Fan shroud diagnosis/help

[Davidf]

1971, 350cid, automatic with a/c. Generally, engine temps are fine except during periods of idling in traffic with a/c on and air temp 90 degree plus. Temps come down immediately upon moving. no boil over, but temp guage climbs to read zone, radiator outlet temp too high (200+). The fan shroud is the obvious issue (gap), I need a new shroud due to the current one being cracked broken in many places. But, the fitment of the shroud is the issue.

I assume shroud should be tight to radiator so air doesn't get sucked into the gap (low pressure zone) instead of being pulled through the radiator. Help on what parts I need to order would be appreciated. I assume I would start with a a/c shroud from LMC (currently on sale for $50).

Here are some photos:
First is top right, showing gap. Radiator side tank fills bracket channel.


Second is top left. Radiator side tank does not fill bracket (shorter than right side), brackets are both same (right and left side).


Finally, bottom gap. Clips hold bottom off radiator with equal gap of 3/4" or so consistently around shroud.

[68is4me]

I think that you have the wrong upper radiator mounts or wrong radiator. Looks like you have the 4 core mounts on the smaller radiator. I believe the front edge of the shroud should sit almost on top of the rad.

[Davidf]

Definitely possible it is the wrong radiator. The right tank flange is wider than the left tank. right flange fills the bracket channel, left does not. It is not a 4 row radiator.

[leftybass209]

While your radiator may not be stock, that gap is pretty normal (maybe a touch too big) for a stock shroud and radiator setup (also for an aluminum rad like Champion). It can be reduced using some clever ingenuity however. You are correct in thinking that is a culprit of reduced airflow, but it shouldn't be the reason why your truck is slightly overheating.

Some things to check and consider:

What condition is your water pump in?
What condition is your radiator in?
What condition is your temp sender and gauge in?
Have you verified the high temps with a temp gun?
Is your coolant fresh and debris/rust free?
Do you have a proper clutch fan, with operating clutch?
Have you added any fix-it-all bottles like water wetter?
Is your system maintaining adequate pressure?

A quick fix to see if somehow that gap is causing your overheating issues, is to cut a piece of cardboard or plastic and fill the gap. If it works then you can go about constructing something that works and looks better, but again, I don't think that minimal gap is causing your issue.

[davepl]

Rubber weatherstripping to fill the top gap and black duct tape to seal the bottom/sides as needed. Or just tape the whole thing up temporary and see if it makes a substantive difference at all.

Honestly it looks like you have the wrong top mounts, as others have likely said. I'd get the rad to match rather than downsizing the mounts, if possible.

[67 chevelle]

Originally Posted by davepl

Rubber weatherstripping to fill the top gap and black duct tape to seal the bottom/sides as needed. Or just tape the whole thing up temporary and see if it makes a substantive difference at all.

Honestly it looks like you have the wrong top mounts, as others have likely said. I'd get the rad to match rather than downsizing the mounts, if possible.

they sell the 2 core mounts , or better yet get a 4 core or a thicker aluminum radiator

[Stocker]

All good advice above. My gut feeling is a weak or failing fan clutch (assuming the fan itself is an adequate design).

[Davidf]

Thanks for responses. In no particular order:
No fan clutch
New water pump
New temp sender (proper sender with proper range). Mounted in intake (won't fit in heads). Normal running temp is lower quarter of normal range.
Radiator cleaned and pressure checked pofessionally.
Fresh 50/50 coolant/distiller water
No stop leak or similiar used. No evidence of same.
Radiator maintains pressure and does not loose coolant.
Never over heated, just veyr close.

Seems I have seen shrouds very close/tight to radiator. Also, I don't trust radiator cleaning effectiveness. I suspect radiator, works perfectly other than idling in traffic on hot days. Probably marginal flow through rad or even block.

[67 chevelle]

A sugestion , try some "blue ice" additive to your radiator , its cheap , Ive had good results with it in the past

[Stocker]

It would probably be more ideal if the shroud sat closer to the radiator, but I doubt that's much of a problem. Since it gets hotter than you'd like while idling in traffic, and does not have a clutch fan, I would install one. It makes a world of difference. The fan blade design is so much more aggressive than a typical fixed-blade fan, to move a greater volume of air when needed.

[Andy4639]

I close up all the gaps in mine with black foam. This forces the air through the radiator. I have AC also which it helped with that also. Make the air go through it not around it.

[MikeyMan]

Originally Posted by Davidf

New temp sender (proper sender with proper range). Mounted in intake (won't fit in heads). Normal running temp is lower quarter of normal range.

Is your temp sender matched to an aftermarket gauge?
If you're using the factory in dash temp gauge with a sending unit in the intake, I wouldn't trust the reading. A stock sender for stock gauge should be in the head. If your sender only fits the intake and not the head, it's not for the factory in dash gauge. It may read close, but don't trust the accuracy.

There are many threads about sending units and factory gauges on this board.
It's very difficult to find replacement senders that work accurately with a factory gauge.

[Davidf]

Originally Posted by MikeyMan

Is your temp sender matched to an aftermarket gauge?
If you're using the factory in dash temp gauge with a sending unit in the intake, I wouldn't trust the reading. A stock sender for stock gauge should be in the head. If your sender only fits the intake and not the head, it's not for the factory in dash gauge. It may read close, but don't trust the accuracy.

There are many threads about sending units and factory gauges on this board.
It's very difficult to find replacement senders that work accurately with a factory gauge.

It is the factory/original gauge. What I believe to NOT be original are the cylinder heads, thus the hole for the sending unit is too small (newer heads). I have two senders: one from auto parts store...reads high on the scale and one from the preferred vendor that claims to have the right sending unit for our vintage trucks...reads lower on the scale. I prefer the low reading sending unit as it gives me more of idea of when I am getting too hot. But, even when it goes to the red, the radiator holds the pressure.

[MikeyMan]

Originally Posted by Davidf

It is the factory/original gauge. What I believe to NOT be original are the cylinder heads, thus the hole for the sending unit is too small (newer heads). I have two senders: one from auto parts store...reads high on the scale and one from the preferred vendor that claims to have the right sending unit for our vintage trucks...reads lower on the scale. I prefer the low reading sending unit as it gives me more of idea of when I am getting too hot. But, even when it goes to the red, the radiator holds the pressure.

OK, newer heads explains why an original sender wouldn't fit.
Radiator cleaned & checked + new water pump, and it's not boiling over, is a good thing. Maybe the sender in the intake is giving an inflated reading on the high end.

You could do what I did on my '72 350, add a mechanical temperature gauge. I got one of those aftermarket fancy chrome triple gauges. The temp gauge has a flexible hard line with a "bulb" on the end that threads into your head, or intake, with various thread adapters included. Mine is in the DS head and gives me a very accurate reading.
You don't have to do a triple gauge, you could just add a single temp gauge somewhere under dash. Run that bulb sender to the small port in your head. You can run both temp gauges and compare.

[67 chevelle]

Originally Posted by Davidf

It is the factory/original gauge. What I believe to NOT be original are the cylinder heads, thus the hole for the sending unit is too small (newer heads). I have two senders: one from auto parts store...reads high on the scale and one from the preferred vendor that claims to have the right sending unit for our vintage trucks...reads lower on the scale. I prefer the low reading sending unit as it gives me more of idea of when I am getting too hot. But, even when it goes to the red, the radiator holds the pressure.

Are they center bolt / vortec heads , if so there is a whole other issue you may be having due to those heads , easily fixed but a serious engine cooling issue if not dealt with . It has to do with the bypass system

[Davidf]

No, not Vortec heads