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79 911SC idle increasing with engine temp
Can someone help with a couple of high idle issues on a 79 911SC?
The original problem was an increase in idle speed as the engine warmed up. The car starts fine when cold and idles around 1500 RPM for a few minutes and then settles down to around 1000 RPM. As you continue to drive, and the oil temperature goes up, there is a corresponding increase in idle speed. On hot days and long drives, the idle gets up to around 1800 RPM which makes shifting into 1st or reverse from a stop a major grinding event. Any suggestions? |
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06-17-2010, 11:10 AM
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Join Date: Mar 2006
Posts: 1,107
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Craig,
A few things come to mind. First reset the throttle stop screw, check the bell crank linkage bushings and check that you are getting WOT. Reset your hot idle to 900 rpm with the large bypass screw and see if there is a difference. Then start looking for vacuum leaks, they generally get worse with heat. The first suspect is the injector sleeve O-rings, then the injector seals, boots, air box and hoses. Use an unlit propane torch with a hose while the engine is at idle. A change in the engine speed means you have found the leak. If the injector sleeve O-rings are leaking, the procedure to replace them is in the archives.
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Paul |
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06-17-2010, 11:53 AM
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Either you've got a vacuum leak and/or your idle isn't tuned right and and your warm-up devices are hiding this.
Also, pro-tip for getting into 1st / reverse... when coming to a stop leave the car in gear until you hit dead stop. This drives the layshaft to zero RPM and should let you shift w/o having to wait for it to spin down on its own. I had to do this when my pedal cluster started slipping and I couldn't fully disengage the clutch... saved the synchros till I could get it to the shop.
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Don't Lift... Don't Lift... Don't Lift  '75 Targa in "Arrest Me" Red, 3.0SC ('79) engine, Bilsteins, Turbo Tie-rods, SSIs into 2-1 M&K muffler... and looking for my next upgrade. |
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06-17-2010, 12:19 PM
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Thanks for the suggestions. I'll give them a try this weekend.
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06-17-2010, 05:02 PM
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Join Date: Apr 2006
Location: Charleston, SC
Posts: 2,359
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Wouldn't a vacuum leak make the idle drop? Like when you remove the oil cap?
How about the decel valve? Does a 79 have one? If so, it could be failing. It seems to be a fairly common thing. It will make your idle increase as the engine warms. |
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06-17-2010, 06:30 PM
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A vac leak could cause the idle to go up, if as the engine warms (and rubber expands) the leak seals itself up. If the engine is tuned around the vacuum leak then when it goes away, the idle goes up.
Of course you may be on the right track with the decel valve (I'm fairly certain the 79 has one)... it could also be the WUR or AAR/AAV not functioning correctly and being affected by engine heat. Vac leaks are the easiest to check, so I'd start there.
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Don't Lift... Don't Lift... Don't Lift '75 Targa in "Arrest Me" Red, 3.0SC ('79) engine, Bilsteins, Turbo Tie-rods, SSIs into 2-1 M&K muffler... and looking for my next upgrade. |
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06-17-2010, 07:54 PM
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Join Date: Apr 2006
Location: Charleston, SC
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Ya ya. Correct. It was past my bedtime when I posted. Tried to watch basketball game but could not stay awake!
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06-18-2010, 04:05 AM
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After checking for vacuum leaks, I disconnected the top hose from the decel valve and noticed a change in the idle speed. I plugged the top hose and the idle speed dropped to around 1050 RPM. Thanks for the help!
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06-18-2010, 03:25 PM
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Join Date: Apr 2006
Location: Charleston, SC
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Nice. There is a fix. You can place it in a vice and squeeze it ever so slightly - pancake style.
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06-18-2010, 03:44 PM
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