only runs warm at higher speeds

[zim]

On my 240Z the temperature gauge stays right in the middle when driving when the outside temperatures are below 65.   

 

When the temps climb into the mid 70's it still runs right on the middle line in town and even on the freeway at speeds below 70.  But if I push it harder and go 75 or do some higher revving passes it will climb up to the 3/4 point.   If I drop back down to a more mellow 65 for awhile it will eventually drop some and if I go really easy on it then it moves down closer to the middle line again.   

If I don't let it cool down sufficiently before getting into city driving it will stumble and want to stall at stops until it cools back down and sometimes that takes awhile.   

 

It has the stock 2-row radiator and it was cleaned by a radiator shop about 5 years and 25,000 miles ago.    It had a small pinhole leak at the time.

 

I added a fan shroud; prior to that it would run warm when idling in traffic so that solved that problem but it still struggles with the higher speed driving.

 

I've tested the cap, it has a stock Nissan thermostat.

 

I did a pressure test on it by pumping it up to 13 lbs pressure and after several hours the needle had barely moved down.  I had thought maybe it had another tiny leak somewhere that was bleeding off just enough pressure to cause the temperature to rise.   A company vehicle once had that issue and would only overheat at higher speeds.

 

Could it still have a pinhole leak somewhere?   Do I just need a 'better' radiator?

Edited February 24, 2020 by zim

[Slowpoke]

I would open the rad cap and check for bubbles or exhaust gas contamination. You could have a small head gasket leak. The system could also not be bleed down sufficiently. Stock engine, good thermostat, and a clean stock rad should cool well in 70 deg. temps. For a band-aid you could run water wetter and distilled water for coolant, assuming its not near freezing temps near you. I would also flush the coolant system, heater core and engine to be safe. I warped a cylinder head in the past to a clogged rad.

[Miles]

My stock 72 240z did the exact same thing.  I did all of the usual checks for : fan clutch, timing, mixture, collapsing water hose at high RPM, thermostat ratings, no thermostat, flow restrictors (in place of thermostat) , water pump, Camaro radiator (marginal)  etc.  Had some marginal success, but the temps would still creep up.

 

My brother's brand new 72 240Z over heated at cruise speed!

 

My take is that the stock cooling system is just undersized. 

 

Your Z gets hot at high speed where air flow is high. The fan and shroud are necessary at low speed stop and go. At speeds above say 30 or 40 mph,  air flow  through the radiator is adequate to cool the engine. At some point the fan clutch releases and the fan just spins like a windmill. At 60 mph air is moving at 88ft/sec or 5,280 ft/min. Lots of air moving through the radiator. So the problem must be in the radiator,  water flow, timing, mixture, belt slippage etc.

 

Take away: You need a better, higher capacity,  radiator, and if using a mechanical fan, make sure that the fan clutch is working. BTW: a mechanical fan is fine.

 

You might want to try the direct-fit radiator from Arizona Z Car and add a shroud to it if stop and go cause the temps to creep up..   This radiator keeps my SBC 350 240z cool in traffic on 100 deg. days.

 

 

[zim]

I think I'll try a new radiator first.   Need to stick with one that the fan shroud will fit on it so will probably get a 3 row 'stock style'.

 

I've played with the mixture, thinking it was running too lean and getting hot.   I used to have the timing too advanced so I backed it off but no change.  I ran an oil cooler for awhile but that didn't make any difference, except it kept leaking oil.

 

My prior Z didn't have these issues and I drove it when it was 100+ and this one didn't seem to initially but since it is so seasonal it is hard to remember exactly when it started.    I also get vapor lock in this one and never had it in the prior one; and these are the same carbs I was running on it but that is another thread.

 

I have heard that the heat from radiator repairs can shrink the tube openings through it so maybe the leak repair caused some restriction.

 

The last time I checked the compression it was okay but I've also wondered if I'm getting a lot of blowby at higher rpms that is heating the engine up.

[Miles]

8 minutes ago, zim said:

I think I'll try a new radiator first.   Need to stick with one that the fan shroud will fit on it so will probably get a 3 row 'stock style'.

 

I've played with the mixture, thinking it was running too lean and getting hot.   I used to have the timing too advanced so I backed it off but no change.  I ran an oil cooler for awhile but that didn't make any difference, except it kept leaking oil.

 

My prior Z didn't have these issues and I drove it when it was 100+ and this one didn't seem to initially but since it is so seasonal it is hard to remember exactly when it started.    I also get vapor lock in this one and never had it in the prior one; and these are the same carbs I was running on it but that is another thread.

 

I have heard that the heat from radiator repairs can shrink the tube openings through it so maybe the leak repair caused some restriction.

 

The last time I checked the compression it was okay but I've also wondered if I'm getting a lot of blowby at higher rpms that is heating the engine up.

 

 

It doesn't hurt to check all of the variables, but ultimately it comes down to how efficient the radiator is. 

[ta240]

I put the 3 row csf radiator in and so far the temp needle stays right at the center all the time even under high revs for extended periods and warmer outside temps than I would have been comfortable driving in before.   It makes driving it so much more enjoyable.