teletekman

I have a pretty stubborn issue I am fighting with my cooling system on my 96’ Accord LX, 2.2L (non v-tech) with 390,000 miles and manual trans which I’ll try to explain in detail the best I can…

It seems that there is a bit of excessive pressure being built up in the cooling system which occasionally will get passed the radiator cap and vent into the overflow tank after a fairly long drive. Ambient temperature outside really doesn’t make any difference as I have noticed this on hot days and cooler nights. At first I figured it was early signs of my head gasket going due to it being the original with that amount of miles but I’m not so sure that is the case.Pretty much my entire cooling system has been replaced since most of my parts had lifetime warranty anyway. The thermostat (OEM), both fan temp switches (aftermarket), Denso Radiator (aftermarket), Lower Radiator Hose (aftermarket), Water Pipe O-ring behind thermostat housing (aftermarket), Both Cooling Fan motors (aftermarket), and Water Pump (aftermarket).

I have repeatedly tried to vacuum fill my system and have pressure tested the system to 16lbs. with no pressure loss and no leaks present. I have tried the OEM method of filling and bleeding by opening the bleed screw at the thermo housing till it ran steady. So far nothing has completely eliminated my excessive pressure buildup issue. I have tried dry compression testing numerous times and am seeing between 180 and 190psi cranking the engine at the same rate every time. I have also tried a leak down test on each cylinder and everything so far has come back negative. At this point I decided to change out the water pump again with an OEM unit as well as new OEM timing belt, balancer shaft belt and both OEM tensioners as I noticed the balance shaft tensioner and water pump bearings on the aftermarket units were getting pretty stiff. At first I thought I might have found the problem with the pump possibly being bad. So after putting everything back together I then filled it using a spill free funnel with the front end jacked up and opening the bleed screw. Temperature control has been set to hot every time to allow the heater core to burp any air bubbles it may have trapped. I can have it idle all day long and it will turn the fans on and off with no problem while averaging good temperature. It appeared at that point all the air had been removed. I decided to rev the engine a few times to push any remaining air out and noticed every time I revved it up there was a very small amount of air bubbles coming up to the funnel. Kind of what you would see coming out of an air stone on an aquarium pump. Again this only appears when you rev the engine up around 2500rpm or higher. At idle everything looks good.

I also did a block test numerous times thinking it might be a small amount of exhaust gases coming through but again these tests came back negative as well. So at this point I’m not leaning towards the head gasket since I can run it at idle all day and everything will be fine. I only really notice it when I’m driving freeway speeds at around 2500+ rpm which after driving a while I’ll pull over and hear the hissing at the cap. Then there are times under similar driving conditions it will be fine. So to add to my confusion this is not a consistent issue. I also changed the radiator cap with an OEM one but no difference. My concern at this point lies around the small air stone like bubbles forming after revving the engine looking down at the funnel. I thought the water pump would have fixed it since I think I have literately tried everything else I can think of. I don’t see it overheating either since my temps seen at my scanner are 185 to 188F driving on the freeway and a normal rise till the fans come and then cool back down while sitting idle.

Sorry for the extremely length posting but I wanted to cover as much as I could think of before asking for help so can anyone shed some light on something I could be missing?

Thanks in advance…

holmesnmanny

Are you only filling the reservoir up halfway ? If you are filling it up all the way it's going to overfill when the system warms up to temperature as the system expands a bit when it heats up to regular operating temp.

teletekman

Are you talking about the overflow reservoir? If so then yes, it's only filled half way. However the excessive pressure buildup I am seeing is coming into the radiator and then vents to the reservoir as it should....I just can't understand where that excessive pressure is coming from.

I have heard of exhaust gasses getting into the cooling system which I have seen build up even at idle, but is there any way a head gasket can blow at or near the intake valves causing intake air to escape into the cooling system? If so then this might explain why my block tests are coming back negative but air may still be getting in. Although I would expect this to be the case at idle also, not just the higher RPM's...

holmesnmanny

I'm not quite understanding exactly what the issue is. The system is going to pressurize as the car warms up.

teletekman

Ok so I took it out on another run for about an hour trying to figure out a pattern on how to get pressure to build up to where you can hear it hissing at the radiator cap. I took it on a known stretch of highway about 8 miles each way. This was at about 92F outside. First run was around 55 to 60mph @ 2000rpm. I pulled it over at various times to see if any pressure had built up. The only time I heard any sound of pressure was just after I got it into 5th gear I pulled over and checked it. The moment I put my ear to it radiator neck it stopped. I took it on a 6 mile non stop trip at the above speeds, pulled over and it was quiet. Turned around and took it back but this time cranked it up to around 75mph @ 3000rpm. After getting it into 5th gear again I pulled over and heard it hiss for a second and then stop. Drove it again on a 6 mile stretch non stop at around 3000rpm and when I pulled over it was hissing pretty good for at least 20 seconds or so and then it quieted down. Brought it back to the house and no hissing when I checked it which I was driving at the slower speeds.

So it appears to be more evident at the higher RPM's which is what I noticed with the funnel on trying to burp the system. If you bump it a few times to about 3000rpm it pushes the small bubbles I described earlier for a few seconds and then stops. If I hold it at 3000rpm for a few seconds and let off, about 5 seconds later it will eventually push a few bubbles to the top. If I bump it around 2000rpm it looks pretty quiet, although you can see the ripple in the coolant which tells me it's at least moving like it should.

So at this point is there any way I can rule out the head gasket being an issue? If there is a leak, even if its a very small leak wouldn't I see bubble up even at idle? Obviously its pushing more pressure out at the higher RPM's, but it seems to be a bigger issue if you bump it a few times rather then hold it there for a little bit. So this is what I am figuring the pressure is coming from. Again I thought it was the water pump since that was pretty much the last thing I didn't replace but even with an OEM pump I still see similar results. Where else could air be pressuring the coolant system from if not the head? Andif it was the head gasket combustion leak or even cracked head for that matter, regardless of the small combustion leak, if that's what it is, wouldn't my block tester see it?

teletekman

Right, I get that. It does build up pressure like it should. The fans come on at 203F since I have a 95C fan switch installed and the stat opens at around 180F. I get excellent heat coming from the vents sitting at idle so having said all that you would think my system is running fine. The problem I am having is at the higher RPM's. I explained a little bit more in my previous post but bottom line there is extra pressure being built up in the cooling system at the higher RPM's which you can hear at the cap and then vent into the reservoir. It does not appear to be combustion gases since there is no smell and my block tester does register it as such. So I am trying to figure out how and where that air is coming from to eventually cause enough extra pressure to do what its doing or what its not supposed to be doing.

holmesnmanny

The cap is designed to vent any pressure at over 16lbs. I cant say I ever tried to watch the system so closely but it sounds as though the system is operating normal.

The faster the engine goes the more pressure the water pump is going to pressurize the system. The cap is designed to vent to keep pressure at that level.

teletekman

Ok so I ran another leak down test but this time I left it on each cylinder for a couple minutes per. All four cylinders are showing anywhere from 15 to 20% leakage as they did the first time I tested it which is good. However I did notice this time that after about 30 to 45 seconds on cylinder 4, very small bubbles were forming in the radiator. I never noticed it before since I only left it on for about 10 to 15 seconds each time.

So now it pretty much confirms that I have either a blown head gasket, warped head or at worse a cracked block (hopefully its not that). It appears the leak is extremely small though given the amount of time it took to be seen in the cooling system and despite the small amount of air but air none the less. I hate to even think this but since this leak is showing to be so small do I even dare and try those mechanic in a bottle fixes or do I bite my lip and just do it right and be done with it? Despite the high mileage which has been 90% freeway I wasn't planning on getting rid of it right now but didn't know if my previous thought of a fix would work or if I would eventually be asking for trouble down the road...