71 350 running hot coolant backup up problem

[harpo231]

Gun i used in my thermoformers took temps 1 inch apart accurately , but it was 600 bucks

[B. W.]

Don't give up on your temp gun yet! I use mine almost daily (I'm a mechanic) If you measure right at your thermostat you can actually "watch" the Tstat open & close (pretty cool) In so doing you can verify it is working & it's opening at the correct temp. Don't get too picky about exact numbers, if a 180° opens at 185° it doesn't matter, close enough.

to confirm your gauge is accurate you need to shoot the sender in the driver's head. From what you posted it's close enough.

So, from what you've posted, your engine & your gauge are doing pretty much what you thought all along. This is good to know.

My opinion (FWIW, everyone else has given theirs!) you've covered all the basics, some more than once. The one item you have not addressed is the radiator. You can look down the tank & it "looks" good but you cannot see down the tubes & these are what get clogged.

Take your new gun & shoot the radiator starting at the driver's top (hot coolant in) then shoot various points to the pass side bottom ( cooled coolant out) In perfect world, you want to see about a 50° drop across the radiator.

Time for a radiator (easy for me to spend your $ but it's still cheaper than an engine!)

side note: 50/50 mix (or any ratio) versus straight water will not make any difference. From the tech articles I've read, water with a "wetting" solution removes heat the best BUT the difference IN A PROPERLY OPERATING SYSTEM is negligable.

[Gregski]

Originally Posted by B. W.

... My opinion (FWIW, everyone else has given theirs!) you've covered all the basics, some more than once. The one item you have not addressed is the radiator. You can look down the tank & it "looks" good but you cannot see down the tubes & these are what get clogged.

Take your new gun & shoot the radiator starting at the driver's top (hot coolant in) then shoot various points to the pass side bottom ( cooled coolant out) In perfect world, you want to see about a 50° drop across the radiator.

Time for a radiator (easy for me to spend your $ but it's still cheaper than an engine!)

I agree, I was always taught to replace parts if needed or suspect starting with the cheapest one first, hence the $7 dollar thermostat, then the $18 water pump, now we're ready to drop the $200 on the radiator, seems logical to me

Originally Posted by B. W.

side note: 50/50 mix (or any ratio) versus straight water will not make any difference. From the tech articles I've read, water with a "wetting" solution removes heat the best BUT the difference IN A PROPERLY OPERATING SYSTEM is negligable.

I totally agree, and was trying to prove that to others in some of my tests

[Steeveedee]

Originally Posted by Gregski

so after she went back all together it was time to test, and to keep things fair and compare apples to apples, I put it all back to the way the General intended

FIRST TEST POST SHORT WATER PUMP SWAP

so 195* Thermostat, Short Water Pump, no bypass hose (heater core delete, remember) and running only water

RESULTS = Favorable but not there yet

the truck did not get hot idling in the garage after 15 minutes like it used to, it used to reach 205* just sitting there, I let it idle for 40 minutes and it just eventually got to 190*

so I took it for a TEST DRIVE...

and gosh darn it I saw it climb to 215* that aint right!!!

Classic symptom of a clogged radiator. Stays cool at idle, and gets hot going down the road, under load.

[Andy4639]

Anti freeze isn't in the radiator for cooling. Its to keep the water from freezing in the winter time and to lube the water pump and keep the system clean that's all.

[Gregski]

so I may have discovered something that may save you some grief or at least save you some money if you ever experience something similar to this

but first let me tell you the last thing I did before this discovery

I went out and dropped $200 on a brand new cheap Spectra "Premium" radiator, of the plastic and aluminum variety

[Gregski]

here is a closer look at the Spectra Premium A369 radiator

it measures almost exactly the same, except the top parts that are supposed to fit into the rubber mounts on both the left and right sides are too wide so I had to cut and use 5" strips of an old mountain bike inner tube to use as buffers between the metal brackets and the aluminum radiator instead of the stock rubber pieces

[Gregski]

mandatory stare and compare of the old brass/copper radiator and the new plastic and aluminum one

[Gregski]

So what are the findings?

So now with a brand new stock 195* thermostat, a brand new short water pump, a brand new plastic/aluminum radiator, running water only (for test purposes only)...

The truck idles fine in P-Park I think it was like 135* F after 15 minutes of just sitting there, it would slowly warm up more, but I decided to take it for a spin

It cruises at 185* F, however and here's what I recently discovered, when stopped at a stop sign or at a stop light, it will climb North of 200* F in just one minute, now this is in D as in Drive

OK, yeah so what? Well I decided to shift it into N for Neutral and sat at an isolated Stop Sign for five minutes and the truck never inched past 195* F, now that has to tell us something! Right?

I also tried it in reverse, I pulled into a parking lot and backed into a space and left the truck in R-Reverse and sure enough in just one minute it reached 200* F

your thoughts?

[harpo231]

Sounds fine to me ,,, fer an ole truck !

[bautino95]

I still think you should put a 180* t-stat

[Gregski]

Originally Posted by bautino95

I still think you should put a 180* t-stat

I think in the long run I will, I am running a 180* thermostat in my '74 but that engine has the Vortec heads on it, which are prone to cracking under heat, just so you know I tried testing with the 180* earlier in the thread and it still ran hot, so the source of the ongoing problem must be dealt with first

[Steeveedee]

OK, let me do the TL;DR version-

You have a new water pump, new radiator and new thermostat. Is this true? If so, check for a blown head gasket/cracked head, etc.

[Andy4639]

I see what your getting to I think. The transmission is causing the rise in temp when in gear. The temp will be added to the radiator as hot transmission fluid passes through it.
I run a external on all of my vehicles.

[Grumpy old man]

Looking at your pictures your bypassing the heater core which would add about a gallon in volume , i would use a 180 thermostat and for a few bucks more the Champion all aluminum radiator that many guys use and like that fits well .

[clay68c10]

Two things I can think of

1. How much is your rpm dropping when you go from p/n to d/r? If the engine slows, so does the water pump. Might kick up your idle and see if it changes the result. The trans front pump is pumping fluid thru the cooler no matter what gear you select, so that’s probably a non issue.

2. If you are running Vortec heads, they have a different cooling passage setup compared to non Vortec heads. I don’t remember exactly, but it has something to do with the water pump bypass and how that is routed thru the block. If those heads are used on an old style block you have to accommodate for that. Might be worth a bit of research to ensure you aren’t chasing that.

[Gregski]

Originally Posted by clay68c10

Two things I can think of

1. How much is your rpm dropping when you go from p/n to d/r? If the engine slows, so does the water pump. Might kick up your idle and see if it changes the result. The trans front pump is pumping fluid thru the cooler no matter what gear you select, so that’s probably a non issue.

You are asking the right questions, I just bought a brand new tach, so I will hook that up and see, I know I can do this with my Mac Tools digital timing light, but I am a one clown circus act, so its easier with a permanent tach. What RPM should it idle at? I read somewhere on automatics you wanna shift it into gear and check the idle RPM that way as well? I aint too versed in automatics, a stick man myself! ha ha

Originally Posted by clay68c10

2. If you are running Vortec heads, they have a different cooling passage setup compared to non Vortec heads. I don’t remember exactly, but it has something to do with the water pump bypass and how that is routed thru the block. If those heads are used on an old style block you have to accommodate for that. Might be worth a bit of research to ensure you aren’t chasing that.

I am not running Vortec heads on this truck. I was talking about my '74 when I brought those up, but yeah you are correct the Vortecs require a bypass hose from the intake manifold to the water pump.

[clay68c10]

I’d set the idle to around 650 in drive and see what happens. What’s it idling at in gear now?

You may have to play with initial timing and idle screw adjustments (throttle plate and mixture) to get it so the neutral to drive rpm difference isn’t too extreme. Like add timing and lessen the throttle opening, or change up ported vs manifold vac to the can. The usual hide the salami carburetor adjustment game...

[Keystone_Hate]

This may sound kinda silly but try jacking up the front of the truck. Get it as high as you can while still being relatively safe. Take off your radiator cap and see if ya get any bubbles. You might have a big air pocket stuck in the back of the engine. Just an idea.

[harpo231]

Do a couple cartwheels and a backflip ,, that'll do it ...

[Keystone_Hate]

Originally Posted by harpo231

Do a couple cartwheels and a backflip ,, that'll do it ...

he's jumped through enough hoops already, this couldn't hurt.

[71cheyennesuperlongb]

Just a thought.Do you have the wire like, a coil spring in your lower hose? I know it wouldnt matter at idle, but when driving the hose could suck shut restricting flow?

[garyd1961]

No matter if you are running vortec heads or not a by-pass hose is a good idea. The factory put one it for a reason. The by-pass keeps the water circulating through the block and heads and eliminate hot spots.

[rockyrivermark]

You can get a 3 core polished aluminum champion radiator on eBay now for $205 with free shipping
I just picked one up last month. Fit very good.
Good deal

https://m.ebay.com/itm/1967-68-69-70...gAAOxyBPZTid8U

[Gromit]

I did notice the 850 RPM idle speed you mentioned sounded about 200 RPMs high but since some people like what I would consider a high idle so I didn't want to judge.. I prefer a putta-put-put - maximum 650 RPM or as low as I can get it to run smoothly in that neighborhood. You definitely don't want your centrifugal advance coming in at whatever idle you wanna use.

Hey I may have been wrong about your initial timing being the cause of your overheating but just maybe I was on the right track with my second choice of the heat possibly coming from your TH350. If so it is too bad I wasn't smart enough to link your high idle with the trans fluid shearing forces maybe creating too much heat in the fluid. Maybe a sniff test of your fluid will help to tell the tale. Luckily you can drive it now just put it in neutral at the stoplights.