I am curious on the logic behind it. I've only had 460's and a 400 (but no ac) so I have no experience with how, let's say a 300, is impacted by the ac turning on at idle if it didn't have a tps. Thinking about the alternator ouput alone, I would assume all motors would benefit from having the bump.
I see Holley makes a simple solenoid to accomplish it if one wanted.

 

I do know on my diesel, when the A/C kicks on you don't even know it. There is no change in the idle rpm when it kicks in. Not like on 4 cyl and v6 cars I have driven. I am going to assume the 460 is the same, it's large enough to where the extra drag from the A/C compressor doesn't have a negative affect on the idle speed.

 

Hey guys, all these philosophical musings are interesting, but aren't necessarily helping with the original query. We should probably steer back towards helping the poor guy... So far, approximately zero diagnostic work has been done, so it would be good to figure out exactly what is going on.

One quick point to clarify: Does "lower speeds" refer to the forward motion of the truck, or the speed of the heater/AC fan? I think you mean the forward speed, meaning less ram airflow through the radiator and AC condenser.

Before digging too deeply anywhere, I'd suggest springing for an infrared thermometer. You can pick up an inexpensive model for around $20, so no whining. These are very handy tools for diagnostic work.

With the engine warm, idle speed, and AC off, point said thermometer at the thermostat housing. You should see a temperature close to the thermostat rating, probably around 180 or so. (I don't know what thermostat you have.) Compare this to your truck's dash gauge. Before somebody chimes in that you should replace the stock gauge with something more accurate, I'll speak up as a card-carrying member of the Stock Gauge Anti-Defamation League (SGADL). The stock gauge is perfectly adequate, and I rely on mine. Make note of where the needle is pointing, and that is your base reference. Now turn on the AC and watch the gauge. Now I'm assuming (Danger! Danger!) that you can duplicate the problem from the comfort of your driveway. If the gauge starts to rise, use your handy infrared thermometer to measure the actual temperature at the thermostat housing. Is the temperature also rising there? If so, you have an actual overheating problem. Or does the actual temperature stay the same, or within a few degrees? If that is the case, you have an indication problem.

I mention the possibility of an indication problem based on my personal experience. I've had two vehicles where the temperature gauge seemed to operate normally until the actual temperature neared the top of the normal range. Then the indication erroneously shot way up. The first instance occurred back in the dark ages, when an infrared thermometer was exotic and expensive. I chased my tail, trying to fix a non-existent overheating issue, when it was really an indication problem. After changing a lot of parts, I finally took it to a shop, where they had an infrared thermometer (probably cost several hundred dollars at the time) and quickly figured it out. When I came across this again, fortunately I now had my own infrared thermometer, and quickly confirmed an indication issue.

So for any problem like this, the first step is to confirm whether you have an indication problem or an actual overheat. From there, you will know how to proceed.

If you've got an indication problem, we can walk you through some steps to take care of that.

If you've got an actual overheat problem, we can walk you through steps for that, too. For an overheat issue, please keep in mind that your engine normally produces X amount of heat that must be dissipated by the cooling system. As long as the cooling system has the reserve capacity to dissipate more heat than the engine can produce, all is good. But if the equation changes, and there's no reserve capacity left, you will have an overheat. This can be caused by the engine producing more heat than normal (such as ignition timing way off, a blocked exhaust, etc.) or the cooling system not up to snuff (scale inside radiator, no radiator shroud, etc.). If in good condition, the cooling system for a 460 should have PLENTY of reserve capacity to handle the moderate amount of extra heat produced by running the AC, but let's not get ahead of ourselves just yet. There's no point in trying to fix the wrong issue, so please do the thermometer check and report back.

 

I did not say bypass, I said shut off. There is a huge difference between the 2, they are not the same.. There is no circumstance you would want a bypass other than to isolate failed heater core.
When the AC is on the heater core is not shedding heat as airflow is minimal over it as the blend door for the heater core is closed.
The coolant that goes through the heater core when the AC is on does not significantly drop in temp and is then fed right back into the cooled coolant coming from the rad, this signifcantly decreases the amount of heat the coolant can absorb. Typically you can see 10 to 15°F drop in engine temps with hearter core shut off.
This is exactly why all modern and many older vehicles use/d heater core shut offs. Most of these trucks are running hotter on today's gasolines and adding a heater core shut off can tip the balance between not having enough cooling capacity and having adequate capacity.

 

I said "I" bypassed the heater core with a 4 port valve because I also use that hot water to heat the intake manifold as I am running EFI exh.manifolds.

If the blend door did seal 100% then it would not shed heat but we all know the door does not seal 100% and why we add the shut off valve to the heater core.
So heat IS coming into the cabin and it has to shed a little heat in doing so. It may not be much but it does.

Yes the OP needs to see what he is starting with for temps and go from there.
He may find all is well and needs to do nothing.
Dave ----

 

I added the 4 port bypass to my 89 and it does help the A/C cool a little better. I just added another one a couple of days ago to my Bronco II. It helped it significantly, not the A/C but the heat coming out of the vents, since the A/C is not working right now. As I am driving in this heat with the windows down, I could feel the heat rushing out of the vents. After I installed the bypass, you only feel a little heat after sitting at stoplights in town.
Like was said, the bypass is just covering up another problem, the doors in the HVAC system not sealing off. I adjusted the cable on the BII till it slammed shut, and it still leaked. The valve is just a easy way to fix it without tearing into it. In both cases I got one from the autoparts store that was for a 1995 Explorer. I think both the Rangers and the Explorers started using these valves in the mid 90's. It operates on vacuum, and there is a slick way to hook it up on the larger trucks since they use vacuum in the HVAC system. But on the older Rangers and BII's, they used cables only, so I have to manually hook the valve up to the engine vacuum.

 

apology, have been away
I have a new 4 core radiator, headers, rv cam, block was .030 over, and everything else is stock. radiator fluid is 50/50
should I try a better fan?

 

Are you saying you added a shut off valve to the heater core line? That is a shut off and not a by pass as it does not bypass coolant flow to the heater core but shuts off coolant flow to it.

 

It depends. How much money you got? Want to keep any of it? If so, please see my previous reply (post #18) for details on how to start the diagnostic process. So far, we still don't know if you have an actual overheat condition or an indication problem. A little troubleshooting is generally preferable to blindly launching expensive parts at the problem.

Glad we didn't scare you away with all our blathering.

A little history will help, too:

1) Was the temp gauge doing the same thing before the engine rebuild?

2) Does the radiator have a shroud for the fan?

3) Does the fan have a clutch coupling or is it direct drive?

 

It looks like the picture below. It routes the coolant around in a loop when vacuum is applied. I had to use this on the Bronco because I have my electric fan sensor inline with the heater core line, so I need the coolant to keep flowing in the heater line near the engine. It came stock on the later rangers and explorers. Autozone sells it for about $21.00.

 

A pretty smart guy I heard one time say overheating in town means a airflow problem, overheating on the highway means a coolant flow problem. I have ever seen a situation so far that contradicted that. So if you are getting hot in town, I would say you need more airflow through the radiator, however you can achieve that. A fan with more blades, and a fan shroud that is working properly. Adjusting the idle speed up just a little bit might help also.

P.S. I just re-read your post that started this thread, and something stands out to me that I hate; You are relying on the stock temp gauge. Who knows what temps you are actually running. I do not like the stock gauges and have seen a lot of money wasted on assuming they are correct. You can at least go to Harbor Freight if you do not have one, and get a infrared temp gun and see what you have under the hood.

 

Um, actually, it will be kind of difficult to go to Harbor Freight if he doesn't have one. He could go to a different store, though...


All seriousness aside, I get it. The odds are slim for a temperature gauge to behave properly for most of the normal range, but then shoot up near the top of this range. But I have seen this exact scenario twice on other vehicles, although not on my truck with its rugged, well-designed and thoroughly reliable factory gauges.

But regardless, it's just basic troubleshooting to begin by confirming the accuracy of the indication system. Even if you've already convinced yourself the indication is fine and I'm full of $#!+, you will still need an infrared thermometer for some subsequent steps. And if you can't spring $20 for one, just put a bandana on your face and knock over the store. Maybe grab a jumbo pack of bungee cords, too, as those are real handy to have around. This pandemic does have a silver lining, as people are no longer surprised to see your face covered.

As much as it pains me to admit, Dave is probably right and you're likely looking at a problem with insufficient airflow through the radiator. That's what I've been thinking, too. But don't get too far ahead of yourself just yet. Start with the basics. And we're still waiting to hear more details about the history of the problem, and if you've got a shroud.

 

This can also be checked pretty easy.
Think you said you are running a fan shroud and if so is the fan sitting half inside from the outer edge?
How far are the fan tips from the shroud?
How many blade fan?
Direct drive or clutch fan?
Clutch fan if you spin the fan, motor off, how long will it keep spinning?
With the motor running take the hanky you been crying into and hold it in front of the grille dose it get pulled into the grille?
Dave ----

 

ok, got some info:
the blades r over 1/2 way beyond the shroud, its clutch fan w/ 5 blades, and the blade tips r about 1/4 inch from shroud.
when the temp indicator needle is at the "m" on normal the temp is 185
when the AC is running, and truck setting, the temp starts increasing when the indicator gets to the "A" on normal the temp is 220
Thanks for the replies

 

All right, making some progress. Per the numbers you posted, coolant temp increases 35F with the AC running. That's a lot.

Next I'd suggest checking the airflow through the radiator. With the engine at idle, hold a rag or sheet of paper in front of the grill. The airflow should be strong enough to firmly hold the paper or rag.

Then break out your spiffy new thermometer. Engine at idle, Measure the temperature at the radiator inlet and outlet. For the most accurate values, take your reading on the metal, not the rubber hose. For any measured inlet temperature, you should see at least a 30F drop at the oulet. Repeat with the AC on and off.

Does the idle RPM drop much with the AC on? I wonder if this could as simple as the water pump turning too slowly. Did you replace the water pump or crankshaft pulley during the rebuild? If so, were the new pulleys the same size as before?