Thermostat (engine) failure mode question

[Mister Bally]

Well boys & girls, will I soon be changing my timing belt??? My real question is what has been your experience with a failed thermostat? My new symptom, and this is on my 2004 A8 w/4.2L while driving; The car always would quickly heat up with the gauge stopping at the halfway point. Period. Now it heats up from cold to the halfway point as always (takes about 5 min from stone cold) and remains there. Then, after about another 5 minutes at the halfway point, the needle rises to 3/4, then another minute to 7/8 of the way to red. Heater core automatically gets filled with the hot liquid, but the A/C remains engaged, however not very effective since hot coolant is running through the heat exchangers. Then the warning video plays on the small screen between the tach & speedo. If I'm on a freeway, the temp stays at 7/8, in stop and go it darn near pegs red. I just want to confirm with others what happened to them. Thanks.

[Vicl]

It looks like it is thermostat problem.
But you also need to check electrical fans. Audi has 2 fans. One is small one. The small one should be UP AND RUNNING all the time when AC is on. If you OFF the AC then it will be off as well.
The big fan can run with 3 different speed settings. Temperature sensor controls it.
So. Check that the small one is working, then check the big fan is running with 3 speeds and if that’s what it is then it is thermostat 100%

[dv007]

Your symptoms sound exactly like mine when my thermostat went on my 2004.

[MP4.2+6.0]

Generally similar, but a few other ideas too.

First, I do not have experience with thermostat problems on either my C5 2000 with a 4.2 or on my D3 W12. In the case of the older 4.2, I just did the whole belt/water pump/idler pulley/tensioner drill so changed the thermostat too. Sure enough, buried deep in there is kind of the same old thermostat back to that 5000 era we both shared, just infinitely harder to get to. The W12 one from what I gather is fairly different and up under the intake manifold area; also a chain drive motor.

To your questions and the responses, yes agreed with what is said here, and clear to me from the boards at least the older ones seem to be having thermostat issues as they age. Don't know if the design changed from the C5 where at least I got to 10 years and 110K miles w/out any cooling issue.

The two other areas not yet identified in the posts are: 1) clutch fan, and 2) radiator core. As other responses said, first be sure all your electric fans kick on as expected. If not, look at the sensor (which I think you changed), any power packs used by the fans (recalling in the old 5000's those things failed like clockwork; I think newer Audis simplified or eliminated that trouble area) and the fans themselves to be sure they power electrically.

Also check the clutch fan though, assuming it has one like both my C5 4.2 and my W12. If the clutch is fried and always freewheels, of course you get little cooling effect there. On my C5 4.2, that seems to be the baseline fan for the motor, even before the electrical fans. Basic test as you probably know is when motor is cold it should freewheel (with some resistance) and when everything is warm/hot underhood it should basically lock up and no longer freewheel. Fortunately, not that hard to change as a standalone part if it is bad.

Now, if you are still looking for issues try to diagnose if the thermostat is opening by feeling the main water line after the thermostat water plumbing exit. If it starts cold but then feels warm as you see the temp gauge going up but before it would logically get heat soak just from the surrounding block and other parts, that suggests thermo is opening some. If it (housing or water line) stays pretty cool well past when thermo should have opened (which is just above the lower part of the gauge), that does pretty much indict the thermostat. However, if the housing/hose is warm as expected, then not so fast to conclusions. Recheck fan functioning, clutch fan, etc. per above. BUT, also then check the radiator itself. Feel it to see if the temp warmth is well distributed or not (or better yet, use one of those infrared type temp. guns they have now to get good spot temp. readings. If not, suspect an internally blocked radiator core. I say all this because I had exactly that in my old 5000. Everything else tested perfectly, yet it overheated like clockwork. But thermo housing was warm, yet not parts of the radiator. Sure enough, bolted in new radiator and problem instantly solved. Apparently on the new plastic wonder radiators that complete internal blockage/failure can happen yet with no external leak. You will remember the old ones used the not so good green fluid, but still. The newer ones with the G12, then G12 +, now G12++ (what you should use now if you do the thermo and belt stuff) seem less prone to these blockage issues as well as heater core issues, or at least that what the newer fluids were trying to solve in part. And a last oh yeah...the radiator on my W12 already failed, but due to a leak rather than any known blockage. There I caught a vey small leak when still under warranty that they confirmed and thus changed the whole radiator pronto.

That's my whole grab bag of experience and tips, and married with the prior input pretty much gives you the whole field I think. Net, probably 80-90% either thermostat or fan issues (including any clutch fan unit); 10% or less the more exotic issues like radiator core blockage, weird wiring stuff, etc.

[Mister Bally]

Thanks for the ideas and experience MP 4.2+6.0. Yep, those thermostats on the inline 5's were simple to replace. Funny how the three I had fail (two on my '87 5000 and one on my '81 4000 with the 5+5 package) failed in the open mode so no full heat up in the winter. That's when all three failed on me by the way. You know I have to do do the timing belt eventually so I'd better not take any more chances.

Now I need to find precious time to do the dreaded timing belt and all. Fortunately, I have spare vehicles.

Thanks again,

Bruce

[MP4.2+6.0]

Get the right tools too. From having done it twice on V8's, the tool I initially skipped was the viscous fan wrench and the fan holder tool. When I improvised the first time I spent 2-3 hours just on that. But the next time with those incremental tools, that part was < 10 minutes. Again, I'm guessing D3 4.2's have that viscous fan set up.

By the way, I found I could not replace the timing belt tensioner (a sealed hydraulic thing that pushes on a pulley mount to maintain tension) on my C5 without removing the forward motor mount that was part of a humongous casting to which the alternator was also mounted. Nix on that. I did replace everything else, including front main (crankshaft) seal.

I also did mine (both times) withOUT pulling off the front bumper cover and all that baloney. And that is in a C5 where it is very tight w/ the V8, even w/ 3 more inches in the fenders. I figure that avoided several incremental hours The only two downsides were I could not remove the driver's side cam gear bolt in full, which meant I couldn't get to the front driver's cam seal [since then I understand if I just levered the motor some I could have supposedly], and it was harder to insert the pin in the timing belt tensioner that is necessary to really set the engine/valve timing accurately. But I did get it more easily the second time when I tore more down to get at the thermostat, water pump and all (first time was belt and rollers only).

[Mister Bally]

UPDATE:

Timing belt, thermostat, every idler roller for the timing belt, water pump and of course, the $20 culprit that started it all, the failed thermostat has been replaced.

I manually rotated the motor several times with no interference felt.

With the front end still pulled away about 10" and no cooling system or serpentine belt attached, (air box was temporarily installed) we started the engine.....CLACK-CLACK-CLACK then huuummmmm and all was perfect. That clacking sure scared the crap outta me. My son immediately shut off the motor. We checked again and everything was lined up properly so we started it again and it was fine so we let it idle for 30 sec. Then shut it off. Belt tension still seemed tight so we put on the timing belt covers and started it up again. This time we rev'd it pretty good and all was well-no dreaded clunk or anything. I'm guessing that since the camshaft on the driver's side head clocked (under spring tension about 90° clockwise once that maybe the hydraulic lifters were compressed or in some kind of undefined state that caused the clacking later.

My other fear was did I pinch the thermostat O-ring or did we torque the water pump bolts enough to allow the cheap, thin paper gasket to seal? Everything says no additional goop or sealants so I listend to them. Problem is that all has to be reassembled before I can pressure and heat test it. The torque specifications seem too light but I did what the manual said to do. I waited a few more days before reinstalling the front end and wheel wells etc.

Changed the oil and filled the cooling system and when we finally started it up, the dreaded check engine light came on but it idled smoothly. Ran VAG-COM and it gave two codes, mass airflow sensor error and intake air sensor low reading. I think the last key on cycle I did was with the air box out so we cleared the codes and the light stayed off. Took her for a test drive on local streets and she ran well. Then hit I-215 and decided it was time to test it under maximum load. I'd rather it die on me now than when I'm trying to outrun a tornado or BMW! Full throttle in sport mode allowed the engine to rev over the 5000 mark. Had it up to 120 briefly and it ran great.

No leaks and I drove it to work yesterday. After using my beetle turbo for three weeks straight (even my '97 Chevy Venture once-it's just kept to haul pinball and slot machines that I collect), the A8 was quite the ride. I didn't realize how much I missed driving it.

I had to properly repair the "parktronic" wiring that I spent a few hours on stripping wires and attaching alligator clip leads to fix a broken wire a few years ago (without pulling the bumper cover off back then). That was part of the decision to pull the front end away even though it has been noted that you can do the job without all of that hassle. Bentley says remove the entire Lock Carrier which means discharging the air conditioner. Since mine works very well, especially considering the car was built over seven years ago, I did not want to mess it up. I do have a big can of R134 and the gauges etc. but figured I'm doing enough already.

Today is wash, clay and a nice wax job day. As Griot's says: Have fun in your garage! The last job was not fun. However, I saved about $1700.00 and most important, my son got to experience a major repair job. He wants a Lamborghini Murcielago. I told him the V10 has a timing chain not a belt so he should be good to go. Not sure if he's going to be a rock star or pro athlete which would be the only way to afford one though.... He has learned, never, EVER buy a car with a large engine that has a timing belt!

As I may have mentioned, the original timing belt that I pulled off looked fine. IMO, It would have easily lasted to 150,000 miles. (car has 119,000 miles on it now) maybe even *200k. Too bad the thermostat sucked. Also, the serpentine belt had millions of hairline cracks in it. The backing seemed good so who knows how much longer that would have lasted before it disintegrated.

* In my opinion-follow at your risk and peril!

[mikesd1980]

I am at 92k on mines. I am waiting to till around 100k before replacing my timing belt. I was curious is there anyway to tell when the timing or serpertine belt will fail before totally wrecking the engine?

[MP4.2+6.0]

It is more commonly the water pump, the tensioner or a roller. If pump or roller freezes a bearing, belt then just buns or tears and it's all over. If tensioner dies, it can skip teeth and then valve crash time. My reading suggests tensioners are likely to go 200K +, but sometimes posts note trouble.

[MP4.2+6.0]

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