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SBC 350 Overheats

4452 Views 86 Replies 12 Participants Last post by  Joe White
I've been battling an overheating issue for quite some time. Now I do understand it is 100+ degrees out right now, but I'd like to believe the engine should still be able to cool itself off.

I've got a big three core aluminum radiator, with dual electric puller fans, and smaller electric pusher fan. Running a 195 thermostat at the moment, was running a 180 (no change). There is ac in the truck, but the overheating problem still exists with the ac off. I am able to open the hood and let it idle and it will cool down to around 210. Doesn't seem to matter if I idle or drive around, if anything heats up faster when driving around.

The trucks radiator appears to keep up for the first 3 or so openings of the thermostat, keeping close to 195. But after than it seems to overpower the radiator, and the fans are just blasting 200 degree heat constantly.

Here are some picture for reference.
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Every vehicle, with any engine combo should be able to completely remove all cooling fans and be able to drive above 45 mph and have zero cooling issues. this is due to the forced air being pushed through the radiator.
I think your right on forcing more air into the radiator. I'll try to seal up the radiator some more. I'm still just worried about it working well on idle. It is a slow overheat, so I'm fine at stoplights as long as I'm not at one for 5 minutes.

Here is what I did the other day trying to see if it would help with my overheating issue. It appeared to help, around 207-210 while driving around like 30mph. Before hand it would slowly climb to about 220 before I would come back home and open the hood to let it cool off.

I'll try to fix up some of the gaps where the air might be recirculating, see if that helps. Just trying to find a good way to do it and it not look too terrible.

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OP i see you are using the 4.3 belt routing. Is the waterpump a reverse rotation pump? I believe that causes all the problems if it is not.

A bit tedious vid below but you get the idea.
I've been wondering this too. I didn't do the original engine swap. Is there a serial number on the waterpump to look up. I didn't see one on top. Also whenever I was toping off the coolant, the coolant appeared to be bobbing up and down quite rapidly. Not something I've really ever seen before.
I am about 98% sure the 4.3 pump will fit the SBC. Around a 90 91 water pump should work and it is reverse rotation. Changing the pump will most likely take less time than putting in the extra fans. I think what you are seeing with coolant rise and fall is the pump impeller causing cavitation.
I'm not 100% ruling out the waterpump, but I don't believe it to be the issue. I drove the truck again tonight, only about 90F outside plus its dark. But I am able to run the AC at somewhere probably about 1/2 max ac. Otherwise at max ac the truck slowly, over a couple of miles, rises about 10 degrees. When I turned the ac down it managed to slowly go back down and start cooling normally, and staying at 195F (the thermostat temperature).

The air dam/scoop that bk2life mentioned will help a lot. Time to hit the PickNPull. You can use backer rod to seal around the perimeter or the radiator....or a slice of foam pipe insulation. It's closed cell so it won't soak up or retain water. All of that along with ditching the pusher, and getting a good set of puller fans and you'll be golden.

I did make some small improvements with putting some of that backer rod around the rad. Glad you recommended that, never would've known what to use. I'm not done putting the backer rod in to seal up some of the air gaps that could be recirculating all the hot air. Also need to close up the big opening in front of the radiator, which is more than likely a big cause the heat "recycling". Might look into getting another set of puller fans, the ones in it seem to work good, but I noticed the shroud is pretty cut up in a couple of places. If you know of any good brands or fans that would be good for this please tell me, I got no clue.

Also just coming to terms that idling in 100+ degree heat might not be that feasible with such a big engine, with limited cooling capacity.
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Did some research on the fans, they appear to be a dual fan kit, Maradyne MJS22K dual 12in fans. This is the first car of mine to have electric fans, to me they appear to move a lot of air, but maybe when it comes to electric fans they are on the slow side. Didn't see anything about CFM but the wattage was 225W. Is that enough? I'd be fine upgrading, it is just another jap into my wallet I don't want to do.

Edit: looked up the fan itself, a single fan appears to be 225W with a CFM of 1565.
Get the Intrepid fans...6 blade.
I’ll need to check on the width. The fans that are in it are almost touching the water pump. With the ac condenser and enormous radiator it’s a tight fit. But if they will fit I’ll get some of those fans.

have you tried a second temp gauge? just to make sure youre not chasing the dragon?
I actually already have 2 temp gauges. One for the normal style gauge and one that goes to the Holley Sniper. I know the normal style gauge doesn’t work correctly. It’ll show around 240 when I’m at 200. But the digital one for the Holley works. I also have a temp gun so I’ve checked it with that.

All in all I do believe it to be the fans not pulling enough heat out in time. Slowly creating the overheating.
You can always recess the radiator into the core support. The downside is your radiator is so thick that you'd have to put the condenser on the other side of the core support. That raises the issue of modifying the hood latch support.

You could get a stock 4.3 radiator and condenser recessed in the core and have plenty of room.
I know yours is a 1st gen, but this is what we did to mine. 2000 Grand Am radiator and condenser that snaps onto the radiator...
Looking at the distance I have between the radiator and engine, I cant get any fans bigger than the ones that are in it. Which are only 2.5in thick.

I'd try putting a kill switch on the fans that you can flip in the cab. I'm willing to bet he'll see his temps go down by shutting them off above 50 mph. Just don't forget to turn them on when you slow back down.
I also think his fans look too shallow to seriously move enough air. CFM ratings from the manufacturer need to be taken with a grain of salt. 225 watts at operating 13.8 volts is 16 amps. At Battery voltage of 12 volts it's 18.75A.
So I gave that test a go, drove around 55-60mph turned off the fans while driving, temperature quickly rose from around 201-202 to 216, within a minute.

Turns out my AC compressor leaks, so I couldn't test it with the ac on as it stopped working. But today being right at 100F outside the truck would hover right around 201 when driving around, with the fans constantly on.
Sorta not a update, but an update. Nothings changed, but I want to know if it is possible to do a mechanical fan, or it is just better to upgrade to better electric fans. I've had the worst time with this truck the past few weeks, needed a new ac compressor, alternator still wont charge after a new one, struggles to idle and start, and of course the overheating problem. But I never really understood how little air the electric fans moved. Seemed like plenty to me at first, but then I went over to my big suburban and with its mechanical fan it was blowing 4x as much air at idle. So in the end I'm putting more money into the truck no matter what, but not sure what option is the best, especially considering I plan to use the ac. Also radiator options, I will try to get back the smaller radiator I had before the one in it now.
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I went to a pick n pull, grabbed fans to which I believed to be similar looking to what was suggested. Turns out it wasn't exactly, and the fans I got only does 800cfm. I a little confused on all the cfm numbers, since there is two would that mean for each, or both. Because the fans I have now show 2600 cfm. Also clarifying the fans I have I have confirmed to be MJS26K Jetstreme series. I'm pretty tired of spending money. If there is something I can find easily online that should be able to cool I'd be very interested.
Get the Intrepid fans...6 blade.
After returning to the junkyard and returning the fans I got, turns out the only intrepid they had the fans were off it.

so…I bought the Four Seasons brand version of the intrepid fans. Saw someone else post the same one I got on the forum.

ive looked into a couple of things more. My problem with overheating I believe consists of 3 things. One being the recirculating of hot air, two the fans not pulling enough, three the fans probably are not getting as much power as they should. The wiring used for those fans appears to be something like 16 gauge. Which did get warm under load. I’m glad I caught onto that.

Upgrades obviously being the new intrepid fans and wiring. I planned to use 12 gauge wires. But that might not be enough for the intrepid fans. Also what amp relays should I use.
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Many of the aftermarket "Intrepid" fans are NOT dual speed as the factory models that is why there was a ridiculously low price on them last year Or so.
I’m not sure if this ones not two speeds. A bunch of the reviews people seemed to like it. I don’t care if it has two speeds though, I could always just turn one fan on later then the other one.
Another update. Got some new fans installed from Summit, Last fans were around 2600 cfm these show 3,700-3,799 cfm. so a noticeable difference. Now I noticed that while it helped some it didn't seem to fix the problem. 100 degree days seems to drive fine without the ac fine and stay right at 195. But when I turn the AC on it seems to come up to 210, now I wasn't on the road for longer than 15 minutes after it warmed up so I didn't really know if it would go any higher or if it was done climbing.

Now I haven't got to test it on the highway, since I don't live super close to a highway. But just driving around at 40-50 seems to sit around 205-207 with the ac on. I did add some sheet metal under the bumper to scoop some air into the radiator while moving.

I'm just a bit lost on what else it could be, think I've tried everything there is, sealed up the radiator, tried bigger radiator, larger more powerful fans, coolant additives.
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I truly believe the rest of your problem is flow related. Ditch the oreily water pump and get a good stewart warner or edelbrock aluminum pump. The oem style pumps impeller has too much slippage and won't move enough coolant through your thick radiator.

When I finally fixed my trucks airflow issue and it was still running "just a bit too hot" my sewart pump dropped the running temps from 210 down to a steady 185-190. Are you running oem pulley ratios?

Does your intake have bungs to add rear coolant returns? SBC's take really well to that modification.
Checked on the waterpump today, it is a reverse rotation. It seems to work great. I changed out the thermostat once more to a 180. I didn't build this motor nor put it in the truck. Which I though was a cool thing to not have to deal with, now I am regretting it since I don't know much of the details of the engine. But from the looks I would imagine the pulley is stock as it is a very large one.

It does have the plugs on the back of the intake.
How did you determine it works great?
I should say the bearing and impeller seem good, bought a new pump, same one suggested from earlier at oreilys. But after looking at the current one there is no reason to replace it so I put it back on.
I should add that if I leave the cap off while it is warming up, it does seem to once the thermostat opens the coolant rises and falls quite a bit, my assumption is that is just the thermostat opening and closing, this causes some of the coolant to overflow out the top when this happens. Overheating problems is new to me, so I'm unsure if this is normal.
Might want to look into this pump on Summit.
They say:
  • Increase pump pressure up to 300 percent
  • Eliminate steam pockets trapped inside the engine block
  • Reduce engine temperature by 15-30 degrees
I will look into putting that on, as it might help with the problem.

All my tests I've done with this truck has been off highway, on roads that go around 45. So no real highway driving. But today I had to drive 3.5 hours with it on the highway, couldn't go above 65 without it slowly moving back up. The overheating is relative to speed. So it will cool slowly back down under 65, but a lot faster at idle. The radiator shroud I have does have the rubber flaps to help move air through the shroud/radiator at higher speeds.

I did remove the push fan so it is only two pull fans.

The only things I can think of are the waterpump and possible air in the system. If air in the system would cause this type of behavior I will defiantly try bleeding it some more. As replacing the waterpump requires me taking the radiator out due to the limited space.

Just wanted some thoughts as this truck is giving me some major headaches.

Edit: Never thought about this until now, but the possibility that the radiator cap could be bad, I have no clue what type of effect that would cause though.
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Yeah try a new cap. I've never really seen them go bad but it could happen I guess. Air in the system is highly unlikely in an sbc, its very hard to trap air in a forward flow cooling system as it naturally bleeds. Ideally the thermostat housing should be the highest point in the cooling system.

If you're overheating at highway speeds you definitely aren't circulating coolant well enough. Airflow just isnt an issue at 65 mph. Seems like flow is dropping off dramatically below a certain rpm.

This is what I meant for the coolant returns, not mandatory by any means but it does help give hot coolant at the rear of the heads a way out.
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I'll go buy a new cap. I bought the pump from Summit Racing, I don't have a place to work on it at the moment so I'm unsure when I'll have time to go change it. But it would make sense at higher speeds that the impeller could be slipping on the shaft. I feel stupid for not checking that, I kind of just looked at it, and it didn't look that old so I didn't even think about that being a possibility.

The cap could very well be the problem though, I noticed that the overflow tank has not changed a bit since I put new coolant in it the other day. Which is defiantly wrong because I know there has to be some air left in the radiator. Almost impossible to fill it completely when the level goes up and down and overflows coolant out of the fill neck.
Well, I call it fixed I guess. don't think there is anything else to replace. I put in the new waterpump, still gets warm on highway. Before the waterpump I was only able to go 65 mph at around 205F, (180F thermostat still) now with the new pump I was going 70-75 mph at around 198F, but slowly..slowly..slowly climbing up. So after 30 more minutes it could've gone to the old temp. Both days I've driven it on the highway, same route. Both days a cool 75-80F outside. I've spent wayy to much on a simple problem.
In post #25, you’d mentioned “struggling to start and idle.” That, plus the overheating, has me wondering if the timing is too far advanced. That would be worth checking.

A flow issue could also be a block issue, especially if a PO used Barr’s or some other miracle in a can, and there just isn’t enough water moving through the engine. Might be worth doing a couple of flushes.

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I believe your not wrong, I think there is something with the flow wrong, so I need to flush it some more. Timing is not set correctly I'll add, but I have adjusted it far from what it has been right now and no change. But both need taken care of, harmonic balancer is off, so it isn't as easy to set timing since I have to set another TDC.

I've been looking into sending it into a shop for them to take a look and perform those two things. Only one that was willing is 2 weeks behind, but obvious something is wrong, because it was 77 out today, at night, and it was overheating.

Thank you for the response.
You've been driving it w/o a harmonic damper?
It has one, it is just obviously offset. Correct timing on it shows around 70 degrees after tdc. I tried setting it on the dampener at 34/36ish, still after tdc, but the engine would spit out the throttle body, when trying to accelerate.
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