How far back does the fan need to be from the radiator?

I was doing fine on the road until these past few days when the temp hit a hundred. Once it got that hot, it was get hotter and hotter out cruising at full speed. I guess the temp of the tarmac was a lot hotter than the air temp and that didn't help either.

The fan is 1/2 out of the shroud.

The hoses are all new and the lower hose has a wire in it. It doesn't collapse.

The t-stat is a 180* unit. You can see on the temp gauge when it opens up because the temp will go back down a bit but then it just keeps going up and up. I've got a Milodon high flow stat in it but it did the same thing with a regular style stat.

I can control the temp some but opening up the water valve to the heater and kicking the fan on full blast. But that's really not that much fun.

I'm really leaning toward a problem with the radiator.

How hot is it getting?

Its sounding form the symptoms that the rad does not have enough capacity or is too small.

Not to hijack the thread, but how much modification can you get away with on the engine before you start having problems with a stock type setup?
How modified is your engine combo?

Check to make sure your exhaust flapper vlave isn't stuck closed.

If turning on the heater helps drop the coolant temp, you have a cpacity issue with the radiator.

double check the clutch, i have one of the jaguar units on my RR and it is leaking with about 2,000 miles on it.

overheating at speed, what Crizila and hemirick said. I would toss the stat for 1 day on the off chance it is not opening 100%, might get lucky

The factory service manual says the clutch should be no more than 1-1/4" from the rad. My new Hayden HD is about 2". I dug through my spares and found a original 7 blade fan and the shaft on the clutch is about 3/4" longer, which should put me at the correct distance, I'll let you know if it makes a difference when I put it on today!

How far back should the fan be from the radiator? How about as far as the shroud lets it while still keeping the fan 1/2 out of the shroud. Check out a V6 Tbird. The fan is about 12 inches from the radiator, but the shroud is also that long. Make sure your shroud is sealed around the radiator and is is doing what is designed to do: allowing the fan to pull the maximum amount of airflow across the entire surface of the radiator. Also check for clutch slippage/seepage, cold spots in the radiator surface, that would show areas of blockage. Does your radiator look good inside ie. low/no corrosion? Take a STRAIGHT coathanger and put it down one of the tubes, does it go in smoothly? Might just need a rod out or a solid fan coupler.

what kind of gauge are you useing (stock or aftermarket) and is it acurate? a friend of mine has a old corvette he said was running hot we checked it with a hand held laser temp gauge and it never ran hotter than 190deg. Im not shure about the history of your truck how long have you had it, but has it run cool in the past? is this a new problem? have you done any hp mods? make shur your gauge is working acurate.

Sorry, y'all for the slow reply. I went on a trip for my 1 year wedding anniversary with my wife to Savannah, GA and we left our electronic devices with access to the internet at home... fun trip.

Anyway, I can't say how "new" the problem is. I never drove the truck in this kind of temperature until this year when my '86 Daytona bit the bullet (going to try to rebuild her but it will be a bit) and I didn't have a great paying job up until last week so a newer car should be in order very soon. In short, I may have had this problem since I bought the truck several years ago and just never knew it. It has been in the mid to high 90s here with heat indexes over a hundred and the air coming up off the interstate is probably pushing boiling.

The details on the build are going to get messy so hold on (but not as modified as Feets):

1968 D100. Engine was recently rebuilt (though not original) and has less than 3500 miles (but is close to 3500 miles). It is a 440, 0.040 over and the pistons are .100 in the hole (reuse - no budget - don't poke fun), with stock rods and a forged crank. Heads are Eddys (purchased a few years ago) that have been milled for a final compression ration of 9.2:1. Cam is a Lunati 60303 with Lunati lifters and Trend custom pushrods because I'm running Harland Sharp rockers (and that's where the budget went). The oil pump was rebuilt to stock specs and I get great oil pressure even without an oil limiter to the heads. I am running full length headers and 2.5" dual exhaust. The torque converter is from Turbo Action. I run a small oil cooler in addition to a deep trans pan. The fan is a 7 blade from a '76 or so Chrysler C-body of some kind. The water pump and t-stat are new Milodon units. The water pump housing is a factory Mopar unit. The fan clutch is a new thermostatic heavy duty unit made by Hayden. It get very stiff when the engine is hot. The fan moves a lot of very hot air. The rad cap is a newer 15-16lb unit that could be found on any Mopar from the 80s and 90s. The carb is the cheapo Holley 750 (74 jets in the front and a plate equal to 75 jets in the secondaries). I run 18* initial timing and 32* total. I have a Mopar Performance dizzy that I've set up based on advice from here. The total timing doesn't come in until 3500 rpm and the vacuum advance has been pulled back quite a bit. Plugs look great (Champion RC12YC).

The radiator itself has been recored. It is from a '68 C-body that had a 440 and AC (according the the part number). Unfortunately, the radiator and engine are not in the same exact orientation in the truck as they were in the car (the rad sits an inch or two higher) and the the fan is a few inches bigger in diameter than what was used in the original application. While I do have a correct fan shroud (in perfect freaking condition), I cannot use it. I ran without a shroud for a long time but when the air temps got over 85 or so I had to add the shroud. I hacked up a shroud from an '80s Dippy. It actually fits very well. I've sealed around the top and bottom with double sticky sided foam tape and the sides are more well sealed than they were in the Dippy.

Because the radiator was a recore (though via a reputable local shop) I am a little worried it wasn't done well and isn't flowing water evenly. I do not have one of those fancy laser guided temp deals so I guess I need to invest in one.

The temp gauge is the stock factory gauge. When things are humming along nicely in cooler weather the gauge points straight up (between the letters "E" and "M" in the word "TEMP). But now that it is warmer moves to the right (past P) and I get nervous. Once again, I guess I need to confirm how hot it is getting but it does want to start pushing out coolant when it gets real hot. I have no puke tank.

Thanks for all the help guys. I guess I have some things to look at.

On my 340 with the factory ralley gauges it runs on the H when I'm going down the road. I check the actual temp and it is around 190. It never boils over but it always make me worry when I look at the gauge. I run a 160 thermastat new heavy duty clutch fan with a factory shroud and the heavy duty water pump. You can one of the new laser at Napa for about 50 and they are worth it. Good Luck with it! Ronnie

It might be worthwhile to make a trip to Harbor Freight and buy an infrared thermometer for about $30.00 then you can point and shoot to get actual temperatures at specific locations and not rely on gages that may or may not be accurate.

Not to hijack, but how close is too close for the fan to be from the radiator? I'm worried about hard braking and a fan munching a radiator.

I saw a friend at a car show Saturday and he reminded me how i solved his overheating problem. Since he had done about everything I could think of I told him to check the coolant. Sure enough, the coolant was almost pure antifreeze. Cheap and easy to check. Just a thought........
Ron

I'm going to run to Home Depot today and pick up one of those laser guided temperature gauges. I'm not running anywhere near full antifreeze. The mix I run only floats about 3 of the 5 balls in my tester (don't remember what that equates to) but is less than a 50/50 mix.

Drove about 15 miles today and stopped at Home Depot to get a infrared temperature probe. It is only 88* out so things were fine. The gauge stayed between "E" and "M" the entire time. When I stopped I left it running and the top tank was 180* and the bottom tank was 166* or so. So I'm getting good cooling across the radiator (I was also thrilled at how well my small trans cooler was working). Maybe it'll really warm up this afternoon and I can try to get it to overheat.

14* drop across the radiator isn't very good. Under the best conditions you'd get around 40 degrees.
If it works, it works. If it's getting hot and you can't see anything better than that, you have a problem with either low air flow or coolant moving too fast.

That was at idle. I think I need an assistant to hold the throttle for me while I check out temperatures.

You should get a high temperature drop at idle. The coolant is moving slow enough to spend time in the radiator dropping temperature.

Is the fan pulling enough at 700rpm to get adequate air flow?

If you haven't changed the pulley ratio and the clutch is in good shape, it should be adequate.

I've been thinking that maybe it was the rad considering I've replaced everything else. Now the data really seem to be pushing that way. I'm going to do a tad more testing today. But no, I have not changed the ratio.

Of course we had rain come through this afternoon and cool everything off. I can't get the t-stat to stay open long enough to do any more testing.

Finally, some data! The top tank is getting to over 205* when it starts to pass "M." The fins are running about 155* and seems very even across all the fins. But the bottom tank is 175* or so. What in the world does that mean?

Bump

175 in means that you are not getting enough air flow or water surface area.

if it cools down while driving at speed it's not enough air flow.

But why would the tubes between the tanks be so cool while the bottom tank is so much hotter?

how many rows does it have?

what are the temps of the back tubes?

there should be some reading differences from the inlet to the outlet.

It should be three row.

I cannot get good readings on the back tubes because there is a fan moving in the way. If I turn the engine off everything soaks to the same temp.

Top to bottom the tubes go from near the temp of the top tank down to about 150* or even lower for some. But the bottom tank is freaking hot.

If the transmission cooler is in the bottom of the radiator that would explain the elevated temps there.

I also have an external cooler after the one in the radiator. The entry temp on it is around 150 and the exit temp is 110 or so. I'm really confused about what's going on.