Aspie-Rocco

I got a new cream puff car a 95 Z28, 62k miles.
low coolant light comes on after hard acceleration, a little or a lot of overflow from reservoir cap happens. It dumps coolant out of the reservoir cap.

no noticeable leaks other than the reservoir cap

after cooling down the reservoir empties into radiator (I assume)

added 1 gallon yesterday, maybe a quart this morning.

fans work when car on AC on, could not get them on with engine off key on.

I suspect a bad radiator cap. My reason is the pressure increases under heavy acceleration, and coolant seems to flow into and over fill reservoir spilling out.
the cap or maybe thermostat?

the car is bone stock except exhaust system as far as I know.

Any input or suggestions greatly appreciated.

Injuneer

Good source of cooling system info.... scroll down to "Temps escalate with speed and fans are working"

4th Gen LT1 F-body Tech Articles


Can also be a problem when when the coolant line from the back of the heads is connected to the wrong point on the radiator. The line (95-97) goes through the base of the throttle body, then connects to the lower nipple on the radiator, under the fill neck (line 5 in diagram). Sometimes is gets incorrectly connected to the overflow nipple and dumps coolant

http://shbox.com/1/95-97_hoses.jpg


Incorrect thermostat - the LT1 t'stat is unique, unlike a conventional SBC:

http://shbox.com/1/tstat.jpg

And the last thing you want to hear - Could be as serious as a blown head gasket. Time to check for hydrocarbons in the coolant.

Aspie-Rocco

Thank you. It appears hose routing is correct. Will check for hydrocarbons this weekend. I grabbed a new rad cap just in case, as rad was low but reservoir was full at break time today.
thanks again for all the info

Injuneer

In post #1 you assumed the radiator is pulling coolant back out of the reservoir as it cools down. Was the radiator cooled down at "break time"? If so, you may have a crack/hole in the hose from the reservoir to the fill neck, that prevents vacuum from pulling the coolant back into the radiator. The overflow hose connects to a metal tube under the battery, and that tube could also be corroded/perforated,

ToocoolZ28

Could be the sensor in the radiator, I replaced the sensor in mine twice, I finally just unplugged it.

Aspie-Rocco

I will investigate this next. Excellent suggestion!

can anyone provide a photo of the LT1 specific thermostat, or even better a side by side comparison of an LT1 thermo and a standard thermostat?

has anyone tried simply drilling a few holes in the thermostat to allow some constant flow? Is this a major NONO for LT1 systems? I have had limited success with a few 1/4” holes in an older center bolt head, chevy 350 thermostat (1990 camaro)

DrewHMS97SS

Fred posted Shoeboxes pic with the LT1 thermostat and a SBC thermostat in #2

Aspie-Rocco

I changed the thermostat, radiator cap, and topped off the coolant yet the problem still persists. However, the coolant loss only occurs under heavy acceleration. Normal driving is fine and no coolant loss happens.
it does not run hot, in fact is runs pretty cool at all times except traffic and stop lights, even at a stop it does not get more than half way up the gauge.
it is pretty frustrating since the intermittent issue practically rules out a bad head gasket.
would a clogged heater core cause these symptoms? The car gets warm air when under normal conditions, but after a hard acceleration no heat, so there must be a blockage or air pocket getting trapped in the core, I assume.
the coolant overfill hose is properly connected and is working as it should, no rips, wear or blockage.

Injuneer

Heavy acceleration increases cylinder pressures. Might cause a small leak to become a bigger leak.

You could look to see if this specific problem as addressed in the factory service manual. Free download:

https://www.mediafire.com/?40mfgeoe4ctti

Germansheperd

Sensor in the radiator as stated above. Replace or unplug.

Aspie-Rocco

how does the sensor cause coolant loss?
does the sensor cause some coolant passage to open or close?
the low coolant indicator does come on but only when coolant is lost. When it is refilled the light goes out.
Either way I will try this, but my problem is coolant loss, not the light coming on. Also, if I unplug the sensor, how will I know when the coolant loss happens (other than seeing the puddle from the reservoir or smelling the spilled fluid)?

Germansheperd

Oh I didn’t see your loosing coolant. Do you have puddles on the floor? Blown head gasket?

Injuneer

The only thing the low coolant sensor does is feed a latching relay that turns on the low coolant light. Unplugging the sensor just keeps the light from turning on. I would leave it connected since it’s working correctly.

Aspie-Rocco

I am beginning to fear a blown head gasket.
the curious part is the oil is fine the exhaust is fine there’s never any big white clouds and when I open the radiator cap there’s no bubbling at idle or low rpm’s. It seems to me that if there is a blown head gasket it’s only leaking into the coolant system at RPM above 5000. So if it is a head gasket it only happens under heavy acceleration which to me defies the logic of a blown head gasket however, I haven’t been able to do a hydrocarbon test yet because I don’t have the test kit I did rent a pressure tester yesterday and I’m preparing to do a coolant system flush today
edit, at parts store now getting a block tester and leak indicating fluid. Hope the gaskets are okay.