MSefi.com read only forum (Jan.15,2018)

Here is a good one. At my last race the engine suddenly detonated badly (and scored pistons) on the first lap after running perfectly in practice and qualifying. After coasting in to the paddock and checking under the hood, I found the IAT (intake air temp sensor, GM) had a broken wire. Any ideas as to whether this could cause a massive lean condition which caused my failure?

Thanks,

James

I guess it comes down to - what was the IAT 'reading' at the time of the detonation? If the MS was being lied to that it's 200F in the airbox when the actual air temp was 100F, then yes, it would definitely lean out.

This is odd - the resistance should increase as the wire came apart, which would signal lower temp / more fuel / richer. Unless the broken signal wire shorted out, thus showing VERY high temp...

Guys,

If the sensor breaks (open) or shorts, then MegaSquirt assigns an intake temperature of 170 or 180 degrees F (depending on the code). If the actual intake air temp is higher than this, then the mix should run rich (rather than lean). That's because MegaSquirt assumes the air is relatively dense at 170F, so it injects enough fuel for that. If the actual temp is 220, it is less dense than at 170, and there's more fuel than needed. However, if the intake air temp is much lower than 170, then it is possible that the mix would run lean.

The GM 'open-cage' sensors typically used in boosted applications have been known to break in the wires to the sensing element. People have used small blobs of epoxy to prevent them from vibrating and breaking.

There is more on this here: http://www.megamanual.com/v22manual/mwire.htm#clt

Lance.

I know it's been a long time but actually the fuel pump circuit in my MSIII failed and I lost fuel pressure leaning out the engine.