Why does this happen? Over heat

[Messi720D]

Simple problem my 81 720 2wd 5spd over heats only when driving on freeway but on on stop and go traffic in the city water pump and fan clutch installed new termostat and radiator flushed. Runs and starts fine No smoke out of exauste or collect in oil what's going on?? No power lost either only thing doesn't work is the heater core belive it's clogged Thanks

[Janksun]

Maybe the radiator is clogged and not letting enough coolant flow around in the radiator to cool the engine at higher speeds when the engine is working harder. I just went through this and the radiator was completely clogged up, replaced the radiator and it runs cooler that it did before.

['70dime]

Also check your lower hose, I had this exact problem a couple weeks back. The higher RPM at highway speed can suck the lower hose closed causing it to over heat but by the time you pull over nothing is out of place.

[TENDRIL]

sounds like a clog somewhere thats not the rad. is there coolant under the carpet on your floorboard?

[izzo]

I've noticed this on the B lately too.

 

Check to be sure you have the right amount of antifreeze and water.  Yes, it does make a difference. Too much antifreeze can cause it to over heat at driving speeds.  It needs water to help keep it cool.  That happened to me, and its been running better since I used 50/50. Never over heats, but does climb a little still while driving, especially up hills. Hasn't reached half way yet, but drives me nuts that it goes up at all.  I have had the rad flushed, lined, fixed, new thermostat, flushed system, new water pump. Mind you I put in a 180 instead of 190 thermostat.  It never over heats, but still raises a tiny bit during highway speeds a little bit more up hills.  Ill be checking my lower rad hose to see if its worn and easily squishable. 

 

Like they said, have you checked to see if its loosing any fluid?  Be sure to use some kind of overflow reservoir, heats up when you drive it and comes out and onto the ground otherwise. Then when its cooled down it sucks air back in.. over time will cause it to be low.  No heat, somethings not right.. low fluid, or something is clogged as mentioned. 

 

I'd open up the petcock on the bottom of the rad, put a hose on the top and flush the shit out of the system.  Hoses on heater core should be easy enough to get to, to try and blow it out or at least check and see if water is flowing thru it.

[datsun 160J SSS]

it could be your CAP I had  the same truck and it was the cap .if the pump and fan are in good order check the cap

[Messi720D]

Thanks alot For the advise I just got this 720 and yes radiator cap was bad, got new one but did not fix problem did a full flush today too raditor and coolant and still climbs up on freeway and I looked at lower hose it does look weak and gummy status will get new one tommarow and give it a shot again after that my next step is to take in raditor for a check up at a radiator repair shop and no I don't have any fluid leaking anywhere did bypass heater core it was bad and clogged I just want it to get it running 100% before it gets 100+ degrees here in arizona will post tommarow regards new water hose and se what happens drives me nuts aswell that the temp just creeps up little by little will post pictures later tonight thanks to all

[datzenmike]

What does 'over heat' mean? Is this the stock gauge or some after market add-on? Generally I see the needle run in the range between 1/3 and just below half way. I've seen over heating problems turn out to be the needle moving up slightly on the highway but still well within the safe operating range. Nothing was wrong.

 

Are you loosing coolant? Are you adding water every week?

 

Once over about 30MPH you don't even need a fan. Enough air is pushed through the rad by vehicle motion. So air supply is likely not the problem.

 

The rad may have been flushed but are the cooling fins and tubes clogged with bugs and dirt on the outside? Can you hold the rad up and see through it?

 

Speaking of rad flushed... what exactly was done? Running a garden hose through it may not be enough. You say the heater core may be plugged and it's also part of the cooling system so if it is dirty the rad might be also. At the least the rad should have been taken out to flush it. Take the cap off and look inside/ Are the vertical tubes covered in lime scale from hard water?

 

Watch the lower rad hose and have some one rev the motor. Does it collapse?

[Messi720D]

Ok peeps So replaced hoses and stuff and Still gets hott so it's time to get a new radiator and hope thats what it needs also how improtant is the temp switch in these trucks?? I have to special order the radiator at autozone they don't have in stock

[Messi720D]

[Messi720D]

[Ira210]

Friction :thumbup: 

[datzenmike]

What does 'over heat' mean? Is this the stock gauge or some after market add-on? Generally I see the needle run in the range between 1/3 and just below half way. I've seen over heating problems turn out to be the needle moving up slightly on the highway but still well within the safe operating range. Nothing was wrong.

 

Is this a stock temp gauge?

Where does the gauge read?

[HRH]

Best tool ever for finding clogs in radiator:  Infrared temperature gun.  Buy one at harbor freight, spot temp all over the radiator, look for the hot spot.

[datzenmike]

... or maybe the cool area where there's less flow.

 

Hell you've ordered one now. Take that one out and lay front side down and fill with CLR to dissolve lime scale. Let sit overnight and flush in the morning.

[q-tip]

Heating up on the highway maybe timing too far advanced?

[racerx]

not sure but does that have a clutch fan?

[Messi720D]

It's a stock gauge and when I drive around city it only gets to half way no more and gets no where close to the H but when I drive it on the highway it creeps up to the H slowly and can get hot at that point I just pull over and let it cool before It continues to get hotter and yes my pops had own 2 trucks like this 84' an 85' and the hottes I ever seen them run is between 1/3 and half way even in hot az days and I will try clr I have radiator out and do that before new one gets here i hardly drive on the highway my job is less than 3 miles and i drive on streets but it bugs me knowing that if I take it in highway it will sure get boiling

[darrel]

Best tool ever for finding clogs in radiator:  Infrared temperature gun.  Buy one at harbor freight, spot temp all over the radiator, look for the hot spot.

this, diagnose, pinpoint problem, then repair or replace. I have a problem with throwing parts at something HOPING it will fix it.

[datzenmike]

not sure but does that have a clutch fan?

 

Above 35 there is enough air forced through the rad by forward motion that you don't need a fan. A slipping clutch fan would cause it to run hot at low speeds or waiting at the lights. 

[Messi720D]

ok guys thanks to all for advice i did get the truck back to normal temp it was the radiator it was junk got a new one for 60 bucks :thumbup:  insted of 140 at autozone that had to be s/o i retuernd so drove from phx to casa grand and no issues at all at 80mph feeling good :sleep:

[spitznoz]

i had a problem like this with my old mazda mx6 me and my dad got  a big Winnebago radiator from the junk yard, people (mx6.com) were saying hey you really didnt fix the problem... others was saying it could have most likely been the radiator i didn't care it was fixed  :thumbup: now i can enjoy the vehicle and you can too congrats