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Old 04-29-2018, 12:18 PM   #1 (permalink)
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YJ SBC swap overheat no matter what

Hello all, I have a YJ that I swapped a TBI 350 in with a Holley 600cfm. My issue here is that it always overheats on the highway but not at idle. The fan properly cycles and cools the fluid at idle but as soon as I start moving the temp creeps up (250) within a few miles. The few trips I took when it was still cold out (30s and 40s) were a little on the warm side (210) but not over heating. My setup is as follows:

350 out of a 92 k1500 with a serpentine belt setup.
New water pump from local store
Aluminium radiator with approximately 18in x 19in fin area 3 core (new) with 16lb cap
Derale 2400cfm electric puller fan sucking air through the radiator properly
180* Thermostat (new and tested to be working with a 3/32 hole drilled to vent air pockets)
New head gaskets (which I'm fairly certain can't be installed backwards/upside down on this engine)
50/50 coolant - burped
SUM-226016 intake manifold and SUM-G2410 gasket
Timing without vacuum is appx 10-12* at idle then maxes out at about 30-35* with vacuum advance connected to a full vacuum port bumps both up about 10* with about 24in of vacuum at idle
I'm running at about 12-14 afr as it bounces around a bit.

I've tested the cooling system for exhaust gases by using one of those fancy vacuum tubes with the color changing fluid (no exhaust in coolant). I have looked at the water pumps by pulling the back off and both pumps spin correctly per a thread on another site that I am unable to link to.

The only thing I can think of is the intake or intake gaskets are wrong or the radiator either isn't getting enough flow or isn't efficient enough

The one thing I've found peculiar is if I start it and stop it after a less than a minutes, pressure immediately builds slightly to the point where if I pull the cap off coolant sprays all over but there's no presence of exhaust in the coolant.

I'm at a complete loss here. Any help would be MUCH appreciated.
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Old 04-29-2018, 06:29 PM   #2 (permalink)
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Originally Posted by deegan26 View Post
The one thing I've found peculiar is if I start it and stop it after a less than a minutes, pressure immediately builds slightly to the point where if I pull the cap off coolant sprays all over but there's no presence of exhaust in the coolant.
Builds press. in less that a minute? I'd take guess at a flow restriction.

Right away I was thinking water pump rotation....but you got that covered.

"Generally" overheating at speed is an airflow issue.
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Old 04-29-2018, 06:42 PM   #3 (permalink)
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Got a pic of your radiator set up. But for me I think 2400 cfm is not enough. I have a 3500cfm pusher in my CJ crawler with a sbc406 and it gets warm when hot rodding it. It's fine normally when wheeling.
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Old 04-29-2018, 07:45 PM   #4 (permalink)
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Builds press. in less that a minute? I'd take guess at a flow restriction.

Right away I was thinking water pump rotation....but you got that covered.

"Generally" overheating at speed is an airflow issue.
It's just enough to spit out some coolant. always thought it was weird. Where could a blockage be? The inside of the block looks brand new. No rust or scale buildup. And the thermostat opens before the pot of water boils when I tested it. and looking at the impeller (when the pulley would be spinning counter clockwise like it should) it looks to be slinging water towards the ports to the block. The intake manifold only has coolant ports up by the thermostat and the gasket looks like you cant mess that up at all.


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Got a pic of your radiator set up. But for me I think 2400 cfm is not enough. I have a 3500cfm pusher in my CJ crawler with a sbc406 and it gets warm when hot rodding it. It's fine normally when wheeling.
I'm still too new to post links or photos sadly. So I'll do my best to explain. Rad Setup is as follows:Looking at the front of the jeep front the outside, 9 inch trans cooler sitting in the lower left quadrant looking with a pusher fan over top and then a 17 inch puller behind it sitting high right to use the pusher to cover the missed area. Can't fit a shroud. Winch covers about 6 inches of lower rad but air should still freely flow over that. Radiator is about 2.25 inches thick all alum. with 3 .5 inch-ish rows.

I ordered a 17 inch flex fan and am gonna try that and see if it helps. I went with the 2400 because the only other one that pushed more air between this one and Spal was meant for paved circle track. I didn't believe the amazon/ebay ones that claimed bigger numbers for $50.

also this motor is just a stock 350 TBI with a carb instead and shorty headers that are wrapped.
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Old 04-29-2018, 07:56 PM   #5 (permalink)
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First thing I would try is move the trans cooler to a separate location. This is just adding heat to the radiator.
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Old 04-30-2018, 07:20 PM   #6 (permalink)
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^^^^ the above. Plus it also sounds like you have a lot of air FLOW restricting articles in the way there too....winch....pusher fan....puller fan. What are they ratings of the pusher fan.

As far as blockage...when you said it builds press in less than a minute, I thought you meant the hoses became pressurized in less than a minute. So, it spits out coolant where, the over flow?
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Old 04-30-2018, 07:55 PM   #7 (permalink)
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Those two fans may be fighting each other.
Try pulling the fuses or turning them off if you have a switch, then try running the truck at speed and see what happens. GM LS factory programming usually turns off all fans at speeds above 45 mph.
Also, try another radiator cap even if the one you have is new. If it's burping that quick most likely it's bad. Check heater core for trapped air.

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Old 04-30-2018, 08:50 PM   #8 (permalink)
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I think its a coolant flow problem.. restriction.
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Old 05-01-2018, 07:41 PM   #9 (permalink)
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https://www.speedwaymotors.com/Water...r-Kit,605.html try something like this, or just take an old thermostat and cut the core out and shove it back in, see if you get any change
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Old 05-09-2018, 06:02 PM   #10 (permalink)
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What water pump, thermostat and radiator cap are you running?


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Originally Posted by deegan26 View Post
It's just enough to spit out some coolant. always thought it was weird. Where could a blockage be? The inside of the block looks brand new. No rust or scale buildup. And the thermostat opens before the pot of water boils when I tested it. and looking at the impeller (when the pulley would be spinning counter clockwise like it should) it looks to be slinging water towards the ports to the block. The intake manifold only has coolant ports up by the thermostat and the gasket looks like you cant mess that up at all.




I'm still too new to post links or photos sadly. So I'll do my best to explain. Rad Setup is as follows:Looking at the front of the jeep front the outside, 9 inch trans cooler sitting in the lower left quadrant looking with a pusher fan over top and then a 17 inch puller behind it sitting high right to use the pusher to cover the missed area. Can't fit a shroud. Winch covers about 6 inches of lower rad but air should still freely flow over that. Radiator is about 2.25 inches thick all alum. with 3 .5 inch-ish rows.

I ordered a 17 inch flex fan and am gonna try that and see if it helps. I went with the 2400 because the only other one that pushed more air between this one and Spal was meant for paved circle track. I didn't believe the amazon/ebay ones that claimed bigger numbers for $50.

also this motor is just a stock 350 TBI with a carb instead and shorty headers that are wrapped.
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Old 05-09-2018, 06:08 PM   #11 (permalink)
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Opinions;

Electric fans aren't appropriate for cooling at highway speeds. Get a good mechanical fan, big, lots of blades, and a quality clutched fan.

Get a shroud. Build one, buy one, whatever.

I've got a TJ and YJ both with V8 conversions.

Also, my stock replacement radiator cools way better than my $$$ aluminum one. Go figure.
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Old 05-09-2018, 06:10 PM   #12 (permalink)
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I had a '96 K1500 and that radiator seems like it's half the size of stock
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Old 05-10-2018, 09:36 AM   #13 (permalink)
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Are you sure you got the correct reverse rotation water pump? AND get rid of the electric fans and go with a fan clutch and a solid blade fan and a shroud. I used to have a big block 454, 4x tow truck with a bad ass dual electric fan set up, heavy wiring, relays the whole thing. Always overheated no matter what I did. Put the factory style fan system on it and no more problems...ever!
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Old 05-10-2018, 09:44 AM   #14 (permalink)
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I had a similar issue on my flatty with a sbc. The lower radiator hose was collapsing at higher rpm. do you have a spring in it?
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Old 05-11-2018, 04:46 PM   #15 (permalink)
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I ran a stock tbi 350 in my 76 cj7 with a sbc conversion radiator, Ford Taurus fan and never had an issue with it overheating. I had a normal re-man water pump from oreillys and 180* T-stat.

If your yj is anything like my cj was, there probably isn't enough clearance to put in a mech fan.

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Old 05-11-2018, 05:14 PM   #16 (permalink)
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If your yj is anything like my cj was, there probably isn't enough clearance to put in a mech fan.
BS.

Got mechanical in BOTH my YJ and TJ with V8s. And a big shroud. Plenty of room.
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Old 05-12-2018, 10:37 AM   #17 (permalink)
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BS.

Got mechanical in BOTH my YJ and TJ with V8s. And a big shroud. Plenty of room.
Obviously yours wasn't anything like mine...
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Old 05-14-2018, 10:14 AM   #18 (permalink)
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Also, my stock replacement radiator cools way better than my $$$ aluminum one. Go figure.
A friend of mine had one of those Champion brand aluminum radiators in his big block Dodge Dart and he couldn't keep it cool. The thing was new and was plenty large enough to cool his car. He swapped it out for a new copper/brass radiator and his problems went away. I don't know what was up with that aluminum rad, but it sucked.
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Old 05-16-2018, 04:59 AM   #19 (permalink)
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A friend of mine had one of those Champion brand aluminum radiators in his big block Dodge Dart and he couldn't keep it cool. The thing was new and was plenty large enough to cool his car. He swapped it out for a new copper/brass radiator and his problems went away......
Simply put.....Copper-brass conducts heat considerably better than aluminum does.

Although, admittedly the argument doesn't stop there, rad design and construction carry it on.
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Old 05-16-2018, 06:04 AM   #20 (permalink)
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Simply put.....Copper-brass conducts heat considerably better than aluminum does.
If the difference between copper/brass and aluminum makes the difference between overheating or not then there is something else that is seriously wrong with the cooling system.
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Old 05-16-2018, 06:34 AM   #21 (permalink)
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The first thing I'm thinking is water pump rotation... if you open the radiator cap are you able to see that the water is Flowing from the top of the engine to the bottom? It's pretty obvious on my truck which is a 72 engine converted to Serpentine.

The pressure build-up really is strange. That sounds more like the wrong head or intake manifold gaskets.
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Old 05-16-2018, 12:20 PM   #22 (permalink)
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Simply put.....Copper-brass conducts heat considerably better than aluminum does.

Although, admittedly the argument doesn't stop there, rad design and construction carry it on.
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Originally Posted by arse_sidewards View Post
If the difference between copper/brass and aluminum makes the difference between overheating or not then there is something else that is seriously wrong with the cooling system.
I agree to both, but there must be something wrong with the cheaper aluminum units that make them suck. When the one my friend bought was new out of the box, it looked like it would keep anything cool but that wasn't the case. I know aluminum radiators can lose conductivity due to corrosion, I wonder of the cheapies have a layer on them, or the way they assemble them doesn't conduct heat to the fins. Don't know for sure, the one thing I do know is to stay away from them.
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Old 05-16-2018, 02:07 PM   #23 (permalink)
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Considering that engine had a 5-6000CFM mechanical fan and a huge radiator in its original home, I'm going to say that it's an air flow issue and a radiator issue. Assuming the water pump is correct and the thermostat isn't hanging up. Let it idle with the cap off and see if you can see the coolant flow when the thermostat opens up. You could also rev it up to highway RPM's and see if the lower hose is collapsing.
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Old 05-16-2018, 05:18 PM   #24 (permalink)
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Considering that engine had a 5-6000CFM mechanical fan and a huge radiator in its original home, I'm going to say that it's an air flow issue and a radiator issue. Assuming the water pump is correct and the thermostat isn't hanging up. Let it idle with the cap off and see if you can see the coolant flow when the thermostat opens up. You could also rev it up to highway RPM's and see if the lower hose is collapsing.
This X2. It comes down to removing the heat from the radiator. You need airflow to do that.
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Old 05-16-2018, 06:49 PM   #25 (permalink)
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^^^ see 2nd post.

He states he has the "correct" water pump (rotation) so I will not question that.
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