3.2 Surging Idle/Tach frustrations
 

My 3.2 Targa has an intermittent surging idle. I had it at my local air-cooled specialist a couple months ago, and they couldn't get it to duplicate the issue when they had it. They checked the usual things like vacuum leaks and idle control valve (I pulled that myself prior and it was working fine). A couple of times since I got it back the car has died when clutching to a stop, but for the most part other than some bouncing idle at cold start maybe half the time, the car runs great under throttle and after running for a couple of minutes it idles smoothly.

A couple of days ago the tach started operating intermittently. It either doesn't move or it moves wildly, but about half the time it works normally. It went dead a few years ago and we sent it to Palo Alto Speedometer for repair and it has been fine since.

Car has a SW chip, M&K cat bypass pipe and sport exhaust. Any chance the surging idle and the tach are both caused by an ECU issue, or somehow related?

Simple first step, install a stock chip!

I've been hunting down this same issue. Finally got it situated, but now have a fuel delivery issue... likely injector(s) related.

I found two smoking guns... One literally.

I bought a cheap smoke machine (and the smoke juice, come seperately)... Found a massive leak in my brake booster.

<iframe width="1661" height="793" src="https://www.youtube.com/embed/l6GoFKhLexg" title="1987 911 3.2 Carrera Smoke test" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" allowfullscreen></iframe>

Got that settled and the idle got a lot better.... then intermittently I had a wild surge, but it only really occurred when hitting hard bumps, which are easy to come by in LA.

This lead me to search out an electrical culprit.

My smoking gun there was a destroyed Cylinder Head Temperature connector (white connector) usually found in the stack of three on the driver's side of the motor.

I installed a new CHT sensor and it solved my massive surge issue. There are quite a few threads on this topic, but a good place to look.

-Ian

Follow-up: a couple of weeks ago I unplugged and plugged in all of the electrical connectors in the engine compartment that I could access, including the plugs wires at the distributor, and it has been running perfectly since. The random on/off behavior of the tach seems to go along with an intermittent electrical issue, so hopefully that’s all it was.

I have an 86 911 3.2 that started surging. I changed the fuel filter, and it went away. Now, I change the filter at least once a year, no matter what.