911 SC running "flat" and no idle

[jimrs2000]

I have had this problem twice now in the past month.

The car is a daily driver. and as I say this happened about a month ago and then again today.

Driving the car for a few miles as it gets warm the engine goes "flat" there is no power and when you put the clutch in the engine struggles to idle at around 250rpm. OAT is around 100deg.

What may or may not be related is the fact that when the engine is cold overnight the car starts first turn of the key but struggles to maintain idle for the first couple of miles. When I have left the car all day in the Texas sun I have then had to crank it for 5 minutes to get it going again, clearly fuel related as it starts up on one cylinder and slowly the rest come in. This has happened every day as opposed to the above.

All of the above has happened in the same timescale.

Last year I replaced the fuel pump check valve and the accumulator due hot starting issues.

Any Idea's?

thanks in advance.

[Bob Kontak]

Check the O2 relay under the passenger seat. It may be malfunctioning or the contacts are corroded/wires loose causing the O2 system to fail intermittently. It is a $10-12 from PP.
It is a little 1" cube that is pretty easy to remove and replace. You can get to it without removing the seat.

Also check the O2 system fuse - third of fourth back from the dashboard - the dome light and front luggage compartment light are powered by this circuit as well.

Either one of these will cause similar symptoms.

[jimrs2000]

Thanks Bob,

I had almost discounted the 02 system as I have had that fuse blow before when I was troubleshooting the instrument lighting and the effect was much more severe than what I have experienced today.......however I have just been out and pulled the fuse....guess what? no difference in running so it must be that.

Coincidentally I have just installed some 964 seats and lunchtime today was my wifes first time in the car since, of course she had to move the seat around until she got the right position......

Looking for a loose conection tomorow I guess.

thanks again.

[jimrs2000]

So I started the car this morning and it was fine? I get most of the way to work (about 14 miles out of 16) and I start to get the problem again but its very intermitent.

I reach down and feel the relay, it very warm but not hot. I will pick up a new relay at lunchtime.

Is there anything else in the system other than the 02 sensor, the relay, the control box and the fuse?

Does anyone have a schematic of the system?

thanks in advance.

[Bob Kontak]

The O2 sensor itself has to be connected properly. There are plenty of threads on O2 problems.

From memory the connections in the control box can become corroded and the frequency valve can crap out but I think you have a more simple issue. Once the car is running warm, there are no big changes in the system like during start and warm up so it sounds like an electrical connection issue.

This trouble shooting chart (link below) shows symptoms and root cause. Note that most of the warm running low power type symptoms have causes that should be present all the time once warm. However, the Lambda system is the cause that has the highest chance of intermittent electrical connection problems - the rest are pressure, vacuum and mechanical related.

For example, if your warm running pressure was too high (lean fuel) then it would stay like that once you reach that state. Make sense?

Pelican Parts: 911 Tech Info - CIS Troubleshooing

Also check this CIS primer.

CIS Primer for the Porsche 911

[jimrs2000]

Thanks again Bob, I had been looking at the troubleshooting on Pelican but I have found the second site of great use thankyou.

I got a new relay at lunchtime so I will try it out on the way home.

Ok, thinking outside of the box I have been trying to find other things that might cause the same problem and have come up with the question " can the engine run if the fuel pump fails?" All logic tells me no but I am now having doubts.

I have headed down this route because I had the engine out earlier this year to fix a broken transmission, before removing the engine I could hear the fuel pump with just the ignition on, now I cant.......I can only hear it when I move the key to "start" Any connection do you think?

[Bob Kontak]

The fuel pump does not run when the key is in the on position. If you turn it to "on" and lift the air sensor then you get some action but not when simply turned to on.

[john walker's workshop]

the pump will run with the key on if the green plug on the sensor plate switch is off. you probably plugged it in when you had the engine out. a totally clogged fuel filter can limit performance, but my money's on the O2 system not working the frequency valve, starting with the relay.

[jimrs2000]

Pluged the new relay in and no difference, I can feel it clicking though.

Looks like an evening in the garage looking for bad connections.........not a bad thing as my 13 year old daughter has a sleep over for 7 friends tonight!!!!

now where's the beer fridge..

[jimrs2000]

Ok back from the fridge....sorry garage.

I refitted the original relay as I checked the new one and although it looks the same the terminals 85 and 86 are reversed from the original.

I have broken the connector at the 02 sensor, cleaned and remade. No change.

Two minutes later I start the car again to turn it around in the garage and its fine

I have removed the passenger seat and checked all the connections to the 02 relay and the control box, No change, I then started the car and it was normal, I shook and pulled at all the wires under the passenger seat and the car didn't show any symptoms.

I refitted the passenger seat and the car was fine.

I replaced the fuse again just for completeness. the car was fine.

I reached back and (I think) found the connector to the WUR and gave it a shake, no change apart from a burnt left wrist!

I took the car around the block and the problem cut in a couple of times just for a couple of seconds each time.

Could it be temperature related? how does the WUR fail?

Off to check out the site Bob linked to earlier in the thread, any idea's appreciated.

[john walker's workshop]

if the relay works like it should, unplugging it will cause the engine to run rough. that's the test. so find another relay. they are not all created equal. and be sure all the connectors in the plug are all the way in and clean and tight.

[jimrs2000]

Hi John,

I did unplug the relay whilst the car was at idle and it stalled, I could get it restarted but it popped and banged, refitting the relay cured the fault.

Intermitant faults are always the most difficult to troubleshoot.

[jimrs2000]

If I jumper pins 87, 87b and 30 on the 02 relay that would effectively simulate a working relay?

This could prove the relay is bad whilst I wait for a new one to arrive from our host.

[Bob Kontak]

I have little electrical background but since you have some time........

Does it matter that the 85 and 86 are reversed on your new relay? Will it make a difference in the functioning? All the boxes and connections look the same where 85 and 86 go through the relay. You would just be running the ground out of 86 rather than 85.

I am prepared for being flamed but would like to understand. Maybe you can use this relay in the interim?

[jimrs2000]

I am thinking along the same lines as you that the reversed pins will make no difference as you are just powering a coil to pull the relay in.

However, when I got to the car last night and started it the problem was not there so I switched out the old relay for the new one and the problem was immediately apparant. I held the new relay and could definately feel it clicking.........so that being the case I thought that if I jumpered across the pins, effectively simulating a good energised relay and see what happens, if the car runs good then its the relay, if not I have to look elsewhere as the relay is not the problem.

Another question sprung into my mind as I looked at the wiring diagram and that is why is there a relay in this position at all? Why not just have a switched power source from the ignition switch or appropriate fuse?

[jimrs2000]

forgot to add the line into the relay labelled III 9 is the 12v source from fuse 18, still trying to find out if this is live all the time or switched.

[Bob Kontak]

Fuse 18 feeds the clock and glove box / interior light.

You are close. FYI - I pulled my relay out last week and reinstalled it and pushed the ground wire back out of the harness. You could not tell by looking at the harness.- had to look down into the female connections to tell but you must have done that.

Check continuity from fuse 18 to the harness.

Regarding why a relay - maybe it keeps the live circuit from going downstream. I 13 taps into the fuel pump relay.

[Groesbeck Hurricane]

Jim,

Mr. Walker and Bob are heading you down the correct path. I would only ask that you take out the fuel filter and blow it out backwards with some air. Just to see if any junk is stuck in the filter. With older cars we sometimes get floaties in the tank and they can clog up the lines and cause issues. I'm always kind of stuck on suggesting this for problems that could be fuel related even if only slightly so...

[Bob Kontak]

Mainly Mr. Walker is heading you down the right path. I just say what he says after he got my car running from reading his posts.

[jimrs2000]

Ok so I think I have proof of concept....I didn't drive the car to work today as I am concerned of the effect running so lean will have on the valves.

So tonight I have jumpered across the 3 pins I noted above.

When I put the jumper into pin 30 I heard a faint whurring sound (Fuel pump?)

I started the car and it ran fine, so I loaded up the dog and off we went, my normal drive to work is around 16 miles, I think I have done about 7-8 and the car ran fine the whole time.

Does this mean that I have proved the relay is at fault? I wont really know until one of two things happen, either the new relay shows up and I do the drive to work and back without issue or the relay doesnt show up and I do the drive to work and back with no issue with the relay jumpered.

I am travelling tomorrow so it's only 8 miles to the airport but the car will sit in the heat all day and get a nice heat soaking, we shall see how it is tomorow evening on the way home.

thanks to everyone for their help so far, it's the first time I have used a forum to get this kind of advice so many thanks for making it so nice and easy.