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Possible MAF issue w/IATS Reading High

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After firing my newly rebuilt engine for the first time yesterday, I noticed that the intake air temp sensor might have been reading 63 C (145 F), I will verify this during the first drive.

The engine oil was allowed to heat up to 111.8 C (233.24 F) and engine coolant to 102.8 C (217.04) before it stabalized (those were high points), I have a hard time believing it was 145 F in my garage. I turned on the high speed radiator fans manually and also had a couple external fans blowing on the radiators, it certainly did get hot in the garage, but not 145 F hot. :)

Could this be a sign of a bad IATS on the MAF, and are there any additional troubleshooting suggestions I should do perform before replacing the MAF?

Thanks in advance.

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Ok, removed the MAF sensor. Between pins 1 and 3 on the MAF I get very brief blips of a reading appx 900 to 1.9k ohms when I first touch the probes to the pins, then infinity if the probes are held onto the pins for any length of time thereafter. Sometimes there is no reading. Replaced the battery in the multi meter and tested several more times, same results.

It's 61F, 7F below the 20C BTW.

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Interestingly enough, engine off of course. I installed it again and it read 19.8C. Then up to 20.3C now (which is a little high, since it's only 60F outside). Then it crept up to 21C. Now 21.8C. Bet it will keep going up if I leave the ignition on. edit, yep now 22.5C.

It's only about 62F in the air box.

Sounds like it's toast.

Looks like there are alternatives on pelican, ebay, or amazon instead of paying $438 from a Porsche discounter.

Edited by logray
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Wow, those little buggers are expensive. Probably a silly question, but have you tried hitting it with MAF cleaner?

The MAF has a hot-film sensor that measures air flow over the sensor. This is the part that get dirty and causes erratic air flow readings - in lograys case the second part of the MAF the air temperature sensor is giving bad readings. The air temperature sensor is enclosed in a glass-like material so it doesn't get dirty.

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Yep tried cleaning it. I think it is shorted out internally. That would explain the sporradic multi meter tests and incorrect readings.

I've got another MAF on order should be here tomorrow or Thursday.

For now, I'm going to delay any further running of the engine since I don't want the DME think it is 145 degrees outside and lean out the AFR too much.

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:( :( :( :( :( :( :( :(

New MAF arrived. Ohmed pins 1 (iats) and 3 (ground) out before I installed it. At 58F it read 1700 ohms and steady. 20C is 68F. 58F is 14C. So I am "supposing" that 2300 to 2700 ohms being a normal MAF at 68F/20C correlates to a MAF reading 1700 ohms at 58F/14C. But I'm just guessing, since I haven't researched what this MAF is supposed to output, assuming that is publicly available information.

So, new MAF installed and it read the correct outside/intake temperature, until about 5 miles into a 12 mile drive, it slowly started reading higher, until it reached 30C, even 48C, unfortunately after I had completed the 12 mile drive. :@

Can someone please help my sanity and put their multi meter on 2000 ohms and ohm out pins 1 and 3 on their MAF? Tell me what you get and what temp it is?

I checked the MAF harness for voltage and read 5 volts on wire 1 with wire 3 being ground, which is normal.

I checked the MAF harness again for voltage and read 12 volts from the supply wire and 5 volts from the MAF sensor wire, wire 3 being the ground in both cases.

I checked for voltage on the ground pin and there was none.

I checked for continuity between the DME harness IATS pin, MAF ground pin and the MAF harness. Continuity is perfect.

I swapped the DME with my spare DME.

IATS STILL reading 48C !!!!


Edited by logray
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Ok doing some reading it looks like on most cars the resistance vs. temp is an inverse relationship (or negative coefficient), so as resistance increases, this incdicates lower temp, AND conversely the higher the temp, the lower the resistance.

Whether this is true for the Porsche IAT in the MAF, I am not sure.

But if it is like most cars, on my NEW MAF, this would mean that it is reading a lower resitance at lower temp, which should mean higher resistance.

I'm really at a loss here, why with both the old and new MAF are the intake temps steadily increasing as the car warms up then once it is hot or warm the intake temp reads very hot, 120-150 degrees F. More than double what the outside air temp actually is..

Perhaps I should just hard wire a 2300 ohm resistor to the DME IAT wires and see what happens.

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"Intake Air Temperature Sensor The intake air temperature sensor is installed in the mass air flow sensor housing and is therefore situated in the intake air flow of the engine. It is supplied with voltage from the DME control unit via terminal 43 and its ground is connected to the HFM 5. The intake air temperature sensor functions according to the NTC principle.

The signal from the intake air temperature sensor is used by the DME control unit to calculate the default load signal if the mass air flow sensor fails. It corrects the load which is calculated by the control unit and dependent on the throttle potentiometer. With higher air temperatures, the default load signal is downwardly corrected.

The falling air density is also compensated. In addition, the risk of knocking during combustion increases as the intake air temperature rises. Therefore, the ignition angle go is corrected towards "late" with high engine temperatures (> 90°C) and high intake air temperatures (> 30°C)."

Have you checked the wiring harness for damage?

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...The falling air density is also compensated. In addition, the risk of knocking during combustion increases as the intake air temperature rises. Therefore, the ignition angle go is corrected towards "late" with high engine temperatures (> 90°C) and high intake air temperatures (> 30°C).


This would explain why it starts to run like crap when the sensor starts to heats up over 30C (which did both times during first firing and the first 12 mile drive), but runs fine when it's cold.

I'll take a look tomorrow at the wiring and perhaps even bypass it with direct wire runs. Maybe even hard wiring a resistor or just unplugging the MAF.

Edited by logray
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I just went out to my car, park it on my driveway and plug in the OBDII scanner. The engine was fully warmed up. When idle, the ambient temp is 54F and IAT is 54C=129F.

My car runs fine. I think I noticed similar thing before but when car is moving though, the IAT should be much closer to the ambient.

My theory is when the car is not moving, the hot air close to the cat is sucked into the airbox via the small holes (for water to drain) at the bottom of the air box below the air filter. So maybe you IAT sensor is just fine?? Have you checked IAT when the car is actually moving?

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Thanks Ahsai, I appreciate it. That does help to see perhaps it's not that unusual, but I would want to see moving numbers to compare with, and perhaps more numbers (bigger sampling) in general to look elsewhere for the problem.

60F outside temp today, the IAT when moving was over 30C (reached over 50 C) while driving with coolant temp over 90C (reached 100C).

Later on, when it was around 55F... doing more testing... Removed MAF from housing, ignition on engine off, plugged MAF into harness, still reading 48C. Plugged in old MAF, still reading 48C. Plugged in spare DME, still reading 48C on either MAF. Makes no sense.

I am thinking there might something wrong with the wiring, or I just got a bad new MAF. What seems to be happening is over time once the IAT heats up, the reading from the DME stays hot. But the mystery here is why I saw these values even after swapping the DME and MAF, even with ignition on engine off and MAF removed from the intake tube. Maybe there is a lot of resistence in the wires as they heat up, causing the wiring to be part of the equation, registering a constant voltage drop from the heat soak, which translates to a high intake temp.

The good thing is I've got a ton of info and things to try tomorrow, such as bypassing wiring, measuring voltage on IAT signal wire, artificially heating/cooling the IAT, installing a fixed resistor or pot, running car without MAF, inspecting the wiring, etc.

"The signal from the intake air temperature sensor is used by the DME control unit to calculate the default load signal if the mass air flow sensor fails"

Based on this above info, one thing I do not know is if the IAT is used at all as part of the ignition timing or air fuel ratio if the actual MAF portion is working OK. I suppose I will find this out tomorrow if I bypass the IAT, or perhaps even unplug the MAF.

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No codes, just the high intake air temp reading which once the car is fully heated up (over 90C ECTS) (and the IAT is very hot) this seems to lead to rough running, seemingly late ignition timing, and perhaps leaned out fuel. I need to do more data logging though to confirm this, monitoring fuel adaption, ignition timing, and rough running values. Once the engine got hot, it stumbled a little at idle (almost like a miss, even though idle was dead on) and seems to be laboring, power seems to be down once it gets hotter.

I really don't want to to be running the "new" engine in this state until I get this sorted out.

When the car is started cold, and the IAT is still reading "coldish" the engine purrs, not a hint of a problem, totally smooth.

It would be great to find what others typically see for an intake air temp.

The only one I could find on a quick search was Richard Hamilton who observed 27C IAT at 90C engine temp, but I'm not sure if he was quoting the book, or if this was what he actually observed. I'll send him a PM to confirm this. But 27C sounds much more reasonable than the 50+ and 60+ I saw, especially on a cool day.

Still though, I'm leaning towards bad wiring or I received a faulty new MAF.

Edited by logray
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I PM'ed Richard to see if this was from an unmodified intake. It looks like where he lives around that time he posted his 27C IAT, the outdoor temps were similar to the past several days here (mid 50F's ish). And his temps were at idle.

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Good grief Logan, I have trouble remembering what I did 7 days ago, let alone 7 years! :blush:

The readings in that post ( http://www.renntech....es-p1123-p1125/ ) were taken with my old PST2. The fault turned out to be a bad MAF, and the reason that it didn't throw a CEL is because my car was a UK model 1998 996 C2, which is not OBD2 compliant.. I seem to recall getting the results by doing a vehicle report, printing it to PDF, and then cutting and pasting the results into the post.

Having changed the MAF rules that out for you. Sorry that this info isn't much help.

Edit: I forgot - Yes, the intake was totally stock.

Edited by Richard Hamilton
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Ok, Logray. I did two logs for you this morning. One 15 min drive started from cold engine. Ambient was about 50F.

IAT started at 79F and slowly creeped up to 84F as I was driving in local streets and the engine was warming up.

Then I had to park the car for 10 min.

Restarted the car, IAT started at 106F, then went up to 118F (~47-48C) and stayed there when I was driving on local streets/highway for 25 min or so. Then I stopped the car and kept the engine running for 6 min during which time the IAT creeped all the way to 144F but stayed there.

My long term fuel trims are 8% and 5% in bank 1 and 2 respectively btw and the engine is quite smooth both idling and on power. If you touch the airbox after a drive, it's actually quite warm. So I'm not too surprised if the air is warmed up to 118F steady when crusing but that's just speculating.

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Do any other input reading go out of limits? When I get readings that are odd I always inspect basics such as ground paths or loose/corroded main terminal connections.

I don't know if it's used in fuel/ignition trim but I would also verify that the engine bay temp sensor is operating correctly and in it's housing on the intake runner.

For comparison I'll take a IAT reading on my Cayenne.

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Thanks, I really appreciate the additional data. After 3 people chiming in I think the IATS temp might be a red herring.

Here's the steps I took this morning.

Disconected battery.

Pulled apart IACV, completely spotless inside and out, reassembled and reconnected.

Re-inspected NEW ground and power wiring, all is spotless and perfect.

Reinstalled old MAF.

Unplugged and disconnected unncessary all electrical loads.

Tested and inspected wiring between MAF and DME and DME to DME. I could find no kinks or breaks in the lines and looked over all of the MAF wires, except I didn't unwrap the tape once it gets past the inner duct/wire mold. By feel and sight though, the tape looks undisturbed.

Continuity was perfect between these wires.


pin 17 pin 5 HFM signal

pin 45 pin 3 HFM ground

pin 15 pin 1 IAT signal

pin 47 pin 4 5V HFM

Tested resistance between these wires


pin 17 pin 5 HFM signal .3 ohms

pin 15 pin 1 IAT signal .3 ohms


pin 17 pin 6 HFM signal to ground. Read infinity, no short to ground

pin 17 pin 45 HFM signal to HFM ground. Read infinity, no short to ground

pin 15 pin 6 IAT signal to HFM ground. Read infinity, no short to ground

pin 15 pin 45 IAT signal to HFM ground. Read infinity, no short to ground


pin 1 IAT appx 5 volts

pin 2 12v supply appx 12.5 volts

pin 3 ground no short to ground

pin 4 5v HFM appx 5 volts

pin 5 HFM signal did not test

Drove for appx 8 miles, until warm.

On this drive the engine seems to be behaving more normal now. Power is better than it was yesterday once it's hot, and the idle seems to have really smoothed out. I am kicking myself because I didn't get very good data on the first drive I took yesterday. Yesterday it was stumbling like crazy once it got hot, especially at idle, I swear it felt like it was running lean. I already thoroughly went over all the intake to check for leaks, also used some propane, but no changes to idle. As predicted, removing the oil filler cap produces a rough idle and then very smooth again once the cap is back on and the intake leak is fixed. I went to some pretty extreme lengths to make sure there were no intake leaks during engine assembly. Also during engine assembly I meticioulsly cleaned everything inside and out, and many of the parts are new.

I gathered some good data this morning, you can see at the very end the computer starts to lean out the idle range from 0.00 to -0.03 for both banks. FRA is pretty much solid on 1 or 0.99. Towards the end they seem to get better as the DME learns.

At the very lowest of idle, 680 RPM there might be a very very slight mis. Hard to say though, feels much smoother now, could just be me being over sensitive. Will take it out for some more drives today and gather some more data.

These numbers look much more solid to me, what do you think, especially towards the end.




Edited by logray
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