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Temp Spike in Coolant (BBC 496)

11K views 69 replies 14 participants last post by  1965ChevyII 
#1 ·
I have a fairly fresh Big Block 496 (built by Lawrence Racing - 660hp/650tq) that is giving me temp spikes right when the thermostat opens.. I didnt have this problem last year however I did have to replace the intake gasket last winter when I pulled off the carb and noticed the intake gasket was squished out into the end ports so I replaced it (I assume it was originally over tightened).. Ever since then when the thermostat first opens the temperature begins to rise quickly until it reaches 220-240 and then it drops back down to 180 and stays there.. It only spies for a few seconds) and it only happens when I initially drive the car and the thermostat first opens, thereafter it will stay at 180.. Other than that, the motor runs perfect! Here is what I have tried so far
1. Pressure checked the coolant system and it held 16lbs for 24 hours.
2. Jacked front of car up, installed a tall funnel and filled it 1/2 way with coolant, fired the car up and left it there until the thermostat opened and until it completely cooled down again. I did this 3 times..
3. Installed 3 different thermostats (the last one I installed is now a 160 so the temp spike isnt as extreme when the Tstat opens)
4. Installed 2 different radiator caps (it never spits any fluid out the overflow into my catch can)
5. Tried drilling two 1/8" holes in my thermostat and it still did it so i put another new thermostat in..
5. Verified there is no coolant in the oil and no oil in the coolant.
6. My radiator never leaks and it never pushes coolant out the over flow
7. My temp gauge is a mechanical gauge located in the intake next to the thermostat housing
8. The bypass hose that goes from the intake to the water pump is installed and was installed last year too..
9. None of my hoses are colapsed.
I know i tried other things, just cat think of them at the moment.
As mentioned, I didnt not have this problem at all last year..

Any help is appreciated
 
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#2 ·
Always go back to the last thing you did.
Are you sure the intake gaskets are correct and have the coolant passages open where they should be.
It is weird that it only does it on the initial warm up. You have done everything thermostat related that I can think of.
 
#3 ·
I have also had 2 friends that own mechanic shops try to figure it out with no luck.. They both suggested a vacuum purge so I just ordered a coolant vacuum pump/purge, its the last thing we can think of trying.. I can only assume there is still a bastard air pocket somewhere. Your right, its so weird that it only happens once.. Its been very frustrating because I don't want to toast this motor, it wasn't cheap so I only make short drives with it now..
 
#5 ·
You're worried over nothing....all you are seeing is that your high HP engine is heating the water, as it should, in the block and heads....the momentary temp spike is from the thermostat reaching it's opening temp, way up there in the top of the intake manifold(and where your temp sensor is...it takes a few minutes for the non-circulated water at the thermostat/water neck to reach that opening temp, meanwhile the block and heads have already gotten warm....once the thermostat opens THEN the water in the block and heads can get moving toward the thermostat and your temp sender.....now it shows a quick warm spike as this hot water rushes past the temp sender on it's path to the radiator.....so your sender reads a brief spike.

It means nothing, you don't have a problem....your overly worried about a new engine. It's good to be alert to everything that a new engine is doing, but don't let it create problems in your head that aren't really there. If the heat spike lasted a half hour then you'd have a reason to be concerned.

If you watch the gauge for several minutes after you shut the car off, you'll see a similar heat spike, of a 15° or so lower rise....caused by heat soak.
 
#7 ·
I understand what your saying and I'm aware that the temp will climb after I shut it down but it never spiked at all last summer, hence the reason for the concern. I also have 2 other members in our car club who run the same motor (BBC 496) that have even more power than me and theirs doesn't spike at all.. Last summer my temp never exceeded 200.
 
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#9 ·
Everything I have is the same as last year when I didn't have this issue, with the only exception of a new intake gasket. My over flow tank is a catch can, not a recovery tank with a tube in it.. I thought about changing out to a recovery type system but then I thought why should I have make changes when I didn't have this problem all last summer, which leads me to believe it must be something else like a possible air pocket or it sucking air somewhere so we did a pressure test and it held 16lbs for 24 hours.
 
#11 ·
I'm going to disagree with Hcompton here. With a functioning bypass for when the t-stat is closed, a spike of more than a couple degrees above t-stat temperature is NEVER normal.

I'm not really sure what's causing your issue though, as the only times I've seen this, it was poor coolant flow in the area of the t-stat and a 1/16" hole in the stat got enough flow past it to make it start opening at the set temperature.

I would change to a recovery type system though, as the worst case is it acts the same, the best case is that it works better (as it'll do better at self-bleeding air pockets out of the system).
 
#12 ·
I'm going to disagree with Hcompton here. With a functioning bypass for when the t-stat is closed, a spike of more than a couple degrees above t-stat temperature is NEVER normal.

I'm not really sure what's causing your issue though, as the only times I've seen this, it was poor coolant flow in the area of the t-stat and a 1/16" hole in the stat got enough flow past it to make it start opening at the set temperature.

I would change to a recovery type system though, as the worst case is it acts the same, the best case is that it works better (as it'll do better at self-bleeding air pockets out of the system).
I was thinking about that last night. I agree, if the bypass if hooked up, the coolant is cycling through the block before the Tstat opens, which should elliminate any air pockets that would be under the Tstat.

Its acting like its developing an air pocket somewhere as it cools down and then the air pocket gets pushed out when the Tstat opens.

My vacuum purge kit will be here today so i wil use it to run a few tests too.. If I cant get any positive results with the vacuum purge, then I will install a recovery type system and mark the fluid level to see if it draws anything in during cool down.

I have never seen anything act like this before and i have owned hot rods and drag cars for the past 35 years..
 
#13 ·
Are you using the original bypass setup with the hose of the intake? You could get an air pocket on top of the tstat if its the highest point in the system.
Have you tried drilling a 3/16 hole in the thermostat, or getting one with the little valve, to aid purging any air?
 
#14 ·
I tried drilling holes in the 2nd thermostat I installed and it still did the same thing so I installed a 160 Mildon Thermostat which already had a small air bypass built into it..

I'm using the same bypass that was installed by the engine builder.. I will post a picture of the motor shortly.. If the air pocket was on the top side of the thermostat, wouldn't it just get flushed into the radiator since that's the direction of flow when the Tstat opens?
 
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#20 ·
I tired 2 different 16lb caps, both are Stant racing caps with the pressure relief lever. I ran the car all last summer, including a big cruise where I was stuck in traffic on a 90 degree day and the temp never exceeded 200.. I even took the car to the strip last year and it never spit any fluid out the overflow and the temp never exceeded 200. That's what I like about the 496 for a street motor, you can make allot of power without running allot of compression, my compression is only 10.5 with mildly modified pro comp heads.

I should be able to use my vacuum pump/purge kit as a temp recovery system because it has a hose you put in a can full of fluid to pull any fluid if it needs it as you release the vacuum. My kit should be here today.
 
#22 ·
One other thing, I just looked at your pictures. have you tried a different thermostat housing. Sometimes those chrome aftermarket ones are not perfectly flat or not machined properly and can effect the thermostat operation.
 
#24 ·
As a matter of fact I did. I tried a CSI housing that has a filler cap on the top so I could top off the fluid at the highest point in the system but it still spiked when the Tstat opened so I took it back off because I didn't like having to 2 separate caps (one on the radiator and one on the housing)
 
#26 ·
A 16lb cap will almost definitely release some amount of coolant unless you're running the thing very cold and in a really hot environment (low temperature difference from cold to fully warmed up).

I typically see an 18psi system start to push a little coolant out to the expansion tank once the coolant is about 90* above ambient temp. I tried to find a chart that would indicate the expected pressure rise for a given temperature increase, but I'm failing to do so, unfortunately.
 
#27 ·
With a 15 pound cap water won't boil until it gets to 220 - 225F. With the 50/50 mix it is even a higher boiling point. Water expands at the rate of .00012 (.012%) per degree F of increase up to the boiling point. So with 100F increase the water expands 1.2%. (this is for distilled water and not a 50/50 mix of glycol)
Water expands 1700 times (1700%) when it turns to steam (completely).

Sorry, my chemistry experience doesn't include a lot of physics so I can't tell you the amount of pressure increase.
 
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#30 ·
Thanks for the compliment on the car.
I didn't change the intake, just the gasket and that's why this is blowing me away.

If my "mechanical" temperature gauge slowly rises as it works its way up to the thermostat temperature (from the initial start-up), that tells me there is no air pocket below the thermostat because I don't think the sensor reads air? Its a few seconds after the thermostat opens when it begins to spike quickly from 160 up to 220-240..
With that said, even if the radiator needed to recover during cool down and it sucked some air, I dont know how that air could end up in the block. Wouldn't it just add some air to the top of the water level in the radiator?
 
#31 ·
Sounds like a sticky thermostat, I would throw it in a pan of water and boil it to see how it opens.

These things are low tech, it obviously opens but in a stick slip manner.

Easy to test and might as well throw another new one in after you test it at the same time. I don't put an untested thermostat in anything I anymore, I have a collection of duds from over the years.
 
#33 ·
I quoted my first post and highlighted that I already tried 3 different thermostats



Here is what I have tried so far
1. Pressure checked the coolant system and it held 16lbs for 24 hours.
2. Jacked front of car up, installed a tall funnel and filled it 1/2 way with coolant, fired the car up and left it there until the thermostat opened and until it completely cooled down again. I did this 3 times..
3. Installed 3 different thermostats (the last one I installed is now a 160 so the temp spike isnt as extreme when the Tstat opens)
4. Installed 2 different radiator caps (it never spits any fluid out the overflow into my catch can)
5. Tried drilling two 1/8" holes in my thermostat and it still did it so i put another new thermostat in..
5. Verified there is no coolant in the oil and no oil in the coolant.
6. My radiator never leaks and it never pushes coolant out the over flow
7. My temp gauge is a mechanical gauge located in the intake next to the thermostat housing
8. The bypass hose that goes from the intake to the water pump is installed and was installed last year too..
9. None of my hoses are colapsed.
I know i tried other things, just cat think of them at the moment.
As mentioned, I didnt not have this problem at all last year..

Any help is appreciated
 
#36 ·
I've had the coolant spike on my Willys since day one. Half a dozen thermostats later it is still there. My latest 'stat is minimal of about 20 degrees so I just don't worry about it.

Even my diesel truck does this. About 20 degrees. 375k miles and it's still going strong.

My son's over the road 2016 truck has done the same since new. Not a problem.
 
#39 ·
I was on my way home from a car show last month on a hot day and it spiked to 240. There after, and until I made it home from the show, it never dropped lower than 220 and I never even got stuck at a light (was moving 45mph until I pulled in the driveway).. In other words, it never fully recovered from the spike.. Where-as last year, on an even hotter day, I was stuck ideling in a traffic jam for a good 20 minutes and my temp never spiked and never increased past 200. So no, these spikes are not normal for my car since it never did it at all last summer.
 
#40 ·
I am going to go back to my original thoughts. You didn't have this issue until you changed the intake gaskets. I think I would pull the intake and make sure something is not wrong with the gaskets or passages that they seal up. i am wondering if something is keeping the fluid from getting to the thermostat but once it does everything stays consistent.
Some of those gasket sets come with inserts for applications that dont want to circulate water through the intake.
 
#41 ·
Yes, some gaskets are blocked off to cover the center coolant ports for heads and intakes that don't have the crossover, which mine dont. Mine only have the 4 large ports on the corners, which are 1/2 as big as the intake ports. Here is a picture of when I was replacing the intake gaskets (the old one is still on the drivers side in this picture). I know for sure the new intake gaskets did not cover any of the 4 coolant ports.

 
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#46 ·
Its a Victor Jr. intake and there are no ports in the rear. After I changed the intake gasket I initially changed the upper radiator hose and thermostat housing to a swivel type but have since changed everything back to how I had it last year to rule out the changes.. Everything is now the same as last year.
 
#47 · (Edited)
I think I may have found the problem.
Looking back when I changed the intake gasket, there were only 3 things I had to unhook in regards to the cooling system
1. Upper radiator hose
2. The Bypass hose (from intake to water pump)
3. Water temperature sensor

Did anyone mention the water temperature sensor "fitting"? What kind of mechanics are you? LOL Just kidding!!!!

With that said, if you watch the below video, especially starting at 1:40 in the video, you will see how a lose fitting can cause a vacuum leak but not a pressure leak... Well I went out to the garage and sure enough I could wiggle the sensor just like the video (not as extreme). The fitting was tight but another 1/8 turn stopped the sensor from wobbling.. The reason it didn't leak fluid is because pressure would actually seal the fitting where-as vacuum (during cool down) would allow it to suck air.. I wont know for sure until I can drive it on Wednesday or Thursday but this all makes sense..

https://www.youtube.com/watch?v=e3FvTLG4uqY

....
 
#48 ·
Well If tightening the bushing doesn't help go back to the intake manifold gaskets. What is the part# of the gasket you used? From the checking I did it should have been Fel Pro 1275 or 12755. The gasket thickness is the difference. Both Edelbrock and Pro Comp recommend the same gasket. Another thing Stant thermostats are not one of my favorites. I replaced the T-stat in my 2000 GMC 5.3 truck twice with their brand t-stat. After refilling I could drive down the road and watch the needle on my gauge swing from about 210 back to 180 when it opened. I couldn't believe my eyes. I checked my fluid levels but they were on the money. I finally replaced it with a EMP/Stewart 308 then my needle held steady at about 200. They also sell an EMP-301 that has three holes in it. If I think it needs a hole I will drill it. BTW just personal preference I guess but I would not go below a 180 degree T-stat. I also agree to change the t-stat housing to an original GM aluminum housing. https://www.summitracing.com/parts/nal-10108470/overview/ Another thing to try is use some steel wool to clean your temp sensor.
 
#50 ·
Well the lose fitting wasn't the problem.. Here is what I have done since my last post
1. Installed new temp gauge, sensor AND new intake sensor fitting.
2. Purchased a coolant vacuum lift pump and the coolant system held 26hg of vacuum for 3 hours without dropping, which measn there are absolutely no leaks.
3. Used the vacuum lift pump to fill the entire system with coolant (this is suppose to eliminate any chance of air pockets)
4. Back timing down to 32 total to see if maybe the extra timing was building heat in the heads.

After doing all this, I did a cold start and let it idle at 1500 and watched the temp slowly rise to the thermostat set-point of 160 and then it leveled off and stayed at 160 for about 2-3 minutes... I initially thought "yay its fixed" but after staying at 160 for about 3 minutes it started to pulse up to 190 and then shot up to 240.. As soon as it spiked to 240 it came back down to 180 and stayed there..

Is it possible the pump could be going bad and with the thermostat closed, there isn't adequate pressure to pump through the bypass but has enough pressure to barely cycle when the thermostat opens? I'm at a loss here.. The only thing I have done yet is replace the water pump.
 
#52 ·
LOL, I double checked that already, the cone is on top and arrow pointing toward the Rad. I have tried a few different thermostat housings (even one with a fill neck on it), all with the same results. I'm currently running the same TS housing I ran all last year when I didn't have this issue.
 
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#53 ·
Once the thermostat opens, it seems to flush the hot spot out of the heads and past the thermostat and then I can drive it the rest of the day without any temp spikes at all (as long as it doesn't cool down to atmospheric temperature again, otherwise it will do it again).. With that said, I'm pretty sure I wont get a temp spike at all if the thermostat is out because its consistantly cycling.. But pulling the TS altogether would be an interesting test to try.
 
#54 ·
My diesel truck does this to although not quite as high over run. I gone through half a dozen thermostats even a couple expensive Cummins ones. Once the stat opens it stays steady at 190.

My car does it to but only 10-20 deg if it is really hot outside. It has the sender in the crossover so it see the hottest temp water.

If your sender is very close to the stat it will,probably over run some because all the hottest coolant is right there. It takes a while to cool the rest of the motor down.
 
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