noise coming out air filter and overheating

[ronger04]

Truck just started doing this.

It started overheating, but it seems to be intermittent. Sometimes its fine, sometimes its boiling hot

It also just started making a low humming noise that seems to be coming out of the air filter. This noise only happens as the RPMs drop on deceleration or reving when parked. The noise starts around 1500 RPM as the RPMs fall and then the noise stops when the RPMs reach idle speed. I think that the noise may be the engine back firing into the intake manifold

Any ideas what this is and what could be causing it?

Thanks

 

[RonD]

Well an overheated engine cylinder can cause fuel to pre-ignite before intake valve closes, but it would have to be REALLY hot.

No mention of coolant leaks.

So could be the rad getting clogged up.
When engine is warmed up, feel upper and lower rad hoses, squeeze both.
Lower should be a little cooler but the same pressure when squeezed.
If lower is softer then have someone rev up the engine while you squeeze the lower hose, if gets alot softer then rad is getting clogged up.

Water pump pushes coolant through the engine and then out the upper rad hose.
Water pump pulls coolant from the lower rad hose.
If rad starts getting clogged up not enough coolant can flow down/sideways, thru the rad, so water pump suction lowers pressure in lower hose.

If you think that may be the problem then with engine warmed up remove the fan shroud bolts and move it back away from rad, feel the rad surface, should have nice even heat, no cooler spots, cool spot means blocked tube(s).

Thermostat could be sticking.
Not really a test for that, just getting a new t-stat.

Most cooling systems use a by-pass when engine is cold, this is usually the heater core or on the heater hoses, if the by-pass gets clogged up you will get random overheating, uphill downhill wouldn't matter, engine temp just goes up and down randomly, never super hot, but hotter than normal, then back to normal.

Noise could be coolant bubbling because of lack of flow.

Also check the overflow hose and tank, a leak/crack in the hose can cause the cooling system to suck in air instead of coolant as it cools down, so you end up with air in the cooling system, this can cause random overheating until air is purged, but will repeat next time engine cools down.
The overflow hose goes to the bottom of the overflow tank so it is always covered by coolant, otherwise air will get sucked in, but debris in the tank can block coolant from coming back when engine cools, so it's always a good idea to clean the hose and tank from time to time.