Temp Gauge And Scanner Don't Agree..

[RustyW]

Hi,

 

On a 99 old body style K1500 Suburban with a new 5.7 Vortec....

 

The gauge reads about 230 most of the time. Today I hooked my scanner to it and the scanner monitor showed engine coolant temp dead on at 195.

 

Anyone have any thoughts as to why the two would be so off?

 

The only change to the cooling system during the engine install was to eliminate the quick connect heater hose port on the intake in favor of a hose barb (that doesn't leak or require special tools) and to put a new oil cooler in the driver side radiator tank.

 

 

Thanks

rusty

[txab]

Most of the time there are two separate sensors. One for the gauge and one for the ECM. ECM often will be near the thermostat housing, the other will be in the cylinder head. Either sensor could also be a little off due to age. Gauge is not meant to be dead on accurate anyway

[RustyW]

Most of the time there are two separate sensors. One for the gauge and one for the ECM. ECM often will be near the thermostat housing, the other will be in the cylinder head. Either sensor could also be a little off due to age. Gauge is not meant to be dead on accurate anyway

 

I was wondering about the two sensors. My Chiltons book shows the ECM is on the drivers side cylinder head. There's another sensor right by the thermostat as mentioned but I wasn't sure if it was actually a temperature sensor or what. I'll see about testing/cleaning/replacing that one.

 

I wouldn't demand the gauge be dead on but 15-20% difference is kinda high..

[travisstorma]

. Gauge is not meant to be dead on accurate anyway

 

 

This. I've never seen a gauge be right compared to a scan tool.

[jblakeney]

Most of the time there are two separate sensors. One for the gauge and one for the ECM. ECM often will be near the thermostat housing, the other will be in the cylinder head. Either sensor could also be a little off due to age. Gauge is not meant to be dead on accurate anyway

 

I was wondering about the two sensors. My Chiltons book shows the ECM is on the drivers side cylinder head. There's another sensor right by the thermostat as mentioned but I wasn't sure if it was actually a temperature sensor or what. I'll see about testing/cleaning/replacing that one.

 

I wouldn't demand the gauge be dead on but 15-20% difference is kinda high..

 

 

Ditto,...... Its on the head not the intake. And they will be off but that is a lot of difference... Sending unit probably..... Unless your having a cooling problem.

 

Jbo

[RustyW]

Well, this is curious....

 

As mentioned here and in my Chiltons manual the ECM is supposed to be the sender on the cylinder head. That leaves the sender on the t-stat housing operating the gauge....

 

So, I brought it up to operating temp and hooked my scanner up. Gauge read 210 and scanner read 187.

 

I then disconnected the sender on the t-stat housing. The gauge still read 210 but the scanner read -40 and the idle picked up when the computer put it into cold run/full rich mode. So I plugged that one back in and the scanner returned to 187/200 and the guage read 215.

 

I then disconnected the sender on the head and the gauge died but the scanner kept reading 200.

 

By this logic what's been posted here and the Chiltons manual are bass-ackwards.

 

So... My suspicion is that the thread sealer I put on the sender on the head (I didn't want ANY leaks on my new engine, and still managed a couple small oil leaks) is inhibiting the ground on that sender and causing the gauge to read high.....

 

 

Any input?

 

/edit - After thumbing through my manual the emissions and control system chapter refers to the sensor on the cylinder head as the ECT sensor, not ECM. It also says to wrap it in teflon tape when installing so I'm guessing its not a ground issue. The manual doesn't state what this sensor is feeding but by my testing the only thing it's sending to is the gauge.

[clemsonblozz]

Well, this is curious....

 

As mentioned here and in my Chiltons manual the ECM is supposed to be the sender on the cylinder head. That leaves the sender on the t-stat housing operating the gauge....

 

So, I brought it up to operating temp and hooked my scanner up. Gauge read 210 and scanner read 187.

 

I then disconnected the sender on the t-stat housing. The gauge still read 210 but the scanner read -40 and the idle picked up when the computer put it into cold run/full rich mode. So I plugged that one back in and the scanner returned to 187/200 and the guage read 215.

 

I then disconnected the sender on the head and the gauge died but the scanner kept reading 200.

 

By this logic what's been posted here and the Chiltons manual are bass-ackwards.

 

So... My suspicion is that the thread sealer I put on the sender on the head (I didn't want ANY leaks on my new engine, and still managed a couple small oil leaks) is inhibiting the ground on that sender and causing the gauge to read high.....

 

 

Any input?

 

/edit - After thumbing through my manual the emissions and control system chapter refers to the sensor on the cylinder head as the ECT sensor, not ECM. It also says to wrap it in teflon tape when installing so I'm guessing its not a ground issue. The manual doesn't state what this sensor is feeding but by my testing the only thing it's sending to is the gauge.

 

 

 

 

temp gauges don't use a ground.. they are a stand alone circuit, and the sensor has a resistor in the end of it that has a varying resistance(at different temps).. Pressure gauges usually are ground dependant, as they have a 5 v reference, a signal, and a return.