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[Juan Pineda]

I spent an hour messing with the ECU and OBD on my 99 SM today. I thought I'd share a few things learned:

 

0) I used a cheap ~$20 Bluetooth OBD dongle (obtained through Amazon) with an Android tablet using the free version of the Torque app. I monitored these six parameters:

 

RPM

Intake temp

Coolant temp

Advance

Intake vacuum (doubles as boos too!)

Fuel consumption

 

This worked great! I will now always monitor these OBD variables on the dyno to eliminate some of the uncertainties.

 

1) I applied the TEN jumper. I found that a good way to verify that the jumper is actually installed correctly is to give it a bit of throttle to try to rev the engine up to say 2K. If the jumper is correctly installed, the engine will cut out as it revs up. (Try driving off with the jumper installed and you will understand.)

 

2) I warmed up the car looking for when the fan and timing would be affected. Fan came on at OBD indicated 204. The ECU backed off timing at that same point.

 

3) At this 204 degree temp, the ECU reported 5 degrees (at idle) where it would normally be 10, so it pulled out 5 degrees. The amount it pulls varies. I saw it pull as much as 6.5 degrees.

 

4) To be clear, the ECU has no way of knowing how much you adjusted the timing wheel to advance timing further. The ECU reports the amount of timing based on the assumption that you have the normal 10 degrees set. The actual timing is greater by the amount of additional timing you dialed in beyond the factory 10 degrees. So if you adjusted your wheel to give you 14 degrees at idle, then you are actually getting 4 degrees more than what the ECU tells you.

 

5) I let the engine cool down to 161 degrees and the ECU reported timing was back to normal at 10 degrees. I'm not sure how the ECU decides to revert, whether it was simply the temp getting below a threshold, or whether it also required some elapsed time. This is the topic for some further study.

 

6) THE ECU backs off timing even with the TEN jumper installed.

 

7) The best place to measure temp with a pyrometer is on the black heater hose at the back of the engine as close to the engine as you can get. I found the measurement there was about 3 degrees lower vs the ECU reported temperature. Measuring the temperature of that same hose but closer to the firewall resulted in a measurement 10 degrees lower than ECU reported. Taking pyrometer measurements is helpful to verify the ECU sensor is working correctly. Beware that the pyrometer reading is affected by the material being measured. It is calibrated for black, not metal!

 

8) Vacuum was at 23.4 in/Hg at idle. This is awesome to have this measurement available as a diagnostic. I intend to keep an eye on this number.

 

 

Remember measure often to keep your car calibrated.

 

That's what I've got!

-Juan