Coolant Temperature Issues (not your typical issue)

alexdinita

Member
Joined
Jan 10, 2018
Location
NYC
TDI
B7 A4 BHW TDI Swapped
So given the fact that my actual car is a Frankenstein, 2006 B7 A4 Avant 6MT with a 2005.5 Passat BHW 2.0TDI swap, this is the best place to ask for help.

So swap is all done and everything is great except 2 issues that I believe are related.
Most important issue is that the car never goes passed 55-59 degreees C, measured through vag com, no matter if idling for an hour or Driving for any given time. The gauge on the cluster will start moving up when the car reaches 50 C and it will go up a bit passed the white line when it reaches 55c which points to a working CTS. I even measured around the motor and it stays around 60C. The coolant pipe that leaves from thermostat seems to always be cold and it measured around 20-30C at most. The thermostat doesn’t even get to open due to the temperature in the block never reaching its 80-95C temps.

What could cause the coolant to never get to operating temperature?
Fans never turn on either.

Problem number 2 the glow plug light is blinking at me continuously which could have something to do with the coolant never getting up to temp.

This is my first diesel and I’m mechanically apt I believe since I finished the swap with somewhat success lol but this issue has me baffled.

Thank you
 

oilhammer

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outside St Louis, MO
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There are just too many to list....
Diesels take a long time to warm up, and really will barely warm up at all while at idle, especially if it is really cold out. You did not say what your current ambient temps are, but you are in NY and it is early March, so I would imagine it is fairly cold.

The green CTS has two outputs, one goes to the ECU, one to the Cluster. Check the data for both, make sure they both match.

Next, feel the temp of the two heater hoses in the cowl area. Both about the same temp? Does the heater work OK? Does the engine still have its EGR cooler installed and working?

Clogged up heater cores are common on that car, no exact idea why, but I have had to manually flush a LOT of them over the years, and if the heater core is not flowing you cannot ever get the system totally filled up.

I doubt the blinking glow lamp has anything to do with the coolant temp. Are there any DTCs anywhere?
 

alexdinita

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NYC
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B7 A4 BHW TDI Swapped
Diesels take a long time to warm up, and really will barely warm up at all while at idle, especially if it is really cold out. You did not say what your current ambient temps are, but you are in NY and it is early March, so I would imagine it is fairly cold.

The green CTS has two outputs, one goes to the ECU, one to the Cluster. Check the data for both, make sure they both match.

Next, feel the temp of the two heater hoses in the cowl area. Both about the same temp? Does the heater work OK? Does the engine still have its EGR cooler installed and working?

Clogged up heater cores are common on that car, no exact idea why, but I have had to manually flush a LOT of them over the years, and if the heater core is not flowing you cannot ever get the system totally filled up.

I doubt the blinking glow lamp has anything to do with the coolant temp. Are there any DTCs anywhere?
I drove the car and beat on it and the temperature still didn’t go passed 55C. Like I mentioned regardless of it idling for an hour or Driving it for an hour temps don’t go passed 55C. Since this is swapped in a b7 a4 the harness I used required a 2 pin grey CTS. But the readings from it are right no matter which way you look at it.
I have perfect heat in the cabin. The 2 hoses that go to the heater core are both warm to the touch and they measure at 45-50 C going in and 20-30C coming out. EGR cooler is installed and seems to be working properly. But even if it wasn’t I wouldn’t see it keeping temps down.
There’s no DTC’s for glow plugs. There were DTCs for all 4 “open circuit” but turned out they were all dead and I swapped new ones in and now no more DTC’s but light still blinking.
 

Vince Waldon

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Edmonton AB Canada
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2001 ALH Jetta, 2003 ALH Wagon, 2005 BEW Wagon
Just for clarification, for whatever reason VW decided to use a flashing glowplug light as a "fix me now" Check Engine light... generally related to safety or fuel system issues as opposed to a problem with the actual glow plugs.

Do all your brake lights work? Cruise control functioning?

It's strange that there are no DTCs... are you using a generic scanner or a VAG-specific one like VCDS?
 

alexdinita

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Joined
Jan 10, 2018
Location
NYC
TDI
B7 A4 BHW TDI Swapped
Just for clarification, for whatever reason VW decided to use a flashing glowplug light as a "fix me now" Check Engine light... generally related to safety or fuel system issues as opposed to a problem with the actual glow plugs.

Do all your brake lights work? Cruise control functioning?

It's strange that there are no DTCs... are you using a generic scanner or a VAG-specific one like VCDS?
If that’s the case, glow plug light used for other things, then my brake switch is out and that triggers a separate code, but I thought glow plug lug light is solely for glow plugs. Cruise control not functioning and I’m assuming it’s due to the brake switch, I have it I just haven’t had time to do it.
Most important issue right now is to figure out and to get the car to operate at proper temperature.
 

oilhammer

Certified Volkswagen Nut & Vendor
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outside St Louis, MO
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There are just too many to list....
OK, so the glow lamp flashing is related to your brake switch... that one is at least partially solved.

CTS, you are using the late style gray two pin type, and it sounds like you have verified the coolant temp is actually pretty cool, but your heat is working OK. And the gauge indicates what? Never gets to half way? Just barely comes up off cold?

What engine was originally in the car?
 

alexdinita

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Joined
Jan 10, 2018
Location
NYC
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B7 A4 BHW TDI Swapped
OK, so the glow lamp flashing is related to your brake switch... that one is at least partially solved.
CTS, you are using the late style gray two pin type, and it sounds like you have verified the coolant temp is actually pretty cool, but your heat is working OK. And the gauge indicates what? Never gets to half way? Just barely comes up off cold?
What engine was originally in the car?
So back tracking a bit here, the car is a 2006 Audi A4 Avant 6MT 2.0TFSI (2.0 gas turbo). The harness i'm using for the swap is a 2006-2009 Audi A4 BPW (2.0TDI 8v) harness from europe with a modded BHW ECU.
We all know that the BHW harness is 4/5 pin and has a green CTS with 4/5 pin. The BPW harness (BPW is BHW's twin brother from europe) is a 2 pin gray CTS. It turned out that the 2.0TFSI also had a 2 pin gray CTS so i took that one out of the gasser and put it in the BHW. The gauge indicates normally (i.e. gauge reads starting at 50C, first white line to 80-100C middle line) and raises a bit passed the white line but never gets to half way. So yes it barely comes off cold.
 

oilhammer

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outside St Louis, MO
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There are just too many to list....
Yes, all the later VAG cars use the two pin gray CTS, so what you did should be perfectly fine I would think. Only thing I can think is something else in the car's plumbing that could cause improper coolant flow. The 2.0L engine that was in there (BWT engine probably?) I think had its CTS mounted on the outlet that houses the thermostat and the BHW has it back at the rear of the head. I do not think that could or would cause any issues.

What is the ambient temps where you are right now? I think my BHWs take a good 15 minutes of driving in cold temps before they get anywhere near 80C coolant temp, but the gauge shows "middle" much sooner than that.

Can you feel the radiator at all? Does it ever feel warm? Are you using the 044 prefix tandem thermostat like the TDIs normally use? I would think if the coolant was never getting over 55C your heater output would be awful inside the car.
 

alexdinita

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NYC
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B7 A4 BHW TDI Swapped
Ambient temps now are about 35F and last week were anywhere 40-50 and temperature still stayed the same even after driving it for 30 min around town/hwy. I don’t think I have a way to feel the radiator itself but the top pipe going into the radiator is somewhat warm opposed to the bottom one that’s a lot colder.
 

vwztips

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Are you using a VW thermostat?
 

Windex

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I'm wondering about the statement he made about "perfect heat" in the cabin.

I'm with Oilhammer - if the car is truly not going over 55c - the heat output from the heater core would be awful.

I wonder if this is a CTS issue.

I'm assuming you don't have the heater fan on full blast all the time?
 

alexdinita

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Jan 10, 2018
Location
NYC
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B7 A4 BHW TDI Swapped
take the paper towel out of the water line
It would overheat if that was the case.
I suspect you're just not driving far or fast enough to make heat. My B5.5's and audi BHW swap with manual transmissions take about 10 miles of highway speed (70mph) to reach operating temperature when the outside temp is below 0ºC. It takes even longer if you use the heater. The engine will cool down quite a bit at traffic lights, often back down to the 70ºC mark. The heater can easily consume more heat than the engine can produce.
I drove it 50-60 miles on the highway, i drove it 10-15 miles abusing it in the city. It was 42F outside today. I drove it with the heat off.
I'm wondering about the statement he made about "perfect heat" in the cabin.

I'm with Oilhammer - if the car is truly not going over 55c - the heat output from the heater core would be awful.

I wonder if this is a CTS issue.

I'm assuming you don't have the heater fan on full blast all the time?
I did some more testing today. Today ambient temps in NYC were from 40-45F. I blocked off the front grille with a plastic bag zip tied at all 4 corners. Driving the car hard i reached a maximum of 70C verified both on the gauge and through OBD2. I popped the hood and measured every coolant hose and pipe and they measure 90-100C (don't forget there was no airflow in the engine bay due to the grille being blocked off). Heater core hoses were at 90-95C. I ripped the bag off and drove it around and took it on the highway for a few miles and tried to keep it above 2000 RPMS, temps dropped down to 60C and stayed there. Heat output in cabin was all the same throughout the day. I popped the hood after the highway drive and all the coolant hoses had dropped down to around 70-75C and the bottom one leaving from the thermostat was in the high 40's C. The top radiator hose going into the hard coolant pipe was 70-75C. I don't even know what to conclude from here any longer. I'm personally stumped. :confused:

P.S.: In conclusion it seems like the CTS readings are on par with the temperature of the coolant hoses when there is airflow in the engine bay.
 
Last edited:

oilhammer

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outside St Louis, MO
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There are just too many to list....
I agree with your method of verification of CTS value and actual temp. That is what I was trying to rule out before, just so we were attacking one problem and not two, or chasing the symptom of a problem that was not part of the actual cause.

Seems we have concluded your engine is, indeed, simply operating at too low of a coolant temp.

I would not think that "too large" of a radiator, if that was even a possibility, would cause this, because the thermostat should keep the coolant from flowing through the radiator regardless.

So it comes back to the thermostat itself. Is it the wrong one? Is it defective? Is it installed backwards?
 

alexdinita

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Location
NYC
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B7 A4 BHW TDI Swapped
I agree with your method of verification of CTS value and actual temp. That is what I was trying to rule out before, just so we were attacking one problem and not two, or chasing the symptom of a problem that was not part of the actual cause.
Seems we have concluded your engine is, indeed, simply operating at too low of a coolant temp.
I would not think that "too large" of a radiator, if that was even a possibility, would cause this, because the thermostat should keep the coolant from flowing through the radiator regardless.
So it comes back to the thermostat itself. Is it the wrong one? Is it defective? Is it installed backwards?
I double checked part number and comes up as PD thermostat. I have 2 of them I keep switching in between. The one that’s in there now locks into the housing nice and snug. The thermostat was tested in hot water and starts cracking at 87C. Could the car be running rich and taking away from the blocks thermal capacity? Again I’m clueless now.
 

oilhammer

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outside St Louis, MO
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There are just too many to list....
Diesel don't run lean/rich in the sense that a gasoline engine does. I assume the engine runs OK, meaning good and strong? I suppose a severely sick TDI would not be able to make enough waste heat to warm up if it was severely underfueling.... but the car would likely barely be able to move if it was that bad.
 

Windex

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Cambridge
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05 B5V 01E FRF
I still suspect the CTS:

I did some more testing today. Today ambient temps in NYC were from 40-45F. I blocked off the front grille with a plastic bag zip tied at all 4 corners. Driving the car hard i reached a maximum of 70C verified both on the gauge and through OBD2
Yet right after:

I popped the hood and measured every coolant hose and pipe and they measure 90-100C
Emphasis mine - If the above is what you intended to write, then the CTS is not agreeing with the actual temperature and under-reporting by 30 deg c.
 

alexdinita

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B7 A4 BHW TDI Swapped
Diesel don't run lean/rich in the sense that a gasoline engine does. I assume the engine runs OK, meaning good and strong? I suppose a severely sick TDI would not be able to make enough waste heat to warm up if it was severely underfueling.... but the car would likely barely be able to move if it was that bad.
The car drives great, pulls great and sounds healthy as hell. No DTC’s besides 2 related to the swap which is one for the fan module due to an extra trigger sensor on the module the harness doesn’t have which shouldn’t have anything to do with car running cold since fans don’t even turn on and one for immo defeat. Everything else is perfect. Replaced the brake switch no more code for that and cruise control works now. Only issue left is the car running cold and the stupid gas restrictor plate on the fuel tank that I haven’t figured out how to take out without damaging the filler neck.
 

alexdinita

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B7 A4 BHW TDI Swapped
I still suspect the CTS:



Yet right after:



Emphasis mine - If the above is what you intended to write, then the CTS is not agreeing with the actual temperature and under-reporting by 30 deg c.
That’s what I thought as well but don’t forget by blocking the front end of the car I blocked airflow in the engine bay, the exhaust/turbo can raise the temperature in the engine bay itself by a great amount and I measured the temps with the laser thermometer at the surface of things so maybe even if everything I measured was 90-100 inside the hoses the coolant could’ve very easily been 70-75
 

Windex

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Doesn't matter - the CTS and the hose temps should be withing a few degrees of each other. Blocking the rad should raise both equally.
 

pdq import repair

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I suspect you have a bypass hose issue that is allowing too much coolant to bypass the thermostat and just circulate around all the hoses. You should have flow to the reservoir when the t-stat is closed, but not a lot of flow. i would study the hose layout of the original car and replicate it as close as possible. In this colder weather there would be enough surface area to keep the car from warming up if coolant is diverted to all components full flow. In the warmer weather i suspect it will act somewhat normal with a little longer warmup time.
 

vwztips

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For your filler neck, "usually" the Quattro tanks have a rubber hose with clamps where the filler hose meets the tank. My A6, AllRoad and Passat 4Mo conversions all had tanks with that set up. I removed the filler neck and drilled out the restriction with a hole saw and blew out all the garbage from the drilling action.

My 2012 Tiguan conversion was stupid easy. They had a plastic insert that I just popped out.
 

Windex

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Remember, he said he has good heat in the cabin even when this occurs. That wouldn't be the case is coolant is bypassing the T-stat to the rad and lowering the overall system temp.

I'm still betting CTS. :D
 
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