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Which should I trust more? Dash gauge or AP?
Which should I trust more? Dash gauge or AP?
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Old 07-14-19, 02:14 PM   #1
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Default Which should I trust more? Dash gauge or AP?

After a protune my stock dash water temp gauge reads high after the car is up to operating temperature. One time it nearly hit the red zone. I allowed it to go that high only because I wasn’t sure which to trust. Radiator fans do not turn on, so I turn on the A/C which in turn switches on the fans. Temp gauge comes down to normal (just above 1/3 mark) within seconds.

All the while the AP shows temps that are normal, anywhere between 160 & 190. Never did it shows 204 or 207, which I believe is the temp when the fans are supposed to kick on.
  • Which would you trust more?
  • Also, I’d appreciate if anyone knows if the fans can be turned OFF via a tune. Maybe this was done by mistake by the tuner?
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Old 07-14-19, 02:42 PM   #2
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Default Re: Which should I trust more? Dash gauge or AP?

The stock temp gauge in an 05 doesn't have numbers and you are splitting hairs . . . except why are you seeing 160? Sounds like a bad thermostat or the wrong thermostat .
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Old 07-14-19, 02:49 PM   #3
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Default Re: Which should I trust more? Dash gauge or AP?

This version sounds like air? Never heard of a bad STI gauge.
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Old 07-14-19, 04:00 PM   #4
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Default Re: Which should I trust more? Dash gauge or AP?

Yep, the AP shows 160 or less while driving. That’s only initially, but still, quite cool. This is why I’m not sure I can trust the AP temp readings.

The stock gauge doesn’t have numbers of course, but I doubt it should be over the 2/3 mark while the AP reads 175-ish. At normal, the stock gauge reads just over the 1/3 mark.

But I swear it ran fine the other day after I removed the upper reservoir cap. I removed the cap, a bit of air rushed out and a test drive revealed all was well. Fans came on normally, dash AND the AP looked normal. So odd.

I burped the coolant today. Lots of bubbles at first, then pretty clear. The car moved toward overheating until I turned on A/C. I’m a bit stumped.
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Old 07-15-19, 04:54 PM   #5
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Default Re: Which should I trust more? Dash gauge or AP?

I'm obsessing now. Tried burping the cooling system again today, though this time with the heater running full blast. I figured that would get the full flow of coolant going thru the heater core. Anyway, a few more bubbles, mostly tiny ones. Then eventually just clear fluid in the spill-proof funnel thing.

Same crap. After letting the engine fully warm up (~4-5 minutes idling), the dashboard gauge rises once I start to hold revs at 1,500/2,000 for a minute or so. I let it move up to about the 2/3 mark. But AP shows 180, 185.

Gotta manually turn on fans by switch on the A/C for the dash gauge to drop. Once fans come on, the dash gauge drops within seconds.

I truly hope this is not a head gasket issue. No reservoir blow ups, no black specs of spent head gasket in reservoir bottle, no milky residue under oil cap.

Maybe I should just spend the $50 on a combustion leak test kit....
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Old 07-16-19, 04:24 AM   #6
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Default Re: Which should I trust more? Dash gauge or AP?

Quote:
Originally Posted by redeye View Post
I'm obsessing now. Tried burping the cooling system again . . I truly hope this is not a head gasket issue. No reservoir blow ups, no black specs of spent head gasket in reservoir bottle, no milky residue under oil cap.

Maybe I should just spend the $50 on a combustion leak test kit....
No you just wrote you aren't seeing symptoms of a bad head gasket. You relate that you have two gauges reading differently - both from the same sensor! It sounds as if the dash gauge moves to outside the normal lines. Is this correct? It does not have numbers.

The AP is displaying the value your ECU is using - so you have no operating issue.
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Old 07-16-19, 07:42 AM   #7
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Default Re: Which should I trust more? Dash gauge or AP?

Trust the AP reading over the gauge. The AP is supplying raw data from the ECU. The gauge is heavily filtered and has a huge deadband in the normal operating range.
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Old 07-16-19, 12:23 PM   #8
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Default Re: Which should I trust more? Dash gauge or AP?

Quote:
Originally Posted by mheyman View Post
No you just wrote you aren't seeing symptoms of a bad head gasket. You relate that you have two gauges reading differently - both from the same sensor! It sounds as if the dash gauge moves to outside the normal lines. Is this correct? It does not have numbers.

The AP is displaying the value your ECU is using - so you have no operating issue.

Yes, you are correct. The dash gauge reads outside of normal lines. I'm going to take the car back to the tuner next week. I am definitely weirded out by the dash gauge acting abnormally. A big part of me wants to let the AP get up to the temp where the radiator fans turn on automatically.
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Old 07-17-19, 04:24 AM   #9
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Default Re: Which should I trust more? Dash gauge or AP?

i would do that. I don't believe you will cause any damage until you see significant signs of overheating - either that or verify with a second temp gauge.
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Old 07-17-19, 09:56 AM   #10
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Default Re: Which should I trust more? Dash gauge or AP?

Decided to drop off the car a few days early to the tuner. Drove it the 30 minutes to their shop and nothing. I mean nothing out of normal spec. Dash gauge was perfect, AP was perfect. It's also lava boiling hot here today (90 degrees @ ~9:00AM). Did a few high RPM pulls, and when I got the shop let it idle while I revved it a bit. When the AP showed 207 degrees, like magic the radiator fans started and pulled the temps down to 190 in < 10 seconds. Dash gauge never moved from normal. Unreal LOL.

I'm thinking something along the lines of the coolant temp sensor or other electrical. Who knows. Maybe when they installed the Crawford AOS something got out of place.
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