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Hi folks. I'm seeing some strange behavior with my FD and I feel like I've covered all the basic troubleshooting procedures, so coming here in the hopes that you all can point out something simple that I've missed.
Overall the car runs and drives beautifully. It starts and comes up to temp properly, and cruises on the highway with zero issues, temp needle solidly in that "just-below-center" position. Heater blows warm, etc. The issue is when I come back home, or into town -- after about sixty seconds of letting the car idle, the following things happen in quick succession:
temp gauge starts to climb rapidly
low coolant buzzer sounds
maybe a quart of fluid spills from the front passenger side, presumably from the overflow reservoir
I've always caught this quickly and the car has never actually overheated on me. And the highway performance seems indefinite -- I went on a 30-mile highway cruise last night and everything was great, and also seemed okay on surface streets on the way home, but the problem happened in the span of time it took my garage door to open and me to move a couple things around in the garage to make room for parking.
All the parts that came off the car looked good, and since the problem persists, were probably fine to begin with. The car otherwise does not leak coolant, aka the garage floor is bone dry if I just pull the car in and immediately shut it down.
Has this happened to anyone else? Am I describing a known issue?
What radiator do you have and how old is it? What radiator hoses are you using? Still have the ast? If so, do you have the pressure cap on the ast or the filler neck? Do you know if your fans are spinning in the correct direction? Pulling air instead of blowing it?
Uh oh. I'm looking at the service history and I see that the previous owner installed a Mishimoto radiator (yay) but also performed an AST delete, maybe that is what's going on?
The pressure cap is on the filler neck and looks to have an aluminum filler neck like this one (mine is black).
The ast delete is fine. You don't need it and it does nothing.
Which mishimoto radiator is it? The slim one thats like 1/4" thick is absolute trash. They "work" until they don't. Start with that. High chance that's your whole problem.
High and low speed thermos, the relays and wiring to and from, might be worth inspection.....if it's fine while moving.
To clarify - he means are the fans coming on? Your symptoms fit perfectly with having them not coming on as they need to when there is little or no car speed to force air through the radiator. The engine is hot from running at speed, and no cooling air flow means the coolant will boil very quickly.
A marginal or partly clogged radiator will exacerbate that issue.
To clarify - he means are the fans coming on? Your symptoms fit perfectly with having them not coming on as they need to when there is little or no car speed to force air through the radiator. The engine is hot from running at speed, and no cooling air flow means the coolant will boil very quickly.
A marginal or partly clogged radiator will exacerbate that issue.
Yes, sorry. Failed to mention that. The fans come on quite angrily, and stay on even after I've shut down (when too hot). They sound like they go through two stages -- high/low/off. I do see what you mean about what could happen if the coolant is hot and there's not much flow. However, the coolant buzzer sounds, which means afaik that there's NO coolant at the sensor. So maybe the overflow line isn't backfeeding from the reservoir tank properly at that point?
Originally Posted by FDAUTO
The ast delete is fine. You don't need it and it does nothing.
Which mishimoto radiator is it? The slim one thats like 1/4" thick is absolute trash. They "work" until they don't. Start with that. High chance that's your whole problem.
idk, seems like a good unit as far as core thickness.
...However, the coolant buzzer sounds, which means afaik that there's NO coolant at the sensor. So maybe the overflow line isn't backfeeding from the reservoir tank properly at that point...
Low coolant could be from several causes.
1. coolant not topped up or as you said, it's not completely returning from the OF bottle when the engine cooled from the last time it got hot
2. coolant boiling so much that there's just steam at the sensor, which could mean the engine was too hot before you stopped, or it could be that the coolant boiling point is too low (not enough AF, etc.)
3. faulty pressure-release (spring) cap not holding at least 15 psi
4. combustion gasses getting past the coolant seals, forcing coolant out, and making the coolant level so low that when the revs drop, there is no coolant circulating or at the sensor
My line of reasoning with this car is Mazda's many engineers are smarter than me, and they were trying to save weight everywhere when they designed and built it. I'm not deleting anything., just upgrading.
You can get an aftermarket AST that is built well and makes a high point for the air in the coolant to bubble up to. Burp it once in awhile and the babbling brook behind the dash is gone. Unless you're running a V-mount or some huge SMIC, there's no reason to throw it away, in my opinion..
I don't own a shop or professionally work on the FD though, and definitely don't mean any offense to FDAUTO. . This is just my experience over the last 20 years of owning one and keeping it on the road..
My line of reasoning with this car is Mazda's many engineers are smarter than me, and they were trying to save weight everywhere when they designed and built it. I'm not deleting anything., just upgrading.
You can get an aftermarket AST that is built well and makes a high point for the air in the coolant to bubble up to. Burp it once in awhile and the babbling brook behind the dash is gone. Unless you're running a V-mount or some huge SMIC, there's no reason to throw it away, in my opinion..
I don't own a shop or professionally work on the FD though, and definitely don't mean any offense to FDAUTO. . This is just my experience over the last 20 years of owning one and keeping it on the road..
I have an aftermarket AST for similar reasons, but there are a lot of experienced FD owners who deleted it with no resulting issues.
1. coolant not topped up or as you said, it's not completely returning from the OF bottle when the engine cooled from the last time it got hot
2. coolant boiling so much that there's just steam at the sensor, which could mean the engine was too hot before you stopped, or it could be that the coolant boiling point is too low (not enough AF, etc.)
3. faulty pressure-release (spring) cap not holding at least 15 psi
4. combustion gasses getting past the coolant seals, forcing coolant out, and making the coolant level so low that when the revs drop, there is no coolant circulating or at the sensor
Those are what comes to my mind.
@DaveW@Natey I hear you and agree with you both. Really hoping it's not #4, although I ran an exhaust test and didn't see any of the liquid changing color. As for #3, I did switch the spring cap without success, OEM .9bar.
Should the line to the OF bottle be full of coolant at all times? Obviously when you crack the cap open, anything there will drain back...just trying to understand what to check next. I'll probably pull that line and inspect it, the port in the filler neck is absolutely tiny, like BB-size.
@DaveW@Natey I hear you and agree with you both. Really hoping it's not #4, although I ran an exhaust test and didn't see any of the liquid changing color. As for #3, I did switch the spring cap without success, OEM .9bar.
Should the line to the OF bottle be full of coolant at all times? Obviously when you crack the cap open, anything there will drain back...just trying to understand what to check next. I'll probably pull that line and inspect it, the port in the filler neck is absolutely tiny, like BB-size.
Checking that OF bottle line is a good idea. The OE setup has a joint in its center that could cause issues if it's not connected.
Do you have the proper AF mixture (50-50 green ethylene glycol and distilled water)?
The OF bottle is full when it is at the full mark on the Nylon dipstick, IIRC, about 1/2 total capacity. If it's too full it'll just run out the vent on its side.
A 1/8 to 3/16" pressure-cap OF port should not be an issue if everything is functioning correctly.
Is your thermostat working properly? If it's malfunctioning it can cause issues.
Do you still have the original temperature gauge setup? That reads normal until it's almost overheated.
Yep, correct mix of 50/50 using distilled water. Brand new pressure cap and thermostat too, with the bleeder pin at the recommended 12 o'clock position.
OE temp gauge, which I consider an idiot light -- if it moves even a needle-width above the "normal" position, it's overheating (or about to).
You're measuring the end tank depth there in that picture.
No experience with Mishimoto, but FDAUTO would be looking at the measurement perpendicular to what you've done.
The overflow line should be full of fluid at all times, otherwise the siphon doesn't work. The tank should have room for the coolant to expand, so make sure the "full" measurement is done at the correct end of the temperature spectrum.
I think there's been several defective new thermostats reported in the past on here, testing before install can be worthwhile.
A bit late to this party but while Mishimoto wouldn’t be my first choice I really doubt it’s a radiator issue or as a result of the AST being deleted. Maybe I missed it but is the belly pan (under tray) in place?
And I second what DaveW said about the line from the filler neck to the overflow. It’s thin walled and kinks easily. That can allow flow out to the overflow reservoir when it’s under pressure, but not back to the engine under the relatively light vacuum that should occur as the engine cools.
Was the new t-stat was installed with that jiggle-on at 12:00? If not that can cause air to be trapped.
IMO one of the first (reliability) mods that should be done to the car is to add an aftermarket temp gauge or linearize the stock gauge. The low-coolant alarm is good, but otherwise it’s pretty worthless as it is. Tons of threads available…many in the FAQs.
I.e., OP needs to measure the thickness of the finned core.
@DaveW@billyboy you're right, derp. I'll give it a look at some point.
Originally Posted by Sgtblue
A bit late to this party but while Mishimoto wouldn’t be my first choice I really doubt it’s a radiator issue or as a result of the AST being deleted. Maybe I missed it but is the belly pan (under tray) in place?
And I second what DaveW said about the line from the filler neck to the overflow. It’s thin walled and kinks easily. That can allow flow out to the overflow reservoir when it’s under pressure, but not back to the engine under the relatively light vacuum that should occur as the engine cools.
Was the new t-stat was installed with that jiggle-on at 12:00? If not that can cause air to be trapped.
IMO one of the first (reliability) mods that should be done to the car is to add an aftermarket temp gauge or linearize the stock gauge. The low-coolant alarm is good, but otherwise it’s pretty worthless as it is. Tons of threads available…many in the FAQs.
Yep, 12-o'clock position. And the original thermostat was correctly installed when I pulled it.
I'll check both filler neck return hoses, as that would be the easiest and most reassuring fix if one of them was kinked or blocked.
What's this about the belly pan? It was installed, but it's currently off for inspection.
That must be a super old mishimoto or not a mishimoto. Their radiators have curved end tanks, not square.... and yes the measurement of the thickness would be the width of the tank, not the height.
Speaking from experience, a friend of mine had a mishimoto installed in his FD for a quite a while when he lived in North Carolina. He moved here to Florida and as soon as summer hit, his car could not maintain temps at all. We changed the radiator and problem solved.
Try pulling the hoses off the radiator and running water through it and see how it flows. Check the temp of the upper and lower hose when it's hot to see if one is significantly cooler than the other. Check that you have oil in the car (sounds crazy I know). Check that your oil cooler(s) are getting hot to verify there is flow through them. Check that you don't have a radiator cap that is new and faulty. We have encountered some bnib parts that were (broken)nib from the parts store. Pull the belt off and check the operation of the water pump.
The pressure test you did rules a lot out so your problem has to be an issue with a component not doing its job or is incapable of doing its job from something else.
...What's this about the belly pan? It was installed, but it's currently off for inspection.
If the BP is not there, hot air will recirculate instead of the fans or forward speed pulling cool air thru the rad. This can and often does cause overheating.
That must be a super old mishimoto or not a mishimoto. Their radiators have curved end tanks, not square.... and yes the measurement of the thickness would be the width of the tank, not the height.
Speaking from experience, a friend of mine had a mishimoto installed in his FD for a quite a while when he lived in North Carolina. He moved here to Florida and as soon as summer hit, his car could not maintain temps at all. We changed the radiator and problem solved.
Try pulling the hoses off the radiator and running water through it and see how it flows. Check the temp of the upper and lower hose when it's hot to see if one is significantly cooler than the other. Check that you have oil in the car (sounds crazy I know). Check that your oil cooler(s) are getting hot to verify there is flow through them. Check that you don't have a radiator cap that is new and faulty. We have encountered some bnib parts that were (broken)nib from the parts store. Pull the belt off and check the operation of the water pump.
The pressure test you did rules a lot out so your problem has to be an issue with a component not doing its job or is incapable of doing its job from something else.
Good suggestions, thanks. The water pump is brand new, just installed it. The previous water pump looked almost new as well, no wobbles, leaks or funny sounds.
Originally Posted by DaveW
If the BP is not there, hot air will recirculate instead of the fans or forward speed pulling cool air thru the rad. This can and often does cause overheating.
Interesting, thanks. I'll reinstall it once I check the overflow tank/lines.
Which mishimoto radiator is it? The slim one thats like 1/4" thick is absolute trash. They "work" until they don't. Start with that. High chance that's your whole problem.
jeez lol, you weren't kidding. this one is pretty thin. how thick is the OEM unit anyway?
no idea but i don't think its far from that. no idea why the oem works and the mishimoto just kind of doesn't. advise installing a koyo and reassessing the issue. n flo or regular is fine. the price difference isn't all that great so if you have it to spend, i would just buy the n flo
The OEM radiator is sufficient for the stock or lightly modded. I think it’s strong point is fin density. It’s just the plastic end-tanks that are problematic and why we change, eventually, to aluminum. I’ve never measured core dimensions but my recollection is that the stocker isn’t any thicker than what you pictured.
FWiW….i think the two big players back in the day were Fluidyne and Koyo N flow. IIRC Fluidyne was said to be about 50% bigger volume and Koyo 100%. But the former was a bit easier for fitment. I’ve had my Fluidyne for maybe 20 years with absolutely no complaints.
Put a belly pan on the car and make sure you have a 16 psi cap on the system. If you have both of those and still have an issue with no apparent leak, I'd throw in the towel and count on a 30+ year old soft seal failure.
I had similar symptoms when my failed coolant seal became noticeable, as it gets worse you may find idle quality deteriorates once hot it it's intake side and it regularly consumed coolant while also filling the bottle.
02-24-24, 11:20 AM
