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496 HO high idle - TechMate scanned

Old 12-24-2020, 04:16 PM
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Default 496 HO high idle - TechMate scanned

I bought a 2008 Daytona with a Merc 496 Whipple about 3 months ago. It has run perfectly. Last month it started idling high, 900-1100. No, it's not the linkage. The throttle plate is closed. I bought a Rinda TechMate. The idle says about 850. There are no faults shown on the scanner. Under the freeze frame I found the following codes:

21 ECT
50 EST shorted
77 Camshaft sensor
121 & 122 TPS low / high
152 IAC output

When I raised the IAC with the scanner it did not raise the engine idle. I already changed the IAC with no change. Fuel pressure was about 43 at idle. IDK if this matters, but under Shift Position it says "In Gear". TPS voltage says .27.

Any ideas?

Last edited by Delta_Dave; 12-24-2020 at 05:25 PM.
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Old 12-24-2020, 04:28 PM
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No expert but

What is the motor serial number?
What is the IAC counts?
What is the MAP reading?

Being a 2008 I'm guessing the fuel pressure should be 39 at idle and 43 at WOT. Check the vacuum line going to the regulator to see if its working, remove line and see if pressure increases
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Old 12-24-2020, 05:41 PM
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Thanks Dodge. The standard serial label is missing. I think the engine is a 2007. The scanner does not give me an IAC count. I raised the IAC to 100% and lowered below zero on the scanner and it did very little to the idle, it still hovered around 830-840. The MAP is about 15. I think I read 44 psi at idle is about right and mine reads 43.5.
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Old 12-24-2020, 06:22 PM
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Which scanner do you have, the IAC count is on the Techmate and others made by Rinda (Diacom)
My 39 psi came from a former expert (MUC)

Trying to determine if there is a vacuum leak with the MAP reading, or the MAP volts may be off

Place a vacuum gauge on the line to the intake and see what your getting

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Old 12-24-2020, 09:24 PM
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Thanks Dodge. I have a Rinda TechMate Pro. Here's more info from the scanner:

Manifold pressure: 10.1
Barometric pressure: 14.7
Manifold temp: 53
IAC duty cycle: 10.7
TPS #1 position: .02
Block pressure: 2.4
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Old 12-25-2020, 08:29 AM
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I think you have a vacuum leak. The IAC is almost closed, the TPS is closed but the manifold pressure is dead on target. So the IAC is doing everything it can but is not able to reduce it further. Just my guess
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Old 12-25-2020, 10:21 AM
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does it still say "in gear" while in neutral?if so,,start looking at the switch on the shifter backet on the exhaust manifold,if its not ther then its on the throttles/shifter at the helm.
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Old 12-25-2020, 10:27 AM
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you have a few codes there,have you looked at the connectors on the ecu?
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Old 12-25-2020, 11:16 AM
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Originally Posted by sutphen 30
you have a few codes there,have you looked at the connectors on the ecu?
Didn't notice these codes with my first comment, and was edited about an hour after posting
21 ECT
50 EST shorted
77 Camshaft sensor
121 & 122 TPS low / high
152 IAC output

Most all these go to a wiring problem or short. There should also be an alarm
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Old 12-25-2020, 12:43 PM
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Thanks guys! The codes were from the freeze frame, there are no current codes. I'm not sure when those codes happened. I did previously find the harness at the TPS had a short so I replaced it. No change. TPS volts are low, Whipple manual says it must be about .65v. I have .25. I need to adjust that today. Whipple manual also says vacuum leak for a high idle issue. I haven't found any obvious vac leak. As far as gear position "in gear", IDK if that's a normal reading for a PCM 555. I pulled the shift mechanism and all I found was the neutral safety switch. You may be right, the IAC is trying but unable to lower the idle. Even bringing the IAC below 0 with the scanner made basically no difference in the idle speed. I guess back to more checking today. BTW, it seemed there was more IAC air noise at idle before this issue.
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