why remove the IAT?

[johnny2can]

Been meaning to ask for awhile...

Bought my rig a couple months ago. When I removed the plastic "TD" cover over the intake to keep it cooler, I found a pipe plug where the IAT should've been. A little more investigation found my IAT comfortably nestled in a wad of fiberglass insulation with duct tape wrapped around it, between the intake manifold and the head.

Anyone know why someone would do that? :confused: I have theories, but would like to hear other's first.

Yes, I put the IAT back in. smile.gif

[rjschoolcraft]

If the IAT goes above a particular threshold, the PCM will begin to pull fuel to protect the engine. IAT rises with high boost... i.e. during high power demand situations. When the PCM pulls fuel, there is a noticeable drop in performance. If the IAT sensor is removed, the PCM is blind to the high intake temperatures and doesn't pull fuel. This could work nicely... for a while. :eek:

Good thing you put it back in. The real way to fix high IAT is to add an intercooler. ;) ... not just pull the sensor.

[Spindrift]

Any idea what the temperature threshold might be?

[mklein]

As I recall, the only temp that will get the PCM to go into fuel cutback is based on ECT. At around 113 degrees C the PCM will start to pull fuel back from the engine until the coolant temp. gets under control.

[rjschoolcraft]

I don't know the temperature threshold. The book doesn't give it and I havn't run with a scanner hooked up to see (pre-intercooler). The book does clearly state on p. 3-6 of the Driveability and Emissions (Diesel) section that the IAT is used by the "control module to control fuel." I've also experienced this phenomenon when towing in the mountains (pre-intercooler). The first thing that the PCM does is reduce the wastgate duty cycle to lower boost and if that doesn't lower it enough, fuel goes.

[johnny2can]

Thanks, guys! That was my theory, that it was removed so the computer wouldn't limit power from high temps.

My seat-of-pants dyno said the max power was reduced, but the turbo whine seemed to be slightly louder.

So... other than melting pistons, what other damage could it possibly have done?

Thanks again!

[jspringator]

Does it really matter what the intake temps are (as far as hurting the engine) if the EGT's are low and coolant is not hot? I always assumed this was a backdoor way to control EGT's.

[GMCfourX4]

I agree with James... This is backed up by a comment Bill Heath made to me once that as long as you monitor EGTs and coolant temps, it makes sense to plug the hole and keep the IAT somewhere its goign to stay cooler.

-Chris

[gmctd]

I remember a poster on here a while back, claiming he didn't need a charge-air cooler - had his IAT sensor removed and wrapped so PCM wouldn't pull fuel when he needed it most.

Wonder whatever became of him and his truck.........

[jspringator]

I know that a hot intake charge is less dense and thereby has less oxygen to burn fuel; hence less power. But I don't see any way the intake could get any where near the heat after combustion. If EGT's are within spec, I don't see how a hot intake could damage the engine, although a hot intake charge would certainly make less power than a cooled intake charge.

[LilRed Turbo]

With the IAT removed and kept cool the ECM will deliver maximum fuel charge for the volume of air going into the engine. If the intake air is hot,(less dense therefore less O2) the engine will be getting a rich mixture in the cylinders. Incomplete combustion will cause a rise in exhaust temps causing engine temps to rise. So if you run without the IAT in proper working configuration and no EGT guage you are on barrowed time? Now,GMCTD,Ronnie Joe,others is this correct?

[johnny2can]

Ok... other than removing the IAT sensor and a K&N filter, this motor is bone-stock.

While true, hotter air is less dense, that's assuming that it ain't compressed. In my case, the whole reason it was hotter was because is was compressed.


ronniejoe
The first thing that the PCM does is reduce the wastgate duty cycle to lower boost and if that doesn't lower it enough, fuel goes.My theory was that removing the IAT sensor a)maintained the total amount of air (because the turbo wasn't wastegated) and b) maintained the fuel (because it wasn't pulled). I've read in other posts that if you reduce intake 100*, typically you reduce exhaust 100* (or at least close enough for this discussion, someone with more exp. please correct me if I'm out of the ballpark).

So, just for arguments sake, if the IAT limits intake temp to 200* and if under a given load the exhaust is 1000*, then if under the same load I remove the IAT sensor and that allows the intake to go to 400*, then the exhaust is 1200*.

This is all hypothetical, as I haven't installed a pyrometer yet.


James Springate
Does it really matter what the intake temps are (as far as hurting the engine) if the EGT's are low and coolant is not hot?Since GM 6.5 doesn't monitor EGT, I'm guessin' it does matter, since IAT is the closest they can get to monitoring EGT.

...but again, mostly theory and edumecated guessin' on my part... :D

Johnny

[gmctd]

You guys are on the right track -

First, closely monitor EGT, which is a direct reflection of IAT.
ECT - coolant - follows more slowly, is more difficult to correct for.

Increase compressed intake temperature by 100deg, EGT increases by 100deg, for same engine loading.

For sustained ~7.5psi Boost, as in towing, expect IAT of over 200deg with no charge-air cooling.

Cannot find where I read it, but PCM pulls Boost, then fuel at 238deg IAT, and\or 265deg ECT.
If wastegate is spring-controlled, where PCM cannot reduce Boost, will reduce fuel earlier, providing for stuck-wastegate scenario.

I'm talking OBD-I, for sure - OBD-II is indicating some differences, which we are attempting to map.

Main thing to remember, without instrumentation, is - be afraid, be very afraid. ;)

[norm]

It seems clear that using the IAT is factory way of keeping EGT from getting too high.
I think the point of Springate's question was if we monitor EGT (and have taken steps to keep it down like exhaust upgrade) do we need to worry about IAT? Will high IAT hurt anything other than EGT?

[On the other hand, since the sensor and programming appear not to use IAT to modulate fuel across the board, but only to pull it at a limit, why pull the sensor? If we are really pushing the motor with towing, mountains, high boost, etc. we might should spring for intercooling or water inj...]

[Jim P]

I have to say that I have had my intake air temperature sensor just laying on the intake for about a year. It has caused no problems that I am aware of.

When I pull my 30' fifth wheel I do have to monitor the exhaust temperature. It will sometimes hit 1250 pre turbo if I let it. Usually it is when I am reving hard in a lower gear like 3 rd or 4th during a take off. I haven't found a hill yet on the highway that I can't pull in high gear once I get it rolling.

Last year we pulled it from Ohio to Cooks forest, PA. and I had the cruise set on 67 mph and I don't think it ever dropped below 65. My gauges are mounted under the dash right in front of the gear shift. A couple times I was not paying attention and my 8 year old son had to remind me that the egt was approaching 1200 but I never did back off on it. The coolant temperature never went over 210. It never backs off on fuel.

[gmctd]

Why would you remove the IAT sensor, JimP - your sig indicates a charge-air cooler..........

My air-to-air seldom gets over 120degF, and that's in slow traffic, with a\c burning full blast.

[Jim P]

I don't know if you remember but I am the one that made the liquid intercooler using a heater core and casting an aluminum housing around it.

The intercooler is technically in place but I have never installed a radiator to cool the water for the intercooler. So actually the intercooler is doing nothing at this point. I made the whole upper intake and at this point there is no place to screw in the intake air temperature sensor or the boost sensor for that matter.

I probably will eventually drill and tap a port for the sensor but I was just trying to show that as long as you keep the egt in check, I don't see what it hurts to run without the sensor. It might get a little better milage with the sensor but so far I have not noticed it.


I noticed an instant increase of power when I installed the aluminum housing with the heater core inside, even with no water cooling it. I am not sure why but I always thought it was because of removing the IAT but it also may have to do the bigger volume of aluminum in this housing to act as a heat sink and naturally get rid of some heat.


One reason I have not taken the time to mount up a radiator for the intercooler is because I wanted to make sure the heater core did not plug up after a while and cause the engine to starve for air. The fins on a heater core are much closer together than a true liquid intercooler and I wanted to make sure this would not be a problem. So far it has not so I need to get busy and get the radiator mounted up.

[ 01-09-2005, 07:44 AM: Message edited by: Jim P ]

[rjschoolcraft]

No argument, that if you monitor EGT, a missing IAT sensor, and subsequently high intake temps, won't hurt anything. It's those trucks that don't have an EGT gage (most of them) that can get into trouble. I thought that was the situation with the truck in question here. As was stated earlier, since the PCM doesn't monitor EGT, the IAT is the only information that the PCM has to try to keep EGT in check. That's why I said what I said earlier.

[Jim P]

Ronniejoe,

I agree that without a pyrometer you could possibly have a meltdown with the IAT removed from the intake.

I wonder why GM did not use a pyrometer in the electronic system to pull fuel when things were running too hot instead of using the IAT.

[johnny2can]

Originally posted by ronniejoe:
It's those trucks that don't have an EGT gage (most of them) that can get into trouble. I thought that was the situation with the truck in question here.Yes, Ronniejoe, you assumed correctly... no EGT gauge, and hence the question about other possible damage.

Thanks, all!

-Johnny