Hot spot between cylinder 6 and 8

[95 Tahoe]

I just got the rebuilt engine in my 95 Tahoe up and running. I had put in a secondary temperature sensor in the passenger head because my factory gauge always read low. Is there a fix for the low reading? The back temperature sensor will hit 250 before the thermostat opens at 180F. My scan tool shows the coolant temp at 190F. Once the truck warms up and everything stabalizes the back sensor still reads about 210 while the front is in the 185 to 190 range. Is this a problem?

 

[Schurkey]

How do you know the passenger-side (rear) temp sensor is accurate?

Everything I've read about the SBC cylinder-head temp sensors is that they tend to read higher than sensors in the intake manifold due to picking up additional heat from the nearby exhaust. What surprises me is not that the passenger-side is hot, but that the driver's side cylinder head temp sensor is low.

I'd be looking for corroded connections, broken wire strands, or other sources of higher-than-proper resistance between the driver's side sensor and the dash gauge; or perhaps a faulty gauge.

Are you using the same wire harness and gauge by swapping the wire from one sensor to the other, or are you using different wire/different gauges?

 

[95 Tahoe]

How do you know the passenger-side (rear) temp sensor is accurate?

Everything I've read about the SBC cylinder-head temp sensors is that they tend to read higher than sensors in the intake manifold due to picking up additional heat from the nearby exhaust. What surprises me is not that the passenger-side is hot, but that the driver's side cylinder head temp sensor is low.

I'd be looking for corroded connections, broken wire strands, or other sources of higher-than-proper resistance between the driver's side sensor and the dash gauge; or perhaps a faulty gauge.

Are you using the same wire harness and gauge by swapping the wire from one sensor to the other, or are you using different wire/different gauges?

I was using completely different gauges. I never trusted the factory gauge because it goes to 160 when hot. The ECT sensor in the manifold appears to read accurate. I also verified readings with a Flir thermal camera.

 

[PlayingWithTBI]

The back temperature sensor will hit 250 before the thermostat opens at 180F.

I had the same thing with my Blue Print Vortec style block and TBI style aluminum heads, which didn't have a bypass system anymore. Evidently the heater hose wasn't enough to equalize the system. Once the T-Stat opened, and the system balanced out, my gauge was only ~10° higher than the ECM saw. I wound up drilling 3 little holes (1/8 and 5/32") in my Mr Gasket 180° high flow T-Stat. The little hole from the factory wasn't enough. That gave me enough bypass to keep the head temp down to acceptable temps while warming up. YMMV

 

[95 Tahoe]

I had the same thing with my Blue Print Vortec style block and TBI style aluminum heads, which didn't have a bypass system anymore. Evidently the heater hose wasn't enough to equalize the system. Once the T-Stat opened, and the system balanced out, my gauge was only ~10° higher than the ECM saw. I wound up drilling 3 little holes (1/8 and 5/32") in my Mr Gasket 180° high flow T-Stat. The little hole from the factory wasn't enough. That gave me enough bypass to keep the head temp down to acceptable temps while warming up. YMMV

I appreciate the info. I was contemplating drilling my thermostat as well. I'll do that when the motor cools down later today.

 

[L31MaxExpress]

This is the exact reason I have drilled SBC thermostats for years and Felpro has a redesigned coolant flow arrangement on their performance head gaskets.

 

[95 Tahoe]

I drilled 2 1/8 holes. It takes a bit longer to heat up but I don't have a high temp swing on the back of the head. It still runs a bit hotter than the front.