Heat cycling

[spacecad3t]

@CarBot How long should it take a late model 944 non-turbo, to go from 80c to 92c on the temp gauge while stationery and idling? If the 944 is sitting idle, should it cycle between 92 and 80c as it heats up, runs the high speed fans, and returns to a lower temp? What might make the 944 never climb past 80c while idling?

[CarBot]

How long should it take a late model 944 non-turbo, to go from 80c to 92c on the temp gauge while stationery and idling? If the 944 is sitting idle, should it cycle between 92 and 80c as it heats up, runs the high speed fans, and returns to a lower temp? What might make the 944 never climb past 80c while idling?

On a late 944 NA, if everything is working and the engine is fully warmed, sitting and idling should usually let coolant temp creep up from the thermostat-regulated range into the fan-switch range within several minutes. Exact time is all over the place because it depends on ambient temp, whether the radiator has airflow, how hot the engine already is, and whether the heater is on, but it’s not typically “never.”

It also shouldn’t normally swing all the way between 92C and 80C just from the fans. On 1985.5+ cars the radiator thermoswitch is two-stage and it triggers around 95C (low speed) and 102C (high speed). So you’d expect it to climb to roughly the first-stage point, fans come on, then it drops a bit and stabilizes, not a big repeated 12C sawtooth on the gauge. Also, low speed can run even with ignition off if the first stage is triggered, while high speed only runs with ignition on and the second stage triggered.

If it never climbs past ~80C while idling, the common causes are a thermostat stuck open (or the wrong low-temp thermostat), a radiator fan that’s running all the time on low speed due to a bad radiator thermoswitch or wiring, or a gauge/sender issue making it look cooler than it is. Quick sanity checks are to feel the upper radiator hose during warmup (it should stay relatively cool until the thermostat opens, then get hot quickly), confirm whether the fans are running when they shouldn’t be, and verify the dash gauge sender (brass sender in the block, wide spade is the gauge) and its wiring/grounds. If you have an IR thermometer, comparing the thermostat housing or upper hose temp to what the gauge claims is a fast way to separate “actually cool” from “lying gauge.”

If you tell me whether the radiator fans are running while it’s sitting there at “80C,” and whether the upper hose gets hot early in warmup, I can narrow it down further.

[spacecad3t]

How long should it take a late model 944 non-turbo, to go from 80c to 92c on the temp gauge while stationery and idling? If the 944 is sitting idle, should it cycle between 92 and 80c as it heats up, runs the high speed fans, and returns to a lower temp? What might make the 944 never climb past 80c while idling?

On a late 944 NA, if everything is working and the engine is fully warmed, sitting and idling should usually let coolant temp creep up from the thermostat-regulated range into the fan-switch range within several minutes. Exact time is all over the place because it depends on ambient temp, whether the radiator has airflow, how hot the engine already is, and whether the heater is on, but it’s not typically “never.”

It also shouldn’t normally swing all the way between 92C and 80C just from the fans. On 1985.5+ cars the radiator thermoswitch is two-stage and it triggers around 95C (low speed) and 102C (high speed). So you’d expect it to climb to roughly the first-stage point, fans come on, then it drops a bit and stabilizes, not a big repeated 12C sawtooth on the gauge. Also, low speed can run even with ignition off if the first stage is triggered, while high speed only runs with ignition on and the second stage triggered.

If it never climbs past ~80C while idling, the common causes are a thermostat stuck open (or the wrong low-temp thermostat), a radiator fan that’s running all the time on low speed due to a bad radiator thermoswitch or wiring, or a gauge/sender issue making it look cooler than it is. Quick sanity checks are to feel the upper radiator hose during warmup (it should stay relatively cool until the thermostat opens, then get hot quickly), confirm whether the fans are running when they shouldn’t be, and verify the dash gauge sender (brass sender in the block, wide spade is the gauge) and its wiring/grounds. If you have an IR thermometer, comparing the thermostat housing or upper hose temp to what the gauge claims is a fast way to separate “actually cool” from “lying gauge.”

If you tell me whether the radiator fans are running while it’s sitting there at “80C,” and whether the upper hose gets hot early in warmup, I can narrow it down further.

The fans are not running while idling around 80c. You are correct, my low speed fans do come on around 95c and the high speed at 102c. So apologies for my incorrect reference numbers. Can a car running too rich hang around 80c while at idle? and how does 11 minutes to go from 80c to 102c in 18c/65f degree ambient weather?

[CarBot]

The fans are not running while idling around 80c. You are correct, my low speed fans do come on around 95c and the high speed at 102c. So apologies for my incorrect reference numbers. Can a car running too rich hang around 80c while at idle? and how does 11 minutes to go from 80c to 102c in 18c/65f degree ambient weather?

Running rich won’t realistically “hold” coolant temp at 80C at idle by itself. If anything, a rich mixture can add heat to the exhaust and cooling system, but coolant temperature is mainly governed by thermostat position, radiator heat rejection, and airflow. Mixture issues show up more as rough idle, fuel smell, sooty tailpipe, poor MPG, fouled plugs, and sometimes hotter running under load, not “won’t warm up at idle.”

Eleven minutes to go from 80C to 102C while stationary in 18C/65F ambient sounds pretty normal to me, assuming you started that timing at a true stabilized 80C and the car is just idling with no extra airflow through the radiator. These cars can sit at thermostat temp for a while, then slowly creep up until the radiator switch hits first stage and eventually second stage if conditions allow. Ambient 18C isn’t cold, but it’s cool enough that the radiator still sheds heat decently even without the fans, especially if there’s any breeze or the undertray/ducting is intact.

If your concern is specifically “it never climbs past 80C while idling” but you’ve also observed it reaching 95C and 102C and cycling the fans, that’s actually a sign the system is behaving. The more useful question becomes whether the gauge is accurate and whether the thermostat is correct. If you want to sanity-check it, compare gauge reading to an IR temp at the thermostat housing or upper hose, or measure the brass gauge sender resistance at the wide spade and compare to the known values (it’s an NTC sender). If the gauge is reading low, you can chase grounds, the sender, or the gauge itself.

If you’re chasing a rich condition separately, the usual 944 NA places to look are vacuum leaks, fuel pressure regulator/damper issues, leaking injectors, AFM and its wiper track, and the DME coolant temp signal from the blue sender (that one affects fueling; the brass sender is just the dash gauge). If you share symptoms beyond the temp behavior (idle quality, plug color, fuel smell, MPG), I can help narrow that down.

[spacecad3t]

The fans are not running while idling around 80c. You are correct, my low speed fans do come on around 95c and the high speed at 102c. So apologies for my incorrect reference numbers. Can a car running too rich hang around 80c while at idle? and how does 11 minutes to go from 80c to 102c in 18c/65f degree ambient weather?

Running rich won’t realistically “hold” coolant temp at 80C at idle by itself. If anything, a rich mixture can add heat to the exhaust and cooling system, but coolant temperature is mainly governed by thermostat position, radiator heat rejection, and airflow. Mixture issues show up more as rough idle, fuel smell, sooty tailpipe, poor MPG, fouled plugs, and sometimes hotter running under load, not “won’t warm up at idle.”

Eleven minutes to go from 80C to 102C while stationary in 18C/65F ambient sounds pretty normal to me, assuming you started that timing at a true stabilized 80C and the car is just idling with no extra airflow through the radiator. These cars can sit at thermostat temp for a while, then slowly creep up until the radiator switch hits first stage and eventually second stage if conditions allow. Ambient 18C isn’t cold, but it’s cool enough that the radiator still sheds heat decently even without the fans, especially if there’s any breeze or the undertray/ducting is intact.

If your concern is specifically “it never climbs past 80C while idling” but you’ve also observed it reaching 95C and 102C and cycling the fans, that’s actually a sign the system is behaving. The more useful question becomes whether the gauge is accurate and whether the thermostat is correct. If you want to sanity-check it, compare gauge reading to an IR temp at the thermostat housing or upper hose, or measure the brass gauge sender resistance at the wide spade and compare to the known values (it’s an NTC sender). If the gauge is reading low, you can chase grounds, the sender, or the gauge itself.

If you’re chasing a rich condition separately, the usual 944 NA places to look are vacuum leaks, fuel pressure regulator/damper issues, leaking injectors, AFM and its wiper track, and the DME coolant temp signal from the blue sender (that one affects fueling; the brass sender is just the dash gauge). If you share symptoms beyond the temp behavior (idle quality, plug color, fuel smell, MPG), I can help narrow that down.

All good, I was just curious... Thank you.

[CarBot]

All good, I was just curious... Thank you.

No problem. Your fan switch points and that 11-minute climb in 18C ambient sound like a pretty healthy, normal cooling system behavior for a late 944 NA.

If anything changes, the two quick “trust but verify” checks are comparing the gauge to an IR reading at the thermostat housing/upper hose, and confirming the thermostat is the correct temp rating and not stuck open.