Hey!

My gf's 740 is overheating and it's really odd. It doesn't seem like the radiator fan is coming on and so after it warms up the temp gauge needle starts to ping pong rapidly between half way and near or in the red zone. As long as it's idling or I keep it below 1000 rpm it stays steady, but as soon as I step on the gas the needle goes bonkers. I replaced the water pump, thermostat and hoses a couple of years ago and gave it a good flush (lots of debris!) and those still look good. The wiring harness that sits on the block looks oily and ratty and I've been needing to find the time to clean it up and maybe replace a few wires.

What I thought was the radiator fan is actually the old AC fan in front of the radiator. The white bladed fan that is attached to the front of the block is called the Viscous coupled fan - can that go bad? It seems like high engine bay temperature and high radiator temperature would cause that to come on, but it did not do that?

Thanks!

 

Yes you are correct - the high radiator temperature is supposed to "turn" the viscous coupled fan on. Problem is when the radiator gets partially clogged, the radiator does not release/transfer enough heat to turn it on. Many times my techs have told me "needs a fan clutch". So - here's the one off my car that works fine.Try that one. Eventually they would change their minds and diagnose the real problem - a partially clogged radiator.

How do you fix a partially clogged radiator? With a new core or a new radiator.

In answer to your question - yes the fan clutch can go bad, but that would not be my first choice.

 

Thanks, that makes sense, except for the wobble in the temp gauge. Why would the needle wobble so much with just a touch to the accelerator? And I did flush it extensively 2 years ago, removing a ton of crud that was bound up near the engine block drain plug. When I drained the radiator the fluid was clean, with no sediment, which doesn't mean I got it all and now it could be clogging the tubes in the radiator. Could the wobble be the fan clutch engaging/disengaging rapidly? The engine did seem hot, but I don't recall the fan clutch engaging when I had the hood open. Of course we ran the heater and fan full blast to dissipate as much heat as possible.

Thanks!

 

1987 models are known for disintegrating wiring insulation and that may explain some of the odd symptoms you describe. You mention "replacing a few wires".
You can easily ascertain a clogged radiator by feeling around it for cool spots when the whole surface should be uniformly hot to the touch.

 

I'll check that out on Saturday. Can you say more about replacing the disintegrating wiring insulation. Just cut and crimp what's rotted along with some shrink tube over the crimp connector? Obviously stranded wiring. Is there and automotive grade more impervvious to heat and oils like buried cable is rated for U/G underground. Is there a standard gauge wire or just gonna have to measure every splice?

 

The cut and splice and shrink wrap doesn't seem to work long term and it's a pain to do wire by wire. The best way is to replace the whole affected wiring loom but it is more expensive. Just use any comparable wire gauge if you go wire by wire. I'd solder rather than crimp.

 

Oops!

I got it up to operating temperature and the whole radiator was a cool spot! That's when I realized the thermostat had seized. Because of how my gf drives her car, I purchased one of the Motorad Fail-Safe thermostats. No one local carries them so I ordered one online!

 

So, it overheated again, even after replacing the thermostat. Digging deeper, I drained the system and pulled the radiator - the bottom third was covered in debris, including a lot of hair which seemed indestructible. There was evidence of corrosion in the upper left so I've decided to replace the radiator. Hopefully this will do the trick!

A couple of questions:

1. I pulled the entire drain plug using a 19 mm wrench - what should I use to put it back in? Red Loctite or something different?

2. I flushed the system with a hose and the car is on an incline (seriously wheel chocked!). I want to run some Evapo-Rust Thermocure DeRuster through the system. My preference is to do this with the new radiator installed as the old one requires so much cleaning and is marginal - thoughts?

3. Anything else?

 

The new radiator comes with a drain plug. If it's just temporary use for flushing, any sealant will do.

How rusty is the system? Water running clear when flushing it? I wouldn't go nuts with the flushing.
Open the block **** to do a good flush: on the passenger side, under the exhaust manifold toward the back, close to the bell housing.

 

Thanks!

Actually, it's the block **** I'm wondering what type of sealant or loctite to use - because it is in the block, it going to be significant hotter than the radiator drain. I took the whole block **** out all the way because I couldn't find the 13 mm threads for the drain from above.

The system ran mostly clear there was a bunch of white gunk in the antifreeze and two small pieces of steel.

 

..."two small pieces of steel"? Better try to find where they came from.
Block plug: I don't remember using anything there but if you have to, use some plummers' tape, that white silicone type. Did it look like sealant was used before on it? May be that's a heresy but I have used that tape before on metal-to-metal engine coolant parts with success, not sure about Loctite.

 

Since there is no oil in the coolant and no coolant in the oil, all I can think of is that it is from corrosion in the coolant pathways inside the block.

To remove the plug in the block, I had to use quite a bit of force to unscrew it, with a 19 mm combination wrench and deep well socket. I'll check locally for a high temperature wet sealant - Permatex probably has some and I'll check to see if there is a high temp teflon tape.

thanks for your help!

 

This is not a big deal. Use teflon plumbers tape. It will not melt. Leave the new radiator out and flush with a hose until clear. Replace thermostat and make sure heater is on.