Occasional CIS problem
 

Car is an 81 911SC, Stock motor except for SSI's and MSD ignition.

I drove it to work this morning, about 25 minutes. After work, about 20 minutes into the drive it started running poorly. I have an AEM air/fuel gauge, as I passed 2000 RPMs the gauge would go to 16 and onto the peg. Upshift and it was better. Idle and low RPM's it actually ran pretty well and the mixture was around 14.7.

I reached my destination after 5 minutes or so of this and parked for at least a half hour.
When I restarted it immediately had the same problem, for about 5 minutes. At some point the problem went away (I was nursing the car on back roads so it was subtle).
Once I realized it was running properly again I tried to make it misbehave - large throttle openings at low RPM, running up the revs - it was mostly perfect "seat of the pants". Occasionally the AFG needle was bouncing around.

This car did this at least twice last fall. I unplugged the O2 sensor and it didn;t do it again until today.

Any ideas? I can't imagine what part of CIS could fail lean, then come back.

So you have SSIs and the CIS O2 sensor? Where is the O2 sensor bung located?

Obviously you’re way lean if your AFR is 16. The car is trying to tell you something. It is leaning out. It probably got to the point where the O2 can’t correct it. You disconnected the O2 so it went to fixed 50% duty cycle. Bandaid. It will get worse and next you’ll have a lean backfire.

First thought is you have unmetered air getting in somewhere. If you let the car warm up and idle in the driveway, take the oil filler cap off. If the rpms don’t change you have a significant vac leak. You’ll need to track it down. Smoke test or carb cleaner/unlit propane around the usual suspects method.

O2 bung is just after the collector on the left manifold. There is a significant RPM drop when the oil cap is removed.

I have done a ton of searching for unmetered air - Propane test and smoke test. I found and fixed some fairly minor leaks last year, replaced the warm up regulator with a rebuild one by Tony and replaced the injectors. All pressures are in spec.

No question - it's trying to tell me something. I thought the O2 controller under the seat was bad.

If it happened while I had the car home I think I could find it. by the time I got home last night it was running sweet.

What could allow unmetered air that gets bad, then gets better? And if it's unmetered air, why is it worse at higher RPM's and load?

I'm leaning toward fuel delivery but I can't imagine what would come and go

Thanks

I'm not an expert on CIS, but I believe at higher RPMs your vacuum is greater, so if there is a small leak, you'll get more leakage and things will get worse.
Also, there are a bunch of components that change size/behavior at different temperatures. So those could explain why the system changes behavior over time.

My experience with vacuum leaks tends to be that they are the most obvious at idle (when vacuum is high and air flow is low) and smooth out at higher RPMs.

Tirwin's comment has me wondering if it could be the frequency valve itself. I'm not coming up with anything else electrical that could affect mixture at higher RPMs.

I had something similar (ish) with my 80 SC five years ago now.

Popped and sputtered at around 3k, then went away after a few seconds. Got to work and left it, came back after work and thing would barely start. Turned out to be a bad lambda ECU - one of the solder joints was completely burnt.

Did you check whether you get a buzz/vibration from your frequency valve with the car off, key in the on position, and the plug at the AFM unplugged?

Could be O2 relay as well...

Lots of diagnosis threads for all of the components above in the Pelican archives.

It's been a while since I fooled with troubleshooting the Lamba system - I will get after that this weekend.

It just seems really weird that the problem comes and goes.

How old is the O2 sensor? Is it possibly time for a new one?

Just to make sure I understand correctly... do you have the stock narrowband AND a wideband O2 for the AFR gauge? Or do you have a wideband with a narrowband output to the CIS Lambda unit? My experience is that the K-Jet box doesn't respond well to a non-stock narrowband output. But disconnecting would eliminate that as a potential problem.

You can check the duty cycle of the FV with an oscilloscope or some meters. With the meter approach, you have to test how the meter to see how it represents the duty cycle. 40% ON and 60% OFF are the same thing -- you need to know which is which though.

If you have poor/weak spark, you would have unburned fuel in the exhaust which would read rich.

Lean condition would suggest poor fuel delivery.

Had another thought.

A quick check would be to test the O2 sensor relay under the passenger seat. Not a bad idea to have a spare in the glove box anyway. You can test the relay to see if it trips. That is what controls the FV duty cycle. Maybe the old relay is failing intermittently.

^ Good call. Get the low hanging fruit first. Interior lights turn on?

The O2 sensor was brand new when I installed the AEM - so a couple thousand miles. AEM has an output to emulate the old single wire O2 sensor. It wouldn't be much trouble to swap back to the original sensor, but then I wouldn't see the mixture on the AEM.

I always used a dwell meter to read duty cycle.

When it's misbehaving it always goes lean. I'm pretty confident that everything on the ignition side is solid.

I had forgotten the o2 sensor relay - I do have a spare in the glove box.

I finally got around to looking at this (busy weekend).
Dead silent when I turn the key on without starting.

I hooked the o2 sensor back up and took it for a 1/2 hour drive - it ran beautifully. Steady state mixtures around 14.3-14.8. excursions during accelration/ decel.

After I got it home I checked frequency. About 43% at idle, 50% with the o2 disconnected.

Well I'm back - but with the opposite problem.

Now the car tends to go full rich - like 10.0 - as rich as the AEM will indicate. When it first happened it came and went. I finally had it happen close to home and started troubleshooting. All indications are that the frequency valve is operating properly (I don't have an oscilloscope). I cleaned the throttle and distributor plates and thought it was fixed - but it came back after a couple days. Now it's not so rich (Like 11.1 -11.5 at idle) but very sluggish and backfiring. once it gets over about 2200 RPMs it comes on like gangbusters and the AFR comes up.

I checked ignition timing and it's dead on, smooth advance, no sign of spark misfire on any cylinders.

Could I be having trouble with the fuel distributor? Possibly try injector cleaner? I don't have a good source of ethanol free gas, but it almost always gets Shell premium.

Thanks guys

cis problem
 

I had a similar problem with my 1982 SC a few months back where it would intermittently go full rich to around 10.0 AFR. After doing some testing it turned out to be a problem with the WUR allowing the warm control pressure to drop drastically resulting in such a rich mixture that the O2 sensor system could not compensate.
I had the WUR rebuilt and the problem was solved.
I will also mention that at one point I had a problem where I was not seeing any enrichment on my AEM air fuel ratio gauge when the throttle went past the 1/3 enrichment switch which directs the system to go to "open loop", I have the mixture set so it achieves 12.7 -13.1 at full throttle depending upon ambient air temperature. It was somewhat difficult to determine the cause due to the fact that the 1/3 switch checked out OK etc. It turned out that the WUR "push valve" return line in the fuel distributor was clogged causing the control pressure to be so high that the system was unable to achieve the mixture enrichment as it should when in "open loop" past 1/3 throttle. I forced some air through the WUR return line to the Fuel distributor which cleared the blockage.
Just as info, I have an addition bung installed in my cat bypass for the for the AEM wide band O2 sensor. I have found that being able to monitor the air fuel ratio has been invaluable in seeing and solving any cis issues.

Dave

I will check the WUR tonight. It was rebuilt by Tony about 2 years ago so I have been assuming it's good.

Thanks!

Drop the WUR to me........
 

Michael,

I will be home the next three (3) weeks finishing my WUR commitments before leaving for Europe late July to early August. The earlier I get your WUR the better and you could have it back after 24 hours. Or wait after my return. Thanks.

Tony

HI Tony,

Thanks for that. I will check it out tonight and if it has any issues I will get it down to you.

Mike

It will be WURth it!

About time to correct this terminology.........
 

Tim,

Why do people still call this relay as OXS relay? This relay controls the operation of the FV (frequency valve) and should be referred as FV relay. This is like calling the FP relay as Control fuel pressure relay or the like. Just my two-cents.

Tony