400CID cooling issues

[1978K5]

Hi, I'm new to this site and am looking for any help I can get regarding cooling a 400 SBC engine. I have a 1978 Blazer which has a 400 V8, TH350, 3.73 Positraction rear axle, power windows, locks, TG window, cruise, tilt, guages with tach, cloth seats, roof rack, rear air deflector, and original aftermarket AM/FM 8 track stereo with fender antenna and CB radio. I just replaced the radiator with a brand new 4 core model, and also replaced the fan clutch. I havent replaced the thermostat yet or water pump, but I checked the condenser fins and they appear ok. I replaced the temp guage that was in it originally and the needle in it would spin freely. That guage always read what I would believe to be 175-195 degrees. The one I put in has a steady needle and it is reading 210-225. Im assuming the first one with the spinning needle was junk and I have a good one in the truck now, but how could they both be so different? I dont want to be troubleshooting a non-existant problem, but I tend to err on the side of not cooking my 400. I know from both what I read and some past experiences, that the 400 has an inherent problem of running hot. So what would be considered normal for a 400 and normal for a 350 then? Any help with this would be appreciated and its nice to see a site like this is out there. Many Thanks

[max75432]

1/2 ton 74 chevy truck 4 speed 373 gears with locker rear end
35" x 12.5 tires & 10" wheels I have ran 38's x 14.5 tires
I have a 350 bored 60 over with a 400 crank installed with heaaders and elderbrock high rise with a spreadbore holley.
I had the same problem with my motor until I changedout my radiator with a 4 tube i ton radiator that I bought from Auto Zone for less than $200.00. I run 180 degree -thermostat. I run a manual heat gage because I donot trust electric gauges. I have tomake sure the cable stays away from the header to maintain a true reading on heat. I also went to a 7 blade direct drive alum. flex fan. But I went back to a clutch fan to make it warm up in the winter. When the thermostat opened the temp droped to 140 and
just jumped from 180 to 140 all the time.

Some 350 and 400 come out with a 195 temp thermostat. The engine when running should not run over 200 to 205. It will rise
when it is off. I also installed a push fan on the front of my radiator but never have to use it unless in the mud very bad.

I have heard of a electric water pump that is supposed to cool even when the motor is turned off. Very high volume for berrtr performance. My problem is rear axles.

[79D604SPD]

Just for your personal reference, my 79 K20 has a 400SB and factory 4 core radiator. It has a new mechanical guage and 195 thermostat, it runs about 195-200 degrees. This is without AC and without an automatic transmission, so yours may be a bit higher. Hope this helps.

[1978K5]

Well with a new fan clutch the temp is down to 215@3000 RPM on the highway, and about 210 in the city. I forgot to mention that it has a trans cooler mounted in front of the condenser, dont know if that would make a difference though.

[frosties snowremoval]

yes your tranny will run hotter than your engine. fyi i would also make sure, if it is a after market trans cooler that the flow of tranny fluid goes threw the radiater and then the external cooler.

[wyldman]

The trans won't always run hotter than the engine.Under normal driving conditions,the trans should normally run a little cooler than the engine,as the engine has a thermostat to maintain a minimum operating temperature,trannies don't.When plowing,towing,or running hard,then you will see the trans temps get up there.

When installing a cooler,you should normally go through the external cooler first,to remove most of the heat,and then through the radiator cooler,to keep the trans fluid temps near normal.This keeps most of the heat out of the coolong system,so the truck will run cooler,and prevents the trans fluid from being overcooled,as if it's too cold,the internal rad cooler will warm it up some.

Running a trans too cold can actually damage it,especially on the newer trucks.Most electronic trannies will not shift correctly,or engage OD until the fluid reaches normal operating temperature.If the fluid runs too cold,these conditions aren't met,and the trans will not shift properly.This can cause lubrication issues,and premature failure.

The only time I would recommend using the external cooler last,is for a vehicle that is used hard in high ambient temps all the time.In that case you want as much cooling as possible.Normally in that case,you wouldn't even use the rad cooler,as it may cause the vehicle to overheat.

[TLS]

Originally posted by 1978K5
I havent replaced the thermostat yet

Do that.

Use a quality 180 degree stat.

The best thing would have been to get a radiator from a 454 or Diesel truck. They weren't only thicker, but larger overall.

How does the engine overall run? Its carb mixture and timing can really make an engine run hot!

Just for $hits and giggles, when you take out that old stat, take it into the kitchen (better done with wife NOT home) pour a pot full of water. Drop in the stat. Turn on the heat. Get a meat thermometer. Observe the temp at which the stat visually opens. Last but not least.....remove ALL evidence!

[frosties snowremoval]

wyldman: i was under the impression that your tranny will run at around 240-260 and that to get most cooling you should run through the radiator, as a prechill to speek, and then threw the external to actually do the cooling. i might be wrong but have never heard of a tranny burning out from runing to cold? just my opinion or knowledge.

[wyldman]

If your trans is running 240-260,don't expect it to last long.Those may be peak temps in the hot cooler line,but way above normal operating temps.

There are a lot of newer trannies that use certain parts of the OD assy to lubricate internal parts.If the trans runs too cold,and never goes into OD,then certain parts will get starved for oil,and damage occurs.

[1978K5]

I replaced the thermostat with a stock 195 degree part, and now temp hovers at just before the center line to just after it. At this point I'm assuming that the center line on C and H marked guages is 200. I'm making this assumption based on when it is fully warmed up and sitting at idle the needle is just before the center line. The radiator is a 4 row original style and is huge. I'm still determining if the A/C works, otherwise I thought of removing the condenser to free up air flow. I would like to keep the truck original so this is sort of a dilemma. I appreciate the replies as they have given me stuff to think about. I also changed the temp sending switch for the heck of it.

[ratlover]

I never trust the factory guages for an actual #, they are just good to see if things are out of the norm.....If you want to know what temp things actually are you will need to use a deccent aftermarket guage. I always try to verify guage readings so I'm not chasing a "problem" that isnt there.

Have you flushed the system and replaced the coolant? Is the radiator plugged?

What kinda stat did you use? Whan in doubt I will throw a stat in a pot of water on the stove with a thermometer and see what point it actualy opens and make sure it opens 100% Did you see a tiny bleed hole on the rim were it seats or on the little closer flap deal?

One snake oil type thing that I actually found to work is water wetter. But only try the stuff when you are sure the rest of the system is up to snuff.

If you are going to get a new water pump(only if the stock one is toast, dont change it to change it) I would recomend stewarts or an actually GM unit......the reman seem to be crap IMO

You got the right shroud?

Please describe the fan that you have on there.

[1978K5]

New 50/50 coolant, brand new Modine radiator, original shroud, original 7 blade clutch fan with new clutch. Replaced stat with GM 195 part. Water pump seems good with no wobble, its the original one. Truck has 120K on it, original radiator didnt have much scale in it either. Yanno what? Im thinkin its that darn metal bowtie grille emblem. Maybe if I cut that out it will solve it...:D

[TLS]

Personally I prefer 180 stats on non computer controlled trucks. 195 is just too hot. Especially in a 400 which run hot to begin with. You get a 195 thats calibrated a little hot, and BINGO, your at that 210 mark on a regular basis. I'd go with a Robershaw 180 degree stat. Isn't that what came stock in 1978? I didn't think they jumped to 195's until the computers took over.

BTW, its NOT the bowtie. :D

[wyldman]

Keep the 195 stat,that's what should be in it.The stat opening temp has no effect on engine temps above the thermostat opening temp.So it doesn't matter if it's a 160,a 180,or a 195,if the engine works hard enough (or the cooling sys is not functioning properly),it will still go above 200 or so.210-220 is not too hot,but you don't want to run it there all the time.

I'd get a mechanical gauge,or use an infrared thermometer to verify the accuracy of the factory gauge.It may just be off.

[TLS]

Wyldman,

Just when DID GM start using 195 degree stats? I was under the assumption that it was when the computer controlled carbs appeared....IIRC early 80's.

My '75 and '76 350's had 180's, as did a bunch of other mid to late 70's V-8 trucks.

Not that its a problem, but why go hotter than you need to, it'll only hurt performance. Plus, if his originally DID come with a 180 stat, his clutch fan would be calibrated to 180 and not 195, or vice versa.

[wyldman]

Not sure when they started using them,but the best all around performance comes from running the engine a little warmer than 180 degrees.

Even on some race cars,we see better performance with hotter temps.

[TLS]

Ohhhhkkkkaaaayyyyy.?? :rolleyes:

Thats a new one to me.

Maybe thats those "Canadian" degrees then ? :eek:

As theres a proven increase in power with a colder intake charge. If air has to pass through a manifold 15 degrees cooler, its going to make a noticable difference in power. One that I could/can feel seat of the pants on several different vehicles.


Still think 1978K5's problem is that he is reading his temps on a C - H gauge, and judging the temps on it.

[1978K5]

I think the 195's started being used once the emissions era was in full swing. For most cars and trucks this was 1973 with the advent of the EGR and other nards they installed on the motors. The hotter it ran the more it cut down emissions, as was evident on my 73 Camaro with an AIR system. It even told you in the owners manual to expect higher temps. I agree with all of you that the factory guages, at least the electrical ones, are more or less giving you an idea at best of whats going on. I know the Bourdon tube oil guages, and the temp probe guages are better and are much more accurate. I'm going to put an aftermarket temp guage on to see where its at and compare it to the original guage. BTW I also had a 76, 77, 78 2WD, and 79 Blazer, and they all had a 195 whether it was a 305, 350, or 400. I know that if you run too cool, it won't allow the impurities that are carried in the oil to burn out. I've thought about installing an electric pusher fan just to help it out in the summer if it needs it, kinda like an insurance policy. However at this point, temps have been stable, and when you park the truck, you can rest your hand on the top radiator hose and not get burned. As far as the electric guages go.....is one whose needle spins freely around bad? That guage gave me perfect readings??? Maybe I should put that one back in for some peace of mind.

[TLS]

The 195's must have came on the LD emissions and Calif. trucks, as both my '75 and '76 came from the factory with 180's and they were both non emission trucks (no cats, egr, or air pump). Matter of fact it wasn't until 1990 when I bought my '90 that I knew that they upped them to 195's. It was soon out for a 160, as I picked up about 15hp with just a stat change alone. Lost almost all winter heat, but she ran great!