temp creeps up at highway speeds

[craigbred]

The rad. cap should have a check valve built in it. It is usually in the center of the bottom side. Mine is about the diameter if a dime. It should let outside air in (or AF from an overflow tank) when the pressure in the cooling system goes negative (cools down), and then check shut when the rad. heats and pressure builds. If that is stuck shut, when the cooling system goes negative in pressure, the biggest and softest hose will be the first to suck flat.
I don't really think this is your problem because the neg. pressure in a cooling system occurs when cooling down (not heating up on the highway), but it is easy to check.
Craig

 

[Hermon Davis]

Simple things first...
1. Belt tight
2. Cooling system bleed
3. Fan clutch
and also make sure your coolant is flowing a visual inspection at temp through the open radiator cap (MAKE SURE IT IS OPEN BEFORE STARTING AND REACHING OPERATING TEMP.)
4. Thermostat, sometimes they open but not all the way... it is easy to remove it temporarily, drive it to see if you get proper cooling and then replace.

 

[JIMMY DIXON]

Hi All,
I have an issue where my temps stay cool, around 195 -200* while driving around town and even just idling for long periods of time. If I start driving on the highway, say 55-70 MPH, the temps start going up. The highest I let it get was 220*. I let off the gas a little and brought it down to 50 MPH and the temp immediately started to come down. I tested in a number of driving ranges and temp days. Same issue always. I read a lot of posts about this all over the internet, but unfortunately, people rarely post the fix. I Think I figured out my issue tonight but I don't know the reason for it or exactly what to do to fix it. at about 1500 RPM the lower radiator hose starts to distort. by 2500 RPM, it totally collapses. so basically all the cooled coolant in the radiator is getting choked off from getting back to the water pump and then is not circulated into the motor. Is the simple fix a new hose or do I potentially have a blockage causing the pump to pull way too much pressure?

Thanks in advance
John

It was not uncommon in the past to see hoses with a big spring in them to prevent collapsing. I haven't seen them recently though. Make sure your thermostat is opening all the way and not causing too much restriction. I think you're on the right path though.

 

[Cudakiller70]

Did you pressure test your cooling system? Does it hold pressure? Free rental at Oreillys.

 

[i_taz]

I would look at the radiator, you can only use cleaners so much and then you'll end up with a leak down the road. Look into this fan clutch or the 2747...

New Hayden Heavy Duty Low Profile Fan Clutch (Jag Style)

 

[Ted70challenger]

I agree with earlier posts . The radiator is suspect number 1 . The hose collapsing maybe happening but unlikely. Get a 4 row , high efficiency core radiator. Or a nice aluminum radiator.

 

[John h]

Update to address some of the questions. No complete fix yet. I do have a clutch fan. I purchased a new lower radiator hose but honestly it feels less firm than the one that is on there so I do not have hopes that it will keep it's self from collapsing. I also picked up a new radiator cap 16LB. for $5.50, I wanted to start with the cheapest and easiest. I went into the computer and limited the spark advance to 36* all in. All I changed was the cap. I took the car out for about a 40 mile trip with freeway speeds. over 3000 RPM It was only 55 degrees outside so not really taxing on the cooling system. It ran cooler about 203 average. but again getting off freeway, it would drop down a few degrees. I got home, popped the hood and reved it up. the lower hose was still collapsing. Maybe at higher revs though, not sure if it had something to do wit he cap or just being generally cooler outside and cooler under the hood so the radiator hose was not as plyable. Last night I drained the radiator and installed the new radiator hose. I also made my own spring using 14 Ga copper ground wire. that is installed but I did not have a chance to test. I will go out for a quick run after work toady and it will be about 70+ Degrees outside so I should know more soon.

Thanks for all the replies.

 

[70chall440]

Good luck.

 

[John h]

All,
I was finally able to test the new hose, new home made spring and new cap all together. outside temp was 70-72 F. ran the car on county roads for 5 miles to the freeway. The temp came up nicely to 180* then to 183* could tell the thermostat opened because it dropped back to 179* for a bit. Got to the freeway, drove about 25 miles at 70 MPH witch is well over 3K rpm. That is the full collapse point of the old lower hose. The temp went up slowly to 215* but held there. I got off the freeway and drove county roads all the way home. was driving about 50-55 MPH and about 2400 RPM. temps fell to 210 and stayed there consistently. I feel like I have made progress but will need further testing when temps are in the 80's next week. in the mean time, I will be looking at New radiator options just in case. IS there a way to test flow other than watching it move with the cap off?

 

[Ted70challenger]

You could get a high volume water pump. On a big block mopar it couldn’t be easier.

 

[volunteer]

I haven't seen all (30) previous posts but get the idea, so will offer a couple suggestions. Apologies if previously mentioned.
A clean, 26" high-density 'two-row' core should be adequate in 99% of cases. Extra heat can occur for several reasons. A small thermostat can be replaced by a 'motorhome' unit (which has 3 attaching points as opposed to 2). A lean fuel mixture will also generate extra heat - combined with a very high 'ambient' air temperature will result in higher coolant temps.
Fans have very little effect on cooling radiators when vehicle speed is over 30 mph. Certain types (mainly 'fixed blades') can in fact set up a wall at highway speeds and actually inhibit the natural flow of air thru your core.
My (340) Challenger has true 10.5 c.r., factory cam, standard (26") two-row core, 5-blade 'flex-fan' and a quick timing curve (10-34 degrees) and runs on 87 octane. Temps. are always good and no 'rattling' whether lugging or passing.
Oh, and it also has coolant recovery system and 195 thermostat. Only change was to (T.Q.) carb by going one size smaller in metering rods. Mileage still 22 on hi-way.

 

[fasjac]

Thanks for all the quick replies. My timing is controlled by my Fuel injection. I have 12* advance at idol and all in at 36* I had badly discolored coolant in the first run of the spring so I flushed it numerous times, with the hose and the radiator hoses off. Flushed it until it ran clear. Then I hooked it all back up and filled with distilled water and radiator flush chemical. drove for about 45 minutes and let it sit over night. Flushed again with water, filled up with distilled water only and drove again another 45 minutes. drained the distilled water and it was still discolored, so I started the process over again with new distilled water and radiator flush. after all that, i filled with 50/50 mix anitfreeze and distilled water. (mixed myself). I check the temp with a IR heat gun at the top of the rad and the bottom outlet. they are 15-18* different so it seems the coolant is getting to the bottom cooler as it's supposed to. I will look at different hoses and see what's available. It seems I have plenty of cooling capacity in the Radiator. It's a 26" core because the car was factory air, even though it was a 318 small block when it was born. The Engine is a very mild crate 360. If the new hose is no help I will look into having the radiator rodded out (if they still do that in these days of disposable everything) in case of a large blockage.

With the IR heat gun, check side to side of the radiator after you run it next time. See what the variance in temp is. Kinda sounds like it’s time for a radiator to me.

 

[John h]

took the car out tonight. it's 89 and humid in Western WI. this time I stayed off the Freeway traveled county roads keeping speeds no higher than 60. mostly around 50-55 mph at about 2200 RPM. the temps went as high as 223* I could bring it down to 219 or even 215 by driving 40 MPH or under. the rad hose is no longer being sucked shut after the new hose/spring insertion. It does distort somewhat. I'm sure the spring is helping quite a bit. I assume the water pump is working if it's able to collapse the old hose and distort a new one with a spring in it. I'm thinking it's possible I have a partial blockage on the inlet side of the radiator, but I have had that hose off while flushing the system. Should I pull the bypass hose and take a look or is that irrelevant when the system is open due to being at operating temp over 180*? do I bite the bullet and put in a modern 3 core radiator? things to ponder.....

 

[Steve340]

You really need to pull that radiator out as posted earlier. A stock one with a good core and fins should work OK however.
If I lived in the USA I would bang one of those Cold Case radiators in it. Get it painted black and all good.
While the radiator is out give the block and heads a back flush with water/compressed air to shift all the flaky gunk out. Make sure you use the correct inhibitor if you go to an aluminium radiator.

 

[Ted70challenger]

Don’t get a ordinary radiator, spring for the high efficiency core radiator they do work better. also make certain the engine is not running lean with the air fuel ratio.

 

[gs73rallye]

You definitely need to get a real spring in the lower hose. The spring is there to prevent collapsing from the suction of the water pump. The one you made yourself is helping but if it's not Spring Steel then it will still collapse.

 

[John h]

thanks for all the great suggestions. I will be dedicating a fair amount of time to this issues this weekend. My computer sets my timing and controls my AFR's but I'm going to start at the beginning and verify base timing and advance timing at various RPM's. I will also manually adjust the AFR on the computer and play with retarding total timing or advancing total timing while I test drive. I will also be verifying the system is properly burped and at the same time verify I can see the decent side to side flow while I'm burping the system. if none of that helps my issue I'm going to assume it's time for a new radiator.

 

[John h]

update: all the suggestions have been addressed as per timing, base and total. I have verified I have a good working radiator cap, (replaced) verified coolant mix is no more than 50/50, I also ran mostly distilled water. I flushed the system 8 times. including the heater core and block through the thermostat housing. I checked the belts are not slipping. I tested the thermostat. it starts to open at exactly 180* and fully opens at hotter temps. I checked both temp sensors. clutch fan seems to spin with resistance when the car is not running. fan shroud is and was always on it's correct to the car and has the hood seal After all of this I got the temps down at cruise to around 210-213* but on highway still creeps up to as much as 232* the car runs well at about 210* and under but starts to feel fat and sluggish much above 215*. It runs at 195-200* for a long period of time, (30-40 minutes) but seems to become heat soaked and can't seem to dissipate heat then goes up to 210-215* and stays there until I hit the freeway. SO, I have talked to Chris at Cold Case radiators and purchased their small block model. As soon as I get it installed and tested, I will report back. I really hope this fixes my issues. I believe my 47 year old radiator just can't keep up.

 

[fasjac]

It sure sounds like it's time for the radiator. You have definitely gone through the list.
Down here in Arkansas, It gets so hot and humid that I started running a 160* (long ago) stat during spring and summer. Not uncommon to see daily temps 90-100 degrees. Mine runs consistently 190-205 after about 30 minutes of running the roads. I know it's not popular with a lot of folks but, it works for my 500 stroker sixpack. I rarely cruise it in the winter.

 

[Ted70challenger]

Typically when a engine overheats on the highway, the radiator is always suspect #1 . Me personally haveing gone through all this , would highly suggest a high efficiency core radiator. The fins are more closely spaced and the rows are more closely spaced to . It really is the best radiator you can get , or get a aluminum radiator. good luck