Heat Soaking MAT - cycling idle on restarts

-nick · Post by **-nick** » Mon Nov 05, 2007 2:52 pm

Hi folks,
I finally figured out what was causing this symptom -

When shutting the car off I might see CLT=180F and MAT=75F. A half hour later I get CLT=150 and MAT=125. On restart my idle will fluctuate wildly up and down and bounce between 14.xx to 19:1 on the AFR.

I can only assume that the heat soaking MAT is the problem. I have it mounted at the rear of my intake near the firewall and I'll admit that it just soaks up all the heat from the engine back there. I assume that anyplace after the intercooler is safe to mount it? I'm just concerned that even further out of the way of the heat source (radiating engine) it's still in the engine bay and can/will soak up the latent heat.

Does anyone else see this behavior?

Cheers,
nick

pb3 · Post by **pb3** » Sat Nov 10, 2007 11:12 am

I may have had a similar problem, my MAT sensor was mounted in my alloy filter tray that I fabricated. The problem was that after running and then doing a hot restart I would get a feeling of running very lean, confirmed by looking at the AFRs, this would last for a number of minutes. What I found was that the air around the sensor was pre-heated by the alloy and it would seem that the sensor was just out of the airflow. This situation thus caused the MAT to read artifically high and MSII would cut back on fuel assuming that the air was actually less dense. The solution was to move the sensor more in the flow and insulate things. The sensor is now mounted on a fibreglass bracket and the filter tray is clad in heat mat.