Hi Everyone,
I just wanted to post a problem I had experienced with my 1992 VW Jetta TDI, and what I finally figured the solution to be.
I read an awful lot of posts regarding engine overheat w/diesels, and almost every one was pointing to cracked head gasket... I also read in a few posts that the fan has high/low speed; this is true. The low speed will come on if you turn the environment control to anything with a snowflake on it (might only be on models equipped with AC). The high speed is controlled by thermoswitches (as I understand).
The problem was that my car would need coolant every once in a while, but seemingly only after long trips on the highway. The problem got progressively worse as the summer came on. As I would drive down the highway, the temperature gauge would be running fine (around the 1/2 mark), and then all of a sudden (usually after an hour of driving), it would spike up to 3/4 (where my coolant boils), until the low coolant light started flashing. I thought, "that doesn't make sense... on the highway I should have plenty of cold air going through my rad". I would then stop at the side of the highway, fill my reservoir up, and start driving. Sometimes, I would have to pour in a gallon of water, depending on how long the low coolant light was on. After I refilled, and started driving, the temperature gauge would spike right back up within 10-30 seconds. Low coolant light came on after about another minute. Needless to say, it made long highway trips even longer.
I was thinking thermostat...
I brought it to my mechanic, and he did some re-wiring to fix a thermoswitch. He said the fan wasn't coming on when the engine got hot, so it would overheat. He said he was 99.9% sure that the thermoswitch wiring would fix it, because the fan would now come on when the rad was hot. Didn't make sense to me (on the highway, I'm going 120 km/h anyway, so why would that tiny fan help?). I told him to replace the thermostat anyway (he already charged me for the wiring).
No noticeable difference. Same problem next time I was on a long trip. After enough troubleshooting, I learned that my car would only overheat AFTER it had lost enough coolant. I couldn't find signs a leak anywhere, except for at my reservoir steam vent, and a pressure test at Canadian Tire didn't show any leaks in my rad (btw: the Canadian Tire mechanic thought I had a cracked head gasket, but I didn't see any discoloration in my oil, or bubbles in my coolant, so I didn't change anything. He also said that my car overheated because I had too much water in my antifreeze... I noticed that when I was running close to 80% antifreeze, the boiling point was just past the
3/4 mark on the temp gauge; with 95% water, it boils right at the 3/4 mark. It boiled either way. Lesson: Don't go to Canadian Tire.)So, I was confused. I read all over the internet that it's usually a cracked head gasket (but I never actually let my engine run past the boiling point of my coolant, and couldn't see bubbles in my water, or water in my oil). My mechanic was stumped (he said thermoswitch is all he could think of, so my fan would come on). My rad was obviously fine, as it passed the pressure test, was fine in the city, and good on the highway for an hour. My thermostat was new. I was even running a good mixture of antifreeze in case there was a glitch in the matrix, and the Canadian Tire mechanic was actually right. I was absolutely stumped...
I put paper towel near my reservoir bowl vent to confirm that this is where it was leaking. It leaked once in the city (only a small amount), and on my next 2 hour highway trip (a whole lot). Should I plug up the steam vent? Why not?
Didn't help -- water still found its way out of the reservoir where I tried to plug it. I finally ran the car in my driveway (3000 rpm) for a good hour, watching the coolant level in the reservoir. After the engine warmed up, the coolant level came up in the bowl, close to overflowing. Then the fan switched on automatically, and the level dropped. The fan turned off, and the water level crept up again, same thing. Again... Uh oh... this time the fan didn't come on, so the level didn't drop... coolant started leaking out of my top of my reservoir (I plugged the steam vent). Leaking BAD. I turned on my enviro control to the snowflake (to start the rad fan). Fan came on, level dropped.
Everything suddenly made sense. When it was winter, I kept the environment control on Defrost the whole time, so the rad fan was always on, and whatever valve the rad fan opens stayed open (so I never had any problems). When summer set in, I ran with my windows down, and my environment controls off (simply because my AC doesn't work). When I turned that fan off, the associated valve wouldn't work properly, and my engine would spew coolant out of my reservoir until it was low, and then I'd *overheat*.
Being as cheap as I am, and frustrated over how much I've already spent, I've decided to run the car as is, but with the fan on all of the time (enviro controls left on defrost). It's funny, but the mechanic was actually right... George Orwell would say he understood how, but not why. He told me HOW to fix the problem, but not WHY (he only knew from experience, and drew his own conclusions). He said that the thermoswitch will stop the overheating, because the fan cools the coolant in the rad when operating properly.
Not entirely true. The thermoswitch will stop the overheating, but because it electronically opens a valve when the engine is warm, allowing the coolant from the reservoir back into the engine for proper circulation. When this valve fails to open, coolant will boil out of your reservoir, because the 1/4" line between the top rad hose and reservoir will still feed coolant into the reservoir, but it can't go anywhere, so it will overflow. I noticed someone else had this problem years ago, but never posted any success.
I'm not sure where this valve is (maybe where the reservoir hose connects to the engine), but it sure caused me problems. It's the first I've heard of a cooling system having 2 valves, but I figure that's the only thing that makes sense. Why else would the level drop INSTANTLY when the fan runs? Why would the level in the reservoir climb in the first place? It MUST be a valve. Nothing else can over-heat an engine full of water in 10 seconds except for a closed valve (which is why I initially thought thermostat). That's my theory anyway. I hope someone saves themselves a few hours (and hopefully more than a few dollars) from this. Thermoswitch or mysterious valve: whatever you want to call it. What a frustrating problem...
Anyone care to comment?
I'm going on a long trip this weekend. I'll post again if my hack solution doesn't solve the issue.
-Randy
(PS: My power steering pump leaked. I didn't want to spend $200 for a rebuilt one, so I disconnected the belt, and get my daily workout steering manually. I was going to run it with the belt on, but apparently, the power steering fluid lubricates the whole pump assembly, so if you don't take that belt off, you'll cease your pump and snap the belt eventually without any PS fluid. Good advice.)