Is my radiator to big

One thing about diesels is that they use very little fuel (and make little heat) at idle because there’s no throttle and therefore almost no pumping losses compared to a gasoline engine.

This thread has been focused on engine temps with respect to larger radiators. I have different question.

While on weekend trip, my radiator went out in my 2007 Toyota Camry and I had to replace it. Unfortunately, the local auto parts stores and the dealership had to order it with a 2 to 3 day arrival. I couldn’t wait that long as I needed to be back home by Monday for my wife’s dr appt. To remain, I would have had to find a way to get my wife home…she couldn’t miss this appt.

I decided to check with a local radiator shop. The shop confirmed the old radiator was clogged badly and a flush was not recommended given the distance we needed to travel to get home. So, upon checking his supply inventory, he said that he had a replacement available now, but it was the tow package radiator which was a little bigger. It was designed to fit my vehicle, so I agreed. Every since, the car has run great with respect to engine temps. Not sure about the gas mileage though…it might have down a few mpgs.

I recent noticed that my heater has been blowing cold air while vehicle is at idle, but blowing hot while moving. Upon internet research, I discovered this could be caused by low coolant levels, blockages in the heater supply lines, heater core, a bad thermostat, or bad water pump. (fyi…I use a failsafe thermostat so that in case of bad thermostat, it fails in open position). So, I checked for kinked hoses (none found). The thermostat was only a little over a year old, so i figured the heater core was the issue. Upon replacing heater core, the heater improved slightly at idle, but not what it should be. So I changed thermostat (again using a failsafe thermostat). At this point, the heater performance is still reduce with no change from previous level. Now I’m down to the water pump. I’ve had no problems with engine temps or loss of coolant from water pump (which is a typical sign of bad water pump).

Here’s my question… Before changing my water pump… could the oversize radiator be why the heater blows cold or less-hot air while sitting at idle?

It is my opinion as stated previously my opinion a bigger radiator does not matter, the thermostat will only allow as much coolant as it needs t maintain running temp. One thing you might look at, I had an experience where I only got cold from AC while moving. It ended up for me the blower motor variable resistor was bad, so somehow I got air blowing through while driving but no air movement when stopped, so no cooling. Just a thought.

Barkydog, you forgot about the heater core flow. It Bypasses the thermostat, and if the core is too large that can be enough flow that the block never gets warm. I know that is the case in my old truck, and I proved it by clamping off the hoses, then the temp went to 220 and was climbing, I unclamped the heater hose and had the thermostat hose still clamped shut and it dropped to 155 Deg. Now I may re-pipe my heater return to go directly to the water pump and thus stop the bypass, or I’ll get the right radiator, however I really want the bigger capacity if I can keep it. Also I put cardboard in front of the radiator but I had to cover 80% of it to get it to heat up, but if the temp outside goes up I have to keep adjusting the amount. Now I have changed the Thermostat 3 times already, and tested it in water and I know it works, what I also know is that with a huge radiator it only takes a trickle to over cool it.

When I was in elementary school, the school buses were all pre WW II. The fleet didn’t get renewed until 1951. The solution to getting the engine to run warm enough room get heat out of the bus heater was to tie a feed sack over the grill.
If you have the proper working thermostat, before I change out a bunch of expensive parts, I would go the feed sack route.
Of course, the school bus was still cold inside in the winter, but apparently the heater put out more heat with the feed sack tied over the grill.


The heater core won’t remove much heat from the engine unless there’s air blowing through it (the core).
That’s not related to the size of the main radiator anyway.
Did you have the blower going when you did this experiment?
The rise to 220+ was due to blocking the bypass circulation.
The coolant needs to circulate through the engine even when the thermostat is closed to even out the temps in different areas.
Where did you get these 3 thermostats?
Some aftermarket ones are junk.

The most a radiator can do is reduce the exit temp to ambient, no matter how big it is.

I am not concerned with the heat remove by the heater core, as I said the heater core allows flow from the block back to the radiator effectively bypassing the thermostat and allowing the radiator to cool the engine via the flow though the heater core. One other problem with an oversize radiator, a thermostat can’t properly modulate between the 0 and 5% range, if the engine heat load and the radiator capacity are not reasonably matched then the thermostat has to operate between closed and just slightly open, this causes the temp to bounce around a lot because flow keeps getting blocked off then reestablished. The gain (or proportional band if you prefer) of the thermostat needs to be adjusted (which you can’t do it’s fixed by the spring rate and other materials inside it) to adjust for the greater cooling capacity of the rad.

You are correct that the Rad can only cool to ambient, but 60 deg coolant entering the block will not come out at 195 with just one pass, and if it keeps getting knocked back to 60 deg before it reenters it will never reach temp. if the flow through the heater core is enough to keep up with what the engine makes it will never heat up.

One more item, my truck does not have a bypass circuit, (1986 Chevy K10) at least not one that doesn’t return to the radiator (and this is factory, just how they were made) the heater core acts as the bypass, coolant exits from the intake manifold just before the thermostat, and then goes to the heater core and then back to the Radiator tank. This means all the flow through the heater core goes from the engine to the heater core to the radiator where it gets cooled, and then back to the engine.

After recently upgrading to a performance radiator, I’m absolutely convinced you can over-radiate. Everyone knows that the thermostat is supposed to have the final say over keeping hot enough, but if once it initially opens, the coolant returning to the pump from the radiator never gets up to a reasonable temp, it continues to backfill the engine too cool, and the thermostat may then start to close back down, but also may never completely get a handle on keeping hot enough- that inefficacy of modulation in the 0-10% opening range as noted above.

And this is before I even thought of the heater-circuit path, as also aforementioned. My car uses a ‘hot’ heater core design, meaning it’s constant full-flow, and cabin temp is regulated by system-controlled air baffles rather than by modulating coolant flow, as I believe is the case with many modern cars. Its outflow bypasses the thermostat to be pumped straight to radiator, in order to allow heated coolant to be drawn up from the engine. This represents an unregulated flow to radiator AROUND the t’stat- so that if it isn’t already struggling to regulate temp from a big radiator by barely being open, this bypass flow could very easily throw it into exactly that condition. This is an absolutely significant amount of coolant movement. Throw in winter conditions calling for a lot of heat removal via the core on top of everything else, and…

Once winter hit (way too early) following my radiator up-sizing, I noticed that after initially coming to normal temp, it wouldn’t consistently hold for the whole trip, and would often drop significantly below normal. On arrival I would find the system not building pressure, and the radiator cap, along with the radiator’s outlet tank, maybe only lukewarm. The solution was simple in the extreme- I stuck cardboard in front of half of the radiator!

All the same here, but the cardboard trick didn’t work for me, I had to keep adjusting how much coverage every time the temperature went up or down, acting as a manual T-stat sort of. however yesterday I was pulling a loaded trailer with no carboard in 40 deg weather and she work perfect the whole trip. The additional heat from the engine brought it right up to temp where it stayed. when I unload the trailer it right back to what you described. up to temp once, then up and down up and down.

The thermostat is an analog device not digital. The amount it opens depends on the heat it is subjected to. So it regulates heat by how far it opens and restricts the amount of coolant going to the radiator. I have never seen any cooling circuit where the heater bypasses the thermostat and allows coolant to the radiator. That makes no sense as it defeats the thermostat. I’d certainly be interested in seeing a coolant path diagram for this situation…



If the heater output temp was fine when hauling, and then cold when not, (and frankly even with some of these other issues in this thread,) I’d look for a locked up fan clutch- more likely on an older vehicle like your truck. Or possibly a faulty fan clutch relay in something newer- on that is keeping the fan on all the time.

If it is constantly engaged, it will pull air over the rad all the time, which could help keep the coolant from getting up to temp at idle or low load instances.

A locked up fan clutch isn’t super likely, but it has been known to happen.

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For many years several GM models had the heater flow emptying into the radiator high side tank bypassing the thermostat. It was very convenient when filling the cooling system because you could watch and see when the heater return reached full flow indicating that the air was purged.

How long ago we talking? Do you recall which model/year or just vintage? I’d like to look up flow schematics if I can find them…

The old trucks being discussed in this thread have the heater hose return going to the return side-tank of the radiator. Cadillac had the same arrangement in the 1970’s and early 80’s.

In the picture below look for the vise-grip pliers.


On a '92, questionable. On an '86 possible. The thread was resurrected by someone with a 2007 Camry. Doubtful the heater core bypasses anything going to the radiator…using an '86 K1500 as an example is not relevant…

In the diagram you show, the heater return flow would go directly to the radiator and not back to the water pump. To the best of my knowledge (which could be faulty since I’m home sick doped up on Nyquil) that was the arrangement for all Chevy V8 engines from the 50’s until the LS series. That’s almost 50 years and a lot of engines to be operating that way.

I also believe that late model Jeep Wranglers use the same coolant flow.

Nevada and asemaster seem to have investigated the issue and so it seems that the plumbing in question was common for quite a while but it wouldn’t have much effect on delaying engine warm up due to bypassing the thermostat. All engines have plumbing to recirculate the coolant when the thermostat is closed and routing the flow into the return tank on the radiator seems convenient.

Also, I have often partially blocked air flow on radiators in extremely cold weather to speed up getting hot air from the heater and got varying results