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Overheating 101

73K views 63 replies 42 participants last post by  95 Legend GS 
#1 · (Edited)
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.

Why?
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

Jetdoc
 
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#4 ·
Hotstartup, are you saying yours still overheats with the thermostat removed?

The water pump hardly ever fails to pump. The bearings may fail and it will leak but will still pump fine.
 
#6 ·
You could try pumping air into the cooling system via one of those hand pumps designed for the purpose. The cooling system should not lose pressure but if it does, eliminate the radiator and cap right away. If the radiator and cap are sound then there is a leak somewhere and most likely at the headgasket or a cracked head.

Remember the radiator cap is designed to open at 16 PSI but if it fails to open you could blow your radiator open so if you use the hand pump be sure the gauge is accurate. A hairline crack in the radiator will reveal itself, at first, by leaking only when hot or under pressure. My experience was seeing drops of coolent on the fan shrouds but I couldn't find any leaks. After reading these boards I changed the radiator and all is well.

There is also a pressure tester available for the radiator cap.

Usually folks don't have these tools and go to shops but shops usually do a quick check and move on. The better way is to leave the pressure tester on for an extended period of time through a hot and cold engine cycle. If you lose pressure you have a leak and that should not be. I just went through a bad engine running hot experience ( not an Acura )
and the problem was that the overflow tank filler cap was not tight. My fault. There is no radiator cap on this car and water is added to the overflow tank, if needed.The tank is part of the pressurized cooling system and I didn't tighten the filler cap as I should have. Fortunatly no damage.

Good write up Jetdoc.

RAP

RAP
 
#8 ·
An excelent synopsis. I would only like to add that having the circulating coolant "wanded" using emission testing equipment will ID the smallest amount of head-gasket seepage.

There are three other sources for coolant seepage which might go unnoticed: (1) The Water (coolant) Bypass Inlet Pipe O-rings connecting the Water Passage to the Thermostat Case; (2) The O-Ring from the Coolant Connecting Pipe (under Intake Manifold) connected to the Thermostat base and the other end connected to a heater hose; and (3) The coolant hoses connected to the Fast Idle valve and the Coolant Connecting Pipe.
 
#11 ·
You must have had an extreme head gasket leak. The thermostat only attempts to regulate the temperature of the coolant by restricting the flow through the cooling system. A sensor sends the temp indication to the indicator in the car. The two items are not connected.
 
#12 ·
"blown" head gasket but on the Legend it is actually a seeping head gasket.
It is always the #3 or #6 (back) cylinders

i think it various , it not always # 3,6

Cool lower hose-temp sensor cool = no fan command on.
my fans is always on , even both lower,upper are cold. BHG Symptom
 
#14 ·
i didnt take the themostat out completely but drilled four 8mm holes around so the w/p could over come the bubble easily.. works really good.. wouldnt even know i had a bhg except for the fact that the rez fills up and i smell coolant when it does.. considered selling the car like that but i am sure the karma would kick me in the ass badly so changed my mind and went for the reco :D
 
#46 · (Edited)
i just bought my legend this weekend. the seller had great mainanence records, and for the record on here, it had the #6 reported as bad with ~109k on the engine. problem is, on the receipts only a $458 purchase of 'gasket kit' was mentioned and billed for. the timing belt and water pump had been replaced at 88xxx miles, on another receipt. what worries me is that no labor was paid for the bhg, just the kit, and no work other than the water pump has been done to the cars cooling system (as far as i can tell by going through the receipts). i dont know how to tell if lucky #6 got replaced properly or even replaced at all.....

how do i tell if the bhg got fixed? im all paranoid and could use some peace of mind.

all the info on here is amazing, but it will take a bit of time to read through everything regarding this. thanks a ton though =)
 
#16 ·
Jetdoc's writeup should be a must read for all new Legend owners.

I think the key thing is to recognize the symptoms of a cooling system problem, and then to fis it *right away*. If you read this writeup, see the symptoms, and ignore them, you deserve an expensive repair!
 
#17 ·
stevieray said:
Au contraire. 99.995% of the time the BHG occurs at #3 or #6 because they are at the farthest point in the cooling system with the exception of the heater core. I guarantee you a poll on this Forum would show the same.
my legend overheated 2 times already. i'm doing the head myself , i'm think it various. the first time was 2 ,5,6 and the second time 2 and 3 .
 
#18 ·
I dont know if these are the same symptons as a bhg, but i hope not. My heat gauge will go up to hot, but either go down by itself when i turn the heat on, or drop when i get over 3k rpms it seems. The heat will come on but it take a long time to warm up.

Any ideas what this may be thats causing the heat gauge to flux as well as my heat to take a very long time to heat up, usually like 20-25 minutes i think.

Thanks
 
#20 ·
Mike95SE said:
I dont know if these are the same symptons as a bhg, but i hope not. My heat gauge will go up to hot, but either go down by itself when i turn the heat on, or drop when i get over 3k rpms it seems. The heat will come on but it take a long time to warm up.

Any ideas what this may be thats causing the heat gauge to flux as well as my heat to take a very long time to heat up, usually like 20-25 minutes i think.

Thanks
Mine was doing almost the same thing. It never went over normal temp, but I didn't have heat unless I ran the engine aover 2500 rpm. Then it worked fine. I eventually got the cooling system full and all of the air out and now it's perfectly normal and wirking fine.
 
#22 ·
Gint said:
Mine was doing almost the same thing. It never went over normal temp, but I didn't have heat unless I ran the engine aover 2500 rpm. Then it worked fine. I eventually got the cooling system full and all of the air out and now it's perfectly normal and wirking fine.
I should update this. After bleeding the cooling system and filling the resevoir bottle, I kept checking the bottle. I found myself adding a few ounces of coolant to the bottle every few days for a couple of weeks. Needless to say, I wathced the temp gauge like a hawk. Once bled, and since I was keeping the resevoir topped up, the heat worked fine and the car still never over heated. But I kept wondering where the coolant was going. No signs of a seeping HG or visible signs of a leak, but a 1000 mile round trip to Salt Lake and back finally brought the source of my coolant loss to the surface. Prior to the trip I could never see any coolant leaks anywhere. But immediately after the trip, I could finally see coolant on the bottom of the rad and it started to leak enough that it was also dripping on the ground. I found the leak in the rad when I pulled it out. Halfway up on the driver's side towards the engine, right along the edge of the core.

I put a new radiator in it last weekend and it's no longer losing coolant and running great.
 
#23 ·
arargh!

I recently purchased a 1992 acura legend with a small amnount of smoke comming from the tailpipe. Well i figured it was no big deal, but now huge clouds come out and it is barely drivable, This BHG problem is very dissapointing. My car only lasted me 5,000 miles and now it will cost more than the price of the car to get it repaired. Well im gogin to fix it because the Legend is an awesome car, good luck too all the other Legend drivers out there! :p
 
#25 ·
great write up .
i must say im in the sec scenario. lots of air getting in to system and cooland getting pumped back to bottle. no smoke ,no leaks. it started overheating 2 times but i pulled over. since than i bleed air about twice a week by opening the bleeder bolt after coming home fron work. all the air get out ( aprox. 3 to 5 sec. of air comes out.) im preatty sure it HG, i got new pump hoses , termo radiator. it stated acurring since this winnter. also a interesting observatino i discovered. with car running rad. cup off and bleed bolt off with full cooland in ssytem there is no bobles coming and cooland stay level in radiator. as soon i turn heat on ,the cooland starts dripping from radiator like is geting pushed out from inside. when running heat my car proly will overheat in maters of 2 days , with no heat i till take aprx 2 to 3 weeks to overheat. tha'ts makes me thing was up with that . im planing to bypass the heater core and see what will be the resolts.
 
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