1980 911SC Suddenly running Rough...Any Ideas?

[BOJAN]

I was driving to work couple of days ago, and all of a sudden car started to run rough as if was running on 3 cylinders(loss of power,slight backfire,when pressing gas it would chocke in beginning the as revs slowly climb it would start toget going slowly). I made it work and parked the car. After i was done working, i attempted to restart the car and it ran rough again. I turned it off and on few times and suddenly it ran fine. I drove home ok. Following day i drove the car without any problems as well , but the next day the hesitation returned, i was barely able to move the car.

This is a stock csi and i really did not have any issues with the car,until now. Prior to this happening i was very very low on fuel(did not runout), then i filled up and drove home. This problem started one day later. I wonder if something was socked from the bottom of tank?? just a guess. Fuel filter is 20k old, so maybe thats the issue? Each time i attempt to start the car, it starts instantly....

Any ideas are appriciated it...
Thank You.

[barndoor]

My guess is it fuel related since you ran low.
Change the filter first,then go from there.

hope this helps !

[Bob Kontak]

Under the passenger seat there is an oxygen sensor relay that can fail. It is a square/cube relay on the Lambda box that can be unplugged easily. Unplug it and see if symptoms change. I bet that is the issue.

Two years ago, a member here in Houston had the same symptoms when it was hot outside. Turned out to be the wiring connection was corroded on the engine firewall on the drivers side that connected the frequency valve to the O2 relay. That connection is reachable by hand and just unplugging and replugging it may help the connection.

[BOJAN]

Thx for the help !

I will try unpuging the relay and see if it helps...When i got the car, the o2 sensor was disconnected..i even passed the CA emissions like this...i connected it a while ago..but dont think thats an issue. The fuel filter is nearly 5 years old, so i will definitevly replace it either way....

I am just puzzled...because one day i was driving 100mph without issues and the next day i can barely move the car....what can it be to come back intermidenly like that?

My mpg have always been bad(at least the city), little too much smell of fuel was present on occasions.....my mechanic tried leaning the mix a little but i dont think it helped....But these are things i dont nit pick on, it is a classic after all, and as i said i never had running issues, started and drove every time....

Thx for the help!

[Flat6pac]

+1 on the lambda relay.
Bruce

[Bob Kontak]

Originally Posted by BOJAN

When i got the car, the o2 sensor was disconnected..i even passed the CA emissions like this...i connected it a while ago..but dont think thats an issue.

With the O2 sensor plugged in or not your car will still run pretty good.

With a bad/failing relay or poor connection between the Lambda box and the frequency valve, the frequency valve does not operate properly. It is not the O2 sensor that may be the issue, it is the functionality of the frequency valve that you need to zero in on. Replace the relay and look at that connection on the back wall. I think it is a 10 or 12 pin connector.

The real test is to pull the relay while the car is running well and see if the same poor running symptoms return.

Also, there is an O2 fuse in the front trunk that powers the whole system. I can't remember the location on the bank of fuses but it should say on the plastic cover. That will create the same symptoms if a poor connection.

[jeffs9146]

Vacume Leaks on the intake!!!

[81 SC Ray]

Hi,

The same thing happened to me on my 1981 SC. I changed - the fuel filter, put on a new coil, new cap and rotor, new Beru wires and replaced the spark plugs with the Porsche recommended ones (look in your manual) and the car ran awesome after that.

I don't know which one of those items fixed the problem, but, the ignition wire harness was original - it was time, so, I just did the whole thing. Not super expensive if you do it yourself.

Ray

[70SC]

H20 in the tank . Drop, Drain & Clean

[Hobbsie]

Check your trunk light fuse, the lambda circuit is on same circuit.

[MotoSook]

Before you go too far with all the CIS stuff, check the distributor. Check to rotor and cap, plug wires, coil wires and the connector to the CDI box. These are all easy to do in 10 minutes.

[Bob Kontak]

Originally Posted by Hobbsie

Check your trunk light fuse, the lambda circuit is on same circuit.

+1

Vacuum leaks / water in gas / Bad wires / skunky dist cap / etc are usually not intermittent problems, right?

OP's issue is it runs like a top then runs like complete shyte and then back to ok.

Here is the link to the post I am talking about where the fellow had the bad connection at the engine firewall relating to the frequency valve failing to communicate with the Lambda box.

911 SC running "flat" and no idle

[BOJAN]

Well its running like shyte all the time now.....

turned it on today, and seemed ok for 10-15 sec, and the chocking returned...

I checked the relay under passanger seat, cleaned it...but it made no difference

terminals inside the rotor cap seemed used up a bit, but dont think its that

I ordered a new fuel filter,cap and rotor, and spark plugs( these i need to do anyways, so il start from there)

If the fuel tank was dirty even a little, i think that fuel filter might reached its limit...its almost 5 years old...

Thx to everyone for their imput, i really do appriciate it!

[john walker's workshop]

if it's already running lean and rough because of a bad O2 relay, unplugging it won't make any difference. you may be onto something.

[Bob Kontak]

Originally Posted by BOJAN

I checked the relay under passanger seat, cleaned it...but it made no difference

Do you understand that if it runs the same with the relay removed you have found your problem? At least the general area?

The square relays are available at the local parts stores pretty cheap.

[BOJAN]

Actually i went to a store local parts store and purchased a new o2 rellay(was not oem but look the same), but it made no difference, ran rough with new rallay

Had no time to swap the fuel filter and plugs....but i have a feeling is something else

When i started it today...it started imeddiatly(always cranks little long on first try), but if i depress the gas...clogging/missfire....the smell of fuel from exhaust was excessive as well....

[john walker's workshop]

the relay needs 12V to the 30 terminal and out of both 87 terminals. does it have that? the domelight fuse delivers power to 30. you should spend some time making sure everything in this system is functioning before getting into anything else. this is a very common problem area. the relay plug can have a broken or pushed out wire or corrosion. the plug by the left rear shock tower can be corroded, broken wire, or unplugged. the O2 computer can be water damaged, or have a bad solder joint. when the frequency valve is rattling and buzzing, you know the system is working. it's behind the big ribbed rubber air boot. it runs when the engine runs and can be made to run with just the key on by unplugging the green sensor plate contact plug. the fuel pump will also run if you do that. the plug is between the boot and the fuel distributor on the sensor plate, near the frequency valve. not the blue plug. reach around with your left hand.

[Dan911/22]

I have the same problem with my 911 C2 1991. It started to happen after I filled up the car for the first time after I bought it... I replaced the DME relay and the oxygen sensor, but still does the same. I will replace the fuel filter this week and see if it's the cause of it. Pretty sure now it's from all the dirt in the fuel tank who got into the filter after I filled it up.

[BOJAN]

Well, no luck so far....

as i said i replaced cap and rotor, and then the fuel filter..which when i blew air through it gas that came out did have have some dark dirt like color to it, i put the new filter on but made no difference. I tried the new o2 relay, but made no difference. Then i tried replacing the plugs, but plug number one is stuck...i was able to move it some, back and worth but dicided to leave it alone...do not want to strip it...maybe there is a method that i dont know about that would help me take it out..????
took number 2 out and seemed fine.....

Car starts every time,but imeddiate clogging...if i press the gas it would take a moment to respond..backfires from exhaust... yes i candrive it..but have to fiddle with the throtle to get going...

I am somewhat out of ideas...to be honest
do not have the csi testing equpment or proper knowldge how to use it

maybe i just drive the s#$%t out of it..

Once again thx to everyone for their imput/ideas

[Bob Kontak]

Broseph, I think I can set you free but there has to be some listening demonstrated.

Are you down with that or are you going to blow off John Walker's and my thoughts?

Please advise as the O2 relay is only one link in the chain. It has to talk to the frequency valve. I don't think it is. If it ain't talking to the frequency valve, you are a fuc*ed chicken. Make sense?

Calm down and advise if you will listen - AND ACT - on recommendations regarding the O2 system.

Call me an a-hole if you like for trying to get your attention but I bet my fat ass I got your answer.

Love you, bro!