where it was rubbing. You can look into the port to see if yours has this.

I think that what was happening on mine was that it was ok when cold but after the engine was warm and the parts of the valve expanded, the shaft started to rub and the valve was sticking mostly closed.

 

[tbomb]

well, i think im gonna get a new IAC and see if it helps, im pretty sure it will after all the stuff ive read about IAC issues.

98fordx24, where did you get your new IAC valve and how much was it?

 

[X24]

I got it at Ford for $80. I'm sure you can find it cheaper some place else. Took me all of 2 minutes to put it on. It's idiot-proof.

 

[tbomb]

Originally posted by 98FordX24
I got it at Ford for $80. I'm sure you can find it cheaper some place else. Took me all of 2 minutes to put it on. It's idiot-proof.

alright, cool. ill call a dealership tomorrow and see if they have one in stock; if not, ill have them order it for me.

thanks.

 

[MaverickDaKid]

Does the IAC make the voltage meter move up and down? I have the RPM moving up and down but I see it go up and down along with the voltage meter.

 

[dogfriend]

The signal sent to the IAC by the computer is a PWM (pulse width modulated) signal. You wont get an accurate reading with a normal voltmeter.

It sounds like yours is trying to control the idle speed by varying the pulse width, but cannot control it for some reason. Maybe a vacuum leak? How much is the rpm moving up and down?

 

[MaverickDaKid]

went from 500rpm to 2500rpm. I cleaned my IAC today with some air intake cleaner, said on the back it could be used on IAC and throttle bodies. I sprayed it on held it in for 5 mins then drained for 30 mins.

My god the thing runs so nice now, except that black thing with the filter thingys in it. When I put it on I get the air horn effect. So im riding with it off now and it runs like a dream.

If I run without that black thing though would it do any damage. I tried cleaning the black part and took it apart and cleaned the filters but still not clean. Can I spray some of that cleaner on those suckers?

Thanks dogfriend

 

[dogfriend]

Originally posted by MaverickDaKid


If I run without that black thing though would it do any damage. I tried cleaning the black part and took it apart and cleaned the filters but still not clean. Can I spray some of that cleaner on those suckers?

I don't know if the filters can withstand the solvent or not. Try using compressed air to knock the dirt out. You could rinse them in water, and then let them dry out for a while before installing them again.

I don't think that it will hurt to run without the filter for a little while, but I think eventually the inside of the vent passage may get gunked up with dirt if you leave it off for a long time.

 

[Hank3rd]

I know that this thread is old and that I am reserecting it, but I have a question. After having problems and cleaning my IAC, I seem to now have hesitation in slight acceleration. Could this be caused by the IAC?

I have a feeling it may be a fuel problem. Does the engine need to be at idle for the IAC to work?

-Hank

 

[dogfriend]

Make sure the gasket between the IAC and manifold is ok, otherwise you have a vacuum leak which will throw off the fuel/air ratio.

Also, its possible that one of the other sensors (MAF) could be causing your problem.

The main purpose of the IAC is to provide bypass air (bypassing the closed throttle plate) to regulate the idle speed. It also provides the control for "no-touch" starting. Finally, it also acts as a 'dashpot' to prevent the engine from stalling when you suddenly close the throttle.