The idea that if some is good, more is better is rampant. Sometimes it's true too. Like with money, when is more not better? But there are things where that ethic needs to be examined.

I'm thinking of radiators and the fact that nearly everyone thinks that if two rows of tubes are good then four rows have to be better. Not always the case. The front row, the one that gets hit with "first air" is the most efficient because the greater the temperature differential the greater the heat transfer from the radiator to the air. So ideally, the best radiator would be a one row radiator if it had a large enough volume to handle the heat load.

Each row back, being exposed to air already heated by the row(s) in front of it is less effective than the first because the air/coolant temperature differential is less.

Also to be considered is that each additional row restricts air flowing through the radiator overall.

There is no rule as to what is the best 'number of rows' in a radiator due to the differences in core design. The most common are the type with the serpentine fins that zigzag between the tubes. These are common because they are inexpensive, but less desirable because of the small 'fin to tube' contact. The more costly, and more efficient is the type where the 'fins' are sheets that go all the way across the radiator and are penetrated by the tubes. This provides the most 'fin to tube' contact. Also, this type of fin design is often punched with a great many micro louvers that increase the heat dissapating qualities.

In any case, the more rows the more restrictive to air flow. Bigger area is more efficient than more rows. Point being, there is a point of diminishing returns when adding more rows to the radiator.

 

No coolant loss. Radiator shop said that old radiator flowed fine, but looked near the end of its lifespan. We ran an IR thermometer and it confirmed the factory gague readings.

I understand that a 4 row is less efficent than a three row- but it does have more heat exchange capacity. In the past we have upgraded grain trucks which have to operate wfo in hight heat- and the extra core has helped on those applications. Because this truck is always heavy, it seemed that I have nothing to lose by increasing the cooling capacity to what was in some model years a Heavy Duty specification.

Still- it was currently heating under light loads, which means that upgrade with out diagnosing the problem is foolish.

So I am back at the fan clutch, even though that does not make sense when the truck heats running into winter head winds.

-Mike

 

New thermostat. Radiator clear. Thanks for the advice on the gaskets.

-Mike

 

After all you have looked at, air flow and circulation seem fine, maybe you should consider putting an exhaust sniffer in the radiator to see if there are traces of combustion in there. Or do a "Block Check" test.

 

Had a 73 Olds with a 455 that did that to me and discovered one of the rad hoses was missing a coil spring in it that prevented the hose from collapsing. There was no flow. Just a thought.

 

Very good point. I didn't think of that, but I have found that on a number of GM vehicles. Hold it at three thousand rpm and pinch the lower hose to see if it wants to collapse. (careful of that fan!!!) Be nice if it was that simple.

 

Bdox:

I think the sniffer is a great idea. I thought that because the coolant was clean and the level was steady that there was no issue here. Wouldn't you see some sign of gasses entering into the cooling system?

twayneb:

You may have hit the nail on the head. I pulled the lower radiator hose during disassembly because it looked a bit tired. I found a bulge on the back side and the whole item just felt too pliable. I'll bet there is a good chance it was not flowing as well as it should have been.

-Mike

 

That lower hose is subject to a huge negative pressure on the intake side of the pump. (contrary to popular belief, the pressure in the cooling system is not equal throughout.)

As far as the sniffer test goes, a very small combustion leak can cause a small problem like you have. Not so bad as to pump the coolant out, but bad enough to cause a hot spot in the block.

The thermostat serves two purposes. One is to restrict flow to insure that the block and heads get hot enough. It also provides a restriction when fully open to increase the pressure in the heads to further increase the boiling point. The highest pressure in the system is between the pump and the thermostat. It all varies with pump rpm.

 

That's interesting. I never would have thought of the pump creating a vacuum on the lower hose. Makes sense. I think all of these issues (bad lower hose, weak fan clutch & possible combustion leak) combined to push a cooling system that was already close to max capacity over the top.

I'll replace the tired and the worn this week, bolt it back up this weekend and take it in to have it sniffed. It has been run so hard that it is entirely possible that it has a tired headgasket.

-Mike

PS. While I have access to the experts... I have another question. Ever since we have had the truck- when shifting from 2-3 or 3-4, the truck stumbles for a moment as soon as you apply throttle in the new gear. It sounds/feels as though you have turned the ignition off and on quickly. I assume that it is fuel related but have found no leaks or suspicious hoses around the quadrajet.

Ideas? And say "throw the Quadrajet away and replace with X."

-Mike

 

Might be a vacuum problem. If the hoses are hard get rid of them as there may be small cracks. PVC also might cause it.

 

I'd check the accelerator pump diaphram. I think it's on the outside front of most quads. Has it's own square cover held on by four small screws and works off of a arm shaped mechanism. The diaphram is only about the size of a quarter/half dollar. Examine closely as it will only take a pin hole to screw it up. I haven't even looked at a quad in forever, memory may be a bit foggy! LOL

I think it comes in most gasket rebuild kits, but you may be able to buy it seperate.

 

good idea- they are all hard w/ small cracks.

-m

 

I'll pull it apart and take a look.

Thanks

-Mike

 

The Q-jet is a great carb. Keep that for sure. Do a careful check for vacuum leaks too.

If you are going to open it up, renew the power valve also.

 

Quadrajets will run with nop or little accelerator pump. In front of the primary barrels, through the vent tube you can see a small hanger with a "tee" shape. Take a small screw diver and see if it moves up and down freely. That is the enrichment system on a quadrajet, it moves the primary metering rods up and down. If it won't move, you'll have to rebuild the carb. Does your truck have a four lead cable coming out from it and disappearing through the firewall? If so you have the ESC system which was prone to failure. You might want to take a good look at the advance system both mechanical and vacuum. HEI distributors are know to develop frozen advance mechanisms. Chevrolet used to use steel shim headgaskets that would leak if ever run hot, then the problem would "feed" on itself getting progressively worse. Bdox, if you find a power valve in a Qjet, I would like to see it. Good luck with it.