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VW MKIV-A4 TDIs (VE and PD) This is a general discussion about A4/MkIV Jetta (99.5-~2005), Golf(99.5-2006), and New Beetle(98-2006). Both VE and PD engines are covered here.

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Old August 12th, 2012, 21:58   #1
cencarnacion
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Default Coolant Temp at 99+ C. Normal?

My 00 Jetta TDI hovers around 99 C when fully warmed up. No CEL. Is this normal? From what I've read in this forum, around 91 C is the normal operating temperature. I am thinking that this is a mild case of overheating and may need to replace the thermostat. Getting 46 mpg. VCDS graph shows a steady temperature rise from a cold start and than plateaus at 99 C. Hovers around 99C at idle. Any thoughts on this? Thanks.
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Old August 12th, 2012, 22:01   #2
Genesis
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It's warmer than I'd like. Both fans working ok on both high and low speed?
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Old August 12th, 2012, 22:34   #3
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Good point! Will check when I make my 60 mile trek to work tomorrow. I'm pretty sure you're on to something. Come to think of it, I have not heard that fan for a while, if at all, since I bought the car a year ago.

LOL, it has been a coolant temperature weekend for me. The elevated temp was a coincidental discovery as I was doing VCDS diagnostics on my other Jetta (02 ALH) that was giving low MPGs. Coolant temp on that car would not get past 79C so I wanted to compare it with the 00 Jetta which seems to be getting decent mileage. I hope to get both cars running normal temperatures next weekend. A new thermostat for the 02 and hopefully just a loose fan wiring for the 00 Jetta. Very fond of working on these cars regardless of the occasional hiccups. Will report back tomorrow.
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Old August 13th, 2012, 12:45   #4
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Usually, if you have highway travel, the fans won't make a difference. In traffic with the a/c on, different story entirely.

Probably the low temperature one has a leaky thermostat, or is in a failure mode of some kind, since they're supposed to fail open. The hot one might just have a thermostat that opens hotter, say 95*C or even 100*C. Or it might just be that the sensor is lying to the ECU.
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Old August 18th, 2012, 19:30   #5
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Okay, finally got done fixing temperature problems in both cars.
In the 02 Jetta TDI with low temperature, I replaced the following:
1) Bad thermostat with a Behr 87 C oem thermostat. Also replaced thermostat housing, torqued to 11 ft lbs. No leaks. Broke the tabs of the thermostat housing, replaced with a new one. Next time I do the same job, I will remove the screws, wiggle the housing a bit to get separation from the thermostat and turn the housing counterclockwise 1/4 turn if at all possible. Not sure if that much turn can be done but if it can, you can keep the tabs intact. This moves the tabs away from the thermostat so that the housing can be pulled without breaking the tabs. Installation of the thermostat is easier by connecting the thermostat to the housing and turning 1/4 clockwise and installing as one piece. Now getting 91 to 94 max operating temperature.
2) Also replaced the coolant sensor with the MTC sensor from Autohaus due to sharp fluctuations of 2 to 3 degrees. Careful with removing the connector, I broke the tab on mine. Hopefully it stays on. Now only getting occasional fluctuations of 1 degree.
Hopefully will start to get good mileage on this car.
In the 00 Jetta TDI with temperature around 99 C, I replaced the sensor with the fluctuating sensor from the 02 Jetta TDI just to detect a defective sensor. The temperature dropped to 87 - 91 range. This indicates a defective sensor (a lying sensor as Powder Hound says) and possibly a slightly failing thermostat. Genesis, I found out that the fans do work so I'm happy about that. Was also worried that the radiator might have a slight blockage causing the 99C temps but that has been eliminated since the temps obviously dropped with a different sensor. Will replace sensor and thermostat next weekend when I get them ordered from Autohausaz. They have good prices as in below..
Behr OEM 87C thermostat: $14.71. Appears to be exactly the same as the one I replaced.
Thermostat Housing: $3.61
MTC coolant sensor: $9.91 Elth Coolant Sensor: $12.60 Bought the MTC which seems to work well for now. Will share experience in a few months.
Sensor o-ring: $0.11
Sensor U retaining clip: $0.11
Thank you Genesis and Powder Hound for the input. I am one happy TDI'er.
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Old August 18th, 2012, 19:35   #6
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Excellent -- glad you're back where you should be!
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Old August 19th, 2012, 00:53   #7
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Quote:
Originally Posted by Genesis View Post
Excellent -- glad you're back where you should be!
Thanks Genesis. I see you're from Niceville. I lived in Valp 1984 to 1988 as a young airman at Eglin AFB. Still have good friends living there and visit once in a while.

Just one thought about the car that hovers around 99C. Knowing that it really isn't overheating, would it be an advantage fuel economy-wise to keep telling the ECU that the coolant temp is at 99C and therefore run leaner? I know that I get 53 to 55 mpg if I stay around 2200 rpm highway (about 62 mph). Just in case, I am saving the defective sensor. If I start getting lower mpgs, I might just re-install it. Any thoughts?
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Old August 19th, 2012, 05:02   #8
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No; lying to the computer doesn't do you any good

I'd love to get 53+mpg but I simply can't keep my right foot retracted far enough on the highway to do it. This summer I did get one 51mpg tank; it was all 2 and 4-lane road drive time (almost all) with 55mph speed limits and scads of cops, so I didn't exceed 60-62mph often. Ah well; my car gets ~46-48 doing 80 on the highway, so that will have to do.
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Old August 19th, 2012, 07:06   #9
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I had overheating issues this summer when my engine was at 109 C at times. I replaced my thermostat with a Stant 160 F. It runs cooler, but too cool 81 C on interstate and 97 C city stops. I've hit 101 C. according to my gauge which is little cooler than what my VCDS indicates. I have a digital that always shows about 4 degrees cooler than VCDS.
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Old August 26th, 2012, 15:42   #10
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Replaced thermostat (behr 87 C) and coolant sensor on the tdi that runs 99C. I'm glad I did, the thermostat was starting to rust. Using a 6 inch wire from a clothes hanger with a hook on one end (some smart TDI'er recommended this), I was able to extract the thermostat without breaking the tabs on the housing. I reused the housing, tightened to 11 ft lbs, no leaks.
Because of the presence of rust, I flushed the system with Bahrdahl flushing liquid and drove for 20 minutes with heater on. I made sure to rinse the system well to get rid of the soapy flush. Since I introduced non-distilled water to rinse the system, with the thermostat and sensor out, I pumped air using dispense mode of a Mityvac 7201 through the reservoir return line to force out liquid. I then dispensed 50/50 pentosin coolant through the reservoir line to push out non-distilled water. When pink coolant started pouring out, I installed the thermostat and and sensor. Filled the reservoir and topped it off after a spin around the block.
Temp is now mostly at 91.9 C. Goes to 93.9 once in a while. It may be all in my mind, but I seem to notice that both cars run much better.
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Old August 26th, 2012, 15:44   #11
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That's right about where it should be.
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Old August 27th, 2012, 02:06   #12
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I need to do a flush on mine. I just purchased a Behr for the same price. I've got an O1M with a Sonnax mode transmission cooler mounted on the radiator. I believe this is creating air flow problems.
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Old August 30th, 2012, 11:39   #13
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Default Overheating!


The 00 jetta is overheating to 108C! I had replaced the thermostat with an OEM Behr 87c and the coolant sensor. This fixed the 99C issue (see first post) which was really due to the coolant sensor. THe car was running normal until yesterday when I noticed the temp gauge go past 190C and the AC would not turn on. This morning, I drove the car until it got past 90C. THe lower radiator hose was cold. I also verified that coolant was returning to the reservoir. Water pump (GRAF with metal impeller) was replaced along with the timing belt about 3000 miles ago. Suspecting a thermostat (although OEM new), I replaced it with one that was failing open. I also tried to spin the impeller and could not so that means the water pump is good. This appeared to have solved the problem staying at around 91C with the lower hose as warm as the upper one. But when I drove it on the highway, temp went up to 101. I quickly returned home. When I tried to open the res cap, steam started coming out (expected as lowered pressure caused the liquid to boil). Lower hose was still as hot as upper and heat coming out of heater vent. When I revved the engine, I could see the coolant level drop and then foam would come out at idle rpm. At this point, I am suspecting 1) a blockage in the radiator 2) and worse, a head gasket leak. Any thoughts?
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Old September 1st, 2012, 07:44   #14
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I'd do a leakdown test for head gasket. I really don't think your head gaskets bad. My engine according to my temp gauge is about as hot. Diesels run a little different then Gassers as far as thermodynamics. Your dash light isn't indicating its overheating is it? At higher engine temp your milage is probably better. This summer when Ohio ambient temps were over a hundred Fahrenheit mine was at 108 celcius. I replaced the 180F Motor Rad stat with a 160F Stant stat. My extreme hits at 101C. If the system has no leaks any vapor will stay in the system, and you will have no coolant loss. I also went with a 30% 70% antifreeze mix to increase the boiling point to 226 F this has seemed to work. I use ethylglycol and not VW pink or purple antifreeze. I am a VW freak and have a gasser as well. I have always had the philosophy that with the metal water pump impeller versus PCP or HCH plastic that any antifreeze can be used. Since several years ago VW said that pink or purple had to be used to protect coolant system plastics. With the aftermarket metal impeller water pumps this problem is gone. this Gasoline engine thermodynamics are different Since VW has a negative pressure cooling system its impossible to look for bubbles in the resevoir which indicate blowby gasses from a bad headgasket. I have been running 101C according to my gauge at times with no problems, but my system is sealed. Heres my opinion and what I am doing. I have been watching my coolant level, so far after a 230 mile trip it stayed at level. The dash light only lights red when coolant level drops below an inche from sensor. I might flush my system if the problem gets worse. I'd make sure all your hoses are tight so you don't lose any vapor, that way the coolant will recondense without any loss. (I'm like you I can't get a fix on it, but I have been told by other members on the forum that 108 C is normal.) My gasoline engine thinking tells me different. I have resolved to watch my coolant level, and determined that if I have no loss its good. Thats all I know. If it helps I'm having the same experiance so please post if you found an answer.
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Old September 2nd, 2012, 03:46   #15
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Update I replaced my injectors and found a leaking one. It was leaking from around the head. I replaced copper gasket as part of the injector change. I noticed oil leaking up through the injector nozzle hole. I'm going to monitor my temps after I resealed the leak. This might be the problem. The side of the injector that recesses into the head had oil on it that was leaking around the copper gasket that seals the nozzle tip into the head. I'm scared a little.
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