imboredx0504

i just rebuilt my 305. it is a 4bbl. i just put on new water pump, new rad, and a 160* thermostat. has mechanical fan with new fan clutch. belts are tight and shroud is on. idling car sits at 170. driving city or highway 160. but when and only when im coming off the highway it creeps up to 250+. i pull over and pop the hood. coolant is bubbling just about overflowing in the overflow. once i release pressure from radiator coolant is slow moving through radiator and is low on fluid. i put my hand over the reservoir and press down to pump fluid back into radiator, but sometimes coolant will surge in the radiator and come out the top. i was wondering what someone might think the problem is. kinda sounds like a bad thermostat. but im not 100%
and once i get it cool, it works fine.
also when motor at operating temp should the radiator be completly topped off while its running or should it not be full. im hearing a few different things. any help would be awesome. thanks

KNBlazer

how long ago did you replace the radiator.. was it tonight, yesterday? are you sure your fans are working correctly?

imboredx0504

it was recently. within the past month and this is only the second day driving it. and yes the fan is working fine. when i raise the RPMs the fan speeds up with the motor, and i never have seen the fan not spinning. i bleed the radiator. parked it on a steep incline and ran the rad for about ten minutes with cap off

KNBlazer

what I would do is double check your timing and if it's set correctly, then take the thermostat out and drive around without it and see if it happens again...

KNBlazer

another thing that it could be is if your fan shroud is not mounted correctly...

imboredx0504

ok. thanks. i was planning on taking out the thermostat to see if that gets me anywhere. just hoping to find the problem soon. i can still drive it on highway because after about five minutes i get the temp down after releasing the pressuring, getting fluid back in radiator, and revving motor to get the coolant flowing. I got a timing light to check the timing just gotta put some chalk on the balancer so i can see it. ill try them both and post a reply with any results. thanks

imboredx0504

fan shroud is fine. sits in there right. no flopping or wobble, and fan is sitting in the shroud just the right amount. since it stays cool really well city driving, i eliminated a few of the possible problems

KNBlazer

do you have the lower air damn hooked up? the skirt at bottom... while it's idling in park temp doesn't creep up to those numbers?

Gregzz4

I like KNBlazer's suggestion of running the system without a thermostat.
This will let you know if you rad and water pump are doing their job.
If your temperature is very low when you do this test, then your T-stat possibly was bad.
Problem is there are many factors with your cooling system.
It's better if you run a 180* T-stat, as that is where any internal combustion engine burns gas the best.
What year/model car is it? I assume it's a 3rd Gen?
Do you have the lower air dam installed?
These cars will not run @ higher speeds without the air dam to direct air into the rad.

imboredx0504

ill be honest i dont know if i have the air damn installed. because im not sure of what it is. and the engine is fine parked and city and while on highway, the only time it ever overheats is within the minute of getting off the highway. and it doesnt shoot up really quick, it creeps up to it

imboredx0504

oh yea and its an 83 z28

4playta

Sounds like bad fan clutch, clutch is not engaged driving down the highway because it is nice and cool, when exiting the freeway it is not engaging fast enough and the temps rise fast. Cruising through town at slower speeds is not cooling the clutch off enough to experience this symptom.

Here is how clutch fan works.
The fan clutch is a small fluid coupling with a thermostatic device and controls a variable-speed fan. It ensures that the fan will rotate at just the right speed to keep the engine from overheating, and reduces drive to the fan when it is no longer needed. The fan clutch has a fluid coupling partly filled with silicone oil designed for just that purpose. When the temperature of the air passing through the radiator rises, the heat alerts a bimetal coil spring to "uncoil" or expand. When it expands, it allows just a little more oil to enter the fluid coupling, so the fluid coupling starts to rotate the fan. If the air coming through the radiator is cool, the opposite happens; the coil spring contracts, the oil leaves the fluid coupling and the fan slows. Slowing the fan when it is not needed reduces fuel consumption, makes less noise and saves engine power. Sometimes a flat bimetal strip spring is used instead of a coil spring; it bows out and in when the temperature rises and drops, letting oil in and out of the fluid coupling.

Now as soon as you exit the freeway and are at idle or driving slow how much hot air is going through your radiator to activate the clutch...see where I am going with this? Throw on a cheap $30 mechanical flex fan and test my theory...I bet it don't do it anymore!

It cant be a thermostat...they either work or they don't, if it don't overheat any other time but getting off the freeway then obviously the thermostat works!

Gregzz4

4playta opened up a whole can of worms here

Thermal fan clutches work with Viscous Fluid
The heat from the air flowing through the Radiator heats up this fluid to engage the fan clutch
If there is not enough air flow through the A/C Condenser or the Radiator, the Fan Clutch will not receive enough hot Air Flow to engage
Faulty thermostats, externally plugged radiators and ac condensers, internally plugged radiators, dirty cooling systems, internal cooling system gaskets and other extenal leaks can lead to a fan clutch not engaging

As far as what you are dealing with.....
If it cools in the city, your Air flow is fine
If it cools on the highway, your Coolant flow is fine
If it starts to get a little warm when you are on the highway, and warm up when you push it, you don't have an air dam

Air flow is critical with these cars due to the design of the grill
The air dam is under the bumper and directs air to the Rad

4playta

I have experienced all kinds of cooling issues with a mechanical clutch fan last summer, everything from sitting idle to getting off the freeway like you described but it was at random times and the clutch was new. When they are working right they are marginal at best on our cars! I decided to install a Black magic Xtream fan on my Trans Am and have not had any issues since.

The newer clutch fans that engage VIA engine temp by the ECM are OK...there is a reason GM switched to that type of clutch fan and later to all electric fans. On my Suburban that was experiencing random overheating problems as well I installed a direct drive flex fan...that friggin thing will blow leaves off the windshield through the cowl when parked, but it is loud as hell. Only reason I am keeping it is I rarely drive it and when I do it is off road, towing, and other conditions where I need a lot of airflow to keep the motor and transmission cool.

Like I said try a mechanical drivin flex fan for a few runs and if it does not overheat you found the problem, then I would switch to a good electric fan. I bought the fan for my truck at Autozone for something like $31.95

tom86iroc

iTrader: (1)

i'd check the lower air damn its a thick black piece of plastic under the car near the front of the nose about 2feet and maybe 4or 5 wide can't miss it under the car that's what brings air into the engine bay on the highway since we don't have a grill area

imboredx0504

thank you very much 4playta. i put a direct drive fan and a spacer on and the engine stays so cool coming off the highway that even with the coolant full, the resevior doesnt reach the full hot line. seems like the fan clutch was the problem. thank you. and thanks for everyone elses input

4playta

Now that you are 100% sure that was the problem take that fan back and get a good electric fan. Direct drive fans eat horse power and gas...if you cant afford to buy a good new fan try finding a LS1 fan from a junk yard or craigs list, I see them on Ebay for $100.