Do I have to drain my coolant if I pull the temp light switch?

Hey everyone,

I was wanting to put a aftermarket temperature gauge in my truck at the temp light switch location, do I have to drain the coolant first?

Thanks

You better drain it or loose most of the coolant. There is a 1/2" NPT plug at the rear drivers side, would be a better place.

you will lose a little bit but i left the cap on tight and the motor was cool and it didn't lose but a cup as i swapped. it really won't drain much unless the cap is off.

itsacrazyasian said:

you will lose a little bit but i left the cap on tight and the motor was cool and it didn't lose but a cup as i swapped. it really won't drain much unless the cap is off.

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This is very true. As long as you have no way for air to be drawn into the system you wont spue loks of coolant. As posted, a very small amount will be released but its no big deal. The new sender should have the sealant on the threads before you start this. Please don't use teflon tape. Way to many other great products to use as a thread sealant... Now if you care to test this no loss theroy remove the sender and crack open the radiator cap. Don't panic when the stream of coolant starts flowing like a cow peeing on a flat rock. Its really simple science in action....

OLDBULL8 said:

You better drain it or loose most of the coolant. There is a 1/2" NPT plug at the rear drivers side, would be a better place.

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Why is this location better? Does the same theory apply about the cow *** on a flat rock That is if you don't open the system to air, you don't have to worry about a loosing a bunch of coolant.

Thanks

What's the worry. Just drain the coolant into a clean container down below the level of the plug you wish to use, swap out the sender then filter the old coolant back through some cheese cloth as you pour it back in. Then you get a nice portion of your coolant filtered in the process Doesn't hurt to take the stuff out and do that once in a while if you don't have a coolant filter on the thing. Helps keep your cooling system clean .

True, I just put the coolant in about 1500 miles ago with fresh SCA so I was looking for a shortcut. So is there much difference between putting the sender in the safety light spot or the head between cylinder 6 and 8?

Thanks

There are plugs in the rear of both heads that are good spots for water temp senders. The drivers side will be difficult to remove without a square plug socket as the exhaust manifold prohibits using a 9/16 wrench. The passenger's side is more accessable, but still a bit of work. The reason for using one of these locations is you retain your factory sender, plus, (in my opinion) the coolant is at it's hottest at the rear of the heads. This will let you know as best as possible the true temp on the coolant.

Photo's below of passenger's side:

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Heath

A quick swap on a cold motor is easiest. You won't loose much if your good.

The rear location on the heads is not going to tell you the correct coolant temp. The flow is from the back to the front of the heads. Then it mixs thru a chamber under the injection pump. then thru the thermostat. Look at the drivers side of the engine at the front. See the sender on the flat area. Thats the dash gauge sender. Now look below that in the head. Thats the check engine lite sender. They are at the hottest location for the coolant. They are not at the back of the heads. Now you are free to place your new senders anyplace you can get the square plugs out of. Just know the differance in where you place the senders. Somebody here has posted a flow diagram of coolant thru these engines...

http://www.ford-trucks.com/forums/1063320-temp-sensor-location.html

I found this pic. I was going off this pic when I posed my original question. So if I am going to do this at the temp light switch the thread pattern will be a 1/2" pipe thread?

Thanks for all the information

icanfixall said:

The rear location on the heads is not going to tell you the correct coolant temp. The flow is from the back to the front of the heads. Then it mixs thru a chamber under the injection pump. then thru the thermostat. Look at the drivers side of the engine at the front. See the sender on the flat area. Thats the dash gauge sender. Now look below that in the head. Thats the check engine lite sender. They are at the hottest location for the coolant. They are not at the back of the heads. Now you are free to place your new senders anyplace you can get the square plugs out of. Just know the differance in where you place the senders. Somebody here has posted a flow diagram of coolant thru these engines...

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We'll have to agree to disagree on this. In my opinion, the reason Ford put those senders there was for convence not really because that is the hottest spot in the coolant. Again, just my opinion.

Look at the flow of coolant:

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The coolant in the front of the head is mixing with coolant that was just pumped from the radiator, passing from around cylinder #2. That coolant isn't nearly as hot as coolant passing all the way from the front of the block to the back of the block and then up to the rear of the head. The rear of the passenger's side head is even further from the rad in the system.

Now, I'm not an expert, but of the overheated IDI's that I've taken down, cylinders 7 and 8 were the most damaged, and head dagmage tended to be on back of the heads, over cylinder 8 inparticular. Again, not a lot of numbers, but just what I've seen.

It would be nice to know what the difference is in temp between the two locations, in the same engine.

Heath

so are these holes a 1/2" pipe thread?

Thanks

I got my gauge in, I couldn't get that stupid plug out between #6 and #8 so I put it in the temp light spot. I guess the question now is how hot should I be running?

Thanks again

Hello Heath. I will agree to disagree with you also. Under the injection pump is the mixing chamber that both heads flow into before the thremostat sees it. Thats also the factory location for the dash gauge sending unit. The rear cylinders are subject to more heat because of the slant the engine sits on. Any oil thats drawn into the intake manifold from the cdr valve ends up back in those cylinders. Also international designed the bores on those back cylinders 7 & 8 to be slightly larger than the front 6 cylinders. Its around 1/2 a thousands larger bore. Its not much but it was the design for the times. Nobody I know will bore those cylinders like that today. So are those back cylinders hotter than the front cylinders.... Perhapes they are. I jst prefer to pick up my temps as close to where the factory designed them to be from. That picture you posted is a 6.9 block. You can tell this by the lower front corners on the block with the triangle coolant holes in the block.... Those are drilled round and plugged in the 7.3 block and heads. I feel we can cool the 7.3 engines better if we remove these plugs and run the 6.9 head gaskets in the 7.3 engines. I have both gaskets and thats the only differance in both of them. All other holes are the same size and number. Even the bolt holes....