Overheating on Hwy 350 SBC

[JPMSK5]

I have thrown this at many other mechanics, but no one has figured it out, 79 K5, rebuilt 76 4bm 30over sbc fresh, new 3core radiator, new fan shroud, new fan clutch, fan fits shroud correctly, new cap, the belt system is off a 76 chevy truck sbc with a/c, there is a small pusher fan that comes on with the high throttle silenoid for a/c which seem to work fine and cured the a/c heat problems, tried different types and temps of thermostates, but on the hwy, over 20 miles and the engine starts overheating, the faster you drive the hotter it gets, the slower you drive the cooler it gets, messed with trans, 700r4 rebuilt, using the lockup and extra cooler, it all ways temps 20-30 cooler than the engine, so I have ruled that out, I installed a vac gauge in the dash and not much vac is seen cursing, like 5 in OD locked, and 12 in D locked, so I am going after the timing with a new dist. now, I have lost my last wit with the HEI weights, the pins come loose, or they are so far out you have to add material (weld) to the center piece and grind to get the correct curve for the engine, all of the reman HEI have been way out by 13-28deg past 20-22deg mech advance needed. I am going to fatten the carb alittle also, and I guess if that doesnt fix it, fuel problem with supply maybe or maybe I am just geared to high, 4.10, 35 BFG AT KO, 16.5, other fuel related parts are stock mech fuel pump, world heads, rpm air gap, rpm cam, 1411 carb, 10/1 compression always, always ran on 91oct.
Any help will be much appreciated, as much money I keep throwing at this, I should offer a reward for who can correctly identify the problem, LOL

 

[jonrpick]

I have thrown this at many other mechanics, but no one has figured it out, 79 K5, rebuilt 76 4bm 30over sbc fresh, new 3core radiator, new fan shroud, new fan clutch, fan fits shroud correctly, new cap, the belt system is off a 76 chevy truck sbc with a/c, there is a small pusher fan that comes on with the high throttle silenoid for a/c which seem to work fine and cured the a/c heat problems, tried different types and temps of thermostates, but on the hwy, over 20 miles and the engine starts overheating, the faster you drive the hotter it gets, the slower you drive the cooler it gets, messed with trans, 700r4 rebuilt, using the lockup and extra cooler, it all ways temps 20-30 cooler than the engine, so I have ruled that out, I installed a vac gauge in the dash and not much vac is seen cursing, like 5 in OD locked, and 12 in D locked, so I am going after the timing with a new dist. now, I have lost my last wit with the HEI weights, the pins come loose, or they are so far out you have to add material (weld) to the center piece and grind to get the correct curve for the engine, all of the reman HEI have been way out by 13-28deg past 20-22deg mech advance needed. I am going to fatten the carb alittle also, and I guess if that doesnt fix it, fuel problem with supply maybe or maybe I am just geared to high, 4.10, 35 BFG AT KO, 16.5, other fuel related parts are stock mech fuel pump, world heads, rpm air gap, rpm cam, 1411 carb, 10/1 compression always, always ran on 91oct.
Any help will be much appreciated, as much money I keep throwing at this, I should offer a reward for who can correctly identify the problem, LOL

Subscibed...

Mine only overheats while going down the road... WTH? The only thing that changed was the tranny, and a whole LOT of time just sitting.

 

[mnstrburbon]

can you hear the fan engage the thermo fan when it over heats and what is over heating what temp?
stock water pump or high volume?
have you tried a different guage i have a brand new autometer and it wont go lower than 140 and says i run at 210 im sure it a littel lower.
Do you have the heter core hooked up?
Sounds like you are circulating the water to fast wrong size pulley perhaps.
Could also be a reverse flow water pump for a later moter?
Do you have a short 3 core or a tall 3 core?

 

[JPMSK5]

stock water pump for 76 truck its correct,

short tall no clue, Its suppose to fit BB applications for 79 trucks.

yes heater core and plumbed from intake to pump not radiator,

180deg thermostate and it stays at 180 till 20 miles down the road above 50mph, then it starts to climb to 215 and climb higher the faster you go, if you slow back down to about 50mph from 70, it will start to go back down after about another 20miles its back at 180 till you pick up speed again. OH OH and it ONLY does it when its above 75 outside, cool days or winter, no problem, winter it takes forever to warm up it seems. right now the outside temp is 95-101, so it is really really bad!!!

never hear the fan, which I have questioned, but what can you hear at 70mph topless??, not the radio or cell phone for darn sure. its not the fan, you can hear it sometimes, LOL

all new autometer gauges!! Its not the gauge because when it shows it getting hot I loose a ton of power, like nothing when you floor it, normally you worry if the u-joints are going to hold up at any speed when you floor it. when its hot, it got no guts at all

 

[dyeager535]

Do I read that right that you are supposedly running a BB water pump on a SBC?

Heater core gets pressure from the intake, through the core, to the radiator. That's the way it should be run, or you recycle coolant from the engine back into the water pump, at least on first gen small blocks. This also causes a "leak" in pressure inside the block, which decreases coolant effectiveness apparently.

Still wouldn't trust the gauges either. New means nothing anymore, although I do understand the correlation between loss of power and indicated temp. I'd still want to at least hit the engine with an infrared thermometer to compare.

You could remove the t-stat to see if that makes a difference, they are quite commonly bad out of the box, 2 bad in a row is not impossible. It would not hurt to remove as a possible culprit for testing purposes.

These trucks do not need an engine fan at cruise during low load, IF the cooling system is up to par. Overheating, unloaded, at freeway speeds has nothing to do with a bad fan.

 

[Jason4x4]

Do you have any KC lights or anything of that nature in front of the radiator? Might need a 4 core radiator...

 

[4X4HIGH]

Sounds to me like you either have a bad thermostat or possibly the water pump is for a later model with serp belt which is reverse rotation. I know you said it was for a 76 but that doesn't mean that the correct pump was in the box when you bought it. Also if you remove the thermostat completely it will overheat since the water doesn't have a chance to sit in the block and absorb the heat so that when the thermostat opens and lets that hot water into the radiator and has the chance to dissapate that heat through the cooling fins of the radiator. It's a vicious cycle that happens over and over again.

 

[dyeager535]

All I'm going to say is that isn't universally true. No reason to discuss it I suppose, it's been plainly obvious for a long time that there are two sides to the debate, and it seems no one ever decides to change their mind.

I found that these guys statements mirrored my experience running a GM engine with no thermostat.

http://www.stewartcomponents.com/tech_tips/Tech_Tips_6.htm

Edit, changing my thought process here. Lack of the pressurized coolant without a thermostat could be a reason some engines overheat without a thermostat and others don't. It could also be due to turbulence caused by faster coolant flow.

 

[POWERMAD]

The impeller could be slipping on the shaft. At higher RPM's it slips.
A quick test is to have the heater on full blast and go for a test drive.

If the vent temp is cold and the gage says it's hot then it's not pumping coolant.

Won't come up to temp and overheats on a pull is a bad t-stat.

I would offer up restrictions & such but you seem to have that covered with a new radiator.

And it only does it at higher RPM's which leads me towards the water pump.
The bottom hose isn't collapsing is it?

 

[gone huntin]

At what rpms are you running hot? I know you said higher speeds, but I've seen no mention of the rpm range. Also, have you corrected the timing issue? Timing can definately cause this problem.

 

[buffblazer]

could there be air locked into it? i know you need to burp those systems after replacing a hose or complete radiator and usually they do it on their own but could that be the problem? take the cap off and let it run for awhile until it starts to boil over

 

[JPMSK5]

Do I read that right that you are supposedly running a BB water pump on a SBC?

No its a SBC pump for a Gen 1 with factory Gen 1 pulleys installed, its correct. I have plenty of heat from heater when it gets hot, so water is moving, yes it comes from the intake to pump, I tried it in the radiator and the engine doesnt warm up at all in the winter.

 

[JPMSK5]

Do you have any KC lights or anything of that nature in front of the radiator? Might need a 4 core radiator...

I have a push bar I made with a winch and lights, hooks, Hi-lift, etc. But I have craned that stuff off the front and it makes no difference at all, if anything the temp got hotter quicker from not having it to guide air into the grill and raised the truck 2inch up in the front.

 

[JPMSK5]

At what rpms are you running hot? I know you said higher speeds, but I've seen no mention of the rpm range. Also, have you corrected the timing issue? Timing can definately cause this problem.

Well in D locked up is about 2900rpms at 70mph and in OD locked up is about 1900 rpms at 70mph, and the engine struggles to maintain speed and has a very low vacuum in OD, also at 6000rpms in any gear the engine starts to heat up pretty quickly, thats why I am attacking the dist and carb thing right now. The cooling system has been worked over and over and everyone local here is convinced its not the cooling system, it has to be timing or fuel problem. But what, no one knows.
New Dist and rods and jets overnighted from Summit just now. So tomorrow I might have a chance to get it in and set up, and find out this Saturday if it made a difference.

 

[buffblazer]

now did i read that correctly when you said 6000rpm, what cam are you running? not that its cooling related but you could be over revving your engine

 

[JPMSK5]

now did i read that correctly when you said 6000rpm, what cam are you running? not that its cooling related but you could be over revving your engine

Yep thats right, B&B engine, pop up pistons, large chamber heads to cut the compression back to pump gas, roller rockers, Scat crank and rods, world heads, etc, etc. RPM air gap intake, RPM Cam, 1411carb that I have ordered parts to jet as a 1407, MSD 6, just ordered a MSD pro billet dist to replace that HEI, afterburn headers, 2.5 dual exhaust, with Flowmaster 40 something mufflers. That engine was balanced at 10,000 rpms and I know the valves float at about 7800rpms, that was a oops!
The short block is the same as the race engines, we build for mud drags, thats why the large 76cc heads to cut back to pump gas. The race engines will hold 8200rpms++ and have only lost one conrod in 6years of racing. The race engines have lots and lots of other HP stuff that this engine doesnt need for the street/trail, but yes 6000rpms is right and it should hold that with no problem at all.

 

[wasted wages]

Well in D locked up is about 2900rpms at 70mph and in OD locked up is about 1900 rpms at 70mph, and the engine struggles to maintain speed and has a very low vacuum in OD,

Race engine in a 2 1/2 ton truck with 35" tires and 4.10 gearing with OD trans
sounds like the problem to me....


but what do I know...


try driving it around in low range at the same RPM's and see if it overheats...

 

[4X4HIGH]

Yep thats right, B&B engine, pop up pistons, large chamber heads to cut the compression back to pump gas, roller rockers, Scat crank and rods, world heads, etc, etc. RPM air gap intake, RPM Cam, 1411carb that I have ordered parts to jet as a 1407, MSD 6, just ordered a MSD pro billet dist to replace that HEI, afterburn headers, 2.5 dual exhaust, with Flowmaster 40 something mufflers. That engine was balanced at 10,000 rpms and I know the valves float at about 7800rpms, that was a oops!
The short block is the same as the race engines, we build for mud drags, thats why the large 76cc heads to cut back to pump gas. The race engines will hold 8200rpms++ and have only lost one conrod in 6years of racing. The race engines have lots and lots of other HP stuff that this engine doesnt need for the street/trail, but yes 6000rpms is right and it should hold that with no problem at all.

So this explains things a bit better now. You have a race engine in a truck serving street/offroad duty and expect it to stay cool at 6000rpm's for extended periods of time. Race engines aren't ran for extended periods of time and thus don't have a chance to overheat. I think you're just expecting too much from what you have in your vehicle.

 

[JPMSK5]

Race engine in a 2 1/2 ton truck with 35" tires and 4.10 gearing with OD trans
sounds like the problem to me....


but what do I know...


try driving it around in low range at the same RPM's and see if it overheats...

Well that is next, to drop gears in it. I have 4.56 for the rear 14ff, and am going to try that if dist and carb deal doenst work out.

 

[JPMSK5]

So this explains things a bit better now. You have a race engine in a truck serving street/offroad duty and expect it to stay cool at 6000rpm's for extended periods of time. Race engines aren't ran for extended periods of time and thus don't have a chance to overheat. I think you're just expecting too much from what you have in your vehicle.

Nope, you missed it, I dont expect to turn 6000 all the time, but in a couple mins in a hole, it shouldnt climb as much as it does, I see the post has lost sight. This is the same engine build, I put in muscle cars, ie, camaros, nova II, etc. So it is a street able engine, because of several other engines I have built are proof, in my case of the K5, yes I know the power band doesnt come in till about 1700 and not idle, and I have a 2000 stall/lock converter, but it shouldnt do what it does. Weight and gears might be it??