Inaccurate temp gauge

[mrskippy]

Greetings, I want to preface this by saying that I know only a moderate amount about cars / repair - and yes, have looked elsewhere on these forums before presenting this question.

My 2000 Focus ZX3 (only 83k miles on the engine) has an odd issue with the engine temp indicator. When I initially start the engine, the indicator sits at cold all the way to the left, and eventually creeps its way up as I start driving (to the middle of the gauge generally).

Every once in awhile - sometimes on highways sometimes on city streets - the temp gauge will drop / not go up - even though the engine is sufficiently warmed up.

From what I gather, this causes the RPM's to sit at about 2000-3000 and causes me to burn excessive amounts of fuel. When this happens it is immediately noticeable that something is "off" and doesn't feel right driving the car.

I have taken it to a few dealers and all they do is plug in the computer and say there's no error codes. I've had them offer to replace the whole gauge dash saying that will fix it; but I'm not certain that is the issue.

From other threads I've gleamed that maybe replacing the engine temp sensor which is fairly inexpensive may do the trick - does anyone have any other ideas on why this might be happening / what is causing it?

Thanks!

[Sponsor]

[my_beautious_ZX3]

an easy place to start is to forget about the temp gauge and look directly at the temp sensor output. You can do this by putting your LCD odometer in test mode, which displays the actual engine temperature in Celsius. Then, you'll have better information to convey to your mechanic.

On this thread, scroll down to Whynotthinkwhynot's post and read paragraph #2:
http://www.focusfanatics.com/forum/showthread.php?t=290621

Play with that Celsius test mode for awhile so that you're comfortable enuf with it to use it properly the next time your intermittent problem pops up.

[elsolo]

Sounds like the computer is not getting accurate temperature information occasionally.
(or the temp is truly too low, and you have a stuck open thermostat)

Could be the sensor, the wiring to the sensor, or even just the connector between them.

If you want to verify actual coolant temperature, just turn the heater on.
If the coolant is hot, the heater will blow hot.
If the coolant is cool, the air will not get heated up much.
Nest time the gauge shows the motor is cold (when you think it is hot), try the heater to verify if the coolant is cold or if the gauge is misreporting data.

[mrskippy]

Originally Posted by elsolo

Sounds like the computer is not getting accurate temperature information occasionally.
(or the temp is truly too low, and you have a stuck open thermostat)

Could be the sensor, the wiring to the sensor, or even just the connector between them.

If you want to verify actual coolant temperature, just turn the heater on.
If the coolant is hot, the heater will blow hot.
If the coolant is cool, the air will not get heated up much.
Nest time the gauge shows the motor is cold (when you think it is hot), try the heater to verify if the coolant is cold or if the gauge is misreporting data.

I have done this before - the heater blows, no issues there ever.

Buddy of mine has an OBD -> bluetooth/smartphone app that I'm going to try and get in there to see what the actual temp in celsius is reporting as.

How expensive / easy to do at home is replacing a sensor / wiring would you say?

[amc49]

If you've 'done this before' you'd know he's not asking you if the blower works, but how HOT it gets.

[sailor]

And that "test mode" referenced earlier will tell you if it's a gauge issue, or an issue with the sensor/wiring.

If the readout is OK, when the gauge reads wrong it's the gauge.

if the readout & the gauge both drop, the sensor or wiring are likely at fault...

hope that helps describe the diagnostics better for you!

[mrskippy]

Originally Posted by amc49

If you've 'done this before' you'd know he's not asking you if the blower works, but how HOT it gets.

Oh thanks - I've never heard passive agression on the internet before, you're so helpful.

Originally Posted by sailor

And that "test mode" referenced earlier will tell you if it's a gauge issue, or an issue with the sensor/wiring.

If the readout is OK, when the gauge reads wrong it's the gauge.

if the readout & the gauge both drop, the sensor or wiring are likely at fault...

hope that helps describe the diagnostics better for you!

Explaining how to get to the test mode was extremely helpful. I've determined that more than likely the CHT sensor is bad. The readout and gauge are in agreement with each other - but the temp will go from say, 40C after 15 minutes of driving to 70C when I pull over and restart the engine.

Do you by chance have a reliable / trustworthy website for parts that you use that I could purchase a CHT sensor from?

Thanks for the help!

[elsolo]

What makes you think the gauge and readout are not correct in their reading?

[mrskippy]

Originally Posted by elsolo

What makes you think the gauge and readout are not correct in their reading?

I drive ~15 miles one way to work a day through both rural streets and highway - roughly 20-30 minutes in traffic one way total.

If by the time I am getting to the end of one of these trips and the readout tells me my engine is at 39C as it did this morning - I think it'd be safe to assume it's incorrect - it wouldn't be that cold after a 15 mile drive (correct me if I'm wrong?).

Occasionally while I'm driving the needle and readout will drop suddenly by 20C, then go back up to where they previously were.

That and when it reads low and I restart the engine it reads normal temps (by normal I mean the range that was mentioned in the post my_beautious_ZX3 linked to).

Additionally when the temp is low - I get revs much higher than normal, which (again correct me if I'm wrong) would happen if the sensor is reporting incorrect temps to the computer, also causing excessive fuel consumption.

Lastly, what led me to this is the fact that my normal MPG calculations every time I fill up my tank was showing that I was getting a lower MPG than normal.

[sailor]

Ah, the added info helps!

We often worry about quick diagnosis just being a guess when we don't have all the info. - I'm SURE that's why elsolo was concerned....

Luck!

P.S. - Tousley Ford in Minneapolis area is closest to you, and one of the two dealers we recommend for internet purchases. They have a good rep here, and I've used them myself a couple times - they'll even try to talk you out of buying from them if they think you can get it cheaper locally! (after shipping) just Google them, then you could call for instant service. Oh Shucks, got it here (800) 328-9552