Engine thermostat stuck open?

[Chuck Jackson]

My engine temp runs normal (190-ish to 205-ish) but on this last trip I notice my engine temp will drop to around 150 when there is no load like when descending a large grade or stop and go traffic. It almost sounds like a stuck open thermostat since it runs normal with a load?

[Gerald Farris]

Chuck,
Where are you reading engine temperature, on the analog dash gauge or your Aladdin or Silverleaf? Also, Is warm-up normal, or does it take a long time to reach operating temperature?

Gerald

[Chuck Jackson]

My VMSpc, Medallion, and analog gauges are reading the same. warm up seems to be about the same as normal but I don't let my engine idle to warm up. As soon as I have air pressure, I start rolling out but never on the boost (no hills or on ramps) until I hit about 150 degrees. I take it easy until it reaches operation temp. So, had I just let it idle, I don't think/know that it would ever reach operating temp.

[Gerald Farris]

Chuck,
From your description, I would replace the thermostats.

Gerald

[Dave Atherton]

Chuck, with your a Caterpillar engine you have a safety feature on your engine ECM called
The cold mode. What this allows engine to warm up until coolant reaches 165 Deg F and will
reduce engine power until the coolant temp allows warm up. Some times the engine stays in
the cold mode and reduce power until reset. Another way to cross check what I’m referring
to make notation of your air intake temperature should be 90 to 105 Deg F. If staying in the
cold mode. Yes failure can occur from jiggle pin on thermostat not seating. Not allowing engine
enough warm up time before driving. This is a very common question with cold morning
travels. This I stuck in cold mode can happen in summer time also. Dave Atherton Retired Cat Mechanic

[Lonne Mays]

Our first trip in our '99 Contessa (a short 100 mile maiden voyage on Hwy 101 up the Oregon coast), I noted that the coolant never reached temperature, and I assumed I had a stuck-open thermostat.  But the problem has never occurred again, and we have driven it many trips and 10,000+ miles.  So I assume the thermostat was acting sticky from sitting months at the RV dealership without being run.  We also changed the coolant after our first year of traveling, just because we don't know when the prior owners might have done that last.  I plan on monitoring the coolant via Cat's sample analysis, just like I do with the oil at oil change intervals (I change the oil every 8000 miles or so (when we get back home after each major cross-country trip)).

[Dave Atherton]

Gentleman, subject Cold mode on Caterpillar engines how it operates. First of all Caterpillar
starting operation consistent of a plate type heater located on air intake on engine. What
operates the intake heater is a 12 Volt relay connect to the intake heater plate. This 12 volt
Relay is called a timer relay that connect key switch, wait to start lamp on dash and intake
manifold sensor and engine ECM and intake plate heater. You will note the wait to start lamp will
Shut off after 30 seconds and you start engine. While the engine is idle and warming up the
wait to start lamp will come back on than few seconds go off. Depending how cold outside air
temperature. Being in a hurry to start moving the engine temperature is being controlled by
the engine ECM. This is how the system operates. Many times you will see post where Dave will
reply to member post saying possible cold engine temperature and air intake manifold temperature
Because engine is stuck in the cold mode operation and many would think temperature regulator
is stuck open and change out the temperature regulator only to find problem is still present.
Over time the intake timer relay will fail for two reason dirty 12 volt connection at 12 volt relay
Or timer relay has failed. Many time hard starting and low power and lamp on dash will not come on.
Can go little deeper on subject at later point. Dave Atherton Retired Cat Mechanic

[Chuck Jackson]

Hi Dave,

Does the C9s have an intake heater? I thought I read somewhere, that it does not. I’v never seen a “wait to start” light even when in mid 30s. My Ram Cummins 6.7 CTD does, so I am familiar with it.

[Dave Atherton]

Chuck , can you give your engine S/N number will look up your engine build when shipped.
Dave Atherton Retired Cat mechanic

[Dave Atherton]

Chuck, looking up information on Caterpillar C9S engines. ( All Cat C-7S and C-9S engines )
Media number-RENR9342-12 last update 08/04/2010. C-9 engine for Recreational Vehicles
Shows intake heater relay, intake manifold air temperature sensor, ECM. The Cat C-9S with
Regeneration system components are located on left side of engine, yes there is a difference
Than the Cat C-7 and Cat C-9 engines without Regeneration system but there is still the cold
mode that operates the same way. Person need to understand with Regeneration system there
are several shut down sensors starting at the DPF filter and on ARD Head, but the starting engine
and warm up does not come into this area. Dave Atherton Retired Cat Mechanic

[Chuck Jackson]

Just noticed that fan (radiator) is on when cold and at idle. Never did that before. Stay tuned!

[Chuck Jackson]

Chuck , can you give your engine S/N number will look up your engine build when shipped.
Dave Atherton Retired Cat mechanic

Dave, my engine serial number is C9S00894.

From the dyno sheet I have this....

Built Date: 11Oct2007
Tested Date: 12Oct2007
Shipped Date: 25Oct2007

[Dave Atherton]

Chuck, the information on your engine is same as I posted earlier. It sounds like sensors that
Has failed of possible dirty connection. Air inlet heater relay, Intake manifold air temperature
sensor, Coolant temperature sensor. Question will you be attending FMCA Rally at Indio in January
Or the BAC Rally at Quartzsite, if you will be attending I can connect Cat ET to your engine and
Find out what you have going on. Dave Atherton Retired Cat Mechanic

[Chuck Jackson]

Dave, yes I will be in Quartzite but not Indio.