iat fault on a uk 4.0 sohc

[neilatkinson1981]

hi guys i'm new to the forums, it's taken me ages to work out how to post so i don't know if this is the right place to post this question. i'm not sure how i'll find this thread? again so any info please inbox me as i can find that

i have a uk spec ford explorer 97 plate with the sohc engine

it has the separate maf and iat

i just replaced my iat sensor as when watching the live data on my snap on scanner it shows the iat randomly jumping from around 7-8degrees C which will be about right in 2-3degree outside temp with a cold air intake

the iat randomly jumps up to 149degrees C then back to normal, the spiking is completely random and is making it run like crap it's up and down almost constantly

i'm in IMI level 3 mechanic and it's got me stumped

i'm either thinking ECU/PCM dying or the wiring has a split somewhere causing a short circuit/ghost feed or the earth is bad creating high resistance confusing the ECU/PCM


it's also difficult to start when warm taking 5-6 seconds cranking to fire up, but when cold starts instantly

 

[2000StreetRod]

small affect

I assume the reason you replaced the IAT sensor was because the reading was unreasonable. It is unlikely that both IAT sensors are defective so you probably have a wiring issue.

Does your "cold are intake" pull air from the engine compartment? If so, the IAT will rapidly rise when the vehicle is stopped and drop to about ambient when the vehicle is moving. If the readings are truly random then is your scanner reliable? The PCM should detect an open wiring path (infinite resistance) and a short to ground (zero resistance) and set the appropriate diagnostic trouble code:

P0109 Intake Air Temperature Circuit Malfunction
P0111 Intake Air Temperature Circuit Range/Performance Problem
P0112 Intake Air Temperature Circuit Low Input
P0113 Intake Air Temperature Circuit High Input
P0114 Intake Air Temperature Circuit Intermittent

To test the PCM I suggest that you disconnect the IAT sensor and see if the PCM reports it. Then clear the codes and insert a jumper across the sensor connector and see if the PCM reports it. My Haynes manual states that the temperature vs resistance for the IAT sensor is the same as ECT sensor. BrooklynBay has the ECT table posted: ECT Chart

When I was experimenting with my Lightning MAF sensor installation I substituted a potentiometer for the IAT sensor. I learned by varying the potentiometer that the IAT sensor output had very little affect on the air/fuel ratio at idle. The ECT sensor has a significant impact. I suspect that your engine warm start problem may be due to a defective ECT sensor or a weak fuel pump. Have you monitored the ECT with your scanner?

 

[neilatkinson1981]

codes

Yeah I replaced it due to getting code
p0112
Circuit low

So hot a new sensor and cleared the code.

Within about ten seconds it was back up with 0112

When watching live data it shows pcm is seeing ect at a steady 80-82 degrees C

At idle
Maf. 5.46g/s
Tps. 12.9%
Ect. 80 degrees C
Load. 23.9%
Spark adv. 18.5deg

Iat. 7-149-7-149-8-149-8-149-7-149 and so on

It's really bugging me as is down on power, using loads of fuel and annoyingly if I floor it while sensor is showing 149 it won't kickdown

 

[2000StreetRod]

intake air temperature limits

Those IAT values exceed the upper and lower limits of the chart. It is almost like the electrical path cycles between an open and a short. The gray/red wire is common with many of the sensors (ECT, TPS, DPFE, etc.). Since your ECT is not intermittent, if there is continuity between the IAT gray/red wire and the TPS gray/red wire it should be OK. The problem is probably with the gray wire that connects the IAT sensor to PCM Pin 39. Maybe the PCM connector is loose or Pin 39 is pushed back and being affected by engine vibration.