Gen II Cooling Systems Woes - Need help!

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John Humphreys

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So my 2003 3.9L Gen II has the fan constantly running on high, without the engine overheating and the temp gauge is on normal. The fan will slow down if I set the heater on full, but comes right back again when I turn off the heat.

From all the threads I've read on here, these are clear signs of air in the cooling system right?

Replaced the Degas bottle & cap, the radiator, all the radiator hoses (upper, lower, etc), all 3 pieces of the Thermostat housing, (Thank Soul!) fresh seals & o-rings. No signs of head gasket problems (No oil in the coolant or coolant in the oil). Note, the DCCV was just replaced 3 months ago, and I'm not seeing any signs of failure from the DCCV.

Bleed the system several times and nothing has changed, the fan is still running on high speed non stop!

I took it to a mechanic, complete flush and pressure tested the entire system. No leaks but the fan is still running on high.

So my questions are:

1) Any help in diagnosing the problem? I know the pressure test didn't show signs of leakage but it shows all the symptoms of it even after replacing all the "plastic parts".

2) Any alternative ideas? Could be the ECT sensors but intake removal is over my head and I don't want to blow $500 and find out that's not the issue. I'm not getting any codes or faults on the MIL. Could it be a wiring or PCM issue?

3) If I do not fix this, what damage is this doing to my engine?

Thanks for any help!
 
When you say you bled the system did you follow the tech article above to the letter?

Also, did you use all motorcraft parts? Did you replace the Degas cap?
 
Thanks for the quick response, yes I replaced the cap as well, all OEM parts except the Degas Bottle was Dorman (Doesn't have the Min/Max markings like OEM).

I have read the bleeding procedure multiple times and followed it to the letter. Like I said, I even took it to a mechanic so they could use their power bleeding system in case I was doing something wrong.
 
"the fan constantly running on high, without the engine overheating" those are conflicting statements. Why do you say that the engine is not overheating? Is it because the gauge hasn't gone past the halfway mark? You can't really go by that. Overheating starts just before the gauge goes past halfway. A decent OBDII scan tool can display the actual engine temperature.

I assume that fan speed is normal at first startup of the day, but goes to high speed soon after the temperature gauge reaches the halfway mark. Is this true?

1. I've passed a pressure test before when I knew I had a leak (I could even see the coolant under the hood after a drive). However, it wouldn't leak during the pressure test. It could be leaking at any of the rubber seals that press against the metal of the engine. It might be that a little of the old seal is still stuck on there and was not cleaned completely off. It could be other things. I've gotten some of the big o-rings that go of the 499 tube that were ever so slightly the wrong size or shape before. It could be something not changed in the heater circuit. How about the aux pump? It could be the water pump (seems rare). How about the throttle body heater hoses?

2. Probably (but not certainly) not the temperature sensors or wiring. (When the plastic pipe from the engine to the thermostat and crossover was replaced, the intake manifold had to be removed. That would have been a good time to check the temperature sensors.) Also probably not the PCM. The fan doesn't run with the key on but the engine not running, does it? Was the thermostat itself replaced?

3. Operating an engine outside of its designed temperature range (hotter or colder) will cause the moving parts of the engine to wear faster. Higher temperatures also bake and degrade plastic parts. The parts on the outside, like the cooling parts are annoying to replace, but the plastic parts on the inside of the engine like the timing chain tensioners and guides are down right inconvenient to replace. Higher temperatures could also cause some oil lubrication issues. Basically, you may be shortening the life of the engine.
 
Thanks, what you said is accurate. I should say the engine is not boiling over. I don't have a scan tool to read the actual temps. I know the temp gage is not linear.

It's good to know that the pressure tests are not 100% reliable, but it doesn't help me diagnose anything. I did replace the o rings and gaskets, but I didn't remove tbe intake manifold to replace the coolant inlet neck to the engine. I used the procedure posted elsewhere on this forum and cut the 2 rear bolts, replacing them with 5 mm shorter bolts.

I would appreciate any advice to isolate certain components like the Aux pump or leaks in the heater circuit.
 
I removed the degas bottle and did a dunk test in a garbage can full of water, the bottle and the cap show no signs of leaking (no air bubbles when dunked under water).

Note, I followed the bleeding procedure as described. The cooling fan does not come on at high speed until the very last step. When I turn the HVAC down from full heat to 75F, after it idles for about 2 minutes. I'm not sure if that helps, any ideas to isolate the potential air leak are appreciated!
 
during your "leak test", was the bottle pressurized and damn hot?
 
As countless (over 1000) threads will describe and I myself have experienced this twice, what happens is due to the paper thin plastic of the dorman degas bottle - it forms hairline cracks which expand when the system is pressurized and introduced to heated coolant return. When the hairline cracks expand - coolant leaks out AND/OR air gets let in causing the system to overheat.
 
It should also be noted that there is a tube that runs inside the bottle down to the bottom. If that tube comes loose and has any other problem, the degas bottle will actually add air to the cooling loop in the engine instead of removing it. The tube inside (Dorman or OEM) can fail without any external leaks on the bottle.
 
I agree about tube. Although I suspect someone broke my return barb on my doorman while working in the area, this tube running through it was in very poor shape and would have likely failed very soon. I know distilled water was not used though during that repair which may have aided its deterioration.

The doorman can certainly get you through a pinch but if you keep having issues, just get the good motorcraft one with new cap and eliminate two more variables.
 
Sorry to restart this thread but I can't seem to close this issue.

I have replaced the Degas with a OEM part and still have the fan coming on high at idle.

The Fan is coming on when the CHT is reading ~149F, which seems extremely low to me.

When I turn on the heater to full, the temp drops to ~130F and the fan turns off.

I don't have a scan tool, so I just reading the CHT sensor voltage and interpolating the temps.

Does this make any sense?
Why would the PCM turn the fan on at such a low temp?
Is there another sensor that controls engine temp and could trigger the PCM to think it is overheating?

Any insight is appreciated!
 
Those temps are too low.
I think that you are misreading the CHT. Get a scan tool and see what the PCM is seeing. (Or take it to a shop/dealer that can...)
 
Joegr, you are wise indeed.

The Gen II has a dual switching resistor circuit. I was using a Gen I look up table by accident.

The CHT reads ~3.0V with the fan on high = ~230F

When I turn the heater onto full, the CHT very quickly changes to ~3.4V = 212F

So I'm back to air in the freaking system.

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