Rough Running 911 SC

[Guardsred911]

Hi, I have been reading quite a few of the posts with what seem to be similiar symptoms, unfortunately with no luck.

Was driving home the other day, and the car seemed to miss a couple times, then coninuously. Was a challenge to get home with little power. (this was after 40 minutes of driving.

I checked the distributor cap for cracks (nothing - seemed normal) - I even threw on the spare rotor I had lying around

I sprayed starting fluid around the intake area- nothing.

I changed out the fuel filter with a spare I had. Prior to that, the fuel pump seemed to whine a lot when I turned the key to the on position. After replacement, there was zero whining from the pump. Still ran rough though. After this though, the CDI Ignition Control Switch was making a super high pitched '"Eeeeeee" sound when I turned the key on to the on position. I wiggled it around and it changed tone, but the connection was good. Could this be the issue?

I checked all the wires to the plugs for connection. reattached firmly

I pulled the O2 control unit relay under the passenger's seat - no change in idle

I ensured all the screws in the fuse block were tight

In my travels I found a green wire going to the AC unit which had seperated (reattached now), a plug on the backside of the air intake which I reattached (no change)

The car runs, but rough at 850 RPM. normal is about 1000 and smooth... when I hit the accelerator, I get back fires in the muffler (not the airbox)...

What else should I be doing


Thanks

J
1980 911 SC, 131 K

[jimrs2000]

Sounds like an 02 issue, pulling the relay should certainly have changed the idle if the system was working.

Check the fuse for the interior light, the 02 system is also wired through it.

[Guardsred911]

Thanks Jim... So I am embarassed to say that I actually did perforn that check and even replaced that fuse the other day with no results. I did it just now, and the car runs fine. Thank you. I must have just not gotten the screws tight enough. Thank you!

In my travels looking for things that are lose, I found this plug coming off the wire harness with no obvious place to plug it in... it's under the drivers side intake runners... I felt around in there and couldn't find anything...


Any ideas?

[jimrs2000]

I have it too, I am assuming its a test port of some form, mine is crusty and old from age.

[tirwin]

Guardsred911,

I am not 100% certain but I think that plug in your picture might be the sensor plate contact plug. If it is disconnected the fuel pump will run with the key on.

-Tim

[Bob Kontak]

Not the sensor plate contact plug. Just went out and checked it on my 81 with a mirror. The sensor plate plug is similar to the plug in on top of the WUR - it's too short vs OP's pic- plus has one of those rubber necks for sealing, is green and the wire goes into the center.

If Guardsred911 or jmrs2000 can give me the wire colors I will find that bad boy. Also, a focused picture directly pointed at the contacts will help.

I think there are one or two wires floating around in the SCs that are not used because they carried legacy wire harnesses forward but deleted some part (e.g., ext V reg - 82 & 83.)

It is definitely not the test port.

[Bob Kontak]

Side note to Guardsred911....


Plug that blue vacuum hose to your distributor - just the hose - car will start better.

Replace the O2/frequency valve relay under the front seat, even if working. A new one works better because the contact points get old and they don't work like the day they were new. It's like a mini tune-up.

[Guardsred911]

Hi Bob, Thanks! Focused picture attached!. It's brown and red with a white stripe.

Roger on your comment about the O2/frequency valve relay! Will do.

With regard to your comment on the blue hose, do you just pull it off the distributor, plug the hose, And leave the male fitting open?

[pete3799]

Does the plug go to the heater blower motor? Think the colors are correct but my plug looks different.

[Scott R]

That plug goes nowhere on the 80'. It's a leftover from the previous year, was for a thermotime valve which you do not have. Jump the connection on the O2 relay and see if that clears things up. As mentioned above pulling the relay should have a pretty dramatic effect on the engine.

[boyt911sc]

Originally Posted by Guardsred911

Hi, I have been reading quite a few of the posts with what seem to be similiar symptoms, unfortunately with no luck.

Was driving home the other day, and the car seemed to miss a couple times, then coninuously. Was a challenge to get home with little power. (this was after 40 minutes of driving.

I checked the distributor cap for cracks (nothing - seemed normal) - I even threw on the spare rotor I had lying around

I sprayed starting fluid around the intake area- nothing.

I changed out the fuel filter with a spare I had. Prior to that, the fuel pump seemed to whine a lot when I turned the key to the on position. After replacement, there was zero whining from the pump. Still ran rough though. After this though, the CDI Ignition Control Switch was making a super high pitched '"Eeeeeee" sound when I turned the key on to the on position. I wiggled it around and it changed tone, but the connection was good. Could this be the issue?

I checked all the wires to the plugs for connection. reattached firmly

I pulled the O2 control unit relay under the passenger's seat - no change in idle

I ensured all the screws in the fuse block were tight

In my travels I found a green wire going to the AC unit which had seperated (reattached now), a plug on the backside of the air intake which I reattached (no change)

The car runs, but rough at 850 RPM. normal is about 1000 and smooth... when I hit the accelerator, I get back fires in the muffler (not the airbox)...

What else should I be doing


Thanks

J
1980 911 SC, 131 K

Either the FV relay is bad or it is not getting power. A spare relay is always good to have. If the idle is not affected by the removal of the FV relay, which should idle rough without one is a sure indicator of your problem. Even worst, the ECU could be bad too. One way to check is to hook up a dwell meter and get the reading. Ideal value is 45-65% range. Keep us posted.

Tony

[esonefour]

Originally Posted by Guardsred911

The car runs, but rough at 850 RPM. normal is about 1000 and smooth... when I hit the accelerator, I get back fires in the muffler (not the airbox)...

Same issue with my 83 sc I thought it was the idle not adjusted properly.
Will look over the fuses tomorrow, and look into the FV relay.
Though nothing is under my seats?

Would love to take it to a P car specialist but they will just winge how non oem it all is :?

[Bob Kontak]

Originally Posted by Scott R

That plug goes nowhere on the 80'. It's a leftover from the previous year, was for a thermotime valve which you do not have.

+1 Just looked it up for grins on the 1978 diagram.

Also, plugging the blue hose cuts the vacuum retard function which is installed for better emissions at low idle. If it makes no difference on cold starts just put it back on. It's a trick I learned here (psalt) as the added ignition advance makes up for some of the minor issues (from age) in all of the CIS components even if they are working ok. Yes - just plug the blue hose with a nail or something like that (German nail preferable). Leave the metal port alone. boyt911sc is not big on the practice as it actually masks CIS "noise" that you may think you don't have. He is correct but just give it a try. If it starts better, you can choose to drill down deeper and find the source.

[emac]

I just went through just about the same issue last week and posted. My issue was the car normally fired up on one click of the key, then it took more cranking to get her running. Bottom line is that from everything I have read here the CDI box either works or it doesn't. I am here to say that mine worked for a few days and quit. I replaced it and she fired right up. Waiting for parts from our host for a complete tune-up

[Guardsred911]

Was able to fix this yesterday...I am embarassed to say that I actually did perforn the Interior light fuse check before and even replaced that fuse the other day with no results. I did it yesterday, and the car runs fine. Thank you. I must have just not gotten the screws tight enough.

[Guardsred911]

Bob, Interestingly pretty rough running this morning.... replaced the O2 sensor relay I had ordered on your suggestion... runs like a champ now. Thanks again. What do you suggest to clip this back in? The previous relay had a metal clip that slid into place... just zip tie?