'81 SC rough idle issues (1st timer needs help)

[NEW911SC2]

Hello all,
I’m having my first start up CIS problem w/ my ’81 911SC (approx 150K). When I initially bought the car back in Nov., I had no problems whatsoever. Every time I would use my car, it would fire up with no problems at all. No cold or hot start problems at all. It would even fire up effortlessly sitting days at a time. Yesterday I had my first bad start (car sat for 3 days). When I started the car, it fired up and idled at about 2K. Then the idle drop below 1K and then proceeded to bog like if it was being starved of fuel. I sat and waited. It eventually evened out and then idled at 1K. I let it warm up and then tried again. It fired up fine. I tried later in the evening and had the same issues.
But one thing that I did notice is that something was missing. Normally, when I turn the key, prior to starting the engine, I usually hear a something that sounds like a click and then a hum (like if something is charging) and then the fuel flowing. This time I did not hear that sound. Once I got the key to that position I hear nothing but I can faintly hear the fuel flowing. I checked the Bentley manual and it suggests that I check the fuel pump fuse and the fuel pump relay.
I’ve done a search on the site, but I’m still trying to figure it out. Many of the things that I came across do not mention the relay or the fuse. Has anyone had a similar situation that was a relay issue? Thanks for any info.

[Hladun]

The fuel pump should run only when 1) the engine is cranking or 2) the engine is running (CIS air meter contact open). The "fuel pump relay" is energized only when 1) the engine is cranking or 2) the air contact closes with the ignition on (safety feature in case of accident). If your fuel pump is running with ignition on, non-running engine then someone has shorted the fuel pump realy contacts or something else is wrong with the circuit.

The CIS is ambient temperature sensitive if it's not set right. Was this youir problem?

[mede8er]

Fuel pressure is essential to proper CIS operation...I'd check that first.....Bently gives you instructions.....

I hear my fuel pump distincly when I turn my key...if I don't I pull/reseat my FP relay and check my fuses.....

[Superman]

Air/fuel ratios are challenging to get perfect in all weather conditions. When new, these cars achieved that. But now, our cars are older, and those components are not working as new. Also, Warm Up Regulators sometimes deliver the correct control pressures, and sometimes they do not. Some guys adjust them, or disassemble and then modify them to make them adjustable. Auxilliary Air Regulators work with the cold start enrichment (WUR's) to give the car some "choke" during cold running.

First thing to do is to make darned sure the ignition system is working PERFECTLY. This should be done periodically anyway, and is the only correct first step in troubleshooting.

That being done, troubleshooting CIS systems is a step-by-step process that basically begins with checking the fuel pressures. The system pressure. And the "control" pressure duing cold running, and during warm running. You will need a special tool for that. Troubleshooting CIS systems without following the step-by-step procedures is ferociously expensive. The procedures can be found in any of several CIS troubleshooting system books.

Finally, my car runs poorly for the first several seconds of COLD running. I know I have a fuel pump that should be replaced. And probably my fuel accumulator as well, but that's a warm start problem.

[NEW911SC2]

After reading earlier responses and after doing some more research om my problem, I would up changing the fuel pump relay, the fuel pump fuse and the dome light relay (to power the FV relay(O2 sensor rela)).
I turned the key to power everything on (no audible humming/buzzing sound like before). Then I started her up. 1st time it fired and died. I turn the key off and then on waited a few seconds and started her up again. 2nd time fired up then dies. Did the same thing again and on the 3rd time she fored up, stayed on, and idled smooth.
I let her idle and warm up for about 5-7mins. There was no dipping of the idle. I did notice now a ticking. Anyway, I turned the car off. Waited a few minutes and then I tried a again. Fired up again and stayed on. There was a slight rough idle but the RPM stayed constant at 1K. The ticking was still there. I accelerated the car and the ticking stoped. I took the car for a quick spin around my complex w/ no problems. The only thing was the ticking. On acceleration there was nothing and at idle I cound hear it. I look under the passenger seat and I grabed the O2 relay and that's whats ticking. Is this relay going out? Also I noticed a thin black wire just laying there coming out of the back of the computer? Should this be grounded anywhere? Thanks for any info.

[NEW911SC2]

Anyone have any ideas on that loose wire I mentioned to the computer? Sould I just ground it? How about the ticking/cliking O2 sensor relay that only does imakes noise at idle. Under load/acceleration, the ticking/clicking goes away. Bad relay maybe? Thanks again?

[john walker's workshop]

that's an important relay. it allows the engine to run properly by activating the frequency valve system. unplug it and see if the idle drops down and runs rough like it did before. it's not supposed to tick, so get a new one. i don't know about the wire without seeing it, but if the engine runs fine, why mess with it. it's not from the relay plug, eh?

[NEW911SC2]

Hey John, that's what I was wondering about that wire. I'm going to try to post a photo and I'll send you one to and maybe figure out what its for. It has an exposed end so I'm going to tape up the end with electrical tape just to be on the safe side. Also I'm going to replace the ralay as soon as possible. Thanks.

[mede8er]

The wire is probably for the emergency brake or seatbelt warning lights.....