Help. 348 overheating fans not going on. Help help help

edit: saw you checked relays

 

Is your coolant level at the right amount. What seems unusual to me is that the bottom and top are not close to the same temp.

Maybe you have an airbubble keeping the fluid high in the overflow even though its low on coolant.

If you can when cold squeeze all hoses and check if coolant goes up in overflow this should kinda confirm no air bubbles.

You can also take out one sensor and put it in boiling water and see if the resistance changes. This should tell you if the sensor is working.

I have a 355 so not as familiar with the 348.

I would also check the coolant temp with a temp gun as the engine warms up with cap off it should read about same as gauge. You only need to confirm at like 130 or 140 degrees. If its the same then you know the gauge is acccurate.

 

Does anyone have a decoder ring?

Almost sounds like a broken water pump impeller *if* I'm reading the post correctly.

 

If all those chck out i agree with above post likely the water pump is not working properly.

 

Your sure your temp gauge is working right? Try a temp gun.
I do know one thing ..keep letting her get up to 250 deg and your going to have more problems than your fans coming on late.

 

The "click" you hear when you push the A/C button is normal. That's the sound of the magnetic clutch at the front of the A/C compressor (lower right front of engine) engaging like it's supposed to.
Would disconnecting a coolant hose after starting the car maybe give an indication of coolant flow rate? Of course you won't have a normal Ferrari to determine what it should be, but I would think it should show a pretty heafty stream gushing out. Do it with the thermostat removed so you can do it on a cold car. Much safer for you.

 

Another thing you could try doing it you got a temp gun, is to get the engine up to running temp, get a baseline reading for one of the hoses, then bypass the fan switches so that you manually get them to run, and watch the temp gun and see if the temp falls. If your water pump is iffy, there probably won't be much change.

 

First thing to verify is that temperature guage, second is the resistance of the sensor at 100C (or there about). If the guage is wrong, you may be chasing ghosts.

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This is good advise and where I would start. Given that you say the fan doesn't come on when the AC is started I would think it electrical in nature because you are correct that fan should start. Heck, it could be something as simple as a bad ground and you are not actually overheating. It's a pain to track these type of gremlins, keep us posted. Hang in there.

 

Are you bleeding the cooling system at the bleeders on the transfer pipe on the firewall ?

 

On my '91 348, the AC condenser is at the front (in front of the right wheel) with its own fan so the engine cooling fans operation is not linked to the AC in any way. As you have a big temperature difference between the radiator bottom and top (cooler top), it looks very much like no-flow, i.e. bad water pump. The coolant flow is from the radiator bottom to top and, if there is no flow, the temp-switch does not get hot enough coolant to trigger it. The fact that you can bleed the system does not mean that the pump must be good; the coolant will flowing out of the bleed nipples even if the pump is bad as it will be pushed by the cooling system pressure created when the coolant heats up.

 

Miroljub, didn't some 348 water pumps have plastic impellers?

 

According to what I have read (and seen pictures of cracked/loose impellers), the 348 pumps did have plastic impellers as factory installed. Perhaps up to a certain year or maybe all (can't remember finding anything about this). Some rebuilders offer the pump with a steel impeller. I do have a new pump at home (from Eurospares, aftermarket I think) and I will check what impeller it has and inform.

 

I bought this water pump, plastic.
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Thought I remembered seeing something about plastic impellers on those pumps. it was tough for me to get a handle on everything the OP was saying but it sure sounded like the pump wasn't circulating water, which is a more reasonable assumption where there is a possibility that a plastic impeller was used. Thanks for the info. Hope things are going well for you.

 

What alternatives do you have other than buying a new pump, pulling the engine, and replacing the pump again?

 

If the fans aren't kicking on the first thing I would check would be the temp sender (aka: temperature switch, part number 158655) on the half radiator, on the right hand side of the engine bay, just above the oil cooler. The sender will be on the top left corner of the half radiator.