JWG

From researching the forums, I discovered AHA apparently has a higher compression ratio 10.3 versus 10 on ATQ. AHA apparently also had a different oil drive pump.

In my AFC 2.8 12v, I know Audi used an 87C thermostat.

I recently replaced the thermostat on my wife's 2000 A6 2.8 30v Avant, but I did not pay much attention to whether I received and installed an 87C or a 92C T-stat. The coolant gauge remains dead center, but average oil temps have increased approximately 15F from 225F to 240F.

I suspect that I replaced a factory AHA specification 87C t-stat with an ATQ specification 92C t-stat . . . Sound logical?

To knock the oil temp down to below 225F while under low and moderate engine loads, does anyone here run an 80C t-stat on an AHA 30v?

My moderately modded 12v runs between 200-225F at under almost all loads in all ambient temps, and I would like to see the 30v run with similar oil temps.

qfrog Ti20

I dont think its any particular help over the normal one. I have one from a V8 of some sort in there its a bit cooler than normal which really doesnt help all that much and iirc it made the car less than totally happy with the HVAC control unit which was expecting a normal thermostat.... I'd just be sure to use the correct one for your enigne. Next time I change it on the 12v its going back to stock.

Yes there was a split in the 30v with the thermostat.

4Driver4

...poor fuel ecomony in the colder months.

JWG

Why would a cooler engine cause poor fuel economy?

I would expect that using a cooler t-stat on a generally hot-running engine to bring the oil temps between 200-225F would have little or no effect on fuel economy. One may need hotter plugs at some point to keep combustion chamber temperatures at the point of carbon burn-off, but oil generally lubricates best between 180-220F (82-105C). Below 180, oil starts to deplete its flow additives and does not get hot enough to burn-off condensation which mixes with additives to cause corrosive acids, and, above 230F (110C), oil starts to thin-out and oxidize. Of course full synthetics exceed the abilities of petroleum and can lubricate at extreme temperatures and non-ideal conditions far longer.

I can only reason that in truly cold temperatures, below 32F/0C, the 80C t-stat causes the engine to run too cool and the oil to flow poorly, causing greater friction and thus poor fuel economy.

What have I missed here with causes for poor fuel economy?

4Driver4

...the engine never comes up to full operating temp. The ECU richens the mixture when the car is cold. Since the car never comes to temp, it runs rich all the time.

Not sure if this true of the A4, but if lowering oil temps is the goal, the themostat is not a good choice for all-around driving. An oil/air cooler is the best bet.

JWG

If installing a colder t-stat results in an engine (which normally runs at the proper temps) running cooler than it should, why could a colder t-stat also cause an hot-running engine to run at the right temperature?

I have considered an oil cooler, but the cooling system on my wife's A6 2.8 30v appears nearly identical to that on my A4 2.8 12v. Why should the 30v's cooling system not have the ability to keep oil temps below 225F and require an additional oil cooler to do so. My 12v is built and makes more power than my wife's basically stock 30v; yet, the 12v runs far cooler.

As I want to lower oil temps across the board (even at low to moderate loads) and not just at high load/high ambient temp conditions, could you explain why changing the t-stat to allow more coolant flow to absorb more heat from the block not allow me to meet my goals better than an oil cooler which, unless you get a thermostatic sandwich plate, has the same purpose?

The engine block has only oil and coolant running through it to cool it down, why not let the coolant do its job rather than install an aftermarket oil cooling system? My 30v sees only commuter and road trip duty--no track or spirited driving time.

I guess my biggest frustration concerns why has my wife's A6 30v always run about 15-20F hotter than my 12v and why, after changing the t-stat, it now runs a full 30-40F hotter than my 12v? Presently, a bad 87C or inappropriate 92C t-stat appears likely, but it even ran hot with the factory 87C t-stat. My only thought: the 30v has an auto-tranny that shares the radiator to cool itself while the 12v has a manual tranny.

4Driver4

I think it is not useful to make comparisons between the 12v and the 30v. Let the coolant t-stat control coolant temp, not oil temp. You are bringing coolant temp down lower than the stock temp to bring oil temps down to a level that you want to see. If you feel oil temps are higher than you'd like, address oil temps directly, not though coolant.

psychobabble

Hot oil will heat up the coolant so that oil temps wont budge. Only things to do would be:
1. EFK with either adjustable thermostat or tied into accessory fan circuit with lower temp auxillary fan switch... or...
2. Front mount oil cooler.

I'm running a PES G2 and saw a similar rise in oil temps. I did both of the above and now see 20-30 degree reductions in oil temps, even under spirited driving. However, with temps rising above 100 degrees in the NE nowadays, I'm running about 1 tick below middle under moderate load.

JWG

Today, we measured the oil temperature independently of the temperature gauge using an infrared heat measuring device and found a 15* difference!!!

Thus, I may have found the problem.

As the oil measured 15* cooler than the gauge read, I just need to change the oil temp sender the next time I do an oil change and then re-check the temperature difference to make sure that the gauge again gives accurate readings.

realmexyaudi

That's for sure.
Realmexy, Nigeria