View Full Version : upper radiator hose blows off


jimboz28
11-29-2010, 01:57:56 PM
I did a quick search and couldn't find a thread with the same problem I'm having. If there is one, I must have missed it, sorry. So here goes- The car is an 81 Z28. I built an engine for it a while ago, and when I was breaking the cam in, after about 10 minutes, the upper radiator hose blew off. I was in the car when it happened, so I shut it off as soon as I realized what happened. At first, I thought the hose clamp was just loose, so I tightened it up, but the same thing happened again. I noticed that my upper radiator hose had a bend in it that went up higher than the radiator, so I changed it. I also noticed that the thermostat housing didn't have the same angle off of the block as the stock one, so I put the stock one back in. I talked to one of my buddies about it, and he suggested I take the thermostat out and see if it helps. I took the thermostat out, and started the car up again. After about 10 mminutes, both radiator hoses were rock hard so I shut it down before they flew off. I'm going to put the t-stat back in, with some holes drilled in it as some of you guys suggested in someone else's thread. I put a new 13lb radiator cap on it as well. I'm not sure what to check next, and I'm afraid my brand new engine might already be shot. There is no oil in the antifreeze, and there is no antifreeze in the oil pan. The only thing I haven't replaced yet is my radiator- new water pump, hoses, t-stat, radiator cap. Sorry for the long post, but I wanted to give as much detail as I could think of. Someone please HELP!:confused: :mad:

tom3
11-29-2010, 02:00:52 PM
Seems like it would have to be a bad cap. Try a lower pressure cap to test it.

Blown Camaro
11-29-2010, 02:15:13 PM
Start the car with the radiator cap off. Once the thermostat opens watch the coolant through the radiator neck. Is it frothy? You may have a blown head gasket or a cracked cylinder.

High Country Z
11-29-2010, 02:16:10 PM
Or a cracked head.

COPO
11-29-2010, 02:17:14 PM
He's already at a 13 lb cap.
You either have a bad cap or bad thermostat.

Regardless of why your coolant system is being pressurized, if you are building enough pressure to blow off a hose, then it's either not clamping tightly enough, or the pressure relief system (the cap and overflow bottle) aren't working properly.

I would pressure test the system to see when this hose pops off. You may need to borrow a tester or have the car towed to a rad shop to test it.
If it pops off at a pressure below what the cap is rated, then it's not clamping tight enough. If it pops off above what the cap is rated for, then the problem would be the cap.

jimboz28
11-29-2010, 02:23:45 PM
Ok, I installed the 13lb cap but I haven't run the engine since then, so I'll start it up with no cap and if it looks ok, I'll put the cap on. I forgot to mention I installed a pressure gauge on the thermostat housing in one of the sensor ports to see how much pressure it's actually making, but I haven't started the car with it installed yet. Assuming I see bubbles or frothing in the radiator, is there any way to tell if it's the block/head gasket/cyl. head? Or am I going to have to take this thing apart again to figure out what it is?

Thanks for the replies, tom3, blown camaro, high country z, and copo. I get out of work at 3, so I'll post an update after I run this thing again.

High Country Z
11-29-2010, 02:27:32 PM
You may have air trapped in the system.

73Camaro383
11-29-2010, 02:29:52 PM
Im intrested to see the outcome of this. Radiator cap would be my thought.

warped
11-29-2010, 03:38:58 PM
Were you monitoring the coolant temp as the engine was running? What are you using for cooling? Flex fan, clutch, electric??? Do you have a shroud? Usually when breaking in a new engine, you need additional cooling to compensate for lack of forward motion and the additional heat generated by new parts breaking in. Make sure you have plenty of air flow through the rad and if possible use a garden hose to spray down the radiator if things start getting too warm. Good luck.

Knuckle Dragger
11-29-2010, 03:48:18 PM
Absolutely has to be a bad cap, bad clamp, or poor fitting hose. Maybe a combination. Read Copo's post again he states the logical thought process.

High Country Z
11-29-2010, 03:53:30 PM
I doubt it's a bad cap, I run a 14 lb cap with no problems.

Knuckle Dragger
11-29-2010, 04:03:07 PM
I doubt it's a bad cap, I run a 14 lb cap with no problems.

Your cap works correctly too. What if it doesn't release pressure?

If the cap is working properly how would the system generate enough pressure to blow the hose off, especially with a 13 pound cap? It should open the cap and vent the pressure to the overflow bottle and never cause pressure over the caps rating. That's why we're saying bad/defective cap. Air in the system would cause pressure build up, but if the cap is working properly it won't blow the hose off, it will vent to the bottle.

High Country Z
11-29-2010, 04:09:55 PM
If he's building up that much pressure and the cap is not blowing off then something else is wrong causing the pressure to build that high. I don't have an overflow tank, that's why I run a 14 lb cap, and my cap never opens, so assuming the cap is accurate, my system never gets to 14 lbs.

jimboz28
11-29-2010, 04:36:45 PM
thanks for all the replies so far.
Warped- yes, I was keeping an eye on the temperature. It got up to about 210-215*F just before the hose came off. I am using a stock clutch fan on a stock replacement water pump with the stock fan shroud. I know the temperature is probably a little high, but I figured it was because everything is new and tight inside the engine. Also, the car is parked nose first into my garage, and there isn't a whole lof of space between the grille and my big toolbox. I am going to turn the car around so the nose faces my garage door as soon as I get it off the jack stands. I'm going out to the garage now, I'll post an update as soon as I have a chance. Thanks again guys, this place rocks.

JONESYFXR
11-29-2010, 05:18:35 PM
I'm thinking a plugged radiator core. Sounds like it's not allowing coolant to flow through it.

jimboz28
11-29-2010, 05:54:00 PM
I just ran the car without the radiator cap on, and I could see coolant flowing pretty good across the radiator, no bubbles or anything out of the ordinary there so I put the new cap on after a few minutes. I ran the car for a while, and the pressure gauge I installed on the water neck never got past 8psi. I'm pretty sure that problem is fixed now- I think it was a combination of air pockets and the radiator cap. The bad news is the engine sounded like it started knocking, but it could just be exhaust noise echoing in my garage or something. I'll have to put my exhaust on so I can hear it. Also, it sounded like it was misfiring around 1,500-2,000 rpms, but as I revved it up a little higher it sounded like it was going away. Any ideas on that?

Cardinal
11-29-2010, 06:06:45 PM
Overheating problems can be a female dog to find.

Lets start from the begining.

1) Thermostat: did you test it by placing it in a pan with cold water along with a thermometer, then slowly bring it to a boil. Watch the thermometer to make sure that the thermostat is fully open by the time it reaches it's specified temperature.

2) Radiator: new, old, flushed? Is there flow with the engine warmed up to at least the thermostat's operating temperature? If the flow isn't good, then the radiator is bad. Have the cooling system tested. The tester kit looks like this ( http://www.harborfreight.com/radiator-pressure-tester-kit-65053.html ). Any good auto parts store has them to rent/loan. Pressure test the cooling system and the radiator cap.

3). Now we have a good thermostat, a good radiator cap, and radiator, is there a good seal around the radiator support and the radiator? If not, make it so.

4). You should have a fan shroud and the fan has to fit close to the diameter of it.

5). Timing: where is the initial timing set? Are you sure that the timing marks are correct (the cam was degreed in and the zero of the harmonic balancer is actually zeroed to TDC of #1 piston and the timimg tab zero is zeroed to all of that? What is the total timing (should NEVER be more than 38 degrees total).

6) Carburetor: what carburetor is on the engine? Is it jetted to your engine? IF you have headsers, can you see if they are glowing red (usually easeir to see at night) because that means the engine is WAY too lean and you need to upjet it. Pull the spark plugs and look at them. They should NOT be white or silver (silver is BAD = burned pistons) or black (too rich). They should be a very light brown.

Now for the BAD news. I had one engine that overheated from the get go. It was a pretty much bone stock 350 SBC that I put in a 1980 Monza. New three pass radiator, water pump, electric fans, everything I listed above was perfect. About ten to fifteen minutes into running it would blow the hoses off of the radiator or water pump. I had professional mechanics look at it and they couldn't fix it. Pulled the 350 out and put a 305 out of a Gen III Camaro Z28 (that a freind gave me) and it can cool as a kitten. Six months later I tore that MF'er of a 350 apart. ALL off the main and rod bearings were GONE! My mistake was believing that the machine shop could assemble the short block and not F' it up! The engine was TRASHED! The only things that I saved off of it were the heads, intake, and oil pan. Cam, crank, everything else was junk.

I'll NEVER trust another machine shop to assemble an engine for me. I always clean and check everything.

I hope that this is NOT what you are up against.

Knuckle Dragger
11-29-2010, 06:29:01 PM
If he's building up that much pressure and the cap is not blowing off then something else is wrong causing the pressure to build that high. I don't have an overflow tank, that's why I run a 14 lb cap, and my cap never opens, so assuming the cap is accurate, my system never gets to 14 lbs.

OK, we're going at this two different ways. You're looking for the reason for the pressure increase, and I'm looking at why the hose is coming off. There is multiple problems with this car. IMO we need to know why the cap isn't venting and allowing the pressure to blow the hose off. Once that's fixed then we can figure out the overheating issue or cause of the pressure. Both issues need to be resolved; we just disagree on the order.

jimboz28
11-29-2010, 06:39:17 PM
Cardinal- I'll answer your questions in the order you asked them-
1- No, I didn't check the thermostat yet, I just got a thermometer for that purpose, but right now the thermostat is out of the car. I do plan on re-installing it after I get this sorted out.

2- The radiator looks pretty old. I also bought one of those almuninum radiators from Autozone that alot of guys on here run, but I didn't install it yet because I was afraid I would damage it from the excessive pressure. I can see coolant flowing pretty good across the radiator though.

3- The radiator is tight up against the support as in it won't move, but it's not sealed up against it with weatherstripping or anything.

4- The fan and shroud are stock, and installed correctly.

5- We set the total timing at 33* BTDC. I didn't use a degree wheel, but it should be pretty close because I installed the balancer and timing mark with the heads off. I turned the engine over by hand carefully until the piston stopped moving up and then marked the balancer with paint.

6- The carburetor is an Edelbrock 1405 I think. Its a 600CFM with electric choke. It was on the 305 that was in the car when I got it, and it worked pretty good on that engine. My buddy has a jet kit for it if I need to jet it up. I do have headers on it, and I didn't see them glow at all. It wasn't night time, but it gets pretty dark in my garage without the lights on. I haven't pulled the plugs out yet, but I will check them. Thanks for your help.

High Country Z
11-29-2010, 06:41:44 PM
OK, we're going at this two different ways. You're looking for the reason for the pressure increase, and I'm looking at why the hose is coming off. There is multiple problems with this car. IMO we need to know why the cap isn't venting and allowing the pressure to blow the hose off. Once that's fixed then we can figure out the overheating issue or cause of the pressure. Both issues need to be resolved; we just disagree on the order.
It seems more times than not, people fail to get the cooling system completely full. They dump a couple of gallons of coolant in the radiator, start it up, run it until the t-stat opens, and top it off. But they aren't getting all of the air out that's trapped in the block. Those air pockets turn to steam and raise hell. That's what I think probably happened.

Knuckle Dragger
11-29-2010, 06:57:10 PM
It seems more times than not, people fail to get the cooling system completely full. They dump a couple of gallons of coolant in the radiator, start it up, run it until the t-stat opens, and top it off. But they aren't getting all of the air out that's trapped in the block. Those air pockets turn to steam and raise hell. That's what I think probably happened.


Could be. I understand where you're coming from now. I hear about air in the system on this forum a lot. I haven't ran into that being an issue on a small block. Obviously others have and your advice is spot on for the overheat.

COPO
11-29-2010, 09:09:18 PM
I'd be installing that new rad too. Some of the tubes could be plugged causing you to overheat. Run your finger down the cooling fins and if they fall apart, the rad is toast. I'd never fire up a new engine with a rad that I know is old.

Damon
11-29-2010, 10:32:26 PM
I've seen some combinations of radiator filler neck and cap that can cause a very weird problem like this even if the cap tests OK. On some combos the little "stop" that the cap is supposed to..... um...... STOP at, when it's turned all the way home doesn't stop it. The cap rides up the notches a little too far, compresses the cap to the point the relief spring stacks solid and you end up with NO pressure relief through the cap. Result- it blows hoses off when it comes up to temp.

I agree that a bad cap is the most likely cause (no hose should blow off at only 14-16 PSI), but you might want to keep this one in your memory banks if you end up pulling your hair out trying to find the cause of the problem.

Cardinal
11-30-2010, 12:40:55 PM
The Edelbrock carburetors are NOTORIOUS for have the float height and drop set WRONG. It may have run OK on the 305 but your new engine is going to demand that things like float height and drop be perfect.


Change the radiator as it will elimate one more potential probem.