[1967Tempest]

So I bought a 1990 GMC Suburban 4X4 with a 350 and 700R4. I have been slowly whittling away all of the repairs/upgrades to turn this into my daily driver. So far so good. One thing I noticed that when it is over 90* here the temps crept up not fast, but consistent.

So I replaced the very small 3 core radiator with a new aluminum 4 core and replaced the WP and the clutch and went from the factory 5 blade to a new 6 blade RR fan. Going down the road, this this is awesome, stays at like 170-180* with AC on and doing highway speeds 70-75 MPH.

Heres the rub. At idle in crazy heat, it will get to t-stat temp and come down 3 or 4 times and then it will heat soak and start to creep up to 200 etc.. I know for a fact that I have only about 10% of the fan sticking out of the factory (trimmed to fit new rad) shroud.

I know that too hot at idle is a fan/shroud issue and too hot at speed is more of a water flow issue.

My q is will trimming the shroud on the engine side to expose more fan help the cooling at idle and help the AC cool better at idle? I feel as if the air might be cavitating near the fan instead of getting pulled through rad etc..

Just wanna make sure the juice is worth the squeeze before trimming the shroud even more..

Thanks..

[Joe's Garage]

Some of the later-style fans throw the air up and away in all directions rather than straight back over the engine. I believe this is to make them quieter.

When they're only partially in the shroud - usually around one-third in - this works because they create suction and that suction (like a lower pressure area) pulls the air (in the higher pressure area) thru the radiator.

If they're too far in (like over halfway in), they just blow the air around and around inside the shroud (cavitate, if you will). This is probably what yours is doing.

Once you get up to speed, it's moot because the air pressure in front of the radiator does the work of pushing air thru the radiator and shroud, even with the cavitation.

Is your fan shroud two-piece? Many Suburbans and Silverados are.

If it is, take the top half off. Start the engine and see if the air is blowing back over the engine or upward towards the hood. I'll bet it's blowing upward.

If the shroud is one-piece, they are a pain to take off. With the shroud in place, CAREFULLY feeling which direction the air is moving at idle will let you know why it's not cooling. If there is little or no air movement back by the thermostat housing, the fan is NOT blowing air back over the engine.

Your idea to trim the shroud should cure the problem if the fan blows air out and away rather than straight back. It might even help if it blows straight back.

Just remember to keep about 1/3 of the fan inside the shroud after trimming.

Good luck!

Joseph in Las Vega$
(yes, we know about heat here, too)

[1967Tempest]

Yes two piece shroud. I’ll work on it this weekend.