I'm currently diagnosing a tough nut of a cooling system problem. I can start and idle it for 20 mins with no problem, but I can only just drive it a few miles before it gets close to overheating. I think this is another mystery/ complex dsm problem with multiple causes, and I've done a few things so far: Replaced the water
pipe o-rings, replaced the radiator cap, and cleaned out some crud that was blocking the flow of fluid to/from the overflow reservoir. I'm planning to install a lower temp thermostat, which was a bit dirty when I opened it up last time. One of the cylinders has lost some pressure (155 psi, cylinder #4) since the engine rebuild 2K+ miles ago, but this is still within the service limits and barely within 20% of the other cylinders, and no misfire codes from the ECU.

BUT: The fans do not go into the HIGH mode. I can turn on the A/C and get both fans to turn, but the A/C fan does not go on with the coolant temperature at 210 F as it is supposed to do. So the manual says, this is either the HI relays or the ECU. But with all the intermittent/funny electrical problems I've seen with this car, I would BET that this has something to do with the wiring between the ECU, relays, and fans. About a week ago, I took out the driver's seat to replace a broken seat back frame and had a tough time with the single wire that runs underneath. I may have pulled some wire that runs into the ECU.

So the basic question: How can I test/ diagnose the wiring between the ECU and relays and fans? The relay box in the 2.4L/Spyder is on the passenger side in the engine bay. Also, the vehicle has a heater control panel issue: The blower motors will only go on with the dial pushed to full blast. Could this also be related to the fans HI mode problem? Also, I know about 1g ECU capacitors, but do 2g ECU capacitors also need to be replaced? There was some 110F weather in socal a few days ago, and I can see how this might have done something to the ECU. Any other hints/ suggestions would be helpful.

Replaced everything but the main component. Your water pump is on it's way out.
It's able to push water while idle, but once you start reving the motor, the blades are in bad enough shape it's not pushing the coolant fast enough to cool your motor.

I had this happen, then about 3 months later total failure of the water pump.

dgdsm said:

It's not an OEM waterpump, and I've seen some threads about lower quality impellers in aftermarket waterpumps....

Click to expand...

To rephrase: It's a new, but cheap 4g63/4g64 waterpump off Ebay. I seem to recall the blades being metal, but it's possible they were actually plastic. I think the cheapest part was the junk O-ring that I replaced with a Felpro O-ring a few days ago.

But even if I took it apart and installed a new OEM waterpump, I'd still have the fans problem.[DOUBLEPOST=1411161774][/DOUBLEPOST]More info. At idle:

The first (radiator) fan comes on at 205F, which is about right.
The second (condenser) doesn't come on until 221F, which is different from this .

Apparently it was the thermostat. Installed a new 180F thermostat today, and the temp gauge needle holds steady at idle and in normal driving.

I'm going to keep testing it though under different types of loads and see how the cooling system handles it. I don't know if the fan temp cutoff differences from the FSM are due to differences in ECUs, or if the one I have is faulty there. The fans in there look like the original 15-yo OEM fans. Eventually I'd like to have some performance system that can handle extreme conditions that can happen in parts of Cali and SW US.

You may want to back flush the cooling system (after running with some radiator cleaner for 10-20 min - follow instructions on can) and/or get a new radiator (old ones do wear out).

How are you accurately monitoring coolant temperature? Just with the dash gauge?

I have an OBD-II bluetooth transmitter, and ScanMaster-ELM on my laptop. I'm having trouble getting a connection today, but should be able to set it back up soon.

About the cooling system problems, I think I acquired the car with a bad thermostat, or maybe it was a 195F thermostat, and it finally got clogged/ stuck closed at some point in the heatwave. The needle would always move up a bit under load. Now, it doesn't budge at all from this spot no matter what load I put on it. That I believe is 198F.