Coolant doesn't 'drain' back into radiator from overflow??

[Tink]

Weird problem with my 4 door, CPI blazer... the coolant won't return from the overflow to the radiator... I've been checking it after every drive (when it is cool) and 50% of the time the overflow is too full and if I open the radiator, it needs coolant. So, I remove the overflow hose, dump some in a cup (it flows easily) and refill the radiator. I don't have oil in my coolant, nor coolant in my oil and the temp is right where is should be. Only thing I noticed, is that it doesn't heat well... but I assumed that was due to large 'cabin' area. Any thoughts on why this is happening? Should I shorten the overflow hose or something? Is the radiator cap faulty?

[SONOMA SS]

It could be the rad. cap, also maybe the t-stat. Is your overflow tank pretty close to the top of the rad.? It should be mounted up high. Check the temp. coming out of the vents. Also, check for coolant on the passenger side floorboard.

[1993blazerlt]

i would bet that you have a leak somewhere in your system, The over flow works this way, when the engine gets warm the coolant expands, the radatior cap allows it to expand a specific amount to raise the pressure and hence raise the boling point of the liquid, when it over pressures the cap allows coolant to flow under pressure into the over flow bottle, now when you cool back down the coolant contracts creating a vacuum that sucks the coolant back in, now if there is a leak somwhere you will suck air in and not the coolant

[bespurcell]

I have the same problem except my heater works fine. I put on a new radiator cap and it stil didn't fix it. I too think I have a small leak somewhere I've just never been able to find it, so every once in a while I take off the overflow like you said and refill my radiator.

[Blazer69]

either that or a clog somewhere...could explain the so-so heat...hope it's not the core.....try a full system boil/flush

[C4Tom]

Sometimes the cap to the overflow tank cracks with age and allows air into the return/suction line. I would check for an air-tight seal in this part of the system first. Since the overflow is always too full, there is a leak somewhere in the system that's pulling air insead of coolant during cool down.

Start with the simple stuff. :D

Not heating very good? Is the blown air at the floor vents very warm to hot? Passenger floor damp?

[Tink]

Passenger's side carpet isn't wet. The heat is just not as hot as the heat in my 2 door 91 - that thing can run you out.

I'll check the overflow cap and swap radiator caps with one I know is OK... so, if you have a air leak how do you ever find it?

[C4Tom]

I would start by swapping caps, then follow the hose looking for leaks. I would try to do a leak down test with a vacuum tester(like a vacuum brake bleeder), plug one end and put a vacuum on the other end, it should maintain a vacuum. Next, look at the cap on the catch can to see if there are cracks. Just do a typical see & feel hunt and find because it's similar to finding a vacuum leak... you won't see any obvious signs.

On your heat problem... it could be the door inside the air box getting stuck, or the diaphram not working, that's allowing hot and cool air to mix. I can't tell you how to test that, but it's a free check if you figure out how to check it. I'd follow up all checking/testing with a new thermostat. Perhaps a previous owner stuck a 160 stat in there. Breaking into the engine is almost always my last option.

[Tink]

Well, I'm trying a new radiator cap today... my first drive to work, so we shall see! The old cap was pretty bad, I dropped it and the gasket fell off. When I stepped on it, it pretty much turned to mush. The overflow cap appears to be OK. Sure don't want to get into the motor on this thing - the freakin water neck is on the front side of the block.

[Tink]

Well, the radiator cap change didn't help... guess I'll try a different cap on the overflow.

[bespurcell]

I don't think the cap on the overflow matters, since the line that goes from the overflow to the radiator is on the bottom of the overflow. This means when your radiator cools off and pulls a vaccuum it will be pulling straight from the bottom side of the overflow. If the cap on the overflow was air tight it would suck the overflow tank flat. This is why the overflow lid has a hose on it, to vent excess pressure when the radiator is hot and to keep the sides from caving in when the radiator cools off. Hope this makes sense. I could'nt think of any better way to explain it.


Well, the radiator cap change didn't help... guess I'll try a different cap on the overflow.

[C4Tom]

That's my fault, I was explaining it like the old style line to cap with a dip hose. My bad! I'm sure Tink knew what I meant. :D She's very smart!

[Tink]

Actually, I checked the cap on the overflow and the holes on the underside were partially plugged with sludge... cleaned it up and put it back on... doubt it will make a difference. Not driving it again until Friday... I'd shoot myself if I had to deal with it everyday.

No cracks in the overflow tank and all the hoses look good... thing is I never lose coolant - it just stays in the overflow - I never need to add.

[Auto-x]

Check the hose between the radiator and the overflow tank.
It is probably cracked enough for the water to pass through on the way to the overflow tank but will not hold enough vacuum to pull the water back.

[bespurcell]

That's my fault, I was explaining it like the old style line to cap with a dip
hose. My bad! I'm sure Tink knew what I meant. :D She's very smart!


I figured you knew better and just wasn't thinking but, I figured some of the people reading this thread might not know better, so that was more for them than it was for you or Tink.

[Craz]

This problem can also be due to a problem that will make you cringe.

Your best bet would be to get a radiator pressure checker. You can rent them from major parts stores for pretty cheap. Pressure your tank to about 20 psi and watch the pressure. If it doesnt drop off to the naked eye within a few minutes, let it sit for about 10 more and just watch to see the pressure. If it drops, you have a leak somewhere you need to locate.

Now the part that will make you cringe. I had this problem with a 93 Vortec motor. Drive the truck for miles and no overheating or nothing. Heated the truck fine, but would lose water and would have to top off every day or so. I found out later when I did an engine swap, that the block had a slight crack in it. This crack was so small, it did nothing when cold (no leaks, no pressure loss), but when it got hot, it would create more pressure in my radiator and cause the fluid to go into my overflow tank. The pressure would release faster than it could suck the fluids back, causing my OF tank to always have extra fluid, and radiator to not have enough. I know it sounds terrible, but it could be a possibility.

You can rule out one thing at almost 90% tho, since the heater is actually blowing out heat, its not a head gasket. Notice I said 90%.... the last and less of a cringe for a leak that may not be identified as easy is a leaky head gasket. Watch for a slight mist in your exhaust when you throttle the engine. I sure hope neither of these last two are your problem. I sure hoped they were not mine as well, but the crack in the block definitely was my problem. Thank god I was already planning and doing an engine swap when mine was happening.

Good luck and keep us posted.

[Tink]

So now that I am totally freaked out - and I just bought this blazer a couple months ago! - I'll check those things.

I have replaced the radiator in it, but I didn't drive it a lot before I did that because it was seeping, so I don't know if it did it before that.

I'm also wondering about the hose going from the overflow to the radiator. Auto-x mentioned it could be cracked, which I didn't see any, but the hose felt more, uh, flimsy than the one on the Baja... maybe it is collaspsing under pressure/vacuum and not allowing the coolant back in the radiator??

[87wildside]

Try changing the rubber hose from the rad to the recovery tank. It may not be sealing tight enough to hold the vacuum to pull the coolant back. I had that problem on mine so I changed the hose and used small zip ties like hose clamps.

Haha, I just read the previous post. The could be the problem too.

[trailblazer_2001]

I had this problem with our chevy lumina. I found out it was the head gasket leaking. Thier was a leak in it.

[Tink]

Replaced the hose, no luck... :(

What about one of these pressure venting radiator caps: http://www.stant.com/brochure.cfm?brochure=204&location_id=179

Could I use that to keep from having to dump the coolant out of the overflow all the time?

Don't have time (or $) to tear into this motor right now... cause if I did, I'd be sending the heads out and putting in a cam. So, I'd rather get by as long as I can.

[87wildside]

That cap works the same as a regular cap. The lever helps keep stupid people from releasing the pressure to fast and burning themselves.

If you plan on getting into the motor latter on way not try some stop leak? I have used it before with mixed results but for the couple of dollars it's worth a try. BTW, it never clogged my rad or heater core as many people say it does.

[C4Tom]

I wish I had more information or suggestions for you. It's really sounding like a motor issue at this point. :( If it's not driveable as it is now, you might try Bars Stop Leak. It looks like guinea pig food in dark oil. I've used it before with good results but I've never tore into the motor after use to check for potential problems, assuming there would be. Good luck... and you might take pics when you tear into the motor and do a write-up. :D

[Tink]

Well, I think I'll just go to driving it once or twice a week until I have time and $ to tear into it... I'm just not willing to replace a head gasket and put high mileage factory parts back in it. Looks like I can get the whole Comp Cams kit for under $700, so I'll hold out for that.

[Craz]

:) Understand this, I do not think its a head gasket. No mist ?? No signs of water in oil ??

Its starting to sound more like a crack in the block itself. Its too small to cause any problems at slow RPM's, i.e. idle and what not. What it does is the pressure from the engine seeps thru the crack. As the engine heats up, the crack expands enough to allow more pressure thru it. The pressure is then entered into your water jackets IN YOUR BLOCK, and works its way to your pressure relief area (the radiator). As the pressure builds above 20 psi, it goes into your overflow tank. Your OFT can only hold so much fluid till it begins to overflow. So while your driving it, fluid is coming out of your OFT. When you shut off the engine, the pressure is too much for the cap to allow the fluids back in from the OFT. The coolant basically is not sucked back into your radiator. It makes you think you have a leak, cause your radiator is low, but your OFT is full. So, you refill your radiator and continue on till the next day, you see the same results again. This crack can stay that way for some time and never bother you. As long as you keep water in the radiator, you will not overheat your engine and blow anything up.

I suggest you drive it around for about 30 minutes, and then put a compression check on it. This should show you what cylinder (if any) has a crack in it. Keep your fingers crossed and lets hope this isnt your issue. I would not continue just changing parts and spending the hard money you have earned on things you do not need. FIND THE PROBLEM.. if not, by the time you change everything, you could have already fixed the rightful problem.

Again... COMPRESSION CHECK... COMPRESSION CHECK.. ensure yourself of the problem before buying anything else.

I will cross my fingers for ya Tink !! :) :)

[bespurcell]

What about one of these pressure venting radiator caps: http://www.stant.com/brochure.cfm?br...ocation_id=179 (http://www.stant.com/brochure.cfm?brochure=204&location_id=179)

Looks dangerous.

[87wildside]

That cap works the same as a regular cap. The lever helps keep stupid people from releasing the pressure to fast and burning themselves.

:)

[Tink]

So a compression test would show a cracked block? And what if it 'passes with flying colors?'

[bespurcell]

Hey Tink did you ever do a compression check? I did one on mine awhile back and they all looked good. I'm just curious because I have the same problem you're having and my truck runs a little hotter than I think it should sometimes. Someone on another thread said mine might be a headgasket but I've never had water in the oil or vice versa.

[Sparky2263]

Builds enough pressure to overcome the 15 lb. cap, push excess coolant into the reservoir, but when the engine cools, and the cooling system creates a vacuum, it won't pull the coolant back from the reservoir.

So, if it doesn't suck all the hoses flat (is it?), where is it pulling air from?

Most likely, the sealing surface of the radiator neck is imperfect. That's where the gasket that fell apart on you seals. The pressure seal is down in the neck, so I'd look real close at the vacuum seal.

I've also seen the little brass tube in the nipple of the overflow tank get a piece of trash right on it. Moves out of the way for coolant coming into the radiator, but seals tight when vacuum forms in the radiator (low pressure).

By the way, I hate the word "vacuum", but am somewhat forced to use it. As the cooling system cools down, pressure is lower in the cooling system and atmospheric pressure "pushes" coolant back into the radiator. If atmospheric pressure can get back into the system other than by "pushing" the coolant back into it, it certainly will.

[bespurcell]

Most likely, the sealing surface of the radiator neck is imperfect. That's where the gasket that fell apart on you seals. The pressure seal is down in the neck, so I'd look real close at the vacuum seal.


Could you explain this a little more? I'm not quite sure where or what you are talking about. I've already ruled out the other possibilities you talked about. I do know all about what vaccuum really is since I'm an EPRI level II certified tech which means I know how to install and calibrate various instruments. Most of these are found at refineries and help automate various processes. Thanks for the explanation though. I'm sure they're are other people reading this thread who find the info helpful.

[Sparky2263]

There are 2 seals in the radiator cap. One to seal the surface on the inside diameter of the radiator neck which is a spring loaded seal to create higher than atmospheric pressure.

The other seal seals the upper portion of the neck and is sealed by the step on the outside of the radiator neck. I'll see if I can find a pic.

[bespurcell]

I know what you're talking about now. I should of mentioned I changed the radiator cap a few months ago and it still has the same problem.

[Sparky2263]

I was actually responding to Tinks problem, didn't see you were having a cap issue. But the same thing goes, just because the rubber is good doesn't mean it's sealing.

There is a ridge at the top of the neck the gasket seals on. Best way to test is to modify the pressure tester and put about 3-5 lbs. of pressure on the overflow hose. That tests the cap seal.

[bespurcell]

Sorry Tink I didn't mean to steal your thread.

[Tink]

No problem, bespurcell - steal away ;) Better we both get our issues figured out!

Builds enough pressure to overcome the 15 lb. cap, push excess coolant into the reservoir, but when the engine cools, and the cooling system creates a vacuum, it won't pull the coolant back from the reservoir.

So, if it doesn't suck all the hoses flat (is it?), where is it pulling air from?

Most likely, the sealing surface of the radiator neck is imperfect. That's where the gasket that fell apart on you seals. The pressure seal is down in the neck, so I'd look real close at the vacuum seal.

I've also seen the little brass tube in the nipple of the overflow tank get a piece of trash right on it. Moves out of the way for coolant coming into the radiator, but seals tight when vacuum forms in the radiator (low pressure).

By the way, I hate the word "vacuum", but am somewhat forced to use it. As the cooling system cools down, pressure is lower in the cooling system and atmospheric pressure "pushes" coolant back into the radiator. If atmospheric pressure can get back into the system other than by "pushing" the coolant back into it, it certainly will.

I thought the hose from the radiator to the overflow was a bit 'flimsy' so I replaced it - didn't help. I checked that little brass tube in the overflow at that time and that looked clear.

Could this be an intake gasket issue at all?

Also, I remember T-man had said in order to check head gaskets, to fire it up and run it with the radiator cap off, and look for bubbles.... well, I did that this weekend and I did see tiny bubbles.

Figured, I tear into it and see what everything looks like... if it is a bad headgasket, then I'll replace the gaskets and run it for a while, if it is a cracked block, I'll put it back together, dump some bars in and save some $ for a reman motor.

[bespurcell]

Thanks Tink. I was hoping you would'nt mind. I'll have to run mine and do the bubble check in the radiator. I had forgotten about that little trick.

[20Xtreme02]

If atmospheric pressure can get back into the system other than by "pushing" the coolant back into it, it certainly will.

This is another way of saying if the overflow hose is not tight at the radiator nipple, when the pressure drops in the rad, it will draw air in at the loose hose instead of water from the overflow tank. That air weighs a lot less than the water... path of least resistance all that stuff. Is there a clamp on the radiator end of the overflow hose? I think you said you replaced the hose so it shouldn't be cracked but it might not be sealing properly.