1968 Extra Coolant Port?

[p51]

OK guys-n-gals. I need some advice/help...

I want to install a second coolant temp sensor. I've read on some other threads on the net that there is (may be?) a second coolant access port on the back of the 1968 302 manifold. But my manifold doesnt *seem* to have one so here are some questions.

(1) Is there a second port on the (stock) manifold? Where to look? Or did some come with and some without the second port?

(2) Are there other ports where coolant temp can be sensed on the engine block? Or elsewhere?

(3) The only other place I can think to place a second sensor is in the thermostat housing where the radiator hose connects to the engine. What are pros/cons of this location?

(4) Is there a way to share a single temp sensor with two gauges?

Thx
James

 

[franklinair]

I don't know of an easy alternate source for an extra temp gage. I have used a radiator cap that has a temp gage as part of the cap. They come with 14-17lb pressures. IIRC, I got it from Summit or Jeggs, $30-40. Worked well.

Neil

 

[CougarCJ]

On one car I had, I ran an electric radiator fan. The radiator shop installed a bung in my lower radiator tank for installing the sending unit for fan switch.

 

[rvrtrash]

Here's my opinions on your questions, for what it's worth.
1) I don't believe there is a second port. The intake blocks the coolant ports on the back of the manifold. You could have the manifold drilled and tapped for another sensor on either side if you wanted. If I had to have a second sensor/gauge, this is the route I'd take.
2)There are coolant drain plugs near the bottom of the block. In front of the motor mounts would be the most accessable. They're small but you could put a bell adapter on to fit a sensor. The down side is that the sensor wouldn't be in the coolant flow path, but the temp would be close.
3)You could put a sensor in the thermostat housing, but if the thermostat fails in the closed position, the engine could melt down and that sensor would never see it.
4)If you tried to use one sensor for two gauges, both gauges would have to be balanced so the load was correct, with diodes and current limiting resistors in each line. If the sensor failed, both gauges would fail to read, so I don't believe you'd be gaining anything with this option.

Steve

 

[p51]

Here's my opinions on your questions, for what it's worth.
1) I don't believe there is a second port. The intake blocks the coolant ports on the back of the manifold. You could have the manifold drilled and tapped for another sensor on either side if you wanted. If I had to have a second sensor/gauge, this is the route I'd take.
2)There are coolant drain plugs near the bottom of the block. In front of the motor mounts would be the most accessable. They're small but you could put a bell adapter on to fit a sensor. The down side is that the sensor wouldn't be in the coolant flow path, but the temp would be close.
3)You could put a sensor in the thermostat housing, but if the thermostat fails in the closed position, the engine could melt down and that sensor would never see it.
4)If you tried to use one sensor for two gauges, both gauges would have to be balanced so the load was correct, with diodes and current limiting resistors in each line. If the sensor failed, both gauges would fail to read, so I don't believe you'd be gaining anything with this option.

Steve

Thanks for (all) the replies..

Sorry, forgot to provide context. I'm doing "what-if" for adding EFI. One coolant port is for the temp sensor for the existing gauge. The second port would be for the EFI sensor. The EFI sensor would attach to where the gauge is currently attached and the gauge would go... ???

#1: Drilling/tapping is likely a higher hurdle than I want to take.

#2: This is something I didn't know about. Thanks. This sounds like best existing candidate for the second port.

#3: Question (possibly silly): In the thermostat housing there is a port on top (for a thermal vacuum switch or ported vacuum switch). Does this allow sensing on the engine side or the radiator side of the thermostat? From the reply above it seems like its on the radiator side but it would seem that if the thermal vacuum switch was on the engine side it would "see" engine temp so that would be better place to actually have the port -

Also, given the large diameter of the radiator hose hookup to the thermostat it doesnt appear that the sensor would always be in contact with the coolant. Thoughts?

#4: I agree. There is no obvious way to balance the impedance of the gauge + EFI load to match that of a single load. Just was wondering if some wunderkind had worked it all out...

 

[Mosesatm]

It looks like it's on the thermostat side.

 

[p51]

It looks like it's on the thermostat side.

Thanks.

One more question (I've never replaced a thermostat)...

Is the thermostat placed in the mouth of the opening to the thermostat housing - at the mouth of the hole shown in the pic? Or is it placed further back where the thermostat housing bolts onto the intake manifold? My (limited) understanding is that it goes where the thermostat bolts onto the manifold - if so, then is the thermostat missing in this picture?

Reason for the question - I'm still not clear on what side of the thermostat the thermal vacuum switch (the plug-like thing sitting just inboard of where the radiator hose connect) sits. I dont use a thermal vacuum switch, so that coolant port is available. But if its on the radiator side of the thermostat then the point about the engine overheating (without the gauge showing it) if the thermostat is stuck closed is very valid.

Thx
James

 

[Mosesatm]

You must remove the thermostat housing to install the thermostat because it rests in a groove on the rear of the housing.

I can't tell for sure if there is a thermostat in that photo or not. I think there is one but the antifreeze is dammed up against it so it's difficult to see.

 

[rvrtrash]

The thermostat goes between the block and the housing. The port is on the radiator side of the thermostat. This is an acceptable spot for the vac switch for it's intended purpose, which was to increase vacumn to the distributor when the engine started to get hot. More RPM's pulls more air through the radiator. If the thermostat (or switch) failed, the RPM's wouldn't pick up but you still had an indicator that there was a problem. If you move the gauge sensor there, and the thermostat fails, you have no indication your engine is overheating until the radiator blows. I thought about suggesting you put the EFI sensor on the housing, but then your car wouldn't run right until the thermostat opened. I still think you should take the intake off, take it to a machine shop and have them drill and tap a hole at the back water passage. Get an estimate first, but I bet it would only cost $20-$30.

Steve

 

[p51]

Thanks everyone for all the great info. I think I actually understand what the options/tradeoffs are...
...at least I've got enough knowledge now to be dangerous :wink:

One of the two coolant ports on the engine block (near cyl 1 or cyl 8) looks like the way I might go.

 

[p51]

The thermostat goes between the block and the housing. The port is on the radiator side of the thermostat. This is an acceptable spot for the vac switch for it's intended purpose, which was to increase vacumn to the distributor when the engine started to get hot. More RPM's pulls more air through the radiator. If the thermostat (or switch) failed, the RPM's wouldn't pick up but you still had an indicator that there was a problem. If you move the gauge sensor there, and the thermostat fails, you have no indication your engine is overheating until the radiator blows. I thought about suggesting you put the EFI sensor on the housing, but then your car wouldn't run right until the thermostat opened. I still think you should take the intake off, take it to a machine shop and have them drill and tap a hole at the back water passage. Get an estimate first, but I bet it would only cost $20-$30.

Steve

Yeah, I see what your saying. If the thermostat failed no big deal if the thermal vacuum switch didn't engage. But if the thermostat failed leading to an incorrect temp gauge reading which then led to overheating - goodbye engine. So the TVS on the radiator side is OK but not the gauge (or the EFI).

 

[tomcwarren]

When I fooled with a Holley TBI EFI system about a year ago, I put the Holley temp sensor in the stock location in the (Edelbrock) intake manifold, and moved the gauge temp sensor to the heater hose that connects to the manifold - I spliced in some pipe fittings from HD w/a tee'd fitting that had the right size hole for the sensor. Worked fine. Whatever you do, I'd put the gauge sensor in the least-sensitive/accurate location, since it's not as important to driveability as the temp sensor for EFI.

Good luck,

Tom

 

[p51]

Just an update (in case someone else ever has a similar question)...

Last week I talked to my mechanic about the best place for a second coolant temp sensor on a stock 302 engine/manifold. He advised not using one of the coolant plugs on the block for a second sensor because (1) they are a real bear to remove (after 40 plus years most are frozen in place) and (2) the temperature of the coolant that low in the block is much lower than the temp that the top of the engine block sees. His advice was (assuming no mods to the manifold) to put the efi sensor in the stock sensor port, move the gauge temp sensor to the thermostat housing, and replace the thermostat with a MotoRad fail-safe thermostat - one that fails open if the engine starts to overheat. The downside is that the gauge reads the radiator coolant temp instead of engine coolant temp but at least, in theory, if the engine starts to overheat (for whatever reason) the thermostat fails in a benign state = open.

Anyone ever used one of these MotoRad (= Murray) fail-safe thermostats?

 

[Powell]

James, I have no personal experience with the MotoRad fail-save t-stat, but have researched several other hot rod sites on this stat. It appears that many of the folks that put them in, found them to fail ( lock open) at temperatures far below serious over-heat.

Ralph

 

[p51]

James, I have no personal experience with the MotoRad fail-save t-stat, but have researched several other hot rod sites on this stat. It appears that many of the folks that put them in, found them to fail ( lock open) at temperatures far below serious over-heat.

Ralph

Ralph

Thanks.

I found similar complaints when I googled threads related to the MotoRad fail-safe thermostat. Interestingly, most (I would guess ~70%) of the people I saw complaining about them locking open (= failing safe) at high but still normal operating temps owned jeeps. Wonder what % of the thermostats fail open under high but normal operating temps - since people who dont have problems usually never post to threads its hard to tell what the fail rate is.

James

 

[rvrtrash]

If the other sites are like this one, the post would be "This thing failed. Anyone else have a problem?" The number of people who say "Mine's ok" versus the number that say "Mine died too" would give you a fair idea of the percentage, I would imagine.

Steve