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Old 03-10-2009, 03:15 PM   #1
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Default Solved my 1.6L idle issue

So I had this idle issue that had been causing my Miata ('90 1.6L) to idle funky. This issue has been going on since I brought the car back online from being totally disassembled. I figured it had something to do with either the rebuild or the installation of the Link ECU and its various supporting components.

Here were the symptoms:

-Idle would stay between 1100-2000 rpm regardless of the car being warm or cold.
-Idle would oscillate between 1100-2000 rpm if the ISC was plugged in. Unplug the ISC, idle would stay steady somewhere between 1500-2000 rpm, depending on conditions, regardless of whether the car was cold or warmed up.
-Adjusting the IAC valve would make no change in the behavior...idle would stay high even if the IAC were closed completely.
-Removing the air valve made no difference in the behavior.

The first things I looked at were possible vacuum leaks. I had modified/swapped most of the intake manifold components as part of the rebuild, so there were plenty of possibilities where this could be the case.

-Multimeter test of the ISC showed it to be good, swapped out ISC in case it was dirty/ change in behavior
-TPS was working properly
-Air intake sensor was working properly
-ECU temp sensor was good.....BUT

BUT, what I ultimately found out was the wiring harness had a problem. The ground wire in the wire harness that was connected to the ECU temp sensor had broken somewhere inside the harness.

-This broken wire made the ECU temp sensor have to work with no ground...
-This in turn caused the ECU temp sensor to constantly give too high of a voltage signal to the ECU...
-Which made the ECU think the car was very cold...
-Which would make the ECU add fuel to the car at idle...
-Which would raise the idle speed...
-And if the idle speed reached 2000rpm, the idle speed fuel cutout would occur...
-and this process would repeat, causing the idle to oscillate between 1100-2000rpm
-Disconnecting the ISC prevented the oscillation, but would make the ECU run the car as if it were cold at all times (thus the idle being steady, but high).

So, at the moment the harness is flayed open and I've found/repaired the broken wire. Once surgery is complete, I expect that the idle issue will be solved.

Lesson learned: If you have an idle problem, there are other things to check besides vacuum leaks. Check your ECU temp sensor signal, too!
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Old 03-11-2009, 02:07 AM   #2
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Schweet! Everything went back together cleanly and the idle problem is fixed. Now I can start tuning the Link and actually saving settings (you can't save settings unless you're idling correctly).
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Old 03-11-2009, 02:09 AM   #3
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gratz man glad u got it straight
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Old 03-19-2009, 07:29 AM   #4
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Mr Woolery,
I had a broken wire in the harness to TPS gnd and every one said “never happen” .
Well done took me weeks to find mine as the break was in the harness that ran across the top of the radiator so it only went open when the car warmed up and the tape used to seal it in the factory softened.
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