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SlowRob

Engine running hot! Didn't used to.

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I recently made some changes to my cooling system and am now having an issue or two.  Before the change my temp ran right at ~180 degrees with little variation.  Whether I was cruising, idling or driving "spiritedly" the temp stayed fairly constant.  I was running a 16" electric fan, set up as a pusher (I know) slightly angled off the rad (3 core) because it was too big.  Not a great setup, but it was working.

 

I have since made two changes: I switched out to a more powerful 14" fan setup as a puller which has a good seal to the rad.  When it runs, it's moving a ton of air, seemingly much more than the old setup.  I also removed the water circuit for the manifold heat to my SU carbs.  I assumed since this circuit ran parallel to the heater core that it shouldn't be an issue; worst case I just open the hot water valve and the coolant system is essentially unchanged.

 

My issue:  The car now runs hot (~210deg) at anything over ~1500revs.  At idle, the temp settles right back down to ~180.  While cruising, if I clutch it and let it idle, it drops down to 180.  And, if I cruise slowly enough to keep the revs down, it rides around the 180 mark.  As soon as the revs go higher, the temp runs up.  What seems odd is that it drops down/up about as quickly as the guage is capable of moving, so I'm pretty sure it isn't actually changing temp by ~30 degrees that quickly.  Even if I open the heater valve so that circuit is active, it behaves the same.

 

I'm thinking that I have an air pocket stuck someplace, but I've done everything I can think of to bleed the system.  Are there any tricks that I should know about?  Does the manifold heat circuit really have an affect on running temp?  Absolutely any help would be appreciated!

 

Thanks!

Rob

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Yes it does. If I push the idle to ~2000, it creeps up. I would mention that at normal idle, the fan moves enough air to bring the temp well below normal.

 

This is essentially a bone stock 2.8

 

Also, the thermostat was replaced recently and is functioning properly.

Edited by SlowRob

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The manifold heat circuit you eliminated was also a bypass that tends to stabilize temp readings at the temp sensor in the bottom of the thermostat housing. Reconnect that circuit and you readings should be more stable.

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Please forgive my ignorance:

So, without it, I'm seeing more direct head temps, since the bypass isn't there to dilute the water from the engine?

 

Is it OK to run without that bypass?

 

Last one: why does a similar setup that I am familiar with, which prompted me to remove the bypass, not have the same fluctuations? Temp sender being in the same location, with the bypass delete being done the same as mine.

 

I'm not looking to be a pain in the @$$, I just want to fully understand what is going on.

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That probably isn't the problem. A sudden rise in coolant temps, due to removing a circuit that isn't even active at the temperatures we're talking about?

 

There is a thermostatic valve that closes, blocking circulation in that circuit, when the engine is up to temperature, according to the FSM.

 

I'd start checking for a water jacket that pressureizes very quickly on startup.

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There was no thermo-valve on the mani-heat circuit.  Pump out goes to a "Y" feeding the heater core - which returns to the block (maybe head - I forget) - and the mani-heat - which returns directly to the t'stat housing.

 

I'm still trying to figure this out.  Adding to the confusion, my electric fan cycles properly regardless of the temp reading at the guage, leading me to believe it's a trapped air issue in the t'stat housing.  The sensor for the fan is at the radiator, so it's completely separated from the gauge sending unit.  I don't even see any fluctuations with an external temp probe placed on the engine at various locations - the values are meaningless as it's a simple thermocouple, but the values stay within a tight range, indicating little to no change in temp.  So the up-side is that my cooling system seems to be adequate and efficient.

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That's the pump INLET...hot water comes out of the cylinder head, into the heater core, then returns to the pump inlet. The thermostatic valve is in that T-fitting that's bolted to the side of the block, in the heater core return line to the inlet of the water pump. (At least that's where it's supposed to be.) I have also seen reference to a thermostatic valve actually installed in the rear manifold half too...not sure on that one, never seen it.

 

The hot water comes out below the thermostat, flows through the manifolds, into the thermo-valve T-fitting, then returns to the inlet of the water pump. Blocking this circuit should, in effect, eliminate heating of the manifolds and the cooling circuit should behave exactly as it would once the engine has come up to 170F and that thermostatic T-fitting closes up.

 

Probably a bad sending unit for the dash gauge if you are reading normal temperatures everywhere else in the engine. Check your electrical grounds; at idle...the alternator isn't really putting out much voltage, as RPM's climb, the voltage climbs. Your gauges will report inaccurately as system voltages rise, which would occur as RPM's climb above 1500RPM. Could be a voltage regulator or alternator issue, could be a bad ground for that particular sending unit. My temperature gauge works fine, and my sending unit works fine...but they won't play nice together in my car due to grounding issues with the sensor.

Edited by Xnke

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After 20 minutes of highway driving, with temp reading high, it dropped and stayed there. I drove it hard to get the temp up and it stayed normal. I'm inclined to believe it was an air pocket. I won't know for sure until I get a few cycles into it, but it looks good.

 

Thank you all for your help.

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