Water temp accuracy not so bad
 

Replaced bad themister on passenger (r) side cooling fan today. Fans now working properly at both low and high speeds (checked via Durametric). Decided to check dash gauge against my obdii app (didn't want to drive around with the computer in the car for Durametric) for coolent temp because I've read a lot about how inaccurate the dash gauge is.

Well, with the dash gauge perfectly centered on the 180F hash, ODB Fusion was reading 178F. Not so bad IMHO. Highest temp I could get the car to reach was 212F (with a 51F ambient temp) and the dash gauge was here:

http://986forum.com/forums/uploads02...1496784946.jpg

The gauge did seem to be 'stuck' here for the 205-212F range, so there may be some step measurement going on above 200F. If the gauge is linear, the hash before 250 s/b 215F, so I should have been closer to it than I was.

Low speed fans were turning on at 212F and turning off at ~206F. Couldn't get it hot enough to kick in the high speed fans, but I expect them at around 217F.

So, YMMV, but my dash gauge is ok for me (for a dash gauge).

Gauge accuracy is in the eye of the beholder! HAHA. My 98 kicks the low speed fans on when the pointer is in the center of the zero. The low temp thermostat (160 deg) controls on the left edge of the 180 hash mark. I replaced the sender since it seemed that all temps were indicating higher and swing was greater than what seemed logical. I'm comfortable with it now that I have verified everything working correctly but initially watching those swings was a bit well terrifying. I still am looking at some mods to minimize the swings though.

The gauge is not linear. Try the same thing, but use a PIWIS or Durametric to monitor the coolant temps, it will be a lot different.

Low speed fans on at the center of the zero seems more correct to me if your low speed fans are kicking in at 212F as mine are. Porsche may have implemented some step increments at some point to keep from freaking out the masses with temp swings.

Thanks JFP, that explains it. Can you confirm that ~217F is correct to kick the fans into high speed?

According to the Bently manual, Low speed = coolant above 96.75C (206.2F) and High speed 102C (215.6 F) or A/C pressure switch closed (refrigerant pressure above 18 bar/232 psi).

I would like to minimize the 35 degree swings since I can be in 100 F temp and 20 minutes later be in 60 F so I can swing through 70 degree +/- swing in less than an hour. Can't believe this is good for metal parts expanding and contracting so quickly.

The real question is whether the coolant temp sensor is an accurate representation of the engine temp?

There are many people who say no.