My family has been in the heavyhaul trucking biz for over 50 years and none of our Cat or Cummins engines in the last 35 have had the thermocouple in the manifold. The only engines I have seen that do have the thermocouple in the manifold are noncycleing engines like boats, gensets, dozers and scrapers. Your reading will only be as accurate as your gauge calibration. I don't know if you can get a set that is calibrated for the manifold. I think you will be around 200 degrees warmer in the manifold. To be honest, and I know this from my biz is that it doesn't what kind of gauges you have, how many or how there hooked up. If your not in tune with your rig, your the one to blame for the problem. I don't mean to sound harsh but you wouldn't believe the excuses I have heard over the years of why the engine melted down or why the connecting rod sawed the block in half. Some of the major trucking fleets in the US know this and don't even get extra gauges. The bottom line here is to know your rig's limits and stay under them.
