I have a 1986 928 S. It has a "surging" problem. Does not necessarily feel like a rough idle, but rather, the car feels like it is surging when I stop at a light. Just for fun, the problem is intermittent, a times the car purrs like a kitten at lights, you can barely tell that it is on and at idle. Most of the time however, 90% it has this surging problem.
Engine cold, engine hot, AC on AC off, does not seem to matter. Timing belt recently changed out along with all the other belts, the surge was present both before and after the belts being changed.
Gauges seem fine, no dramatic drops in any of those. No smoke, no unusual smells. Has never died, never quit running, never faltered when pulling away from the light.
As mentioned above timing belt and other belts recently changed. Now, to the chagrin of a much more experienced owner I am getting a tune up with spark plug wires. Hopefully this will solve the problem.
If it does not, any ideas?
Thanks in advance for your response.

Recondoc

 

TPS is not making contact is the best guess. The harness at the front of the engine gets brittle and chances are it was bumped. Check the harness first. It's up there by that circular rubber part above the fuel line.

 

Can you give us a pic Sean? I get that every now and again.

Thanks.

 

It is the three pin Bosch connector that mounts in the metal loop right behind the Diag Plug. I think you can do a test for it working from that diag plug.

On the surging problem, yes could be a mis and a Tune-up is a good start.

 

Surging may often be caused by a leak in an intake manifold gasket.

 

Or that you drive to slow!

'89 S4 GP White/Black
'76 912E Silver/Black sunroof

 

+10

If the idle switch is not closing/connected, then the ignition advance at idle will be >20° (should be 10ish) which does make it stumble occasionally.

 

LOL!! You were just waiting for the opportunity, weren't you??

 

Brian, you don't have a TPS so yours will be something else.

The connector we are referring to is right above the new fuel hose where I marked with squiggles. It's brown, left side. If the harness is shot (likely) Rog has new ones made up.

 

See Dwayne's intake r&r thread for TPS test. I cut down two crimp blads to fit into the EZH/LH. Pretty easy with a multi meter. You'll find the pin out pdf's on Ken's site. Liftbars.com

 

Guilty as charged!

'89 S4 GP White/Black
'76 912E Silver/Black sunroof

 

Hi Sean,

I have a similar problem after a top end refresh. Here are the symptoms:

When I first start the car, the idle is fine.
After a few minutes it creeps up to 1800.
I can sit at a stop light at 1100RPM, then it would go back down to 750.

Below is what was done. I had another thread but was too long and convoluted. Any guidance and help you can provide is greatly appreciated.

• Replace ISV with new
• Replace front TPS connector (plastic crumbled in my hands)
• Replaced CPS (Plastic crumbled in my hands)
• New injector seal kit
• new intake gaskets for the tubes
• New Fuel lines
• smoke test for vacuum leaks (none)

Thanks

Boaf