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  1. #1
    FEP Power Member 85stanggt's Avatar
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    Default Coolant Overflow Tank filling and overflowing

    Hey guys. After getting my intake gaskets and heater core done, I've been driving the car a bit more. I took it on a 1.5 hr round trip highway drive the other day and it ran great. Everything was fine.

    Then I went out the other night when it was cooler out and when I got home, I smelled coolant. Popped the hood and the overflow tank was 3/4 filled and spilling out the top on turns. It also didn't seem like the radiator was sucking all the coolant back in once the engine cooled down.

    Decided to take it to work this morning. It's pretty cold here ~40 degrees this morning. My drive is like 6 miles to work. I notice halfway there that I have like zero heat. Then I look at the temp gauge and it is getting up to H. I've never seen it that high. I put it in neutral and turned the engine off as I coasted down from 60. I watch as the temp gauge starts dropping immediately. Put the ignition in run and pop it into 4th to start the engine up again, and I have heat!

    Pull into work and see steam. Fearing the worst, I pop the hood and the overflow tank is full to the brim and has overflowed onto the frame and my exhaust.

    What is going on?? How could I have no heat, yet the car is basically overheating? Then I get heat back and the overflow tank is overflowing? Air bubble in the system? Bad radiator cap? Overfilled it with coolant?
    1985 Mustang GT Convertible
    Stock and original @ 213k, except for dynomax ultraflos.

  2. #2
    Moderator wraithracing's Avatar
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    It sounds like you either have air trapped in the system, a blockage in the system, and/or a stuck thermostat.

    I would first recommend putting the front end up on ramps/jackstands to get the radiator cap as high as possible. Open the radiator cap and start the engine. Turn the heater on highest heat setting and turn the fan on high. Allow the engine to run and completely come up to temperature.

    If you have air trapped in the system, be careful as the air bubbles can cause hot coolant to spit and pop out of the radiator with the cap open. You should be able to see the coolant flowing well thru the top of the radiator once the thermostat opens completely. If you can see the coolant flowing well thru the radiator at that point, then allow the car to run for at least 15-30 minutes to help clear any air bubbles in the system. If the car will not get up to temperature on its on sitting there, you may need to throw a towel or blanket over the nose of the car to limit air flow and help the radiator/engine build heat. You can always remove the towel/blanket once the thermostat opens if the car begins to overheat. If it doesn't just leave the airflow limited, but make sure you don't overheat.

    If you don't see coolant flowing well in the radiator when the gauge is in the middle or so of the gauge, then you may have an obstruction that is preventing coolant flow. Unfortunately if that is the case, then you are going to have to start feeling heater hoses, radiator hoses, etc. to try and determine where the blockage might be. Carefully feel the radiator hoses, the top will most likely be very hot and the bottom hose cool or only slightly warm if beginning to over heat and no flow. Also feel the heater hoses as the inlet may be hot and the outlet cool which could show that either the heater core has a blockage or even the restrictor does.

    Once you have the coolant flowing and everything working properly, you have bleed the system by running it with the front end jacked up, then you can replace the radiator cap and take the car for a test drive. You should see the coolant temp gauge rise and peak, then the thermostat opes and the gauge will often drop just a bit, but should run around that point most of the time. If you have overheating issues while driving the vehicle then you need to check your fan, airflow across the radiator, belt and pulleys, and even your ignition timing both initial and total advance. Even a lean condition with your carburetor can cause over heating issues.

    Hope that helps a bit and good luck!
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  3. #3

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    Sounds a lot like a head gasket. If you drive 1.5 hours after the intake gaskets I don’t think you have an air bubble. Idle the car and pull the radiator cap. If it keeps pushing coolant out and bubbling it’s a head gasket.
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  4. #4

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    Quote Originally Posted by Lowetlx View Post
    Sounds a lot like a head gasket. If you drive 1.5 hours after the intake gaskets I donít think you have an air bubble. Idle the car and pull the radiator cap. If it keeps pushing coolant out and bubbling itís a head gasket.
    +1. Pressure check the cooling system and the radiator cap after the engine has cooled down and the coolant level has stabilized.

    Based on your description of the symptoms this sounds like a head gasket problem. I've seen this many times as a tech., and it seldom isn't a head gasket, however, there's always a very slim chance that it could be one of the aforementioned things to check. Just curious, how many miles on the motor?

  5. #5
    FEP Member Dave9052's Avatar
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    Could it be wrong intake gasket or installed wrong? Maybe blocking coolant, causing overheating. Why was intake gasket replaced?
    Last edited by Dave9052; 01-02-2018 at 12:46 PM.

  6. #6
    FEP Power Member 85stanggt's Avatar
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    Ok, so I went out at lunch and checked on things. The radiator is like empty and the overflow tank is full. The car is not taking the coolant back into the radiator when it cools. So, my guess is that the cap is bad? I barely made it home for lunch to swap out the car. I stopped halfway because it was nearing H. Made it the rest of the way home coasting and turning it back on. Hope I didn't damage anything. Car still turns over fine though.

    The upper radiator hose is hot, the heater core inlet is hot. The lower radiator hose is cool as is the heater core outlet. Not sure what the heck the deal is with that, except that it is really low on coolant.

    I can't imagine my head gasket magically went bad after the work I did. I had it open for probably over a month, so I'm not sure if that has anything to do with it. I think the thing is just out of coolant from puking it out constantly and not taking it back in.

    Motor is all original (original owner is my dad) and has 215k on it. Runs like a top.
    1985 Mustang GT Convertible
    Stock and original @ 213k, except for dynomax ultraflos.

  7. #7
    FEP Power Member 85stanggt's Avatar
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    And just another update to confuse things. Before I left home to go back after lunch, I popped the hood and the overflow tank is empty so it took everything right back in this time. I wapped on the cap before when it was overheating in hopes it was stuck or something. Anyway, I pulled it in the garage and parked it. I will refill with coolant tonight and watch it warm up with the cap off to see what is going on.
    1985 Mustang GT Convertible
    Stock and original @ 213k, except for dynomax ultraflos.

  8. #8

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    Try replacing the cap.

    Bad news is that it really sounds like a head gasket. When those go the engine compression pressurizes the coolant system and blows all the coolant out the overflow.
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  9. #9

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    My 88xr7 did this. I kept having leaks and random overheating. Finally got it all worked out. Previous owner started removing the heads for some reason (without unbolting anything but the exhaust) and broke off a head bolt, gave up and put everything back together. When I romped on it, it would spray water into the exhaust. Normal driving and idle, no leak. Under throttle it shot into the bottom of the exhaust manifold and evaporated instantly.

    Before this, it had a hole in the radiator and a obviously new water pump kept blowing gaskets.

    I probably drove around 1500 miles with that broken head bolt. I knew I had a random water leak, but I couldnt find it. Had to have a buddy brake torque it while I put my head in front of the tire so I could see the steam.

    It did not leak any oil or mix coolant. Took a while to figure out.
    2 1986 cougars (both 4 eyed and 5.0)
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  10. #10
    FEP Senior Member Matt J's Avatar
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    Quote Originally Posted by 85stanggt View Post
    Ok, so I went out at lunch and checked on things. The radiator is like empty and the overflow tank is full. The car is not taking the coolant back into the radiator when it cools. So, my guess is that the cap is bad? I barely made it home for lunch to swap out the car. I stopped halfway because it was nearing H. Made it the rest of the way home coasting and turning it back on. Hope I didn't damage anything. Car still turns over fine though.

    The upper radiator hose is hot, the heater core inlet is hot. The lower radiator hose is cool as is the heater core outlet. Not sure what the heck the deal is with that, except that it is really low on coolant.

    I can't imagine my head gasket magically went bad after the work I did. I had it open for probably over a month, so I'm not sure if that has anything to do with it. I think the thing is just out of coolant from puking it out constantly and not taking it back in.

    Motor is all original (original owner is my dad) and has 215k on it. Runs like a top.
    You can't read too much into the hoses being cool when the system isn't full of coolant. You also can't take much away from the gauge reading "H", as the stock gauges are notoriously inaccurate. If it's higher than normal, that's about all you can take from that, I'd suggest hooking up an aftermarket gauge, even if temporarily, until you have the problem under control. Keep in mind that the sensor on the intake is assuming that there's coolant running behind it, so your readings will be off. The cap works by reading the pressure of the radiator, which increases as the coolant heats and expands. The cap opens and lets the coolant flow into the reserve tank. As the engine cools, the coolant contracts and draws coolant back into the radiator. If the radiator is very low on coolant, there isn't enough to contract and draw the coolant back in, so it becomes a one-way trip. Same deal as was already mentioned, if the gasket is bad, it's pumping pressure into the system so more of the coolant goes out than can be drawn back.

    Why did you replace the intake gasket? Did you have a leaking problem already? Was it running hot before? If this is a completely new condition since you did the work, it's logical to assume that they would be related. It's possible that you just have an air pocket in there, but if you drove it for over an hour I think it would have overheated if the thermostat never opened. Are you sure it was the right gasket, you lined everything up exactly right, and that it was torqued down properly? Did you replace the thermostat as part of the work? If you try the easy stuff, burping the system as described above, then it's most likely you missed something when you were working on it. It doesn't take much to make a problem like this. Oh, since you're in FL and not too likely to freeze up even with the weird weather we're having this week, I'd suggest just using water until you figure out what the problem is. You're going to end up dumping $100 of antifreeze by the time you figure it out!

  11. #11
    FEP Power Member 85stanggt's Avatar
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    I agree, the stock gauge isn't precise, but it is consistent. It never goes above the A, and that's with A/C blasting and 90 degrees outside. It was low on coolant, so that was also causing concern for me, because I was thinking the gauge wasn't reading right because there wasn't even coolant up there for it to measure.

    I replaced the intake gaskets because the car was leaking out coolant from the back of the intake. It started with a little puddling in the front next to the distributor. It would seal up when things warmed up. Then it started getting bad to the point where it would puke out on the ground when the car cooled. That's when I said time to do them.

    I replaced the inlet heater hose at the same time because the hose looked old and a little ballooned. There was no restrictor, so I added one of those. I never had any issues until after these two jobs. I had the car open for about a month while fixing this stuff.

    I think my thermostat is semi-bad from the system being opened and drying out. I don't think it is opening all the way or not soon enough. I'm thinking there was some air in the system, and some coolant puked out that first time as the engine warmed while the t-stat slowly opened. But then the extra coolant never got sucked back in. Then I drove it again, it did it again, leaving the car with only a gallon left.

    I have since filled it while letting the car warm. Bubbles came out as the radiator filled and air got purged from the system then they stopped. It's filled all the way up. It took a long time for the upper hose to get hot. The car didn't overheat or overflow, but it took way longer than I remember it taking. I've driven it a couple times since, and what I notice is the gauge goes up a lot higher than it used to before dropping back down when the stat opens. So I think it is something with the thermostat. The housing is also badly corroded under that upper hose, so I have a new 192* t-stat and housing on the way. So far it hasn't puked anything out, though.
    1985 Mustang GT Convertible
    Stock and original @ 213k, except for dynomax ultraflos.

  12. #12

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    A bad thermostat can lead to a host of other problems

    Get a nice new thermostat from a reputable manufacturer. The parts store are big into fail safe thermostats these days and they seem like a good idea until you try them. I wouldn't give you 3 cents for every one of them now.

    Even at highway speeds a worn out fan clutch can be the source of big frustration as well. I've had good luck with the OE style fan clutch Autozone carries for what it's worth.

    Make sure your fan shroud is in proper position and held too and bottom. These can move at highway speeds and disrupt fan rotation.

    Intake gaskets could have something to do with the problem. Be sure they are correct and in properly. I always take pictures when I'm doing stuff so I can go back and look if I have problems. Works for me.

    If you do replace head gaskets make absolutely certain you have the right ones and install them correctly. Coolant passages at the rear or you'll end up with a damn hot motor for no explainable reason.

    Also toss the head bolts and buy new ones. I only trust ARP myself. AFAIK head bolts are often torque yield and not intended for reuse.

  13. #13

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    +1 on making sure the radiator is flowing. A quick trip to a radiator shop is money well spent. They can cook them out for about an hour then blast them with pressurized steam inside. It gets all the scale and crap out and makes them work like new again. With mine back in the day they got out a bunch of stray solder and flux from original assembly of the radiator. It was better than new when they were done with it.

    That being said I run an FB169 these days. I couldn't find another stock style 3 row brass radiator for a decent price when mine went bad so I went with a 3 row all aluminum. Supposedly it's good for up to 750HP. Now car stays cool even though it's pretty lean up top and timed more aggressively than ever, etc.

  14. #14
    FEP Power Member 85stanggt's Avatar
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    Radiator is a brass 3 row. Not new, but not clogged from what I can tell. Intake gaskets are on right.

    I bought a 192*F Motorcraft t-stat. I don't like using aftermarket stuff. That plus a Ford housing and a new gasket hopefully straightens things out.
    1985 Mustang GT Convertible
    Stock and original @ 213k, except for dynomax ultraflos.

  15. #15

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    Forgot to mention - double check your base timing vs where the motor wants to be.

    Personally I would not trust the timing marks unless I've verified them.

    the way I've always done it is mark where it is now. Loosen up the hold-down and remove the vacuum advance line then plug it. Turn the distributor until the motor just starts to speed up then go back just slightly. Lock it down. Unplug and put the vacuum line back on. Warm it up. Turn it off. Try to restart it. If it starts hard (turns over slow, engine kicks against the starter) reduce advance just slightly.

    Too much retard will cause big power and torque loss and a lot of fuel consumption and heat production. seems like it makes tons of noise and really isn't going anywhere, etc.

    Good luck getting things sorted!

  16. #16
    FEP Power Member 85stanggt's Avatar
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    I timed it to my old mark, but maybe that shifted. I'll have to try your method. I do get some pre-ignition now though when running on regular gas, which makes me think timing is too advanced.

    Going to try to replace the housing and t-stat this weekend.
    1985 Mustang GT Convertible
    Stock and original @ 213k, except for dynomax ultraflos.

  17. #17

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    The other thing you could do is the old obstruction down the spark plug hole on #1 to approximate TDC. Put some tape on the balancer. Bar it over until it stops you before TDB. Mark it. Go the other way until it stops you. Mark it again. Split the difference on the tape marks. That's likely TDC. Give or take about 1/4 - 1/2 a degree anyway.

    If your advance on your distributor goes wacky and sticks of you have too much vacuum on it that can cause trouble too. Try disconnecting the advance and plugging the line just to see if it continues to have heat problems.

    Too much base advance causes the hard cranking that stops the starter. Too much vacuum advance because it's on full vacuum or it's sticking would not but is just as bad and harder to diagnose.

    good luck!

  18. #18
    FEP Power Member 85stanggt's Avatar
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    So I pulled the housing and old t-stat. That was harder than it looked with that bolt behind the water pump!

    The old stat is a Motorcraft 192F, same PN as my replacement (XR3Z-8575-BA). I had the bleed hole lined up at 3 o'clock .

    I decided to run an experiment. I boiled a pot of water with my old and new thermostat in it. The new thermostat starts opening sooner than the old one. At about 180į degrees I start seeing about 3mm of opening. At 190 degrees it is pretty much fully open with about 8mm opening. My old stat was open about 3mm max. Even up to 200, the old one never opened more.

    This little experiment clearly showed how little the old thermostat opened. My engine has probably been running hot for years because of this, but it never overheated. For a long time it has run in the mid to upper middle of the gauge and used to run in the lower middle. I'm interested to see how much that changes and if my pinging goes away now.

    I'll hopefully have it back together tomorrow after work.
    Last edited by 85stanggt; 01-22-2018 at 02:02 AM.
    1985 Mustang GT Convertible
    Stock and original @ 213k, except for dynomax ultraflos.

  19. #19

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    Sweet! Sounds like progress!

  20. #20
    FEP Power Member 85stanggt's Avatar
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    Alright so some positive news...

    Got the thermostat in, new housing on, and everything sealed well. Topped up with coolant and the car has been running smack in the middle of the gauge. It used to run between the M and A. Now it is right in the middle or slightly below and will only climb up to the M when the A/C is on Max.

    No overflow, no overheats. Hopefully it stays this way. May not have a head gasket issue after all

    Also, I got on it yesterday and didn't notice any pinging under hard acceleration. Car is nice and responsive, and timing is set right at 12 degrees. Like a whole new car running cooler.

    Just a small side question...my expansion tank lid is held on with black goo from the factory (butyl?). That isn't sealing anymore. Am I right with the butyl or what exactly is it and where do I get it?
    1985 Mustang GT Convertible
    Stock and original @ 213k, except for dynomax ultraflos.

  21. #21
    FEP Power Member 85stanggt's Avatar
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    Need to post here again.

    Drove the car to the beach on Sunday a good 60+ miles round trip. Was running great.

    Got off the highway ramp and decided to have a last fun run up to 4k rpm in 2nd and 3rd (I don't really beat on this thing and rarely wind it up that much...it is 33 years old after all with 214k miles). Car pulled great.

    Roll into DQ for a late night snack and notice the temp gauge climbing up and up. Cut the engine before it went too high and popped the hood. Sure enough, the overflow tank was 3/4 filled with coolant and there was a good splash of coolant around that area of the engine bay.

    So, maybe it is a head gasket. I ordered a tester to check for exhaust gas in the coolant and will check that out this week to get a definitive test on if my head gasket is bad.

    BTW, after cooling down, the level in the expansion tank dropped to half and the radiator is low about 3-4 inches of coolant.

    One last thought I had (grasping at straws here, but this all started after the intake gasket job):
    When I did my intake gaskets, I placed a heater core restrictor in the line leaving the intake for the core. So if anything, the water pump is pushing more coolant to the radiator now. What happens if the lower hose collapses as the pump is sucking more coolant into the engine at higher RPMs all while pushing more coolant into the radiator? Could that just be pressurizing my radiator and causing the coolant to blow out into the expansion tank? That seems like a very plausible scenario...


    Also to recap...

    New intake gaskets properly installed with no RTV on the steel core gaskets, but RTV in corners of silicone end gaskets
    New in/out heater hoses with restrictor
    New heater core
    Thermostat (Motorcraft 192*F), thermostat gasket, and housing new (Ford Racing)
    Good radiator cap
    Healthy radiator (brass 3-row radiator from LMR back when it was 50resto.com)

    Water pump, upper and lower radiator hoses, bypass hose, and radiator have been in the car for a while without issues until system was drained for intake gasket replacement and left open for about a month.
    Last edited by 85stanggt; 08-28-2018 at 10:21 AM.
    1985 Mustang GT Convertible
    Stock and original @ 213k, except for dynomax ultraflos.

  22. #22

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    Check for signs of coolant in the oil. Is there any more than trace amounts of moisture at the oil fill cap?

    Around 200K on my 86GT I had a radiator cap go bad. It would only hold pressure sonetimes. Other times the hoses would never get firm and it would run hot and boil **** out the overflow.

    I put on a new radiator cap with pressure relief lever from the Mc parts store. Slant I think the brand was...

    Anyway - it held up until I did my front end redo and radiator swap at ~450K. We this old but known good cap on my son’s car when his was giving similar symptoms.... about 10K miles later symptoms returned and were resolved with a new cap. You can bet we put an identical twin on.

    Something to try! Hate to see car get disassembled without verifying the source of the problem first.

  23. #23
    FEP Power Member 85stanggt's Avatar
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    No oil-coolant transfer at all from what I can see. I will get the exhaust tester today and try to test sometime this week.

    While I was at home for lunch, I felt the lower hose for a spring inside. None. It is quite easy to squeeze. I'm seriously wondering if it is collapsing under high WP load, causing a lack of coolant to the engine, over pressurization of the radiator, and all these issues.

    I definitely don't want to take the engine apart. I'm finally at the point where I can drive it...

    I called LMR to ask them if the new lower Motorcraft hose comes with a spring in it and the guy on the phone couldn't tell me. Has anyone bought one recently and got the spring inside?
    Last edited by 85stanggt; 08-28-2018 at 03:16 PM.
    1985 Mustang GT Convertible
    Stock and original @ 213k, except for dynomax ultraflos.

  24. #24
    FEP Senior Member Matt J's Avatar
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    Consider too that this might be a new problem and similar, but unrelated, to the old one. If the car has been fine for months and just started with this, maybe it's something different. The fact that some of the coolant went back into the radiator when it cooled means that the cap is working in one direction at least, but if it allows too much to spill out at too low of a pressure it would account for too much coming out. The fact that you had coolant splattered around the reservoir sounds like there's a leak someplace, or it's possible maybe some came out and hit the fan or a belt and it sprayed over there. Check for a leak in the hose that runs to the reservoir, it's not pressurized, but if it has a split in it there's the possibility that it's sucking air back into the radiator. Maybe even a small leak in the radiator itself. It could have come out around the cap seal if the cap is bad, that's a cheap and easy fix to try first. If the head gasket was leaking that much, I'd expect to see quite a bit of coolant in the oil, I mean you're talking about at least half a gallon here, even if only some of that went in, you'd see it.

    I strongly recommend temporarily mounting a cheap mechanical gauge in the car so you can see the actual real temp during actual driving. If it doesn't go over 205 or so, you have nothing to worry about in terms of overheating, at least. You can easily remove the mechanical sender and replace the electric one and plug it back in when you're sure everything is fixed.

  25. #25
    FEP Supporter SECESH's Avatar
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    I could be wrong, but wouldn't a compression test help spot a bad head gasket?

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