Engine (ATQ) overheating? Replaced CTS, still overheating

[Black A4 2.8]

2000 2.8 ATQ

Started getting the 3 beeps with the coolant symbol in the center display. My coolant temp was pegged on the dash gauge, so I pulled the codes with a VAG.

16502 - Engine Coolant Temp. Sensor (G62): Signal too High
P0118 - 35-10 - - - Intermittent
16815 - Warm Up Catalyst: Bank 2: Efficiency Below Threshold
P0431 - 35-00 - -
16805 - Warm Up Catalyst: Bank 1: Efficiency Below Threshold
P0421 - 35-00 - -
16518 - Oxygen (Lambda) Sensor B1 S1: No Activity
P0134 - 35-10 - - - Intermittent
17831 - Secondary Air Injection System: Bank 1: Insufficient Flow
P1423 - 35-00 - -
17819 - Secondary Air Injection System: Bank 2: Insufficient Flow
P1411 - 35-00 - -
Readiness: 0000 0000

Replaced the coolant temp sensor (CTS) today, but the coolant temp gauge still pegged after a couple miles of stop and go driving (along with the 3 beeps and coolant symbol in the center dash).

Could it be my water pump or thermostat? How could I check? I had the water pump and thermostat replaced during a timing belt job 3 years and 30k miles ago.

The old CTS was the dark green version, so I'm assuming it had already been replaced. When I took off the old one, the old o-ring had completely melted and gunked up the area. I tried to clean it up as best I could, but could this have clogged the cooling system?

[walky_talky20]

- No, the o-ring wouldn't cause that.
- What brand of water pump was installed. This is important. If it was a plastic one, the water pump is suspect.
- Does the heat work?
- Does the secondary (electric) cooling fan kick on when the temp gets high?
- Do both rad hoses get hot (upper and lower)?

I assume you shut the engine off as soon as it started to overheat each time, yes?

[Black A4 2.8]

- No, the o-ring wouldn't cause that.
- What brand of water pump was installed. This is important. If it was a plastic one, the water pump is suspect.
- Does the heat work?
- Does the secondary (electric) cooling fan kick on when the temp gets high?
- Do both rad hoses get hot (upper and lower)?

I assume you shut the engine off as soon as it started to overheat each time, yes?

Water pump - Not sure, but I believe it was remanufactured
Heater - Works great
Secondary fan - Where is it, on the radiator?
Rad hoses get hot - I'll check, good idea
Shut engine off - Nope, drove the 5 miles home with the windows down and heater on full blast. How can I be sure the engine is actually overheating, and it's not an electrical problem? The coolant isn't bubbling or flashing from the dump valve on the expansion tank, shouldn't it be if it was actually overheating? I suppose not if the water pump isn't circulating the coolant.

BTW the oil temp is just above 250 F.

[walky_talky20]

You can also check the temperature of the air that is blowing off of the rad. If the top hose is super super hot, but the fan is pulling cold air off the rad, odds are it isn't flowing.

Secondary fan is on the radiator. It runs on low speed whenever the A/C is on or at a certain coolant temp. It runs on high speed during an overheat situation. The temp switch for this fan is in the lower rad hose, so if the coolant isn't flowing the car can overheat without this fan ever turning on. This is why you check the temp of the upper and lower rad hoses.

[Black A4 2.8]

You can also check the temperature of the air that is blowing off of the rad. If the top hose is super super hot, but the fan is pulling cold air off the rad, odds are it isn't flowing.

Secondary fan is on the radiator. It runs on low speed whenever the A/C is on or at a certain coolant temp. It runs on high speed during an overheat situation. The temp switch for this fan is in the lower rad hose, so if the coolant isn't flowing the car can overheat without this fan ever turning on. This is why you check the temp of the upper and lower rad hoses.

Looks like the coolant flow is intermittent.

After letting the car sit for a few hours, I felt the upper hose and it was super hot (not sure which is the lower hose, do I need to crawl under the car and take off the shield?). I turned the car on and immediately rechecked the upper hose. It had cooled off significantly. I figured this meant the water pump was working and moving the coolant. I turned the A/C on and off a few times to verify the secondary fan turned on, which it did. After about 15 min of idling, the temp shot up to the max, then warning beeps sounded and the coolant symbol illuminated in the dash. I then checked the secondary fan and it was not on. I turned the A/C on to turn the fan on, verified the fan turned on, then felt the air blowing out - lukewarm.

It's almost as if the coolant circulates initially, then stops for some reason. Is this a tell-tale sign of a bad water pump, or could it be the thermostat?

[walky_talky20]

Usually with these type of plastic water pumps, when they fail, they fail completely so there is no flow whatsoever. In this case you won't have much heat either as the water isn't moving. This probably isn't as likely for you. Even with a plastic pump, they very rarely fail as early as 30k miles.

In the case of a bad thermostat (which is stuck closed) you will get the most excellent, amazing heat in the cabin, but almost no heat off the rad. The water is being pumped, just not through the rad.

I'd probably proceed in this order:
- check temp of lower rad hose (on passenger side bottom of rad).
- Check flow of radiator (drain coolant, run water through it)
- Change thermostat
- Change water pump

[Black A4 2.8]

Usually with these type of plastic water pumps, when they fail, they fail completely so there is no flow whatsoever. In this case you won't have much heat either as the water isn't moving. This probably isn't as likely for you. Even with a plastic pump, they very rarely fail as early as 30k miles.

In the case of a bad thermostat (which is stuck closed) you will get the most excellent, amazing heat in the cabin, but almost no heat off the rad. The water is being pumped, just not through the rad.

I'd probably proceed in this order:
- check temp of lower rad hose (on passenger side bottom of rad).
- Check flow of radiator (drain coolant, run water through it)
- Change thermostat
- Change water pump

Talked to the mechanic who did the timing belt, water pump, and thermostat 30k miles ago. He's hesitant to blame the water pump or thermostat, especially after only 30k. He said to check the hose splitting off of the upper rad hose that runs into the top of the expansion tank. Coolant should be flowing through it and into the expansion tank. If not, there might be an air bubble in the system, preventing the coolant from continuously flowing.

I'll try to bleed the system and see if that changes anything. Finding the bleed screws sounds a bit difficult though:

http://www.passatworld.com/forums/sh...8&postcount=17

That rear bleed screw is hella hard to find. Should anyone ever read this thread in the future the best reference point I can give you is to look at the breather on the back of the driver's side valve cover... its almost directly below that, about 6 inches down, and is obscured by a hose. You need a 12" extension with a socketed allen to reach it, and taping the socket to the extension will save you an enormous headache should it fall off. It very hard to see anything, so after I was sure it was seating in the screw head I used a mallet to tap it down just to make sure it was not going to strip out b/c of a bad seat. Both screws were TIGHT, so ensuring good grip and straitness is crucial. The front screw also had the low pressure inlet line for the power steering pump partially blocking the alignment of my wrench, but b/c it is flexible you can gently push it out of the way to get the bar lined up strait before cranking on it to get it open.

[Black A4 2.8]

Bled both plugs, one at a time with the coolant tank cap off. Managed to get a few bubbles out of the front plug, but nothing out of the rear. Didn't seem to help though. The temp shot up after 15 min so I shut the engine off. Initially, both the lower and upper hoses were hot. Suddenly, the lower hose cooled off completely at which point the temp shot up.

Didn't notice any coolant shooting into the tank from the upper hose split off while running the engine, unless I squeezed the upper hose. Didn't seem to matter if I had the coolant tank cap on or off.

[Black A4 2.8]

Ended up doing the complete timing belt job including water pump and thermostat. No longer overheating. Water pump looked fine. I plan to boil the thermostat to see if it opens at temperature.