Temp difference between front and back of block

[Flanders 46]

I'm a bit stumped.  I've got an OEM-style temp sender at the front of my intake manifold, and a new Holley temp-sender at the back of my intake manifold (Jim Inglese 8-stack EFI system).  180F thermostat.  After 20 minutes of driving around, using my infrared thermometer I can see the outlet of my water pump is 180F and my dashboard factory temp gauge is right in the middle.  However, the infrared thermometer on the back of the engine reads around 140F, which matches the Holley ECU readings.

I've replaced the Holley coolant sender, but this didn't change the results  (I wasn't really expecting it to, as it matched the infrared thermometer reading).  Coolant blooped out of the sender hole so I don't believe there is an air bubble in there either.  40F seems like a large temperature differential between the front and back of engine!  

Any thoughts?  I could burp the system again but not sure what else could be going on.  Since the ECU only thinks my engine is at 140F, it's always doing cold-start adjustments and never thinks the engine is warm.

[Dave R. 85]

I read several years ago that a temp sender that isn't in contact with coolant gives a cooler reading. The sender can't read hot air. I understand that you burped the system but air is the first thing that makes any sense to me. Maybe crack loose that rear sender ever so slightly, on a cold motor of course, and start the engine. After the engine is up to temp for a few minutes maybe any air will seep out around the rear sender.     Dave R.

[aslanefe 333]

Can you change the temp setting in EFI to 140 degrees?

[Mach1 Driver 560]

Is this a recent rebuild? The head gaskets can be assembled with the water passage to the front or rear. If the passage is positioned in the front, then the rear of the engine won't get any water. If its positioned to the rear, then the entire engine gets circulation.

[Flanders 46]

I'll burp the system again, good idea bout keeping it loose and letting coolant (and hopefully air) escape.  I've gone through two temp senders and burped the system a few times, so maybe there's just a stubborn air pocket in there, due to the nature of the intake design (??).

I can change the temp setting in the EFI to 140, but I'd prefer not too.  This is me being stubborn I guess, but if my temp needle is right in the middle, I want the ECU matching.  This isn't a recent rebuild, aside from intake manifold, so I was hoping to get to the bottom of this.

I could swap the Holly sender with the factory one too, but then my dash needle would always read 'cool' which would drive me bonkers!

[aslanefe 333]

When I installed Holley Sniper on my 302 last year, I installed the Holley supplied temp sender on the back of the intake and kept the factory sensor for dash in stock location. Checked the Holley temp sender by reading the temp of intake where the sensor is mounted with IR temp sensor and found Holley sender to be reading low. So I replaced the Holley supplied sender with a Delphi TS10075 and the readings on Holley display matched real close to my IR readings at the manifold by the sender. There was a difference in temp readings of front of the manifold (where factory sender is installed) and back of it (where Holley sender is installed) with the IR temp sensor, but don't remember the difference being as much as you have.

[jpfll 11]

On 11/24/2020 at 11:57 AM, aslanefe said:

When I installed Holley Sniper on my 302 last year, I installed the Holley supplied temp sender on the back of the intake and kept the factory sensor for dash in stock location. Checked the Holley temp sender by reading the temp of intake where the sensor is mounted with IR temp sensor and found Holley sender to be reading low. So I replaced the Holley supplied sender with a Delphi TS10075 and the readings on Holley display matched real close to my IR readings at the manifold by the sender. There was a difference in temp readings of front of the manifold (where factory sender is installed) and back of it (where Holley sender is installed) with the IR temp sensor, but don't remember the difference being as much as you have.

I had a similar issue with the temp sensor that came with my FiTech - replaced with the Delphi and the readings have been perfect

[eric69 0]

 Coolant cooled by rad. enters water pump then through block to rear of heads then front through heads  to intake to thermostat  then to top hose returning to rad. Coolant cools as it passes down through rad. to lower hose coolant in rear of intake has not picked up all the heat yet maybe why its cooler. sender would be more accurate in front of intake.

 

[TexasEd 188]

Yeah, my question is how much heat does the coolant pick up from the back of the intake/heads to the thermostat?  I think Eric69 might be right.

 

[Dave R. 85]

22 hours ago, eric69 said:

 Coolant cooled by rad. enters water pump then through block to rear of heads then front through heads  to intake to thermostat  then to top hose returning to rad. Coolant cools as it passes down through rad. to lower hose coolant in rear of intake has not picked up all the heat yet maybe why its cooler. sender would be more accurate in front of intake.

 

I doubt that the coolant temp exits the radiator at less than 140* which it would have to do in order to register 140* on that rear sender. It has to pick up some heat between the radiator exit and the rear sender.

What is the temp of the coolant as it exits the radiator as checked by a IR thermometer? I would guess more that 140* and the coolant temp can only go up from that point.    Dave R.

[Mach1 Driver 560]

I think the right question here may be: where did Ford CHOOSE to measure engine temperature? It is in the front of the intake on the side opposite the thermostat. If you look at the circulation path, Eric69 is correct that the front of the intake should be hotter than the rear. Its the last place in the engine where water circulates before exiting via the thermostat.

Water leaves the bottom of the radiator, and is pumped into the block, and around the cylinders. It exits the block through the head gasket passages. Most of the water is forced to the rear so the entire engine gets cooled. DON'T reverse the head gasket- this is a common and disastrous mistake. No water is allowed through the front red passages, some can go up into the heads through the green and blue, but most is forced to the rear. From the heads it goes into the intake manifold to the front where the thermostat is located, and through the top hose into the radiator.