Cis issue on rebuilt engine

[gomezoneill]

I just rebuilt my 82 sc engine and trans. Car runs great from cold to warm then engine will hunt then die from time to time and the fuel mixture seems to change back and forth.Warm and cold pressures are spot on system pressure spot on. No O2 sensor plx wide band set to 13.0 at idle. It's seems as though the frequency valve is controlling the mixture even though the o2 isn't hooked up. The one thing that doesn't seem right is the idle air screw is almost all the way in. When I remove the oil cap the engine idles down as it should. Any help would be appreciated.

[gomezoneill]

Okay I found that when I twist/grip or otherwise manipulate the harness plug at the rear of the engine bulkhead (lambda harness) the problem seems to change, the frequency valve oscillation seems to increase and decrease. Okay what can you do the clean the connection there at the harness connection re solider the terminals? What else would make the frequency valve oscillate with the o2 disconnected if this is not the issue?

[Walt Fricke]

Assuming all else is hooked up, the frequency valve will oscillate without the O2 sensor input. In that case the freq valve is controlled by the throttle position switch. It has three positions, so to speak: idle, part throttle, and full throttle. If I remember correctly, the valve runs at a richer frequency (don't worry which way is which) at idle, a neutral frequency at part throttle, and back to richer at full throttle (or close to it).

What does not happen is any part throttle adjustment to keep the typical driving mixture at its best compromise between economy and emissions. For that you need the O2 sensor working. When the sensor says too rich, the frequency valve goes to lean. When, soon enough, the sensor says too lean, the valve goes to rich. Back and forth, with the average being where it should be. But you won't notice this directly in driving or engine response, just in measures like MPG when traveling, or at your friendly local emissions testing station perhaps, depending on how they test.

Hunting is apt to be due to not setting the little 3mm Allen screw adjuster quite right. Though air leaks maybe could do that also.

I thought that removing the oil tank cap, which lets some extra unmetered air get to the throttle body and thus lean the mixture, usually speeds up the idle. But I may have that backward.

With the idle adjuster screwed almost all the way in, perhaps you have an air leak? Not a big one, and the adjuster compensates?

Bosh gives a nifty tip on setting the metering screw to an initial value: Remove one of the fuel lines at the fuel distributor. With the engine not running, run the fuel pump. Adjust until fuel just starts coming out of the open line/port. Back off a half turn. Use that as a starting point for adjusting the screw with the engine running.

One method (other than with exhaust gas measurement instruments)for the final setting of this mixture screw involves getting it to where, if you pull down on the air measuring plate arm just a bit, the idle - otherwise smooth, stumbles. And, when you push up on it a similar little bit, it also stumbles. This is often said to be the just right position - at least if everything else is al fine.

you could try electrical contact cleaner and shop air on the multi-pin harness plug for the lambda harness, though just connecting and disconnecting the plug several times ought to clean it up enough.

[bpu699]

Wow... how odd to see someone with the same problem I have .

Love the PLX by the way. If you are using it, you do in fact have a working O2 sensor. The PLX O2 sensor reads the AFR, and then passes this info onto the car to process. This prevents the need for 2 O2 sensors like all the other AFR meters need. Huge plus in my book.

I was having wacky AFR readings. Car would start cold at AFR 13... Then, sometimes, go stupid rich with AFR less than 10... and stall. Traced that back to a misfunctioning relay that was intermittently shutting of 1 of the 2 930 fuel pumps... Only found this after replacing the ignition, cleaning the WUR, fuel pump, etc, etc, etc...

Then, a year later, started to have the car warm up at an AFR of 13. And after a couple minutes it would go to super lean... This was intermitent... Disconnecting the O2 sensor from the harness fixed it. I think. As a relay seemed bad too. Reconnecting, made the problem occur. I checked fuel pressures, etc. It looks like the issues is the O2 connector to wiring harness (the 3 pin round connector in the engine bay. At least I think I have convinced myself that it is, as jiggling it seems to make a difference.

I cleaned that connector, and all is well again... for now....

The PLX setup only uses 1 wire to connect to the O2 sensor. The two heating element wires aren't active. So its just 1 connection out of the 3 that carries electricity...

The part that makes diagnosing everything difficult is that issues always seem intermittent... It drives me mad...

Hope that help. Replace the relays, clean the conectors (especially at the relay!).

Bo

[javadog]

You need to put the correct O2 sensor in, ditch the PLX crap and set the CO using a gas analyzer, then see where you are.

With a warm engine and no sensor input, the duty cycle reverts to 50% until the throttle opens 35 degrees, at which point it goes to 65%. Normally, the sensor would control the mixture when the engine is warm and the throttle is under 35%.

With a cold engine, or a partly warm engine, it does other things. If you want to learn the details, grab a copy of the '81 technical training booklet.

JR

[gomezoneill]

I use the PLX just to measure the mixture with the sensor in the exchanger I do not have it installed in the loop. When done I'll hook up a new single wire and set the duty cycle. I found the problem was ignition related to a loose connector near the CDI causing intermittent connection compounded with the corrosion on the Lambda harness plug at the fire wall that caused the frequency valve to shut off and turn on. Tightened and cleaned and all is fine. The engine and transmission were out of the car for three years while I completed the rebuilds and I guess the connectors got a little moisture on them and lost conductivity. The engine runs great now thanks for your input guy's.

[javadog]

Your car will run better if you run the original type of O2 sensor, especially if you have to modify a plug on single wire sensor to make it fit.

JR