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Author Topic: Up & down idle 88 Duke when warm  (Read 33046 times)

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Donster

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Up & down idle 88 Duke when warm
« on: December 14, 2018, 03:15:35 pm »
Did the first long distance test drive with the Coupe today (+100 miles).When I got back and it had a good temp, the idle would go up and down between 800 and 1300 RPMs rapidly.Any idea?I know that since the cat replacement (when it ran rather rough and had a bad idle), the ECM has not been reset (battery disconnected). Could that be it?
Thnx,
\D
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GTRS Fiero

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Re: Up & down idle 88 Duke when warm
« Reply #1 on: December 14, 2018, 05:53:49 pm »
Resetting the ECU is quick and easy, so you could start there.

So, the idle hasn't been good, as long as you've had the car?  You need to check the readings for the CTS & IAC.
« Last Edit: December 14, 2018, 10:56:58 pm by GTRS Fiero »
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GTRS Fiero

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Re: Up & down idle 88 Duke when warm
« Reply #2 on: December 14, 2018, 10:57:58 pm »
I think the MAP and O2 sensors could also be suspects.
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Donster

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Re: Up & down idle 88 Duke when warm
« Reply #3 on: December 15, 2018, 04:25:06 am »
BTW: No error codes.
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GTRS Fiero

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Re: Up & down idle 88 Duke when warm
« Reply #4 on: December 15, 2018, 08:03:08 am »
Right.  I havve found that the sensor readings are sometimes wrong, causing erratic behavior.  In my case, I checked the readings using WinALDL.
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TopNotch

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Re: Up & down idle 88 Duke when warm
« Reply #5 on: December 15, 2018, 08:53:41 am »
Check all vacuum lines. Try removing the large vacuum line that supplies vacuum to the brake booster, and plug the fitting.
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Fierofool

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Re: Up & down idle 88 Duke when warm
« Reply #6 on: December 15, 2018, 09:08:24 am »
When I had the 88 Duke, I found a small vacuum hose underneath the filter housing, to the right side of the throttle body, that had come loose.  I later found that it was common to miss that hose after having the filter housing off.  It was also common for it to get blown off if the engine ever backfired.  I think it went to the MAP sensor.  Use a mirror to look underneath the filter housing and around the throttle body for any vacuum nipples that don't have a hose on them. 
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Donster

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Re: Up & down idle 88 Duke when warm
« Reply #7 on: December 20, 2018, 06:37:12 am »
OK, reset the ECM, got the temp up on the engine, took off the filter housing and tested all vacuum lines. Every line I disconnected, had an effect on the engine (either high or low idle, or engine stalled). When I disconnected the EGR valve vacuum line, nothing changed.
I have already replaced the MAP sensor, but it seems I replaced a good one with a good one. I also have a spare TPS, but wanted to ask first.
So, does it sound like it is the EGR valve? Should I just make a cover/gasket for it and block it? Have heard other success stories of blocked EGRs on Dukes.

Also a question: I noticed that the nipple for the vacuum line on the top of the EGR valve rotates/spins freely. Is that how it should be?


Thanks,

\D
« Last Edit: December 20, 2018, 06:40:56 am by Donster »
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Fierofool

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Re: Up & down idle 88 Duke when warm
« Reply #8 on: December 20, 2018, 03:12:09 pm »
You can test the EGR valve by placing your fingers underneath and feeling the diaphragm.  Push up on it and cap the nipple with a finger.  If the diaphragm moves down, the valve is bad. 

It sounds like you have a universal EGR valve.  I've often wondered how a vacuum can be applied to the diaphragm if the vacuum chamber cover can rotate.  If it can rotate, seems to me that there's not a seal between it and the body of the valve, so there's no seal of the vacuum chamber.  But I'm no engineer. 
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GTRS Fiero

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Re: Up & down idle 88 Duke when warm
« Reply #9 on: December 20, 2018, 06:30:37 pm »
Did you plug the vacuum line for the EGR, when you unplugged the line from the EGR?
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TopNotch

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Re: Up & down idle 88 Duke when warm
« Reply #10 on: December 20, 2018, 09:32:48 pm »
BTW, the EGR valve on a duke is both vacuum and back pressure operated, not vacuum only like the V6 EGR.
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GTRS Fiero

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Re: Up & down idle 88 Duke when warm
« Reply #11 on: December 20, 2018, 09:34:48 pm »
Where does the back pressure originate?
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TopNotch

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Re: Up & down idle 88 Duke when warm
« Reply #12 on: December 20, 2018, 09:39:40 pm »
From the exhaust.
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GTRS Fiero

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Re: Up & down idle 88 Duke when warm
« Reply #13 on: December 20, 2018, 09:43:21 pm »
I'm feeling dense, tonight.  Does the back pressure make the EGR open or close?
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Donster

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Re: Up & down idle 88 Duke when warm
« Reply #14 on: December 21, 2018, 05:56:37 am »
Quote from: GTRS Fiero on December 20, 2018, 06:30:37 pm
Did you plug the vacuum line for the EGR, when you unplugged the line from the EGR?

Plugged both directions, no change.
Turns out EGR valves are rather difficult to find new here in Germany.
Again, does anyone have experience with blocking out the EGR on an 88 L4? I can have a cover-plate and gasket made within minutes and permanently plug the hoses.
Thanks

\D
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