someone11

So after doing a bunch of reading on the forums I think I am more confused than I was before I started reading. I have an 84 z28, 5.0 4bbl with single fan setup, all stock. Car sat for 20 years and we just went through it with a fine tooth comb and got it back on the road. Went for a 30 min ride a couple hours ago. I noticed the temp gauge went up to 220 stopped at a light. (its been warm here today ~90F). I kept an eye on it and it did not get much above 220 and would drop down to around 200/190 when driving. Got back to the garage and checked for coolant leaks but there was none. BUT, the coolant was bubbling back into the reservoir and ended up cracking my reservoir as it sat in the garage after the trip. I am not sure if my fan turned on during the trip as I never heard it, but I know it turned on at cold start, which I thought was odd. Not sure whats going on and looking for pointers on what to change. From the reading ive done it sounds like since I dont have a dual fan there is no sender between cyl 6&8 but I have not checked yet (car still too hot). There is a sensor above the thermostat and the wires are exposed there-this is where im assuming my issue is. Is there another sensor or sender I should be looking to maybe replace? Any help appreciated.

Go Blue

someone11

Radiator is basically brand new with less than 100 miles on it. The only thing we did with the cooling system was replace the heater core. I replaced the radiator when I replaced the fuel pump a few years ago.

Thermostat could be an issue but it drove an hour and a half yesterday with the temp never getting above 190.

I realize 220 isn't above average for these 3rd gens but the coolant shouldn't bubble back into the reservoir.

sofakingdom

iTrader: (1)

The fan turns on at some temp slightly above 230°. So, 220° while sitting idling is COMPLETELY NORMAL.

Yes there is (was, anyway) a fan switch on the pass side head. That, and the AC connection, is the ONLY WAY the fan turns on. Make sure it's present and hooked up.

The bubbling and all that, is your radiator cap. Nothing more. Replace it.

Make sure also of course, that your coolant is between 30 and 50% antifreeze. Remember, the old adage about "water boils at 212°" is only PARTLY true: it leaves out 2 critical pieces of info. It REALLY should say "pure water at sea-level atmospheric pressure boils at 212°". A coolant mix in the above range, at 15 psi, boils at around 270°, with about a third of the increase due to the antifreeze mix and the rest due to the increased pressure. The reason you have boiling into the reservoir is because your rad cap isn't sealing up and holding pressure like it should.

someone11

So it sounds like a new radiator cap is what's needed? Now that you mention it I don't remember if the new radiator came with a cap or not. The one on it may be the original one, I can't remember.

sofakingdom

iTrader: (1)

someone11

Thanks guys, dont know why I didnt think of that. I might still change the sensor at some point so the fan comes on sooner.

sofakingdom

iTrader: (1)

The operating point of the sensor is just fine. I'd suggest leaving it as-is. Make sure it actually WORKS of course, and isn't defective in some way, or the wiring pigtail is bad, or the relay or high-power wiring has any trouble (bad connections mostly). The stock electric fan system works great when the rest of the cooling system is also working properly.

The car should be able to sit and idle; temp should rise slowly and steadily; the fan should come on at 230ish°; temp should begin to drop; fan should shut off at 205ish°; temp will begin to climb again; fan should come on at 230ish°; ... forever, or until the car runs out of gas, whichever comes first. It should NEVER boil, steam, or otherwise misbehave.

someone11

So the fan seems to be operating correctly. Took a drive before work this evening and it kicked on about 225/230 and shut off around 200ish. But I still have coolant pooring back into the reservoir when the car is shut off. I don't think it's doing it while driving but hard to say. I'll get a new cap in the morning and see if that solves the issue.

someone11

New radiator cap did the trick, thanks guys!

Jazzyjeff87

You can run throttle body bypass, and low thermostat. Throttle body required is to bypass the heated antifreeze threw throttle body heating unneeded air for useless cooling it heats it.. Tpi runs hot to start.