The BHG phenomenon
Why does the Legend cooling system give such confusing indications when we have problems?
Air can become “trapped” in the system.
Small amount of air
If you have a tiny coolant leak a small amount of air will enter the cooling system replacing the coolant that leaked out. When enough air gets into the system, it will usually show up as an erratic idle because the coolant temp sensor is not submerged in coolant but sitting in an air gap. It will also cause you to hear coolant flowing in the heater core area, a "gurgling type sound". This sensor tells the computer to adjust the idle according to the coolant temp. This erratic idle problem usually goes away once it warms up due to the small amount of air. This sensor doesn’t work correctly in air causing the erratic idle. Bleed the air out, top off the coolant, find the small leak and repair it. Occasionally, slightly low coolant can't be explained. It just disappeared, evaporated, went to that place where missing coolant and socks go. That's why we check the coolant regularly.
Large amount of air in cooling system
If a large amount of air is introduced into the cooling system you will notice several things.
1) No heat- temp selector on hot but blowing cold air from the vents.
The heater core is full of air not hot coolant.
2) Temp gauge fluxing- The temp gauge will creep up close to hot then quickly drop back down, but not all the way back down to the normal position, but just above it.
You pull over and open the hood, you notice
3) The cooling fans are not operating but the temp needle is on hot. Most people at this point wrongly assume that the fans are causing all these problems. You cannot troubleshoot the fan system until there is no air in the cooling system. If it only overheats when stopped it may be a fan problem, if it overheats when the car is driving at normal speed it is not likely to be a fan problem.
4) The upper radiator hose is hot and swollen; the lower radiator hose is abnormally cool.
This large amount of air becomes trapped around the thermostat causing it to close, restricting the flow of coolant (this causes the hot upper hose, cool lower hose). The thermostat only works correctly when hot coolant contacts it directly. Hot air will not open it.
The fans are not working because the coolant temp sensor is also in trapped air. It is mounted close to the lower radiator hose outlet in the radiator. Cool lower hose-temp sensor cool = no fan command on.
Why the flux?
When you increase the engine rpm’s enough (+/- 3500) the water pump will eventually overcome this trapped air, pushing hot coolant to the thermostat causing it to open. The temp needle drops, the heat comes back and fans come on, but only for a short time because the cycle will repeat itself quickly.
How did the air get there?
1. A leak, if coolant leaks out air must go in. Many places for a leak. A large leak will usually result in coolant on the garage or driveway floor.
2. Cracked radiator top where throttle cable bracket attaches to radiator-ALL Legends have this problem at one time.
3. If you have no leak in the system there is only one other way to get air in the system.
INTERNAL - Head gasket leak. It could be a very small seeping leak (most common) or a large leak. The more times it gets overheated the bigger the seep/leak will get. We call it a “blown” head gasket but on the Legend it is actually a seeping head gasket.
It is always the #3 or #6 (back) cylinders. It develops a tiny seep allowing air to be pumped into the cooling system by the combustion process. At first it’s so small that only air can be pumped out of the cylinder and no coolant will be sucked into the cylinder. Like a one way check valve.
In most cases the engine will run great, there will not be any white smoke out the exhaust (until later stages) and no coolant in oil/oil in coolant, just the overheating. Some people with advanced stages of BHG will experience some white smoke.
What can I do?
If you remove the thermostat it will eliminate the “trapped” effect but the engine will not operate at the proper temperature (will be too cool). I do not recommend this in a cold climate. This will usually keep it from overheating until the head gaskets can be replaced. Some people have cut the guts out of the thermostat and installed just the outer portion only. Repeated overheating events will eventually result in warped heads and a cracked cylinder wall.
Why did this happen to me?
Most BHG’s are the result of a single overheating event (failed hose, ect.) beyond the control of the operator or continued driving with a known small leak. Do not drive the Legend with a known leak!
Many people spend months trying to figure out why there Legend is overheating so strangely. Most of us have been preprogrammed with a different background of cooling system theory. The Legend is a little different.
MY BHG STORY HERE