IAC valve operation

BillC · 30-05-2008, 04:20 PM

Here is my best guess on how the IAC valve operates after reading my manual and a bit of detective work. As we all know the IAC valve controls the idle. It does this by opening and closing an air bypass to the throttle butterfly. My manual says that the idle is factory set but to adjust it you should disconnect the IAC valve cable and set the idle to about 700 rpm. This suggests that when the IAC is closed the idle shoule be about the 700 RPM mark. When you turn your steering wheel the power steering sensor tells the IAC valve to open and increace the revs slightly to compensate for extra load. Next is the temp sensor. When the engin is cold the temp sensor tells the IAC to open slightly to allow more air and therefor increace revs till the engin heats up. Then the temp sensor allows the IAC to go back to norm. The aircon sensor works much like the power steering and activates when aircon cycles.. Can somebody tell me what the TMAP and temp sensor does ?

**Stav** · 30-05-2008, 04:58 PM

Tmap sensor measures manifold absolute pressure and air intake temperature. It is used as a reference for ignition and fuel.
Stav

EvilChief · 31-05-2008, 08:50 AM

there are two temp sensors, coolant temp sensor at the back of your head, which does ur temp gauge on the dash, coolant temp correction (ie cold start etc) and intermittend cylinder shut off when over heating. second is teh air temp sensor under ur throttle body, which controls the air tem fuel and ignition correction (ie colder air is denser and req more fuel etc)

BillC · 02-06-2008, 09:23 AM

Quote:

Originally Posted by EvilChief

there are two temp sensors, coolant temp sensor at the back of your head, which does ur temp gauge on the dash, coolant temp correction (ie cold start etc) and intermittend cylinder shut off when over heating. second is teh air temp sensor under ur throttle body, which controls the air tem fuel and ignition correction (ie colder air is denser and req more fuel etc)

Thanks for that. I replaced the temp sensor under the thermistat on Saturday and this made no difference to my idle problem. The temp sensor you say is under the throttle body could be the problem. When my car heats up the revs stay high for about one second. When the car is cold its fine. Thats what led me to change the temp sensor under the thermistat. I might look at the other one on the weekend. Thanks

sly · 02-06-2008, 01:59 PM

Quote:

Originally Posted by BillC

Can somebody tell me what the TMAP and temp sensor does ?

If you're working on the car in your profile, you don't have a TMAP. The TMAP in the I6 (in conjunction with engine speed) does something similar to what the MAF in your V8 does.

The temp sensor under the TB on the I6 is the TMAP. Maybe the V8 has an IAT? (Intake Air Temp sensor)

BillC · 02-06-2008, 03:25 PM

Sorry Sly, I also have an 98 model XH ute 4L 6 cyl that has everything exactly the same under the bonnet as my mates AU ser 1 ute. I would be swapping parts with his car to find out whats wrong with mine but he was t boned the other day and its in the panel beaters.

sly · 02-06-2008, 06:09 PM

I'd have thought the XH had separate IAT and MAP sensors like the other E-series engines. I know the '98 EL's used a "hybrid" engine that was the AU head & valvetrain on the EL block, but AFAIK this engine still used the EL ECU and sensors. The coolant temp sensor under the thermostat isn't there on an AU. As EvilChief said it's behind cylinder #6 in the head.

BillC · 03-06-2008, 01:13 PM

I should be able to tell you after this weekend. My mate gets his ute back and I will be swapping everything that might control the IAC valve. I can tell you that it is nothing like a 97 model XH under the bonnet and from a quick glance it was the same as the AU