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Tech Tips
Manifold Air Temp Sensor (MAT) ~ Relocation Mod
The MAT Sensor is responsible for sending air temperature information to
the ECM. It is located underneath the Plenum and it is theorized that because of this location picking up engine
heat that perhaps the air temp readings are incorrect or to say the least "not optimal". By purchasing
a sensor and locating it to a "cold air" duct, it is further suggested that the air temp is corrected
and driveability is improved. If this interest you a MAT relocation kit is available from TPIS for $34.95 Part #300-164, which includes the sensor, harness,
and a grommet. It is suggested that the sensor from a 92 or later be used for more accurate readings.
Photo and Car by Tom Keliher (Moderator) 3rd Gen ID
This is a reprint from ThirdGen Member "Glenn91L98GTA" that contains some valuable info on the MAT relocation mod.
1) The MAT while it may have an effect on the EGR it does not control it at all. The 40°F theory is incorrect.
The EGR functions when the car goes into closed loop and the vehicle is driving at part throttle. The EGR does
not function when the car is idling, accelerating (> 75% throttle) or decelerating (0% throttle). The EGR still
functions when the temperature is less than 40°F as I recently tested it with the air temperature around 30°F.
2) The MAT inside the plenum picks up latent heat from the plenum, thus giving a higher reading than it should.
If your car been moving on the highway for an extended period of time, this will result in the lowest temperature
inside the plenum. But even with the TB coolant bypass, it will still be 30-40°F higher than the ambient air
temperature. If you have been in "stop-n-go" traffic or moving slowly in traffic, the plenum temperature
can rise significantly higher. I've seen readings over 150°F and it can take 30 minutes of constant moving
on the highway to get the plenum temperature back to the 30-40°F above ambient air temperature reading (normal).
3) What the relocated MAT really
causes, is an increase in the injector pulse width due to the lower temperature reading. This is not noticeable
@ part throttle, but becomes very apparent on WOT. I have tested this on the same day, at the same spot, on a 0-100
MPH run and the relocated MAT results in a 10% increase in injector pulse widths on my car.
4) As for performance increases, it has the same effect as increasing the fuel pressure, but with different results.
I have done comparisons between the stock and relocated MAT sensors at various fuel pressures. As overall performance
goes, they both yielded the same max performance but the stock MAT required about 2 psi more pressure. But you
can obtain your best performance with either mod. If someone does not have an AFPR and their fuel pressure is less
than ideal (which most stock cars don't), a relocated MAT can increase your performance at WOT.
5) The relocated MAT gives BLM/INT readings closer to 128/128 (the ideal) when driving at part throttle. The closer
the ECM has readings to 128/128 in normal operation, the smoother the engine behaves. This means better driveability.
With the stock MAT, my BLM/INT readings are further away from 128/128 (with my fuel pressure adjusted for maximum
performance in all cases). My car tends to have a slight hesitation or stumble when driving (especially when the
MAT temperatures are high). Reviewing the readings of my Diacom, I noticed the ECM was searching/hunting the BLM/INT
readings with quick variations. But, with the relocated MAT had readings much closer to the 128/128 and was much
smoother in operation.
BTW, doing the above tests with a 4.7K resistor in the connector causes the ECM to think it has only 55°F air
temperature. Again, I could get the performance to equal the other two, but the driveability was the worst. It
had the most errant readings from 128/128 and hunted/searched the most. I do not recommend this mod to anyone,
unless it is strictly for the strip. AND, as the air temperature changes, you have to adjust the fuel pressure
to maintain optimum performance.
Overall, both the stock and relocated MAT can yield the same maximum performance. The stock MAT with the slightly
higher fuel pressure actually gave slightly better fuel mileage and part throttle response, but had worst driveability.
The relocated MAT obtained the same max performance but with much smoother driveability.
Don't bother with the 4.7K resistor. It eliminates an important reading from the ECM and causes the ECM to solely
rely on the O2 sensor as the "umpire" for the proper 14.7:1 Air:Fuel Ratio. This is where the 128/128
and hunting/searching comes in. The more the ECM has to rely on the O2 sensor than the other sensors, the worst
your driveability becomes. The closer the other sensor readings are to yield a perfect 14.7 without the O2 "umpire",
the closer your BLM/INT will be to 128/128 and the less you'll experience hunting/searching, resulting in hesitation
and surging in part throttle operation.
Bottom line, for smoother operation, do the relocated MAT, but you do need an AFPR to adjust the fuel pressure
for max performance. Max performance is the same for both, but at slightly different fuel pressures.
One final note, with a TB Bypass and a 160°F thermostat, the relocated MAT retains that "cold car feel"
in hot weather operation. This is especially noticeable if your engine is hot, you turn it off for about 15-30
minutes and restart it. With the mods I mentioned AND the relocated MAT, it feels the closest to the "cold
car feel". This is because the plenum picks up a lot of heat when stopped on a hot engine. Your readings will
be well over 130°F and the WOT injector pulse widths will be the lowest.
I have also noticed that the hunting/searching "hesitation" occurs more often in the summer than in the
winter.
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