Tippy

You think the engine coolant temp sensor is out keeping the RPM's up, falsely thinking the engine is cold?

pat056

Was looking at my idling rpms last night. They were 800 with minimal surging. Needle is almost steady.
Why would 84 degres C be impossible? if the thermastat opens at 83 and I'm driving down the road at 70 mph with good air movement and the ambiant temp is 30-50 degrees? It will bounce 84-85 degrees, unless I get into traaffic or slow down below 50, then it will rise to upper 80's, maybe even 90's depending on how much traffic and how slow. If I sit still long enough, it will drift to 100 and the fan will cut on. The temp guage on the dash coincides pretty close w/the digital readout, just past the 180 degree hash mark. Needle actually rests just right of the hash mark.

1999Porsche911

Don't know what to tell you and I don't want to get into the laws of physics, but the coolant cannot maintain such a low temperature even if the outside temperature was zub zero using the stock setup. Based on your theory, installing a 65C thermostat would allow the engine to maintain a coolant temperature of < 70C. There is not enough coolant flow with the thermostat installed and the radiators are not efficient enough. The coolest I have been able to get the coolant was about 86C once the engine was warmed up. This was obtaiined at 70 mph, outside temperature of -10C and NO THERMOSTAT INSTALLED.

With the stock cooling system setup (thermostat installed) you may see 88C periodically if your speed is fast enough, minimal load on engine and < 0C. In SC climate, your coolant temperature is more likely to be 90C+ at highway speeds and often will rise above 105C on warmer days and in traffic.

However:

IF, your readings are close to being accurate, then you must have increased the flow of the coolant. Either a different pulley was installed or the thermostat is not installed. Your comment stating that it can take as long as 20 minutes to get the coolant temperature to 84C makes me suspect you are running without a thermostat.

The 3.4 stock cooling system will reach a temperature of 90C from a standing temperature of 0C in less than 15 minutes, easily.

The reason I asked about the idle speed was because there have been some problems with the sensor on the 99's. Typically, the problem shows itself with inaccurate readings but also includes a delay in idle speed drop when coming off a higher engine rpm.

pat056

1999porsche911
I'll check my idle closely and let you know. Does the relatively quick temperature increase when I'm not "cruising" seem indicative of no thermastat or bad sensor? I always assumed it was a function of poor temperature control on the 1999's due to just the one radiator.

1999Porsche911

Your car has 2 radiators and 2 fans. A missing thermostat will increase the time it takes to warm up idling or cruising.

pat056

I meant 2 radiators, not having the 3rd.

1999Porsche911

If there is a substantial increase in the time it takes for the engine to warmup when cruising versus idling, that typically means you have a water pump that is going bad. Not enough flow at low rpm's.

You made no mention here that it takes longer to warmup at idle than it does when cruising. (unless I missed it)

pat056

I checked last night. Ambient temperature was 61 degrees:
Start-up engine temp 17 degrees c
After 7 minutes at 70 mph engine temp 84 degrees c
next 15 minutes at 70 mph engine temp fluctuated between 84-85 degrees
Went thru residential area for 2-3 minutes at 35-45 mph, engine temp goes to 87-89 degrees
Back on HW at 70, mph, engine temp 84-86 degrees.
Pull in neighborhood for 4-5 minutes at 35 mph w/2 stopsigns, engine temp increases to 90 degrees.
Since my only stops are at stopsigns, I sit for only a very short time at idle, but when I do, it idles at 800.