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Old 04-03-2011, 01:01 PM   #1
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Lightbulb Why Legend Head Gaskets Blow & How to Prevent it!

Summary & Disclaimer
This document will attempt to explain the mystery behind why the Legend’s head gasket blows so frequently and mysteriously and what to do to prevent it. There are various causes to blown head gaskets – it could rot away from neglect (IE: never changing the coolant), it could fail because of severe engine overheating or simply fail from old age. Although I have seen Legends with those problems, the vast majority of failures have been a “mystery” fire ring blowout of cylinder #3. This hypothesis of failure is drawn from my experience doing countless head gasket jobs and 3.5 swaps in conjunction with my full code reverse engineering of the Legend’s engine computer (ECU). I am not an engineer nor do I have a full understanding of the thermodynamics of a cooling system. This document is based on facts that I have collected from years of working on Legends and my recent discoveries of how the EGR system is operated by the ECU.

This is *MY* hypothesis, although it’s not proven - and I’m not sure it COULD be proven - I think I have enough facts to back it up with a fair amount of credibility. To be fair, a fully scientific test would require studying 3 different Legends with factory new engines for a 10 year period of time – something that’s just not feasible. I have done my best to explain relatively complicated concepts in laymen’s terms, if you don’t understand something please inquire and I will do my best to further explain. I am open to debate, but I refuse to argue my hypothesis with someone that is disputing it without hard facts of their own.

Pattern Failure
Just about every Legend I have seen in my shop with a seeping head gasket has failed on the passenger's side rearmost cylinder - #3. The fire ring in the gasket herniates out of the cylinder into the coolant passage on the exhaust side of the head. We will discuss later in this document why the gasket blows like this all the time – pattern failure.

Difficult Diagnosis
What makes them difficult to diagnose is that the head gaskets don’t blow out per se, they “seep”. At the very early stages of failure, the seeping could be intermittent - sometimes even seeming to rectify and re-seal itself. What makes the diagnosis complicated is that the pattern failure of the fire ring acts as a check valve. Small amounts of toxic combustion gasses from the cylinder enter the cooling system on the exhaust stroke, but under vacuum on the intake stroke the fire ring is drawn back into the cylinder preventing coolant from being drawn in and burned. This explains why the stereotypical white smoke from burning coolant isn't present on a Legend with an SHG – making a difficult diagnosis to the uniformed mechanic. I have come across many cars that have had radiators, thermostats and water pumps replaced in the failed diagnosis of a seeping head gasket. Pressure tests, compression tests and block tester kits don’t always give accurate results. The only real way to tell is with a leak down test with a twist – a video of which is stickied in the 2nd generation main section “The Definitive Way to Tell if Your Head Gasket is Seeping

The Chain Reaction - What Happens with an SHG
When combustion gasses enter the system they displace coolant with air, causing problems with thermostat operation, long term damage to the coolant and corrosion to the aluminum block, head and other metal parts of the system. When the exhaust gasses enter the system under pressure, they displace the coolant, cause the radiator pressure cap to lift and coolant to exit the radiator into the reservoir which usually overflows. This loss of coolant causes large air pockets, and here is where the situation gets really hairy and the car starts to overheat – the thermostat only opens for hot coolant – it won’t open for hot air! At idle the air pocket gets trapped around the thermostat forcing it to say closed. The fan switch is in the lower radiator hose, which is now closed off from seeing the hot coolant from the stuck thermostat (this is why the lower radiator hose is cold), so the fans aren’t running even though the motor is overheating! This causes further pressure in the system and further loss of coolant into the reservoir. When you take off with the car from idle, the increased flow from the water pump pressure causes coolant to flow around the thermostat and it to open again, causing the temperature to drop (this is why the removal of the thermostat will greatly improve the situation and even allow you to drive the car for a time with an SHG). This chain reaction takes place over and over again which is what causes the temperature needle to fluctuate and the heat to come and go. Eventually, all of the coolant is forced out of the system and the engine will severely overheat.

The combustion gasses entering the cooling system are toxic and corrosive. As the gases are passing through the blown gasket, they further corrode and scorch the block and head in that area which allows even more gases to flow into the cooling system. The gases also turn the coolant acidic, causing corrosion to metal and weakening to rubber hoses and seals in the water pump. I have seen the aluminum cylinder liners inside of the motor completely corroded away in severe cases. A bloated, soft upper radiator hose is one of the first bad signs of coolant acidity and is why I recommend replacing all of the hoses when I do a head gasket repair. Other evidence of corrosive gases within the cooling system are dark, gassy smelling deposits underneath the radiator cap and on the overflow hose in the reservoir bottle. There is a very unique smell to the deposits and acidic coolant and I seem to have a nose for it – it sounds weird, but I can almost “smell” a BHG before I even open the hood!

How The Gaskets Blow
Most mechanics will tell you that head gaskets blow because of engine overheating. I don’t believe this is true with the C32. I believe just the opposite – that the gasket blows first which *THEN* causes overheating problems. I believe the gasket blows due to a combination of weak gasket design, the relatively high compression of 9.6:1 and detonation caused by a flawed (and clogged) EGR system and its associated timing maps. This is evidenced by the vast majority of Legends that come in without other cooling system problems - there was nothing that caused the blown gasket - because the gasket blew first! By far, the first symptoms of head gasket problems in a Legend are the forcing of coolant out of the reservoir without any other precursors. I have done many budget head gasket repairs in which only the head gaskets were replaced and EGR cleaned and the car returned to normal operation.

The C32’s pattern failure is herniation of the fire ring in the weakest (lateral) part of the gasket - cylinder #3. If you study any head gasket, you will notice that the holes drilled in it to pass coolant are smaller towards the front cylinders and much larger towards the rear. You will also notice that the holes are larger on the exhaust side of the head than the intake. Since the water pump is at the front of the motor, the varying size of holes drilled in the gasket cause the coolant to circulate evenly to the front and rear cylinders and direct the most coolant towards the “hot spots”. If all of the holes were the same size, coolant would never circulate back to the rearmost cylinders. Since the cylinder is the farthest from the water pump, it has the largest holes drilled into the gasket on the exhaust side. These holes are drilled directly next to the fire ring and there are 3 in total, one round hole in the middle and two larger oblong holes on either side of it. These large holes drilled in such close proximity to one another directly next to the fire ring significantly reduces the gaskets lateral rigidity in this area allowing the fire ring to herniate easily out of the cylinder – the “weak link in the chain”.


In this photo there are two gaskets laid on top of one another. The top gasket is blown while the lower gasket is a new fel-pro gasket with the fire ring painted green for visibilty. You can see how the steel fire ring has herniated out of the cylinder directly in correlation with the large holes drilled into the gasket. The damaged fire ring is still relatively round except in the area where it is herniated.



In this photo you can see clearly that the graphite portion of the gasket has been deformed and extruded from the fire ring blowing out of the cylinder. It is clear in this photo that the cause of the gasket failure is excessive cylinder pressures forcing the fire ring out of the cylinder.
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Old 04-03-2011, 01:01 PM   #2
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The Legend’s EGR System
It is necessary to have an understanding of how the EGR system works and what it’s function is as well as the oddities in the way that it is controlled by the Legend ECU in order to fully understand my hypothesis. EGR simply means Exhaust Gas Recirculation and it literally does just what it says – it recirculates a controlled amount of exhaust gas into the intake manifold. Its intent is to control NOX gas emissions from the tail pipe by lowering combustion chamber temperatures. You would think that hot exhaust gasses entering the intake manifold would cause cylinder temperatures to rise, but in fact the recirculated exhaust acts as an inert gas and cools them. Don’t get confused, its intent is *not* to re-burn unburnt fuel! The lower cylinder temperatures cause a reduction in NOX emissions and also lower the motors tendency to detonate. The EGR system does not operate at all times, it only operates at light throttle and at cruising RPMs. EGR operation at idle will cause a very rough idle and engine stalling and EGR on rapid acceleration will reduce the engines power output without any help to emissions.

The Legend’s EGR valve is vacuum operated and controlled by a pulse width modulated (PWM) solenoid valve. The engine computer controls this valve lift from a 16x20 table within the ECU that is referenced by RPM and engine load (MAP). This means that instead of the valve being either all the way open or all the way closed, the ECU switches the vacuum to the valve on and off at high speeds so that the valve lift can be controlled in percentage of lift. If vacuum is present 50% of the time, then the valve is 50% opened.

There is a valve lift sensor on the EGR valve that is connected to the ECU, its function is to verify to the ECU that the valve is actually opening as commanded. If the valve isn’t opening, is stuck open or is becoming lazy the engine computer sets a code 12 - EGR valve lift sensor. Once code 12 is set, the check engine light is lit and the EGR system is disabled until the problem is rectified. EGR compensation maps are not used, nor does the ECU command the EGR on anymore with the code set.

Legend EGR oddities
From my research in other car’s engine computer design I have found that the Legend’s handling of EGR flow is unique. This is most likely due to the relatively high compression (for the time) of 9.6:1 and Honda’s desire to make the engine run as efficiently as possible. I have the Legend’s engine computer code fully disassembled and reverse engineered, and have spent quite a bit of time analyzing the EGR control code. There are a total of 2 ignition maps and 3 fuel maps that the Legend uses for spark and fuel control. The presence of the 2nd fuel and ignition maps were a mystery for quite awhile until the EGR code was examined. I discovered that whenever the EGR is active the ECU instantly switches to these 2nd maps. A quick analyzation of these compensation maps shows that ignition timing is greatly increased and fuel is slightly reduced. These maps, which I’ve named EGR Compensation Maps, provide a high resolution way for the ECU to compensate for EGR gasses entering the manifold and even a way to gain power with the increase in ignition timing and lean mixture! The intent of the compensation maps is so that the engine still outputs high power and efficiency with the EGR active. If they did not exist the engine would have a noticeable decrease in power and efficiency whenever the EGR was active.


This photo shows the primary (1st) ignition timing map of a type II USDM motor


This photo shows the EGR compensation ignition timing map of a type II USDM motor. Note the increased values within the window in the middle of the map – these are the RPM and load ranges where the EGR is active.

Why the EGR clogs
The problem is that that the Legends EGR system has a tendency to clog, SOLID! There is a pipe that runs from the EGR valve to the lower portion of the intake manifold – a chamber of sorts. I believe the clogging is caused by the intake manifolds design. Excessive oil vapor from the PCV valve collects over a long period of time on the bottom of the intake manifold in an area below the runners in the head. The vacuum never really rises high enough to suck this oil into the cylinders and burn it. Instead, it eventually runs into the EGR chamber and pipe at the back of the manifold where it is carbonized by the high temperature of the EGR gases and forms a solid mass blocking the chamber and pipe. This is evidenced by the very oily nature of the deposits found when scraping out the EGR chamber; it just isn’t possible for that much unburnt oil to flow through the EGR system into the chamber. In an almost direct admission as a design flaw, the 3.5 RL’s lower intake is revised with a baffle in front of the opening of the EGR chamber. The 3.5’s also have a much lower tendency of EGR clogging.


This photo shows a clogged EGR chamber on the back of an intake manifold. Note the intake manifold is upside-down in this picture.


This photo shows the legend lower intake on the left and the 3.5's on the right. Note the baffle in front of the EGR chamber, the elimination of the passage on the left and the addition of the small hole in the center rib to pass EGR gasses from the right to the left side. Also note the oil buildup in the lower manifold, this oil buildup is actually much more because the manifold has to be turned upside-down to remove the lower plate.

How a Clogged EGR Leads to a BHG
Once the EGR is clogged there obviously isn’t any EGR flow into the intake manifold anymore. Yet, the ECU is still commanding the EGR valve open and is seeing it open successfully from the valve lift sensor - blissfully unaware that there isn’t any EGR gas actually flowing into the motor! Remember how the EGR compensation maps work above? The ECU thinks the EGR system is operating correctly, even though it’s clogged and is using the EGR compensation maps, but there’s no EGR flow! The ignition timing is greatly increased – almost 10 degrees in some cells – and fuel is decreased by 5-10% causing a lean mixture, yet there is no EGR flow to be compensated for! This is the perfect storm for severe detonation to occur. The Legend’s flaw is it’s lack of EGR flow detection. Newer engine management systems check for slight changes in the manifold vacuum to detect whether the EGR gasses actually made it into the intake manifold. The Legend has no such verification method and that is the root cause of the problem.

I have presented this theory in bits and pieces in a prior thread and it was argued that the knock sensors should see this detonation and retard timing accordingly, and oxygen sensors should see the lean mixture and adjust the fuel accordingly. I don’t believe that the knock sensors or the circuitry can react fast enough and/or pull enough timing in order to rectify the situation due to detonation happening near instantaneously. I also believe that the damage occurs to the gasket over a long period of time, steadily weakening the gasket until it blows. The knock sensors can only pull about 2-3 degrees of timing at max and the EGR compensation maps increase timing almost 10 degrees in some areas. On a hot day with the A/C on, I can audibly hear a Legend with a clogged EGR detonate. Note that even though detonation may not be audible it could still be occurring.

To further elaborate on why the knock and oxygen sensors aren't capable of compensating, I will give you an example. First, you need to understand that the knock sensors are completely ignored by the ECU after 4000 RPM (the knock sensors are basically microphones and the engine is making too much noise for them to work correctly at high RPM) and the oxygen sensors are ignored at high throttle positions. So, imagine a scenario where you are traveling on the highway with the EGR clogged. The ECU doesn’t know its clogged and is happily operating in it's EGR "loop" - sending the PWM vacuum signal to the EGR valve, seeing the valve lifting, and using the EGR compensation maps. The lack of EGR *AND* the increased timing from the compensation maps are causing super high cylinder temperatures, but not high enough to damage anything immediately. Now, you come up to a slow moving car and decide to floor the gas to pass it, the cylinder temperatures are already extremely high from the cruise situation and now you are putting a high load on the engine. The latent heat left over from the cruise situation above combined with the instant high load placed on the engine causes a brief moment of severe detonation which causes an almost instant blow out of the gasket. I believe this is precisely the scenario in which the fire ring is blown out of the C32's head gasket.

There is no question in my mind that continued and/or severe detonation is the only cause of a fire ring blowout in any engine. The combination of long term detonation from a clogged EGR system, as well as a gasket that is laterally weakened from having too much material removed are what causes the fire ring to blow out in that area. On the other hand, it could be that the head gasket was purposely engineered this way to act as a “fuse” – that way the gasket gives out before the piston rings and lands do. I have carefully measured block and head warpage in the area around the blown gasket and have never found any warpage or other inconsistencies on any of my customers’ cars. This indicates to me that the gasket didn't blow due to overheating, but from detonation.

Preventing BHGs
The resolution to this problem is simple - clean, test and inspect the EGR system every 60,000 miles as recommended by Acura! I recommend upgrading the lower manifold plate with the revised 3.5 RL plate that has the baffle in it. This baffle helps to prevent the oil from getting into the EGR chamber. I would also recommend replacement of the PCV valve yearly or the installation of a PCV oil catch can to reduce the amount of oil that enters the intake. Unfortunately, correctly cleaning the EGR system requires removing the intake manifold. Solvents and other methods wont remove the solid mass that is carbonized in the pipe and in the EGR chamber. You could remove the pipe and the cover of the EGR chamber and dig out the chamber with the manifold on the car, however there is very limited access to the chamber. Also, you risk pushing the deposits into the manifold where they'll be sucked up by the motor. Also without removing and splitting apart the manifold, you can’t remove the buildup of oil in the lower manifold – causing it to clog again!
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Old 04-03-2011, 01:02 PM   #3
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Testing the Operation of the EGR Valve
The simplest way to test that the EGR is operating properly is to apply vacuum directly to the diaphragm while the engine is running. You can use a handheld vacuum pump to do this, or you can just "jump" a vacuum line directly to the valve. The smaller vacuum line running to the fuel canister is convenient. With the valve open and the EGR ports clear, the engine should run VERY roughly and/or stall. If it does not, then the EGR is most likely clogged.
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Old 04-03-2011, 01:03 PM   #4
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Old 04-03-2011, 01:52 PM   #5
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That some good ish Matt..
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Old 04-03-2011, 02:30 PM   #6
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Great info for all!!!

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Old 04-03-2011, 03:07 PM   #7
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Great info Matt, on my 95 the Hg's were indeed blown on the #6 and slighty on the #3. Lol think ima clean my egr on my 93 and we shall see how she dose... thanks man
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Old 04-03-2011, 03:12 PM   #8
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Is there a type of oil or gas that would least likely cause it to clog up? After cleaning it would regular fuel injection or throttle body cleaner help it to stay clean?
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Old 04-03-2011, 03:49 PM   #9
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Is there a type of oil or gas that would least likely cause it to clog up? After cleaning it would regular fuel injection or throttle body cleaner help it to stay clean?
It probably has more to do with age of the motor and normal blow by from wear and tear than with any specific type of oil. I think the ultimate prevention would be a PCV oil catch can. Fuel additives wont help at all, the fuel injectors are directly above the valves and very little if any fuel vapor gets into the manifold. I don't know what you would use to get the oil out of the manifold, but I would suggest GM foaming top engine cleaner.

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Old 04-03-2011, 04:02 PM   #10
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Not really wanting to take off my I.M. is this something I can ask the stealership to do?
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