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Join Date: Apr 2000
Location: San Francisco, CA
Posts: 571
73.5 911T With Unstable Idle

My mechanic is in the process of troubleshooting a rough idle problem in my 73.5 911T. 73.5's have the first version of the CIS injection system. He's tried several WUR's and a has also tried a different Accumulator. No improvement. Ran a smoke test as well to check for vacuum leaks. Nothing. Tested all fuel and vaccuum pressures and all are in spec.

I test drove the car today from cold and the engine felt like it was loading up while idling at a stop light. Also it wouldn't stay running without the throttle lever pulled up to raise the revs. Even after engine warmed up, the idle was rough. I managed to take it for a short sprint on the freeway and it felt strong. One thing I noticed was the tach needle would drop pretty fast between shifts. Almost like fuel was being cutoff between shifts? Don't know what would cause that. Perhaps a clue to the problem?

Looking for help from you CIS experts on the forum. Thanks!

Old 03-06-2018, 10:37 PM
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Join Date: Dec 2015
Location: West Richland, WA
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Garage
Is the air flow sensor plate moving properly?

Try using this trouble shooting guide to isolate the problem. Notebook Spiral
Old 03-07-2018, 12:00 AM
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Join Date: Feb 2001
Location: South of the Mason-Dixon Line
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Oh boy, these first CIS cars can drive you nuts!!

I have the same car now going on 18 years! I must have replaced every component in that CIS line chasing problems like your having, so lets start with the easy stuff.

Usually, if the tach goes nuts you need to check the point gap on the points. Points not gapped correctly will give you problems! While you are at it check the distributor cap and rotor for wear or cracks. From there, the ignition wires, which if old should be replaced anyways.

My car after a long ride would just DIE at a stop and not restart until cooled down. The culprit? That wire harness between the distributor cap/coil and the CDI unit. When I replaced that it felt like a renewed kick in the ass and now trouble free driving. Check the deceleration valve hose line between the decel unit and the distributor cap. If its old replace it! Trace the line up to the bell shaped unit just to the left of the airbox.

Remove the rubber bellows on top of the airbox (two clamps to access the sensor plate). Clean it up as its probably oily and could be gummed up. You can get to the underside when you take off the air filter unit.

I can on and on, but go the technical articles for 911 and find the troubleshooting guide you can download, which can help better identify the cure. Often its best to find a mechanic who knows these systems well and can easily troubleshoot. Mercedes used the same CIS system in their 1970's cars as well.

Keep us posted on your progress. We all learn on this forum.

Bob
73.5T
Old 03-07-2018, 04:12 AM
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Can a 2.7 fuel distributor and 2.7 WUR work fine on a 2.4 CIS engine?

Old 03-11-2018, 12:12 PM
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