The more throttle, the higher the temp

[Bormie]

I got my Jetta together recently having the head pressure checked and replacing the head gasket, rings, rod bearings and 80C thermostat along with other stuff.

I noticed that it runs hotter the more fuel I dump into it. In a nice middle throttle cruise it runs right in the middle of the gage or lower.

In traffic the cooling fan kicks in a little above the center close to where the warning light is mounted in the gage and temp comes back to center.

When I am into the throttle at 3/4 or full to pass or going up a hill it sometimes goes about 3/4 of the way up the gage and close to one tick from full hot on long hills at 100F outside. It's like the cooling system can't deal with the higher EGT's.

Back off on the throttle and it slowly cools back down. I opened the coolant tank (slowly) and there is a nice steady stream of coolant flowing out of the radiator. The previous owner had replaced the radiator not to long ago so It should be ok.
Is this normal for the temp to fluctuate this much?

[tylernt]

Is this normal for the temp to fluctuate this much?

Nope.

Your lower radiator hose may be collapsing from the powerful suction of the water pump at high RPM.

[VWCaddy]

Coolant temperature will rise with high RPM and throttle operation when climbing a long hill in a diesel, at least the VW diesels I have driven. Heat output of the engine is a function of how much energy (fuel) is going in plus how much air volume is being compressed. At higher RPMs, you are compressing more air faster and that makes heat since the compressed air is hot. With my old 1.6D, I found that I needed to drive long grades by the temperature gauge. I would flip on the radiator fan at the base of the climb (with a manual override switch) and then run full throttle up until the temperature got a few marks from the red, then back off to hold it there. I found that the heating was more pronounced at say 4000 RPM or higher than at lower RPMs.

My 1.9D engine in my '82 did the same thing when I first got it. But then after advancing the timing and turning up the fueling I was able to go up the same hills in a higher gear at a lower RPM and had no significant temperature problems. Gauge might only rise a 2-3 ticks above normal. And if I drop to a lower gear and hold the revs up higher, I get the old higher temperature readings.

I also had the radiator rebuilt with a 3-row core and put on an overdrive pulley for the water pump. Those two things increased the cooling capacity of the cooling system and that took out about one tick out of the maximum coolant temps (on the factory gauge). Since upgrading the cooling system made the system run cooler, I would suspect that the stock cooling system is somewhat undersized with respect to maximum cooling needs for the diesel engine.

[Bormie]

Would it help cool things down by modifying the intake to allow more air into the air filter box? The engine seems restricted by the small plastic tube feeding the airbox.
On the larger turbo diesels, a larger turbo pumping more air volume along with more fuel keeps the EGT's down.

[tylernt]

Actually, you might try a cold air intake:

http://www.4crawler.com/Diesel/CheapTri ... dAirIntake

[82vdub]

Sounds to me like everything is working and acting like normal. If you expect to pull a grade without any additional load put on the engine (load = more heat put into the cooling system), then this isn't normal. Every engine, gas diesel or whatever, will put more heat into the cooling system based on engine load, speed, fuel delivery etc. As long as your radiator and fan is working fine, everything seems normal to me with the temp gauge fluctuating as you say. The gauge sensor is installed on the upper radiator hose bracket on the head and reads the water temp coming out of the engine. It does not read the temp the cooled water going back into the engine.

[tylernt]

Sounds to me like everything is working and acting like normal. If you expect to pull a grade without any additional load put on the engine (load = more heat put into the cooling system),

Oh, pulling a grade will put more heat into the system, sure. But the radiator should be able to deal with it so the engine doesn't overheat. The radiator should be able to take everything the engine throws at it, and then some. If it can't, the cooling system is malfunctioning or underdesigned.

The gauge sensor is installed on the upper radiator hose bracket on the head and reads the water temp coming out of the engine. It does not read the temp the cooled water going back into the engine

Good point. However, the engine should not be able to redline the coolant with just one pass through the engine if enough cool water is circulating properly. It sounds like what's happening is the coolant being brought into the engine is slowly getting hotter due to inadequate flow or cooling, so the exit temps increase as well. If you keep the inlet temps down, the exit temps shouldn't rise very much -- just a few ticks.

Maybe the stock Rabbit radiator is simply too small. My Mk2 Jetta has a rad that's like 50% bigger so maybe VW realized their mistake.

[Bormie]

The radiator is a BEHR and looks to have been replaced at one time. There is a sticker on the tank that looks to have a date with an '02 year of manufacture. The cooling system didn't look like it was maintained very well though and I think that some corrosion might have been at least part of the cause of the head gasket blowing. The gasket areas that were in contact with the coolant were brown and corroded. I will take the radiator out this weekend and check the flow and also test the new thermostat. Reading some old posts showed some new ones are sometimes bad right out of the box.
Thank you to everyone for all the replies and all the help getting this car back together. This was my first diesel project and it has been quite an educational experience.

[surfcam]

This has been mentioned before. The OME water pump is way more efficient because it has curved ember blades. A lot of the after market pumps have strait blades. This can have a major effect on flow and maybe even cavitation. Another problem is the distance between the blades and the body of the pump. If its more then a couple and thou eternal by-passing will also cut into efficiencies. Other that switching to a OME pump. I don't know how you could check flow rate cheaply. This is not a positive displacement pump so the flow and pressure can very tremendously (engineering terminology dynamically).

[82vdub]

Sounds to me like everything is working and acting like normal. If you expect to pull a grade without any additional load put on the engine (load = more heat put into the cooling system),

Oh, pulling a grade will put more heat into the system, sure. But the radiator should be able to deal with it so the engine doesn't overheat. The radiator should be able to take everything the engine throws at it, and then some. If it can't, the cooling system is malfunctioning or underdesigned.

The gauge sensor is installed on the upper radiator hose bracket on the head and reads the water temp coming out of the engine. It does not read the temp the cooled water going back into the engine

Good point. However, the engine should not be able to redline the coolant with just one pass through the engine if enough cool water is circulating properly. It sounds like what's happening is the coolant being brought into the engine is slowly getting hotter due to inadequate flow or cooling, so the exit temps increase as well. If you keep the inlet temps down, the exit temps shouldn't rise very much -- just a few ticks.

Maybe the stock Rabbit radiator is simply too small. My Mk2 Jetta has a rad that's like 50% bigger so maybe VW realized their mistake.

I didn't read that it was overheating, just that the gauge was raising to almost the red line when climbing a long hill when it's 100F outside, which to me, seems fairly within the rhelm of normall possibilities given the conditions. Fairly consistantly when I travel, my 82 Rabbit temp gauge rises to two lines below red when pulling a grade, or if it's hot out and I'm working the engine pulling a grade with the AC on. I know my system's serviced and working well, so I consider this normal operation. To ultimately know what temp water is circulating to and from your radiator, you would need to add a temp gauge in the hoses going to and from the radiator. Bormie, I'm definately not saying that you don't have any problem, but I'm saying that to me, the symptoms you describe seem fairly normal to what I see.

I also don't believe you have a stuck closed thermostat. If you had one, I think you would have found out the gauge would run hot nearly all the time, and especially if it's 100F out and you're climbing hills.

[Bormie]

It hasn't overheated yet, but it gets up to the second from the highest dot on the gauge. The last dot is at the max on the gauge.
My gauge has no red zone. just white lines and small dots across the top along with the flashing red light built into the gauge just past the center.

I did replace a leaking OEM water pump with a stamped steel GMB unit that was a spare that came with the car. Is there evidence that these pumps are inferior and would cause flow problems? If so I will change it out.

[surfcam]

I wouldn't say these stamped steel pumps are inferior. Maybe not as adequate. Its very hard to tell how much better the OME pump is but the feeling around hear is that it is. Changing your pump may or may not solve your problem. You may have one problem or a several. All you can do is slowly rule out problems with an open mind.

[Bormie]

I went and bought a digital meat thermometer today and did a temp check at the reservoir bottle.

After reading numerous posts, it seems that some gauges read higher than others. I had to find out where mine was at.

The fan would cycle at about 200F and drop the temp down to about 185F.
I am running a 80C thermostat so the fan switch must be the higher temp unit?
The middle of my gauge is about 185F and the fan would cycle just as the needle is touching the red light built into the gauge which is at 200F.

I also noticed as the temp would rise, the flow out of the small hose would get weaker and would pulsate.

If the gauge is proportional then I would say the max I have seen climbing a 6% grade is maybe 220F or so.

Does this sound about normal?

It would be nice to be able to pull over and check the temp when it is high to see exactly where it is at, but I don't want to pull off the cap at that temp to stick in the thermometer.

I don't know at what temp the light starts flashing but one post mentioned around 245F.

[82vdub]

To me, 220F when it's 100F out and you're pulling a grade isn't excessive. I've run up that high pulling trailers on flat ground with my truck. By the way, great troubleshooting and testing techniques.

If you really want to know what temp it's running at, you can install a gauge into your upper hose similar to what was posted in this link: viewtopic.php?t=8629 You can either perminantly install the gauge in the car, or just temporarily install it for troubleshooting purposes.

[Bormie]

The last time I pulled the steepest hill on the way home from work, the temp went up to the second from the last dot again and the red light flashed all the way up. This light has flashed before even when the temp was only half way so there is some kind of electrical problem also. My car is an 82 Jetta so it has no resevoir low coolant sensor. I found the wire going to the temp sensor connector was close to breaking so I replaced the connector and covered it with some heat shrink tubing. I took the radiator out and it looks like new inside. I checked the thermostat on the stove with my thermometer and it's good. The only thing left I might do before adding an extra fan is change to a factory waterpump and see if it increases the flow rate and improves my cooling.