[702ron]

Hi everyone. Im new here but Ive been searching and searching and Ive tried numerous things I found here and appreciate all of the info, but Im stumped. Ive got a 1998 Beetle with an overheating problem. Technically, the fans are not coming on but I think its because the temp sensor is not getting hot enough.
Ive replaced the radiator, hoses, thermostat(2x), FCM(2x), fuse block on top of the battery, both fans, coolant temp sensor(in radiator). The fans come on with the AC and I can jump the harness for the temp sensor to make the fans work both low and high. The lower coolant hose is much cooler than the upper, even after the thermostat is open. I have a little IR temp reader that tells me that the lower hose is 120ish and the upper is 160ish (When the engine block is up to 190-200...so open thermostat). The thermo switch reads around 130 degrees before the tank starts boiling. The water pump doesnt spin by hand, it turns when I crank it, and all of the blades look fine. I have plenty of water coming out the return hose when I rev it up and the hoses get hard too so it seems like there is plenty of pressure. Miles are 106 and I think its the original water pump so I know its due but its seems to be working correctly.
If I unplug the thermo switch in the splitter thing on the drivers side of the motor the fans shut off (the fans work with the ac on) and my meter tells me that there is continuity on the switch so it seems to be working correctly. All of the wiring for the FCM and thermo switch test correctly according to what I could find.

So Im stumped. 2 things I was wondering...1. What is the sensor farther back on the splitter with the 4 wires and could that be the problem 2. I think the coolant tank cap is bad, could that be it? What tests should I do next? When it idles, it seems that the coolant return line is gargling, then when I rev it, it takes 5 seconds or so to spit solid. Could it be pulling air from the cylinder(Bad head gasket??)

Any input is appreciated. This thing is driving me and my wallet nuts. Thanks

 

[red99]

The sensor with 4 wires is for the hot/cold lights on the dash, so nothing to do with your problem. And if the radiator hoses are getting hard I doubt the cap is the problem. The system is pressurizing.

Are you using a coolant/water mix and not plain water?

You should only get air coming from the return hose to the reservoir when the system is being filled. Continued air from that hose probably means a leaking head gasket. That could be the problem if there is enough air getting in there to create the blockage. Does your overheating happen while driving or just at idle?

There could be an obstruction in the engine block too. If the original plastic water pump came apart the bits of it could be stuck inside the block.

 

[702ron]

Thanks for the reply. I am using 50/50. I also noticed when I first filled it up and started to work the air out of the system with the cap off the coolant steams a little. Now that I'm thinking about it I don't think it's hot enough to steam yet...could that be exhaust coming out through the return? Is there a way to check for pieces stuck in the block without removing the head? Would a pressure test of the coolant system tell me anything? Or a compression test? The impeller looks in tact but I think a few pieces of the cap might have fallen into the tank and might be somewhere in the system. Also, when it starts getting hot and overflowing, I can turn the ac on(turns the fan on low) and it will pull the fluid right down to normal. It overheated while driving because the fan went out which melted the fuse box. I haven't driven it since I put the new fans, fuse, fcm, and thermo sensor in because it keeps boiling over in my garage.
Thanks for you help.

 

[red99]

"Steam" coming out before the engine is hot certainly would be evidence of a head gasket leak. If it was overheated on the road at some point it makes it that much more likely. You should do a compression test to make sure.

With a '98 you should definitely not have the original water pump now. It should have been replaced years ago when the timing belt was replaced, so the condition of the current pump may not tell you about the possibility of a broken pump.

It is probably the melted fuse box that caused the fan to go out in the first place, not the other way around.

 

[VDubYah]

A leak down test will diagnois a bad head gasket faster than a compression test.As it is looking at holding pressure in the cylinder. The gauge is a lower pressure gauge with smaller incriments.

 

[702ron]

So I fixed it. It was the cap. It was holding some pressure but not enough. Thanks for all of the input.

 

[red99]

Glad to hear that solved the problem. I've read where others had trouble burping the air out of the cooling system after refilling because they were trying to do it without the radiator cap on. Apparently it is important for the system to pressurize to be able to get the air out.