1981 SC Will Start But Wont Idle - Setting Mixture
 

A couple weeks ago I won a 81 SC on a popular online auction site and was super excited about getting back into older 911s after being away for over a decade. It showed up quite different that what was described, however that isn't the purpose of this post.

First project is to get the WUR, mixture, and idle tuned because it is obvious that the car was setup without pressure gauges. Originally the cold pressure was .93 bar, don't remember the temp but it is California, but that isn't even on the WUR chart. I believe due to the pressure being so low that they had leaned out the mixture and the idle was almost all the way screwed in to just get the car to run.

Currently adjustments since my car was delivered to me:
- Cold pressure at 1.9 bar @ 20.7c, which is the middle upper range
- Idle Screw counter clockwise 2 1/4 turns (@ 1/4 turn at a time), which is a little high, 1400 - 1600 rpms
- Mixture Screw clockwise 3/4 turn ( @ 1/4 turn at a time)

The problem is that I can get the car to start but it won't idle. If I stay on the gas it will run however when it idles it almost immediately studders and dies. I set the idle high to make sure that it wasn't a low idle situation knowing I would tune it down later.

This is the interesting part. After I set cold pressure and didn't touch idle or mixture, I started the car with my fuel pump jumper in place. It was a total accident, got too excited, and when I realized it immediately shut the car down. However when that jumper was in place the car started and ran well, albeit a little rich by testing via lifting the place and hearing a stumble. I know the car isn't meant to run in this condition but I wanted to add this as an accidental data point.

I believe I still have a mixture problem, I assume too lean, so what is the best method to attempt to get the mixture in the ball park with a car that won't idle? I know the mixture screw is very sensitive but do I need to keep going counter clockwise? Could this be indicative of a bad fuel pump relay or other problem? Any tips or advise would be greatly appreciated.

You have to unplug the loxygen sensor to make changes in the system.

Leaving it plugged, the system will not permit adjustments.

Bruce

This is stated in a way that is confusing.

You can adjust your 3mm screw and it will impact the system. However, with the O2 system functioning, it will compensate for your adjustments to the point it is able via the frequency valve adjustment to control pressure.

There is a good thread on how to dial in your CO without an analyzer. I would kill the O2 connection for now as every variable you can remove will make things easier. You have a can of worms now.

I agree that removing variables is the name of the game. I will disconnect the O2 sensor and report back.

(The problem is that I can get the car to start but it won't idle.)

Turn the mixture screw clockwise until she keeps running.


Cheers,

Joe

Does your car have the cube relay for the sensor system under the passenger seat? My 1980 SC does. The idle was terrible. I went through the recommendations of checking fuel pressures, WUR, etc. Before I touched the mixture screw, I replaced that $10.00 relay and the car started right up and ran perfectly.

Sounds familiar.

These engines are very sensitive to false air, incorrect fuel pressures and weak spark.

My car ran like crap when I first got it. A tune-up with new plug wires, plugs, cap, rotor, etc improved the spark quality. It did not fix all the problems, but it was a piece in the puzzle.

At one point I took it to a shop to do a smoke test to look for vac leaks. Without going into details on the whole story, I am not certain they actually did it. Here's why. They told me absolutely no issues found. A few months ago I dropped the engine and replaced the intake runner boots and gaskets, fuel injectors sleeves and o-rings and all the vac hoses. When I started the car for the first time after doing all the work the idle AFR was 10! The point is that there are lots of opportunities for false air and it has a huge impact on AFR. After resetting the AFR to 13.2-13.5 (CO 2-3%), adjusting the idle, adjusting the timing and plugging the idle retard vac line, it is like a new car. Now it pulls like a freight train!

+1 A possibility but not a slam dunk.

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He already touched the mixture screw but gave up too soon and if it works, it won't cost him cent. I just hate throwing money at a problem before doing the simple things first.

I say try your recommendation in post #5.

Can't hurt a thing and may be a remedy to get it in the ball park.

If he can get it to idle, just pull the cube relay and see if it makes a difference. If it does make a difference, that variable has been put to bed.

I thought we always started with the dome light over the passengers side door as that's on the same circuit as the Lamda box relay.

So the circuit is live and therefore that guarantees the relay is working, and the twelve pin connector is connecting to the freq valve and the O2 sensor is functional?

Is that what you are saying?

A dwell meter is the best way to adjust your mix.

It is roughly equivalent to a real CO sniffer and it's cheap. I have ONLY used a dwell meter in 18 years of ownership. Like Bruce says in post #2, Lambda system will make it right even if off a bit.

OP is not there yet.

No, what I am saying is that it is my understanding that the passenger' side dome light is on the same power circuit as the Lamda box relay (under the passenger' side seat) and that is the "usual" first place to start to see if there may be power to the relay. Never said much less "guaranteed" anything about the rest of the Lamda system functioning from the relay socket onwards.

OK. First check would be see if the circuit is working.

I was wr...

What the hell. I can't finish that sentence. Must be the Pelican server.

some things here are confusing.
you say it is too lean but you keep adjusting the mixture CCW, that leans it out even more. CW riches the mixture.

the PO had the idle screw ALL the way in to make it run? that could be a sign of an air leak.

you started it with a jumper in the FP relay and it ran better? it does not hurt to run it like that. you can also remove the connector on the back of the AFM to make the FP run with the key on. does not hurt to leave it like that when working on it.

here is what i would do:
make sure fuse 18 is good. thats power to the relay for the freq valve. make sure the FV is vibrating.
check warm control pressure and system pressure.
have someone start it and while pushing UP on the sensor plate behind the air filter.
if pushing it up makes it run better, it is too lean. adjust mixture CW.
this will at least get it running.

you need to check for air leaks.
does it have a popoff valve? is it seated? is the air box blown if it does not have one.
check intake bolts. injector Orings. vacuum hoses. the hoses tot he oil tank are under vacuum, make sure they don't leak including the oil cap.

check ignition:
cap, rotor plugs and wires.
set timing
make sure the advance works

once all this is done go back and set mixture and idle.

T77911S, you caught a huge error on my part... I have turned the mixture screw 3/4 turn clockwise. Sorry for the confusion.

So I unplugged the O2 relay from under the seat and tried to start the car. It would fire then immediately die. I tried that a couple of times then tried plugging the O2 sensor in and had the same behavior.

I then put the jumper back in to run the fuel pump for 20 seconds without trying to start the car. Then I unplugged the O2 sensor and tried to start it. It fired right up and ran really good. I went about making small tweaks to the mixture and idle to bring it into spec, the idle was high and was a little rich. At a point the car died. Then I attempted to start the car and was right back at square one. Cranks and dies. No matter what I did it behaved the same, O2 plugged in or not. Given this new info any tips?

Post #5 references leaning out the mixture.

Slow down a little. You are doing many things without feedback closure. Example, you pulled the relay a couple of times then pulled the O2 sensor. OK. What happens when you plug the relay/O2 sensor back in? Same?

Bob, I appreciate the help. With the O2 sensor unplugged or plugged in I didn't notice a difference in its ability to hold idle. It still started then died immediately.

That's because it takes time to warm up, maybe 3-5 mins before it has any influence on the mixture. Therefore it has no function when starting.

Fair point but I think people get overly worked up about playing around with the mixture. If a car was starting and idling correctly (we don't know when that situation existed for this car so it may be that the PO had been messing around with the mixture) and the next day you went to start it and it wouldn't idle, I'd tend to think it's a mechanical or electrical component failure not something going out of adjustment. Besides, if it's not the cube relay, keep it as a spare.

+1

Given a better description of symptoms, O2 sensor, O2 relay or both unplugged will not cause the engine to die. If any of these are an issue, they are secondary.

Smarter guys than me are suggesting lean so give the 3m screw some CW attention.

I wanted to be clear, I have not unplugged the sensor just the relay. Should I go unplug the sensor and relay?

I have also played with the mixture screw with the relay unplugged. I went 1/2 turn CW and black smoke started the come out the tail pipe but it would start and immediately die. I then reverted it back 1/4 CCW and still black smoke, start and die, then another 1/4 CCW and the black smoke went away but it still started and died. I think I am at the top end of rich right now.

You are GenX. I am an early-ish baby boomer. Pretend I am your Dad. Just for sport.

Dad says slow the eff down.

Do you need the car to go to Best Buy to buy a gizmo? I didn't think so. Slow down.

Eff the relay and O2 sensor. You are not there yet. Your car can't even run.

Given the CW turn of the 3mm screw is not the answer you need to get really fundamental.

Explain exactly how the car ran when you first received it. Step one.

Full disclosure son (even though you are probably in your 40's):D

Below is from your first post. So the car did "kind of" run?

Mixture screw was screwed clockwise "a lot"
Idle air bypass screw was screwed almost all the way in
CCP is low even at the 2nd figure you provided (1.9 @ 20.7C) - see graph

All three of these are set to deliver a more fuel/less air than is normally required. I am guessing vacuum leak or fuel delivery problem.

Given your cold start valve is working and car fires, you can get six baby food jars and put your injector tips in them and lift the air sensor. Eyeball the flow and check for consistency. Not a perfect test but if they are close in intensity - and spray looks healthy - and fill jars evenly across time you probably do not have a fuel issue.

However, you don't mention system pressure. Something like the fuel filter could restrict flow.

Here is the graph and the red dot is where you say the CCP is. That means rich as the plunger in the fuel distributor does not have to fight as hard to allow more fuel in. (81 - 83 US use same WUR)

http://forums.pelicanparts.com/uploa...1433009594.jpg

Possible given the car runs some with the relay holes jumpered. (well it did)

What about borrowing one of the black relays in place of the red one?

Pulling off the air sensor switch plug as t77 suggested isolates that variable.

So I have more data. I agree Bob, it is time to slow down but if you continue reading I have been tweaking. The car came to me in the morning and it started as the guy pulled it off of the trailer it two started in two tries. I then went inspecting the car for a couple of hours with doors/trunk/decklid open which consequently killed the battery, the battery was connected for the entire trip on the trailer. I then put another battery in and started the car. We went for a drive but the power felt down at upper limit RPMs. Then I was working on the car and the battery died again. Since then I had an issue starting the car, the behavior described in this post.

At that point I checked the WUR system, cold and warm pressure. I got 5.1 bar for system, .93 bar cold, and 3.44 bar warm on the first test. I then went to adjusting the WUR because it was low. I ended up after adjustement yesterday with temp at 20.7c , cold 1.9 bar with system and warm 3.44 bar. Now I thought that it was perfect right in the range... before I saw the chart you posted above. I was working off this chart.

http://jimsbasementworkshop.com/CIS/..._089_specs.jpg

My chart is different from yours which really has me questioning the setup. Specifically I was leery of my warm temp which happens to be right in the butter zone of your chart and off my chart.

Now here is the strange part. Yesterday at 6pm I was doing leak down tests of the fuel system. I did it with the gauges in places, once with the valve closed and once with the valve open. Valve closed 10 min 1.65 bar, 20 min 1.55 bar. Valve open 10 min 1.34 bar, 20 min .65 bar. I am not sure if that specifically means the check valve vs the accumulator is the problem but I am having an issue with leak down.

But after the test I tried to start the car and it fired right up. It ran great, I did a little tuning with the mixture and idle, but it was running strong. I came out this morning and started it right up again after two cranks. Ran great again. This time I checked timing, which was perfect, then replaced the cap and rotor, started it again and it ran great. We then had to do some errands and I came back this afternoon with the intent of taking it for a drive... Won't start worth a damn. Same problem, crank and dies.

Now I am thinking I need to work off the chart you provided and adjust the WUR pressure in your chart. I am also going to report back tomorrow morning when I am going to try and start the car. That sound like a good plan? I am also going to need your address Bob... you got a bottle of tequila and a cigar heading your way for getting me through this.

Your graph is the graph for the Rest of World SC's. They run richer.

You can confirm by snooping around your WUR for a Bosch number that ends in 090. 089 is the RoW WUR.

Yes, you have a residual pressure issue. It is not the "big" issue but it is an issue. Today it may have been if it was hot in CA.

Cripes, don't say check valve in the accumulator around boyt911sc. It's sort of a spring diaphragm that pushes down on the fuel after you shut the car off. I just learned this a few weeks ago after Tony administered a verbal tune up. The system does not "hold" residual pressure. The system receives residual pressure from this spring pushing down on the fuel. There are check valves but not in the accumulator.

Also, all you need is one injector dripping to taper off residual pressure. I don't think it's worth worrying about right now as you go through trouble shooting. As long as you restart within a few minutes when warm, you should be able to get it started.

You have a leak downstream now that I read this. Guessing an injector. It may even be in your connections with the fuel pressure gauges.

Updates: So I adjusted my cold pressure to 2.27 bar @ 22.6c. Car started right up and idled great. My buddy and I took it around the block... then promptly took it for a walk home a block and a half away from the house. After it was home we tried starting it and it would go for a second then die. I then jumpered the fuel pump relay. Car started right up and idled great. Put the relay back in, car wouldn't start. I got a new relay, put that in, car wouldn't start. Jumpered the fuel pump relay, car started right up and idled great. I am going to do some research but it feels like it is the component that tells the fuel pump to run is going awry. Any ideas where to look?

Based on your having to bypass the relay:
Disconnect the plug directly behind the air metering plate. Also known as the fuel pump safety switch, and see if it starts and runs correctly. There may be an intermittent short in the switch or in the wiring within the engine harness that is grounding out the fuel pump when running causing it to cut out.

If the problem goes away you have found the cause.

I believe I have solved the problem. This morning the symptoms were the same start then immediately die. I jumpered the relay, works fine. Plugged the relay back in and unplugged the sensor behind the air metering plate, car would start and die. Plugged it back in and then went and unplugged the rev limiter box that is right behind the fuel gauge. Car started right up and ran awesome. Did some other unrelated projects. Started it up and went for a drive and everything was smooth. I am not sure if there is repairing this box or just replacing but it was obvious that it was cutting out the fuel at incorrect times. I am stoked that I can finally drive the car I bought.

In the end it is obvious that my wur was grossly incorrect but with that sorted out there was still a problem. Thanks to everyone that helped me get through this.

Good find!

Outstanding job. Plus you now know after a few weeks what I knew after a few years with the help of the guys here.

May have been as simple as some corrosion on the sensor plate switch.

I am going to look into that switch. It has been explained a million times, I just forget. It either creates a ground or eliminates a ground so the red relay can work. Don't understand fully how the kill-switch works.

Just keep that tequila on the shelf for if I ever get out your way. :-)

Beer = Good - only make an ass of myself
Booze/wine = Great - Prove beyond a shadow of a doubt I am a drunken idiot.