I tapped the top of the water pump and installed a sensor for the digital gauge on the dash. It shows 20 degrees lower than the HP tuner, which is getting it's signal from the factory sensor near the headers. I know the tuner is reading good because the fans do come on at their sitting. Question: Is is possible the top of the water pump normally has a lower temperature?

 

Yes, different places around the engine have different temperatures and I see this on my 5.3 with Dakota Digital gauges. For me with the passenger side head rear hole using the DD temp sender, I also see 20 degrees different than what the PCM is seeing from the ECT probe in the driver's side front hole. I've gotten used to averaging the two readings / subtracting ten degrees from what the DD gauge is seeing as my actual, overall engine temperature.

This topic comes up often around here and usually draws the same conclusions.

Rick

 

Thanks,

 

See the graphic below on how coolant moves through the engine. Pretty common rule of thumb is that an efficient radiator will drop the temperature of the coolant about 20F degrees between inlet and outlet. Parts of the water pump on the LS see the coolant temp that exists right after the coolant leaves the radiator. Other parts of it bypass a bit of the hotter coolant leaving the head. The factory water temp sensor for the PCM (driver's side front of the head) sees the coolant temp that exists right after the coolant has picked up all the heat from the engine that it can. If you tapped the pump in a location that sees the coolant as it leaves the radiator, then of course the temp you see at the water pump is going to be lower than the temp you see at the OEM sensor. That also means that the temp seen at the rear of the passenger side head will be usually be in between the PCM sensor and a sensor in the cool part of the water pump -- as at that location the coolant has picked up some heat moving through the block and entering the head; but it hasn't picked up all the heat it eventually will as it moves through the head from the back to the front. So subtracting 10F or 'averaging' the two is pretty much a meaningless math effort. The fact is - the temp at the back of the head is cooler than the temp at the front of the head. If you're using the gauge to tell you if you're at a 'safe' operating temperature, then the one to pay attention to is the sensor that's in the OEM location. I mounted the sender for my temp gauge in the radiator tank right beside the inlet hose from the engine. So it sees the same temp as the sender in the OEM location. My gauge and the coolant temp in the PCM are always within a degree or two of each other.

 

The sensor in the OEM driver's side front location is the lowest for me usually by about 20 degrees, according to the OBD2 PCM data. That sensor was replaced with a new one during my swap because I thought the original was not accurate but the two sensors read about the same temperature.

My Dakota Digital sensor at the passenger side rear is always the hotter of the two and the discussion around here has also been about the design/depth of that sending unit. I promised myself the last time we had this discussion here that I would call the DD people and see if they have a different sensor. Tomorrow will be that day.

Rick

 

Interesting. I'm using DD gauges in my 85 Chevy truck which has a carbed LQ4. I have dual electric fans controlled by a DD fan controller which uses the DD sender installed at the passenger rear head port. The factory sending unit however is used by an MSD ignition box. Now that you mention it, I might have to confirm the readings and see if there's a difference. I can also swapped out locations to confirm if the senders themselves read out differently.

Sorry for going off topic.