quiksilverly

Mishar, Bally, MP4, Brozee this one is probably directed at you mostly cause you guys explain things well. So my 2004 D3 has been running on the cool side but the coolant in the reservoir is steaming hot. The nipple on the back left of the reservoir partially cracked and steam was coming out of it. I temporarily JB welded it back. I did a flush and fill and tried to burp the system with the ports on the heater lines (i think) at the back of the engine bay below the windshield. The drivers side port pisses coolant with no air right away....the passenger side port barely pushes any coolant out then air then coolant, then air, then coolant....really really weak, barely noticeable flow. Ran the temp up to cut the fan on but that took forever and as soon as I let off the gas it only runs the fan for a short bit and doesn't seem to change the amount of flow at the ports either. I'm not sure what's happening but if I drive for a long period the system forces hot coolant out at the reservoir at that same line that the nipple was cracked on...coming from the back of the block. Then I get a coolant low light and an "engine temp low" fault.
I guess I was just wondering on these if there was something to do to verify the pump is working and also the thermostat. Part of me wonders if the thermostat was stuck open??? But if the pump is working shouldn't I get a constant flow from that bleed port at some point? (waited like 45 minutes).

I'm in OH on vacation and have an 8 hour drive ahead this weekend so any info would be awesome.

Jack88

I don't know where the temperature sensor is offhand, but the only thing I got is that you are not getting coolant flow to the engine and where the sensor is, overheating the coolant on the reservoir side while the coolant around the sensor never cycles through the engine. My first thought would be a failed thermostat. I hope I'm wrong, because if I am not, running an overheating engine for 45 minutes can do all sorts of bad things. If you have VCDS, see if you have any coolant temperature sensor faults. I wouldn't drive the car in it's current condition, in any case, boiling coolant in the reservoir indicates a serious cooling system fault.

mishar

Jack is right if symptoms you gave us are all accurate. As they rarely are there is hope. As a first step set your heating to hi and try bleeding again. That should open that circuit. Add coolant to max before that. Check the engine temperature. Overheated engine would boil drops of water sprayed on the surface instantly. If temperature is low you can drive it but your heating won't be great. Thermostat is buried behind timing belt. Not easy to replace it.

MP4.2+6.0

I can't be sure, but in general my instinct is thermostat. If it is closed or mostly closed the flow will be in the heating circuit with the cylinder heads, but little to the main cooling system, radiator, etc. It also fits what sounds like coolant overflow from the reservoir when it gets very hot in that circuit. Possible too there is some obstruction of the heater valve; it also has a helper pump that may not be up to snuff from what you describe involving the heater bleeds.

A further diagnostic is when you know the heating circuit water is really hot, check the apparent temp (by feel, or better with an IR temperature device) of the main radiator return hose. If it feels lukewarm comparatively, another strong indicator of a thermostat. Also possible it is actually an internally failed or plugged radiator core, but once again, low probability and little reported on 4.2's. I did have it on my 1985 5000 (after a lot of frustrating diagnostics), but that was a 40 year old less reliable set up.

Given your physical location, perhaps take it to a local radiator type shop and get them to diagnose better, including paying them a modest amount. Probably not where you want Audi work done, but they should know the general diagnostic theory, have the IR thermometer, easily put it on a lift, etc.

Unfortunately the thermostat and water pump are both buried with the timing belt on a 4.2. Thus you kind of back into a timing belt job. Look at the miles on the belt to assess that job. Near 75K, do it all. Way short, then maybe just thermostat surgically, plus I would still change belt since it comes off and isn't very expensive in isolation. On water pump my instinct is probably not. A bad thermostat really obstructs normal (hot) water flow through the system, which sounds like the issue here. It's possible the impeller gets loose from the shaft, but that is really low probability with modern Audi pumps; a little more likely if a belt job was previously done with junk no name parts and a stamped steel type impeller. But regardless, really not reported here in discussions beyond the old theoretically possible type scenarios.

Again, since you are probably going to need to get into the timing belt area, look at the history of that repair if you have it--both miles and for extra credit if you know it, the quality of parts used even if meaningfully less than 75K.

quiksilverly

I'm leaning toward the thermostat myself. I may have falsely characterized the water temp in the reservoir. It was not boiling but extremely hot and if I understand the theory of a cooling system the reservoir is not fluid that is cycling through but fluid at hand and space for expansion....but there shouldn't be any steam trying to push it's way in there. I feel like the eng temp sensor should be in a place that measures coolant near the critical parts of the engine. The engine temp gauge seems to be accurate it's just that the temperature of the coolant in the expansion tank seems to be closer to what you would find in the radiator. That line returning to the tank spits steam once the engine is at operating temp. I feel like if I knew what that line was supposed to have in it I would be on my way to understanding the problem. I have been in far enough before to do the belt/pump/thermostat before so that doesn't scare me anymore. Just want to have a good idea what the easy things to check for are first. I can verify the eng temp gauge is acurate as mentioned earlier but I feel like it is. It moves like it should about when it should, it's just while driving along with airflow the temp dips a few ticks below center (maybe 1-2 needles widths). To me, if the temp sensor is right, this means that fluid from the radiator is flowing into the engine with the engine coolant in the engine still below the "open" temp of the thermostat. That's why I thought thermostat stuck open. I just don't know if that explains the steam reaching the expansion tank.

mishar

If there is gas bubbling in your expansion tank it is not steam. It is exhaust gas and your cylinder head gasket is broken.

quiksilverly

Yeah, I thought of that but it's not bubbling from the main line going out of the reservoir. it's definitely steam coming from the smaller line and only once it reaches operating temp. When I had that line off on the left rear of the reservoir it was steaming from the line....the small line that runs into a little metal neck at the back of the engine. There are two small lines aside from the main one at the bottom of the tank and I assume they are vent lines from either end of the cooling system back to the reservoir. I will troubleshoot a little more tomorrow and see which lines at the radiator are hotter than others etc. Enjoy the rest of your Christmas and thanks for the help guys.

Shark_V8

How cold is it (ambient) and what is the condition of your coolant? Sure sounds like a system that is 1/2 frozen

Jack88

I can say most definitely that a high temp of the coolant in the reservoir is not a symptom of a stuck open thermostat. In fact, a stuck open thermostat would mean that your coolant would tend to be cooler than normal, not hotter. Vapor of any kind in most cars means either excessive coolant temperature has resulted in boiling, you have air in the system, or a head gasket leak, but an A8 is not most cars and I believe it takes care of a small amount of air in the system on it's own and overheating the coolant to the point of boiling (creating the steam) means it has usually dramatically overflowed first. Check your exhaust for white smoke, it would be slightly sweet and oily if it were coolant indicating a head gasket failure.

MP4.2+6.0

Returning back to where I was before (stuck closed thermostat, not open). The temp sender is in the line behind the motor that runs between the heads. When thermostat opens, the coolant starts mixing with the rest of the coolant down in the block, and is now reading an "average" of all the coolant. By then the block coolant would also be warmer as well from the engine block heating up, just more slowly than the heads. Net, no different opinion about the temp sensor sounding like it is working okay.

Meantime, if thermostat is fully closed, or close to it, you can be having a localized boiling condition--hot spots in the head area and related coolant, even if rest of system isn't as hot. By definition the coolant is being restricted flow wise at the thermostat. But if head area gets really hot without thermo opening, the closed thermo position has outlived its cold start "usefulness" if you will. Thus, you can get a weird condition of localized boiling up in the head area, even though the whole system isn't so hot. Hence some overflow, steam, etc. This isn't new fangled D3 or 4.2 either; I've seen it back to my 1970's C1 with a really basic thermostat set up right in front of the head in plain sight; it was just back then with a wide open engine area you could reach in and feel every hose and the radiator itself, and then have a good sense of when thermo was open and when not. Not good for motor obviously if head has hot spots and local boiling. Also may be a fair amount worse boiling point wise if reservoir has been topped up repeatedly with water/non 50:50 type mix.