DocHoliday

Hi, I'm helping my friend fix his Z-28. He is having cooling problems and tells me that his car is overheating badly. He says its a 350, but I have my doubts. It has the looks of an LG4. But anyways back to the problem. His engine heats up way fast. Then he tells me it starts knocking when ever its really hot. It does this just before it overheats. His temp gauge is pegged. Meaning its all the way up past 280. Even when the cars off. So I will start the questions.

1. How do I fix the pegged needle?

2. Should I replace the thermostat?

3. Other than hearing it, which I cant on his car, how can I tell if his fan is coming on? I read to turn on the AC, and you will hear it, but he doesn't have AC. He has the blower still, but not the compressor.

I need help with the anatomy of the fan system really fast. Where is the fan that comes on when your car starts to get really hot? It does this on my Z. Thanks for the help!

DH

88blkbird

have you pulled the rad cap and started when cold? if fluid comes back out when running ( just started ) or there are bubbles it would indicate blown head gasket
sometimes they show overheating but aren't yet because of an air pocket around the coolant sensor...

DocHoliday

No I haven't done that. How long after running is the car cool enough to do a test like this?

88blkbird

once it's cool enough to open the cap you should be ok to give it a shot.. coolant should be full, if not fill it first and try to get it to bleed any air out.. could be difficult to tell if the coolant is low. there are a couple of actual tests to determine if there is a gasket issue or the cylinder is cracked but most people dont have access to an emission anylizer, co in the coolant is a sure way of telling but you can see your snap-on distributor and they can sell you a tester with a quart of fluid to do this test.. it's not cheap though and doesn't make sence unless you own a shop and can charge your customers for it.
----------
oh and by the way if you disconnect the coolant sensor ( with the engine cold ) the gauge should drop back to zero with the ignition on, if not he has a bad gauge if it does it could indicate no coolant touching the sensor which would cause it to rise quickly but should not cause it to be pegged at all times . if it is not pegged when disconnected but goes back to being pegged with the sensor plugged in and the car is cold he has a bad sensor. make sure by the way that the cooling fans are coming on.

DocHoliday

How do I find out if the cooling fans are coming on? Where are they at?

88blkbird

in the likely event that he has electric fans ( you didn't mention the year ) they are mounted to the radiator just in front of the motor, behind the radiator..

Daniel U

Also (If you haven't already) do the basics: Check/flush(There is a tool that you can get to tell you if your fluid is still good, forgot the name though?) fluid seasonally. Also a good time to replace the thermostat as cheap as they are. Check if the radiator cap is working properly. Do you have any leaks anywhere? Is there a small leak from the water pump. This usually indicated the bearing is going out which means your losing water(Good time to upgrade to alum.!). When was the last time the radiator was replaced/professionally cleaned out? I had an 84 that was identical to the H.O in the picture, and when I had the radiator cleaned out it helped the cooling. I'm running a Griffin aluminum radiator now. If you have a TPI car you can adj. you fan settings through minor PROM work. I have my 88's dual fans set to come on at 195 deg. You should be able to see the fan come on if it idles long enough to warm up beyond the current fan switch setting. Also you can check your running to lean or rich also. This to will over heat a motor. Hope this helps out! Daniel U