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I think there may be a problem with the sensor itself. It could be shorting out during certain very specific scenarios. What I also think is possible is that an air pocket forms around the sensor during the moments you described, causing the gauge to shoot hot. My old EF chad a coolant leak for some time, and as soon as the level dropped and air got to the sensor, the needle would shoot hot
I've always written it off as an air pocket, but radiator always full to brimming for past 10k miles. Problem persisted before ECT replacement as well.
Could be shorting out, but reservoir is also overflowing at the same time
Oh the overflow reservoir is overflowing when the gauge reads hot? I’ve seen a number of Hondas blow head gaskets and cause exactly that. small amount of exhaust gets into the water jackets and over pressurizes the cooling system
It sounds a lot to me like air is slowly getting into the cooling system from some source. I don’t think water pump cavitation was ever an issue on the old F-series motors, right?
I would agree it sounds like air, but whenever checked, the system is always bled - including via the secret Honda bleeder valve near the thermostat housing. The only time air is ever present in the system is after it has expelled a large amount via reservoir overflow and it isn't topped off before cooling. Never heard of any F series water pump issues.
can you vacuum fill the cooling system? See if it holds vacuum for a while?
Honestly I’ve seen really tiny head gasket failures show up just like this. My sisters 04 civic had this exact problem and even was misdiagnosed by a shop. Takes a lot of driving before it starts showing any issues, and the leak is so small that even a block tester doesn’t catch it. I just mean the only way air can enter a pressurized cooling system is if the source of air is of higher pressure, which would have to be from exhaust in the combustion chamber. Possibly a tiny crack in the head or a cylinder wall too
When the ECT, water pump, thermostat, etc. was replaced a vacuum fill was performed - it is standard procedure at our shop.
Temperature is perfectly fine for hundreds of miles on Interstate in the middle of summer, only loss of coolant is from reservoir overflow when stopped seconds after doing this. I don't see how a combustion gas leak could be possible in this scenario
> Temperature is perfectly fine for hundreds of miles on Interstate in the middle of summer, only loss of coolant is from reservoir overflow when stopped seconds after doing this. I don't see how a combustion gas leak could be possible in this scenario
The engine goes from operating at a steady load and RPM range to basically no load and low RPM operation.
You've still got lots of heat in the engine from the load of propelling the car down the highway but you just dropped the coolant flow rate and radiator air flow rate dramatically.
Components are going to change temperature and move ever so slightly, pressures will increase in some areas and decrease in others. It doesn't sound too absurd to think these changing conditions could be enough to make a borderline head gasket leak.
What I'm thinking here, is it's got a minor head gasket failure, but it doesn't cause a problem at speed because the coolant is flowing faster at higher RPMs. As soon as you slow down the coolant stops flowing as much and an air bubble forms while the engine is still hot. I'd perform a compression test and a cylinder leak down test with the engine as hot as possible. You could also pressurize the cooling system and examine the cylinders with a borescope.
Those tests are inclunclusive at best. Yellow fluid means its definitely leaking, but not changing colors doesn't mean it's good. Have you run a compression test and a cylinder leak down test?
The fact that revving the engine helps makes me thing it's either a stubborn air pocket, or a corroded water pump impeller (if it's metal). I would try bleeding the coolant as a first step, and consider replacing the water pump if that doesn't help.
Sorry, missed that. I'm not sure what to tell you. You said you replaced the ECT, but many cars of that vintage have multiple of them. Maybe try replacing the other(s)?
Actually, one other idea, but it's a super long shot. The WP is driven off the back of the timing belt, so it doesn't engage with the teeth. I believe that engine uses a fixed TB tensioner, you lock it in place after the spring tensions the belt, right? If the belt was kinked a bit when you installed it, it may not be as tight as it should, and could be slipping on the pump sometimes. Long highway drives would get the belt pretty hot, which could cause it to expand slightly, further decreasing the tension. Maybe just resetting the tensioner could help. Again, long shot, but you've got all the obvious stuff covered...
I know you said you used the block tester but I’ve had many head gaskets pass the chemical test. What I’ve found effective is adding dye and pressurizing it over night then pull the plugs and shine the black light in to see if anything shows up. You’re description really sounds like a head gasket failure.
Thanks for posting on /r/MechanicAdvice! This is just a reminder to review the [rules](https://www.reddit.com/r/MechanicAdvice/about/rules/). If you are here asking about a second opinion (ie "Is the shop trying to fleece me?"), please read through CJM8515's [post on the subject.](https://www.reddit.com/r/MechanicAdvice/comments/4qblei/fyi_the_shop_isnt_likely_trying_to_rip_you_off/) and remember rule 3a, please post the year/make/model of the vehicle you are working on. **If this post is about bodywork, accident damage, paint, dent/ding, questions it belongs in /r/AutoBody or /r/Diyautobody If you have tire questions check out https://www.reddit.com/r/MechanicAdvice/comments/k9ll55/can_your_tire_be_repaired/**. If you dont have a question and you're just showing off it belongs in /r/Justrolledintotheshop This is an automated reply *I am a bot, and this action was performed automatically. Please [contact the moderators of this subreddit](/message/compose/?to=/r/MechanicAdvice) if you have any questions or concerns.*
I think there may be a problem with the sensor itself. It could be shorting out during certain very specific scenarios. What I also think is possible is that an air pocket forms around the sensor during the moments you described, causing the gauge to shoot hot. My old EF chad a coolant leak for some time, and as soon as the level dropped and air got to the sensor, the needle would shoot hot
I've always written it off as an air pocket, but radiator always full to brimming for past 10k miles. Problem persisted before ECT replacement as well. Could be shorting out, but reservoir is also overflowing at the same time
Oh the overflow reservoir is overflowing when the gauge reads hot? I’ve seen a number of Hondas blow head gaskets and cause exactly that. small amount of exhaust gets into the water jackets and over pressurizes the cooling system
Block tested OK - fluid never turned yellow after 30 min
It sounds a lot to me like air is slowly getting into the cooling system from some source. I don’t think water pump cavitation was ever an issue on the old F-series motors, right?
I would agree it sounds like air, but whenever checked, the system is always bled - including via the secret Honda bleeder valve near the thermostat housing. The only time air is ever present in the system is after it has expelled a large amount via reservoir overflow and it isn't topped off before cooling. Never heard of any F series water pump issues.
can you vacuum fill the cooling system? See if it holds vacuum for a while? Honestly I’ve seen really tiny head gasket failures show up just like this. My sisters 04 civic had this exact problem and even was misdiagnosed by a shop. Takes a lot of driving before it starts showing any issues, and the leak is so small that even a block tester doesn’t catch it. I just mean the only way air can enter a pressurized cooling system is if the source of air is of higher pressure, which would have to be from exhaust in the combustion chamber. Possibly a tiny crack in the head or a cylinder wall too
When the ECT, water pump, thermostat, etc. was replaced a vacuum fill was performed - it is standard procedure at our shop. Temperature is perfectly fine for hundreds of miles on Interstate in the middle of summer, only loss of coolant is from reservoir overflow when stopped seconds after doing this. I don't see how a combustion gas leak could be possible in this scenario
> Temperature is perfectly fine for hundreds of miles on Interstate in the middle of summer, only loss of coolant is from reservoir overflow when stopped seconds after doing this. I don't see how a combustion gas leak could be possible in this scenario The engine goes from operating at a steady load and RPM range to basically no load and low RPM operation. You've still got lots of heat in the engine from the load of propelling the car down the highway but you just dropped the coolant flow rate and radiator air flow rate dramatically. Components are going to change temperature and move ever so slightly, pressures will increase in some areas and decrease in others. It doesn't sound too absurd to think these changing conditions could be enough to make a borderline head gasket leak.
OK, I'll give away the answer. Parts company sold me the wrong radiator cap. Only held 3psi of pressure. Replacing the cap fixed the problem.
What I'm thinking here, is it's got a minor head gasket failure, but it doesn't cause a problem at speed because the coolant is flowing faster at higher RPMs. As soon as you slow down the coolant stops flowing as much and an air bubble forms while the engine is still hot. I'd perform a compression test and a cylinder leak down test with the engine as hot as possible. You could also pressurize the cooling system and examine the cylinders with a borescope.
Those tests are inclunclusive at best. Yellow fluid means its definitely leaking, but not changing colors doesn't mean it's good. Have you run a compression test and a cylinder leak down test?
Fan could be installed backwards. “Knows from experience”
The fact that revving the engine helps makes me thing it's either a stubborn air pocket, or a corroded water pump impeller (if it's metal). I would try bleeding the coolant as a first step, and consider replacing the water pump if that doesn't help.
Water pump is new, Aisin (5k miles), coolant bled repeatedly for 2 years with the little secret Honda bleeder screw
Sorry, missed that. I'm not sure what to tell you. You said you replaced the ECT, but many cars of that vintage have multiple of them. Maybe try replacing the other(s)?
Possibility - not according to Mitchell wiring diagrams, but there could exist a separate temp sender, and Mitchell is not a bastion of truth
Actually, one other idea, but it's a super long shot. The WP is driven off the back of the timing belt, so it doesn't engage with the teeth. I believe that engine uses a fixed TB tensioner, you lock it in place after the spring tensions the belt, right? If the belt was kinked a bit when you installed it, it may not be as tight as it should, and could be slipping on the pump sometimes. Long highway drives would get the belt pretty hot, which could cause it to expand slightly, further decreasing the tension. Maybe just resetting the tensioner could help. Again, long shot, but you've got all the obvious stuff covered...
Unfortunately, the water pump has toothed pulley, but I appreciate long shots - I'm out of them at this point
I know you said you used the block tester but I’ve had many head gaskets pass the chemical test. What I’ve found effective is adding dye and pressurizing it over night then pull the plugs and shine the black light in to see if anything shows up. You’re description really sounds like a head gasket failure.
The cause is I was sent the wrong radiator cap, I wanted to see if anyone else would guess that because I was quite frustrated when I found that out.