markroberts

Hi, my 964 has had an idle/running issues for some time. It runs great then randomly starts missing a beat and bucking. I have replaced the Fuel pump, Fuel Filter, DME Relay, checked Battery Leads, Checked Ignition leads...any insight would be greatly appreciated..1989 964 C4 Convertible...Thanks

Kappi2907

Clean ISV, AFM, check distri belt, coils.

markroberts

Thank you Kappi2907.
ISV, AFM, cleaned 6 mths ago.....Dis belt and coils were replaced 6 mths ago as well unfortunately.

John McM

Have you checked the crank sensor?

Feudal Serf

Sounds like an ignition system problem, our dual ignition system is very sensitive. Often will run better on a single coil when compared to both if the ignitions aren't perfectly in sync.
Try unplugging one at a time the upper and lower coil at the harness. Drive it on a single coil and see if the bucking goes away.
For example, my lower plugs on 4-5 cylinder was crossed upon return from a shop.
Car ran fine sometimes, then acted up, felt sluggish on throttle response, sometimes bucked.
cheers
FS

Silvertarga

If your new coils are silver then hopefully you have the old black ones, swap them back in and see how she runs

I had the bucking issue after putting the New Bosch silver coils in...once car warmed up it was horrible. Coils bench tested ok but when hot they were not..I then changed to the 993 style coil pack and no issues since..

Jacke2c

There are about a dozen ground points on your car... even one behind your coils. Have you taken any of them loose and looked for a white powder? (corrosion?) I went around an cleaned all of my grounds and solved a lot of different issues. Make sure that all your wire harnesses are fully plugged in also. As a last resort, there is an issue about the soldering on the back of the clock circuit board that can cause this issue.

markroberts

Thanks guys ... I am thinking the crank sensor and the O2 sensor are next on my hit list... I went to start her this morning and
she carried on like a pig. Seems the issue is quite random so I doubt its a vacuum issue.

John McM

If cold starts are a constant issue then check the cylinder head temp sensor as well. If it's defective it can default to the car's warm start protocol.

Super90

Read the TSB on the AFM sensor connector. It will cause that. Ask me how I know....

markroberts

Cool... whats the TSB please? How do you know?

Jacke2c

+964 on the TSB service bulletin on the AFM sensor connector

I thought.. really, and then I cut the tie wrap and sure enough, I started having intermittent problems. Refitted the fold in the wire and re-tie wrapped and problem went away.

markroberts

Ok, is there a link as I cant find the TSB on the AFM sensor?

John McM

Electrics section of the workshop manual. 97/07. Sorry I can't post the images from my iPhone as the Garage Talk app is giving a server error.

Spokes