Unstable idle - new fuel filter to blame?

atc_marty · 08-31-2012, 06:26 PM

Hello all,

I just got my 87 Carrera back from the shop where they installed a new fuel filter. Drove home, long enough to get the engine hot, and when I pulled in to my parking spot the engine idle RPMs started to drop almost to stall, then surge up to 1200 or so and repeat. I read a few of the threads on here about unstable idle and I am slowly learning about AFMs, o2 sensors and eaten gaskets. But could the new filter be responsible? Ran fine before, so pretty big coincidence, right?

wildcat077 · wildcat077's Garage

Most likely a vaccuum problem ... which shop did the work on your car ?

Cheers !
Phil

kidrock · 08-31-2012, 06:51 PM

sure, anything's possible....my first question is, is the new filter a Mahle? If not, this could be your issue.

Other than that, you'll need to go through a few testing procedures to make sure....there's several things that could cause the issue you described (CHT sensor, throttle microswitch, Idle Control Valve, O2 sensor, vacuum leak).

kj9438 · 08-31-2012, 10:51 PM

i had same problem on my 80 sc . i just fix last weekend . by tune the air and fuel adjusment .

E Sully · E Sully's Garage

Start by removing the oil cap when the car is warmed up and idling. The rpm should drop if there are no vacuum leaks. Look to see if any vacuum lines might have gotten disconnected, especially the 2 on the cruise control unit since it is close to where the filter was being worked on. Also check the O2 sensor connector, they become brittle with age and can call apart.
Start by setting the base idle. Make sure the micro switch on the throttle body is closed. Put a jumper wire in the test port by the electrical panel and set the idle to 880 rpm, as this is the rpm programed in the DME. The jumper disables the ICV and it should not surge with the jumper in.

atc_marty · 09-01-2012, 04:24 AM

Just drove the car to work this morning, the idle was a bit unstable at startup but idle seemed fine at every stop sign. Hey that rhymes! I'm thinking there could have been some air in the new fuel filter. Will keep an eye on it.

Phil, I just bought the car and I deal with GTA Performance in St Hubert. Haven't been with them long enough for an informed opinion, but they seem to do good work. Any thoughts? Where do you go?

kidrock, the fuel filter is OEM purchased from our host. Any known issues with that filter? Hopefully not...

Thank you all for the replies, will advise if the issue comes back. I've only driven it 20 minutes since it occurred, will be taking the car out for a longer drive after work tonight.

86 911 Targa · 09-01-2012, 09:00 AM

Have a look at these:

Hope this helps.

Gerry

kidrock · 09-01-2012, 09:40 PM

Sully has a good point....sometimes setting the base idle will solve the described problem.

atc_marty · 09-02-2012, 06:05 AM

I just went for a beautiful 100 mile drive east of Montreal and the problem has completely disappeared! So many problems have been solved simply by driving... I love this car!

Sully, I will try the jumper today and see if it does anything. I'd like to be sure this isn't an electronic issue.

Gerry, thank you very much for the pictures! I've downloaded them for safekeeping in my Porsche folder. I'm sure they'll come in handy sooner or later.