Engine running too cool

Hey all, new problem. The engine is finally running very well, so of course the other shoe dropped. My gauge is always reading cold. After driving any amount of time to thoroughly warm it up, it’s staying to the left of “N”.

I’ve temped the rad hose (upper) and the area around the thermo housing. Thermo housing tends to read in the right area, ~180 to 190 depending on a variety of factors. I replaced the thermo last year with the right one for a 351m. I took out the old one and tested it, along with a new one. Both open close to 195.

At this point I have to assume nothing is wrong other than my gauge or sending unit. Can I test my sending unit??? I found the 1981 EVTM and my sender would be wired with a r/w wire. Now what

There are several ways to test the gauge.  One is to ground the r/w wire to the engine block and if the rest of the circuitry and the gauge are good the gauge will go to full scale over a period of a few seconds.

Another way is to swap the r/w coolant temp wire with the w/r oil pressure wire.  The gauges and senders have the same properties so the gauges should read the same, meaning the oil pressure should show what the temp gauge has been showing, and vice versa.

And if you really want to test it further you can replace the sending unit with a 10 ohm resistor and the gauge should read on the full scale mark, and with a 73 ohm resistor and it should read on the mark below cold.

The pedant in me says your gauge is reading to cool, and the engine is just fine.  😁

Gary, are these gauges linear?
IOW, would a 42 ohm load put it right in the middle?

I've had a 351M in my life, for a good part of my life.  351M's are cold natured beasts.  Everyone i've ever owned.  Here in the south it isn't such a bad thing.  my 78 LTDII i could sit in gridlock with the ac on.   The only bad thing is in the winter, it takes forever to warm up.   My temp gauge always sat to the left of N.  

Jim - I agree that the gauge is probably reading too cool.

But the gauges aren't linear.  I remember seeing somewhere that a 22 ohm resistor is supposed to put them at the midpoint, but I couldn't find that to quote last night so quoted the end point resistors.

George - The M-blocks and Clevelands have a different bypass system than the rest of Ford's V8's, as described in Documentation/Engines/351 & 400 and then the Thermostats tab.  Most people put the wrong 'stat in them, which makes them overheat more easily than they should.  But I've not heard people say they are cold-blooded.  That's interesting.

I know Gary, I had some friends that had them as well with overheating problems.  I had one in a 79 bronco, 78LTDII and a 76 Elite.  all 3 ran very very cold.  Almost as bad as a chevy 350  couldnt keep them lit when it was cold outside.  The elite had an engine block heater, this was when i lived up in maryland.  it helped out alot.

My mom had 2 400's, one in a 72 wagon and then in a 78 wagon.  The 72 ran hot like a scaulded dog.  The 78 after my father de-smogged it and took the cats off ran cool all the time.  go figure

That's what I thought.
I remember you showing what resistance was apropos at what reading.
Maybe with regard to fuel senders, maybe when you were tuning your ICVR replacement?

I'm good with numbers shapes and patterns because of my autism.
-those 'which is the next number in this set' questions were always more a reflex than a thought process-
I'm also very conscious of incongruity.
So I DO remember picking up on that, but not the value itself.

Well thanks for the reply’s all! I tinker with antique radios so I have plenty of resistors that I could try that test trick. I won’t waste too much space here hypothesizing on possible reasons but it is winter now. That being said I don’t recall this happening last year and it seems like it’s been reading progressively cooler and cooler Over the past month. I’ll poke around and see what I can find!

If you tinker with radios I'm sure you have a few potentiometers to play with, and know the difference between linear, audio and logarithmic tapers.

I still want to dig and find what three resistors Ford suggested.
IIRC, they are called out in the test procedures of my EVTM.

Christopher;  you say you mess with antique radios.  Would you have any idea where i can get some dial string?

Hi Nick,

Thankfully I’ve never needed dial string yet  knock on wood. I did a quick search of antique electronic supply out of Arizona(Tubesandmore.com) and too many off hits occurred although I’m sure I’ve seen dial string on the website before.

I personally have bought capacitors from Sal at his website located below, he is a very nice older gentleman from New Jersey as I recall. And he has quite a few dial string options.

http://www.tuberadios.com/capacitors/

I’ve also bought from Dave at justradios.com. He’s out of Canada as I recall.  Very nice and responsive and what a selection of caps and resistors! Looks like he has some dial string, too.

https://www.justradios.com/dialbelts.html

Also, if you havent found it yet, Antique Radio Forum is an amazing site for tube and record enthusiasts.

Great, thanks so much, i'm going to check it out.

So I tested my gauge. I first connected the wire through a 10 ohm resistor and based on the feedback I heard on this forum I was expecting the needle to register hot. However it didn’t even go halfway up normal, resting around the O or the R.

Since it sounds like that is not whats supposed to happen I connected it straight to ground. Slightly higher but still not the right response. During these tests I noticed that the sweeping of the needle was not very smooth. Then suddenly the needle climbed all the way to the hot side, or the right side of the gauge face. Finally the correct response!

Afterwords I went back to the 10 ohm resistor and it seems to be functioning and responding as expected. Based on this and the somewhat erratic sweeping of the needle I think it might be more of a physical problem - I’m guessing the needle is very delicate and any dirt or corrosion could be a problem. There was also a spider in the gauge yesterday so maybe a spider made a home nearby which I think would be enough to disturb the mechanism.

I’ll run some errands to see if it behaves any better today. I also cleaned the connector boot that slips over the sender. Let’s hope it’s not the sender as that looks very hard to access with a bracket in front of it ( for the alt?).

The needle is VERY delicate, so if it is dragging, or hitting a spider, that could be the problem.

How hot did your 10 ohm resistor get?  The wattage formula is IxIxR, and it should be pulling .5A, so it is .5 x .5 x 10 = 2.5 watts.

delco1946 wrote

So I tested my gauge. I first connected the wire through a 10 ohm resistor and based on the feedback I heard on this forum I was expecting the needle to register hot. However it didn’t even go halfway up normal, resting around the O or the R.

Since it sounds like that is not whats supposed to happen I connected it straight to ground. Slightly higher but still not the right response. During these tests I noticed that the sweeping of the needle was not very smooth. Then suddenly the needle climbed all the way to the hot side, or the right side of the gauge face. Finally the correct response!

Afterwords I went back to the 10 ohm resistor and it seems to be functioning and responding as expected. Based on this and the somewhat erratic sweeping of the needle I think it might be more of a physical problem - I’m guessing the needle is very delicate and any dirt or corrosion could be a problem. There was also a spider in the gauge yesterday so maybe a spider made a home nearby which I think would be enough to disturb the mechanism.

I’ll run some errands to see if it behaves any better today. I also cleaned the connector boot that slips over the sender. Let’s hope it’s not the sender as that looks very hard to access with a bracket in front of it ( for the alt?).

Not a 400  but my 300 six also runs cold on the gauge but the motor & hoses feel hot and the heat is great. When I painted the needles I am sure there were no spiders in there LOL

I think I did ground the wire when I first put the truck together to make sure the gauge & wire was ok.
I will have to ground the wire again to see if it will fix my gauge LOL.
Think I will also run wires to the oil / temp to swap them to see what out come that gives.
I knew of this but forgot I could use it as a tool.
Dave ----