AMC 304 Overheating Issues

adman02

Well-Known Member
Joined
May 15, 2009
Location
Durham, NC
Took my Scrambler to Uwharrie yesterday (first time I've been) and struggled with an overheating 304. Over the space of about 15-30 minutes, engine temps would steadily increase until the radiator would puke its contents (I just have the stock temp gauge, so I can't give a specific temp reading). Removed the t-stat on the trail, but that made no difference. One seemingly odd thing - when I would loosen the radiator cap to release pressure, it would take a loooong time to release it all

Pertinent details:
- AFAIK, the motor is stock; running a 2150 knock-off
- new (less than 100 miles) aluminum radiator
- no coil/spring in the lower hose
- no shroud
- 8274 on the front
- I have used Bars Stop Leak to seal up leaking freeze plugs (which may be the cause of all of this)


My current theories:
- lower spring is collapsing
- head gasket leak

Would appreciate any tips or advice on diagnosing/fixing this issue.
 
No shroud is a big deal. Depending on your fan setup, you could just be pulling air around the radiator rather than thru it.

Also, when you have the cap off do you see coolant flowing? Need to make sure your water pump is good.

Stop Leak is also bad, it plugs up passages. I don’t even recommend using that stuff in an emergency
 
Jody Treadway said:
What fan?
Click to expand...
Stock seven blade.
IMG_1627 2.JPG


77GreenMachine said:
Also, when you have the cap off do you see coolant flowing? Need to make sure your water pump is good.
Click to expand...
Will check and report back.
 
I’d say a shroud would make a big difference for you. So long as your water pump is in good working order.
 
77GreenMachine said:
I’d say a shroud would make a big difference for you. So long as your water pump is in good working order.
Click to expand...
Yeah, I'd look at that. I ran the stock pump until about a year ago. I was shocked to see how small the inlet and outlet was. I think I posted a picture somewhere about it. You may want to think about swapping that out.
 
Aside from the obvious (lack of shroud, does the the fan clutch have resistance when hot, & use of barstop)... how does it run otherwise? Appears you have a "new" HEI, so where's the timing set?
 
Definitely needs a fan shroud. Had a buddy try this. Hauled all the way to Moab, first trail his started overheating. Spent the whole evening hand making one out of some aluminum we picked up at a fab shop out there.
 
I'll also say that for many years, I ran a flex fan with NO shroud on my 304 (Commando's use CJ radiators) with no overheating issues once I replaced the swiss cheese radiator... which shouldn't be a factor here
 
I ran the engine today and am feeling more and more like it's a head gasket.

I started it cold and removed the radiator cap – saw water flow away from the top inlet (seems like the pump is working) but also saw steady "burps" and bubbles in the water. Put the cap back on and let the engine idle for about 10 minutes and kept an eye on the lower hose and it never collapsed or looked misshapen. Then steam started coming out of the relief outlet. I turned off the engine and slowly opened the radiator cap and the water/coolant was VERY frothy. Also, I apparently roached the temp gauge or sender because my gauge didn't move an inch the entire time the engine ran. Is it worth buying a combustion gas tested from Harbor Freight?

Also, I am not sure how to test the fan clutch. I spun the fan after turning off the engine and there seemed to be resistance. Dunno.
 
Compression test will tell ya head gasket easier imho.
Also something as dumb as a bad radiator cap can cause that as wll
 
I test fan clutches with a foot to 1.5 foot long heater hose. With engine idling, touch fan with it. If it sounds like an a10 "brappppppt" and the fan doesn't slow down clutch is usually good, if the fan slows or can be stopped completely the clutch is wasted. This doesn't test actual lockup that happens when "hot" but generally a fan clutch that's not locked up will cool at idle. The actual locked up clutch is more under a load.

I have no experience with the actual hf combustion tester but it looks identical to the kit we use at work from Napa. It should work fine. If you get it you can test it by filling it up then sampling from the tail pipe, if it changes color you know the fluid is still good and works. Then you can empty it and refill to test actual radiator. Drain a little coolant first, if it's bubbling up it can contaminate the fluid in the tester.
 
Update:
Checked the oil - no water/no milkshake; normal level
Did a combustion gas test on the radiator and the fluid stayed blue; then tested it on the tailpipe exhaust and it turned yellow.
Tried to stop the fan with a rubber hose with engine running - couldn't stop it
Disconnected hoses and flushed the system until water came out clear; added radiator flush and burped it
Let it idle for about an hour and drove it a bit; it ran slightly warm, but otherwise good.

My new theories...
Air was in the system
Faulty china radiator cap
Bad tuning (per @Caver Dave's suggestion)

My next steps...
Leave the flush in for a few days
Buy a shroud
Buy a new radiator cap (it popped about 5 minutes after shutting the engine down; maybe that was the problem all along?)
Take a closer look at the carb adjustments and timing

Big hat tip to @amcjeepman for text support; thanks also to the smarties on this thread.

Before:
IMG_1446.JPG

After:
IMG_4796 2.JPG

Exhaust:
IMG_4952.JPG
 
Last edited:
One last update on this. I’ve been having trouble with the engine under load or at WOT. I narrowed it down to the power valve in the Chinese 2150 (hat tip @amcjeepman). The valve was good but I think the gasket was leaking cuz the bowl was full of fuel. Anyway, I think that may have been causing a lean condition thus making my engine run hot.
 
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