I just bought a 98 f150 xlt 4X4 and the check engine light was on. I cleaned the MAF and IAT sensors and reset the computer. The truck then had no hesitation and idled better but not perfect. It had stalled a couple times on a very hot day while idleing in gear at a stop light. Now the rpm does fluctuate but very little when the engine is warm. About 625 to 675 rpm which is annoying because at 675 it makes a small vibration in the cab. Now after about 500 KM the check engine light has returned with no change to the engine operations. I cleaned them sensors again and reset comp again and the light is now off again. I checked the reference voltage on the IAT and it shows as 4.69V. Is it not supposed to be exactly 5? Would this affect the engine running/idleing? (the temp gauge in the cab has hardly ever/ or barely even got up to the normal range.) If it's not 5V, doesn't this give the comp a wrong difference in reading when it is returned from the IAT? If this needs repaired or something replaced how would i do this?? Hope I am making sense... I'm far from an expert.

 

What you really need to do is get the codes read out instead of resetting the computer when you get a CEL. Here in the US, Autozone and other auto parts stores will do this for you free. Then you can research the codes and get a MUCH better idea where to go from there.

 

I had them read at auto value and he said it read that the intake was running hot... i didn't get to see the actual code, that's just what he said... i will go to another shop and see if i can get the actual codes later though.

 

Intake running hot? Temperature gauge reading low? I'd probably flush the cooling system, replace any shady looking hoses, and put in a new thermostat.

 

Acceptable range of Vref is 4.0-6.0 Vdc.

You need the actual fault code instead of poking around. Each code has a specific diagnostic connected to it; the fault code is not the diagnostic itself.

It is pointless to clean an IAT in any event.

Steve

 

I just wondered that if the comp. used Vref of 5.0 for its calculations but at the IAT it was only recieving 4.6V for some reason, then the final output would be off??

Yes the temp gauge is reading low, it hardly ever even makes it up to the normal range and I did have the cooling sytem flushed because it looked pretty cruddy, with no change to the temp gauge. (the hoses and connections all look like new). The heater throws heat but it seems it should be more than it does so the next plan is to change the thermostat because of it, could that be a cause for it to stall at idle in very hot weather??

(this is how the truck has run since I got it recently and upon cleaning the MAF, is when I saw that the IAT had been disconnected)

Thanks for the help!

 

IAT is intake air temp sensor though, isn't it? This should have nothing to do with the coolant temp. Your coolant sensor may be malfunctioning. More reason to get an aftermarket gauge. The stock Ford gauge is really a "replace engine light" - when it shows hot, time to replace parts.

Since the truck uses a MAF sensor, I assume that the IAT is just another stupid OBDII-required sensor that's never used or referenced. I would be really surprised if Ford used that sensor in any way, shape or form. If it's giving you a check engine light and you want it to go away, then replace it. I highly doubt it would cause any driveability problems, even if you threw it in the ocean, so if you don't care about the light, just clear it and ignore it if it comes up again (obviously, pay heed to other codes that might arise).

Now, if the truck had a MAP sensor, then the IAT's input might be used.

 

They really can't overheat -

if the temperature exceeds 154°C (310°F) , the powertrain control module disables all of the fuel injectors until the engine temperature drops below 154°C (310°F) . If your system is really dirty , you might be flirting with the cut off point..

 

The intake air temp sensor is refereced for cold start and also contributes to fuel and timing changes as the air changes temp while driving.
It's not there just for kicks.
Being disconected dosn't result is 'great things' happening like stopping the motor but it is needed as part of overall drivability.

 

The MAF can also "see" all of these things as well. Most manufacturer's programming of MAF equipped cars uses MAF voltage and RPM for the timing and fuel tables. The IAT is required on OBDII cars. Most manufacturers just put it on there and other than throwing a code, there isn't any change in output, driveability, etc.

I could be wrong with the F150, the only thing I've played with Ford-wise was a Mustang, but I don't see why they'd be very different.

 

Change the thermostat and see what that does for you.

 

I just put in a 195 thermostat and had found that there wasn't one before?! Why would someone remove a thermostat and unplug an IAT sensor?? I now have great heat in the cab and the temp gauge shows about 1/3 of the way into the normal after warm up. But...

The RPMs idle at just over 700 in park, then when I put it in gear they drop to about 500 and jump back to about 600, then stay there while stopped with the brakes on. While driving, when I come to a stop, the RPMs drop down to 400 and seems like it is about to stall then quickly jumps back up to 700 then back to 600 and remain there. It didn't stall through my 10 minute test drive but it sure seemed like it was gonna.

It is very hot today outside 30 something celsius, which it was before when I had trouble with stalling at a light (but only when I first stop, then fires right back up and idles good). On cooler days I seem to have no trouble. Weird.

Could something that is causing this problem be the reason for the previous owner to have removed the t-stat and iat? The check engine light hasn't returned but I will have a scanner in the next couple days to make sure... think I have heard of codes that do not throw the check engine light?

 

That is strange about the thermostat.

Sound like you need to clean the Throttle Body and elbow, IAC and MAF - Just need some elbow grease.

Throttle body - http://www.fordf150.net/howto/throttlebodyclean.php

MAF - http://www.fordf150.net/howto/clean-...low-sensor.php

Nope - Codes will always illuminate the light . The only type that does not is from the misfire monitor which will blink until you have Primary failure.

 

thanks for the info!

i'm gonna clean the throttle body etc. in the next day or two so long as i get time to do it.

the MAF I cleaned already which made a huge difference in removing the hesitation on startup. felt like i had much more power.

thought I would also mention that when i switch out of gear the RPM surges to 1000 then settles back down to 700. Even when I am idleing to a stop the engine surges when it shifts. Also in neutral if i rev the engine and then release the peddal, the RPM dips down to 400, then surge back up to rest at 700. It does feel like the engine is surging very little at this time but doesnt change the RPM at all that i can see on the gauge usually, except for every couple minutes I will notice the gauge fluctuate as it takes a larger RPM dip.

Driving down the highway it runs perfectly smooth so hopefully the TB cleaning will help with the idleing. I will post back after I have it cleaned up.

 

yikes! guess I am going FAR off topic in this thread now