280ZX Highway Overheat

[danc]

My SBC/700R4 83 ZX conversion has been running a little over a year now, and I have a pretty consistant "higher-heat" (240-250+ degrees) problem only after I exit the highway. I can run all day in city traffic at near normal engine temps (180 degree thermostat), and the outside air temperature has no substantial effect on how hot/cool it runs. My engine/conversion piece parts only have 4K miles on them, and I'm running the JTR suggested 84-86 Aluminum/plastic Camaro radiator, and the Black Magic 150 Electric fan. I'm also using Dayco #70651 Upper and #71217 lower radiator hoses. I did have a problem with the upper "sucking closed" at higher RPM, and installed some stainless coiled wire. I have no reason to believe the lower one is any better, so I plan on replacing both of them this winter anyway with something more substantial. (Any part numbers you guys have had success with would be appreciated). I do have an A/C evaporator and the transmission cooler out front of the radiator. I would have thought they would have caused a consistant overheat problem in most driving situations as opposed to what I'm having, but I have little experience in overheating causes/solutions. At any rate, any thoughts would be appreciated. Hope you all have great holidays, and I appreciate the insight I've gleaned thru all the varied and great threads this year.

[Guest Aaron]

This may seem obvious, but I saw this once when I was working on cars. Make sure your fan is wired up properly and spinning the correct direction. A car (80's Ford Tempo) came in to the shop doing the same thing. It was find below about 50MPH, but above that, the AC would stop cooling and it would overheat. Someone had replaced the wires to the fan and wired it up backwards. So below 50MPH, the fan could move enough air out of the front of the car to keep it cool, but above that, the air pressure from the moving car would not allow the fan to pass any air through the radiator.

[Pop N Wood]

The Black Magic fan has received some sharp critisim on this site (don't believe me, check the arichives). Most guys recommend dumping the black magic for a Taurus fan.

 

BUT a bad fan usually spells overheating at idle, not speed. My guess is you are either having airflow problems or have an inefficient radiator/water pump.

 

The Z's are notorious for not getting proper air flow through the radiator. Install a belly pan (I think the MSA catalog calls them chin pans). This smooths out the turbulence underneath the car that prevents air from flowing through the radiator at speed. Beyond that, many guys recommend sealing all possible air routes around the radiator. Especially the areas between the sides of the radiator and the body.

 

Check your thermostat by placing it in a pot of water and see what temp it opens up. Also make sure it fully opens. Flush the radiator. An old car can be gummed up with stop leak from previous owners. Also check or replace your radiator cap. These things try to maintain about 15 PSI of pressure in the system. A bad cap will not do this.

 

Now what has me confused is why is your upper radiator hose collapsing? The water pump suction should be on the lower hose (right? I am not sure) so this should be the one collapsing. Must be one hell of a pump to suck all the way through the radiator and collapse the upper hose.

[Guest greimann]

Jim P. has good advice about standard over heating problems, but Dan, your case has some unusual conditions that may need some other questions answered:

 

1. When running on the highway your temps are normal and only shoot up after you have slowed down, is that right?

 

2. When they shoot up do they eventually normalize after a period of city driving?

 

3. Is this temperature problem happening right now in the cool Autumn air?

 

4. Where is your temperature sender for the gauge?

 

5. Does your catch tank ever overflow or do you have other symptoms of over heating like a fluid smell or hot engine smell?

 

6. Do you have a "normal" 350 or is it a late model LT1?

 

7. How did you determine your upper radiator hose was sucking closed?

[Guest Darthmentos]

Had a similar problem with a Road Runner. The electric fan was causing a flow restriction due to its size at high speeds. We put a larger fan with shroud on it, no dice.

then trimmed square ports into the shroud,placing spring loaded "windoors" over the holes, so that at high speed, the wind could push these doors open, gave much better airflow and the overheat was resolved that way. When we stopped the sping hinges pushed the windoors closed and the electric fan could do its work Again this was for a big block mopar so I don't know if this helps very much.

[Guest Z-rific]

Check the archives. It seems like somebody had this exact problem a few months ago.

 

Do you have the higher temp thermostat in? This may be a cheap way to cool down a little.

 

I think the guy who had this problem was advised to insulate the area around his radiator, so that air would not move around it. I think at high speeds, your air may be moving around the radiator more than through it. And the fan may not be enough to cool the engine at high rpms.

 

What rear end and tranny do you have?

[BAlford]

This is a common problem that is posted here from time to time. Here is how I solved the problem.

 

http://www.waskomtexas.com/zcar/re_car/engine_cooling.html

 

You may prefer a more expensive way or more complacated way, this worked for me.

[danc]

Thanks for the thoughts guys. Here is a clarification of the facts I left out. Nothing is too obvious Aaron, and the fan is wired correctly and working properly. I had one problem with the Black Magic fan on-off temp rheostat control vibrating and changing the on/off times. Other than that, it definitely reduces the engine temps pretty quickly when it kicks on. By the way, I had also modified the rheostat switch turn-on-temp, thinking it might be coming on too late, with no change. I thought maybe I'd gotten the wrong water pump on my crate motor, and talked to the tech guys at GM Performance Parts. They said it was the right one, and without a doubt more than adequate. I'm not sure about the adequacy of the Camaro radiator other than a lot of guys have used it successfully on this conversion, so I wouldn't think that would be the problem. (These parts are all new by the way). I do have the factory belly pan on, but have not sealed off the small gap between the radiator and front core support sides. I did check the thermostat, and it is opening at the right temp, and have also tried a new 15 Lb radiator cap. Yep the upper radiator hose was sucking closed when warm about half way between the radiator and engine. I discovered that problem when I was testing a change to the carbuerator throttle linkage. The running temps on the highway are normal (fan cycles on and off as needed) until I slow down and come to a stop. They shoot up pretty quickly but the good news is they go back down about as quickly too. I would have thought the overheating temps would have been less exagerated in the colder weather, but no real discernable change on that one! My temp sending unit is mounted on the right side of the intake manifold Griemann. The catch-tank does fluctuate as normal, and I do occasionally whiff the unmistakeable sweet antifreeze smell, but it doesn't overheat to the overflow point/hot engine smell. It's a Fastburn 385. I've installed the 430 H.P. LT-4 "Hotcam" kit, and as expected, it had no effect on the problem either. I'm going to change the mounting locations of the A/C evaporator and transmission cooler to see if perhaps they are impeding the fans ability to move "needed- air" and fabricate something to close off the gaps between the radiator and core support. I know Mr. Heat is not our friend, and although this is not a major problem today, I can see it becoming one as the piece/parts become less effecient with age. Any other thoughts would certainly be welcome, and again happy holidays.

Dan

[Guest greimann]

From your additional information Dan, I propose that your cooling system is operating fine and there is really no problem. Here is why:

 

An inadequate system cannot remove heat as quickly as it is put in by the engine. The most severe condition is usually driving in city traffic where there is no ram cooling from forward motion. Your car does fine in this situation.

 

Driving on the highway you benefit from a high degree of ram cooling that keeps the system at the minimum temperature. The thermostat will actually be cycling open and closed to keep the engine at its minumun temperature. When you come off the exit ramp and slow down, you remove the ram cooling effect, the heat load in the block temporairily spikes the water temperature until the thermostat can open fully and increase the flow.

 

If your cooling system was inadequate, you would not recover the temperature so quickly. You might want to verify that you have no trapped air in the system, because ideally the hoses should not suck down if they are full of uncompressible water.

 

I know that my cooling system is fully adequate and my temp gauge goes higher than normal a few minutes after starting from dead cold, until the thermostat opens fully. It is just a spot of heat around the upper manifold and not an endemic problem.

[jt1]

I think Greimann is right on about the spike situation. I also think you have a slow/sticking thermostat that's making it worse, and probably some air in the system adding to the problem. The upper hose should never suck shut. Try a new thermostat and drill a 3/32 hole in the flange so it won't trap air when closed. This will let air bleed out of the block as you slowly fill the system. Also make sure your filler cap is at the highest point in the system.

 

John

[danc]

Greimann/John:

Thanks for the insight. I did think it odd that the spike and corresponding drop happened in such a relative short period of time; and understand the point you guys are making about reasonable heat dissipation. I'm not sure how I might still have air trapped in the system but your comments about the upper hose sucking closed makes sense. I'll syphon some of the coolant off and refill to try and remove any trapped air. Either of you guys running the Black Magic fan, and if so have any problems?

Dan

[denny411]

Try filling the coolant level with the front of the car raised and the engine running. this will allow the cooling system to BURP, and helps to insure that the radiator cap is the highst point of the cooling system, which will allow any trapped are to escape.

[jt1]

Dan:

 

I'm running the Black Magic, with a 24 x 19 Griffin. My 355 is in the low 400hp range, and even at open track events in the summer, I rarely go over 210deg. I'm a big fan of the hole in the thermostat flange because the radiators in our cars sit so low. I also have the moroso filler neck that mounts in the upper radiator hose. This makes the fill point the highest point in the system and the air gets out a lot easier. Denny411's suggestion to jack up the front of the car and fill it while running is also a good one, as the thermostat cycles that will help purge the cooling system. Try to jack it up enough the cap is the highest point. Hope this helps.

 

John

[Guest greimann]

I ran the Black Magic on a Griffin 26 x 19 for many years without any overheating issues. I recently switched to the Taurus fan because I wanted to have the best cooling I could get to help control detonation on a 10.5:1 compression motor.