Overheating sbc - Chevelle Tech
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post #1 of 138 (permalink) Old Apr 21st, 19, 3:03 PM Thread Starter
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Gerrett
 
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Overheating sbc

Motor is a 350 .60 with 882 heads. And a comp thumr cam. Motor turns on easy , but less than 20 mins into idle the temp climbs to 240, if I put it into OD it immedalty drops the temp to 180. I have a new 18” fan blade, new clutch, shroud and 4 row champion radiator. 160* stat with a hole drilled to relieve air pockets. and new hoses. I’m scratching my head.
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post #2 of 138 (permalink) Old Apr 21st, 19, 3:15 PM
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jerry
 
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Re: Overheating sbc

Try draining your block coolant. It's sometimes pretty tough to get to a drain plug in the car with headers but it's a must if you have coolant issues. So often these blocks have been left sitting and really get mucked up with scale and rusty water. If the water comes out looking good report back. If you get dirty water.....I've had to not only drain, but flush and reverse flush AT the drain holes. Adapt a barbed fitting to one side of the block and leave the other open, change sides and repeat until you get virgin water and full flow. Now flush the radiator and pull the t-stat and gravity drain the block once more. Fill with 50/50 mix.
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post #3 of 138 (permalink) Old Apr 21st, 19, 4:19 PM
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Re: Overheating sbc

I'd start by replacing the thermostat. A good flush couldn't hurt. Include a back flush.

Sounds like your system is up to task but even new parts fail.

If your fan clutch is not cycling on or locking up it may also be the culprit.

Water pumps usually fail at the seal and start leaking, but it's not unheard of to have a broken/loose or corroded impeller.

Shifting into over drive would slow the engine. My guess is that you are shifting once you get over 35mph. At that point airflow from forward movement usually supplies enough cooling. So again this may point to the clutch fan while idling or under speeds sufficient to supply air for cooling.

From a cold fire up, you can remove the rad cap and watch for the thermostat to open. Once you reach 160ish, you should see flow through the radiator through the cap.

Also check your timing. Too far retarded may cause over heating.
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post #4 of 138 (permalink) Old Apr 21st, 19, 5:10 PM
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Re: Overheating sbc

What year Chevelle and are they stock pulleys below or reduced size aftermarket ones?
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post #5 of 138 (permalink) Old Apr 21st, 19, 5:17 PM Thread Starter
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Gerrett
 
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I’ve added fluid to the block at the intake and waited an hour and topped off. I’ve drianed the rad and all of my fluid is green. It was brown because of rust from a clogged water pump as mentioned earlier by someone. But now it is green as radiator fluid should be. I forgot to mention inside of my fan shroud I have an electric fan hooked up to the ignition switch so I know there’s extra air flow. I did just do my timeing and I thought it was a retard timing that creates over heating not advanced?
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post #6 of 138 (permalink) Old Apr 21st, 19, 5:20 PM Thread Starter
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Gerrett
 
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69 chevelle, with stock pull-ups yes. This over heating issue started right after getting the timing set. Before the timing was set it would take three four key turns to start and then it would drive like crap and backfire. Now that I did the timing it turns on great no hesitation but now the heat issue I’m having.
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post #7 of 138 (permalink) Old Apr 21st, 19, 5:31 PM Thread Starter
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Gerrett
 
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I’ve put two new thermostats and a drilled for bleeding
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post #8 of 138 (permalink) Old Apr 21st, 19, 6:58 PM
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Re: Overheating sbc

Quote:
Originally Posted by Mrfresh352 View Post
but less than 20 mins into idle the temp climbs to 240, if I put it into OD it immedalty drops the temp to 180.

I am not sure I understand this statement ??? Is it overheating sitting still or driving ?
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post #9 of 138 (permalink) Old Apr 21st, 19, 7:16 PM Thread Starter
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Gerrett
 
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Sorry let me clarify. Idle in park it climbs above 220+. But if I put it in OD or D without driving it and my foot on the brake it stays at 185
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post #10 of 138 (permalink) Old Apr 21st, 19, 9:12 PM
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Rick
 
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Re: Overheating sbc

Do you have an electric gauge or a mechanical one?

Sounds like a gauge problem.
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post #11 of 138 (permalink) Old Apr 21st, 19, 9:50 PM Thread Starter
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It’s an anolog gauge. And I’ve put a new gauge in also. Temp Gauge is in the passenger side of the heads. I have a infrared temp reader and it shows 220-250 at the gauge threads but I figured that was heat soak from the header pipes. I aim it at the thermostat housing and it 160-180. Car idles perfect at 950. The recommended rpm for the car I have.
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post #12 of 138 (permalink) Old Apr 21st, 19, 10:11 PM
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Re: Overheating sbc

Is the electrical system properly grounded??
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post #13 of 138 (permalink) Old Apr 21st, 19, 10:48 PM
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Re: Overheating sbc

Are you running a serpentine belt system or the stock V belt system. A serpentine system rotates in the opposite direction of a v belt system. A mix and match pump/belt system could cause issues. You had a pump issue, you replaced the pump? Remember the part # of the replacement pump?

How old is that radiator. What temps do you record at the radiator inlet and outlet? If the outlet is higher than the inlet, the circulation is reversed. The inlet to the radiator flows from the thermostat to the radiator. What do you show at cruise? Say a consistent 55 or above?

Leo might be onto something with the lack of a good ground to the engine or sending unit. When you put it in drive you might get a better ground. The way you describe it going down immediately is hard to explain and may be ground related. It's definitely strange. You sound pretty confident that your gauge is correct.

Your temp at the heads is a bit high but your outlet temp at the thermostat is in range. You're not that far off from normal. Typically a sender in the head will read 10-20 degrees warmer than one mounted in the intake manifold. There might a blockage in the coolant system heads or wrong head gaskets or a flow issue. Only you know the history of the engine and the type of blockage you found in the pump. That blockage may have nested in the heads or coolant system creating hot spots.

All speculation. Just throwing out all I can think of.
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post #14 of 138 (permalink) Old Apr 21st, 19, 11:24 PM Thread Starter
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I’m using a vbelt. The motor was rebuilt in 2012 and the radiator was purchased new maybe a year later. The reasoning I replaced the water pump was due to the weep hole leaking brown. I figured it was from the block. Drained everything and did a flush of the rad, and block. You might be on to something with the sludge moving around and blocking flow in my heads. Although all of my fluid now is true to the color. I’ve added 4g grounds to all of the main negative wiring.

I haven’t driven it now that I’m overheating, I’ve left the rad cap off to watch flow and it does flow but as my gauge goes up I shut it off and I either burp because of expantion or I’m boiling idk the difference.
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post #15 of 138 (permalink) Old Apr 22nd, 19, 1:37 AM
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Re: Overheating sbc

The burp you are seeing maybe from a steam pocket or head gasket leak. To rule out a head gasket leak do a compression test of the cooling system. You maybe able to borrow the tester from your local auto parts store. It could also just be the thermostat opening.

The steam pocket could be caused by a blockage. A chemical cleaner and back flush might remove debris from a blockage.

You actually have me stumped as your symptoms are quite strange. Your coolant temp should not instantly drop just because you put the car in drive.

By analog gauge, you mean a nonelectrical mechanical gauge? Ground would not affect that. So there goes that theory.

Your water temp out of the thermostat is 180 so your radiator is cooling properly. It seems the issue is in the head or gauge. Maybe move it to the other head if you have enough lead and see if you get the same results. If it shows normal the debris in the other head theory proves true.????
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