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Join Date: Jul 1998
Location: Santa Barbara, CA
Posts: 5
low idle on 1.7

I have a 70 1.7, all stock, complete rebuild 10,000 miles ago along with a new pressure sensor. Car has 119,000 miles. Just had the valves set, new points.... timing and fuel mix checked at 2 different porsche shops...the car runs like a dream...except at red lights, stop signs, etc...feels like it may stall at anytime, and has on occasion, especially when hot. The car will restart easily and as long as I keep the rpms around 900-1000,it runs. I have tried to adjust the idle screw and that seems to have zero effect on the idle. I just added a "new" used aux. air valve...any suggestions on what to look at next? Thanks for any help. Randy
Old 07-30-1998, 04:15 PM
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john rogers's Avatar
 
Join Date: Dec 1969
Location: chula vista ca usa
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These have been posted here and on other sites and we had the same thing happen to our '74 2.0. several things were wrong:
- several vacuum leaks were messing up the mixture, mainly the molded fitting that the oil system vents to. Replaced hoses & fittings.
- the advance on the distributor was stuck somewhere between retard and full. The timing had been set as per the spec book, but it didn't retard correctly. Got distributor overhauled.
- the aux air valve was not shutting fully which acted like a vacuum leak. removed it, soaked in Wd-40 overnight and it works fine.

Old 07-30-1998, 06:01 PM
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There are usually three things that will bog down your idle.

One is the timing. If your timing is retarded, or *really* advanced, the idle will slow down. (If it's only *somewhat* advanced, it will actually idle higher.) Check the timing at idle. You should be right around 5 degrees BTDC. You'll either need one of those dial-in-the-advance timing lights, or you can just guess that 1 fan blade from the TDC mark is close enough.

The next is the fuel mixture. Too rich a mixture will bog the idle down. This actually sounds like it might be your problem, since it gets worse when the engine is hot. Measure your head temp sensor's resistance. When the engine is cold, it should be around the 1500-2500 ohm range. When warm, it should read much lower--perhaps 100-200 ohms. Higher resistance means the mixture is richer.

Let's see, a 70 doesn't have the idle mixture adjustment... OK, you can lean out your mixture by tweaking the fuel pressure at the fuel pressure regulator--left-rear corner of the engine compartment. Put a gauge in the fuel circuit, and drop the pressure about 3-5 PSI. If that brings your idle up, you know the car is running rich. (Stock setting is 28-29 PSI.)

Finally, there is airflow. Not enough air going into the motor will bog the idle down. You should be able to affect that by using the idle speed adjuster, the screw on the Throttle Body. It's possible that the passages inside the TB are blocked. Take the air filter (and the big hose from it to the TB) off, and check to see if any air is going through the idle passage in the TB. If not, take the TB off the car, and clean out the idle passage(s) with carb cleaner and maybe some pipe cleaners.

Good luck, and let us know what you find!

--DD
Old 07-31-1998, 09:40 AM
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Thanks to everyone for the advice. I took out the throttle body and throughly cleaned it with carb cleaner and q-tips and pipe cleaners....put it all back together and now my idle shot up to 1600...but, the good news is that the idle adjust screw on the TB now has an effect when turned and was able to set the idle at 900. Car seems to be idle great now. Did have one scare after putting it all back together...idle was perfect, thought i would take it out for a spin, pushed in the CLUTCH and instantly red lined the motor...nearly had a heart attack. Each time I depressed the clutch the engine would scream. I looked in the engine and had a friend depress the clutch pedal and sure enough the accelerator cable was moving. Looked under the pedals for any clue, finally was able to look inside the center channel between seats...some repair was made in the past and there is a place to bend open a section..could now see and feel both cables...both were independent at this site..checked the TB again and now it was all working fine. Guess there must be spot underneath where the 2 cables were getting hung up on each other and then worked back to original placement...ahh..well, the story ends on a good note as my car purred like a cat on the way home! Thanks again for the advice. Randy
Old 08-05-1998, 03:25 PM
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