CIS troubles
 

Rebuilt my CIS in 2017 and its been running like a top ever since. It's sitting on my full engine rebuild from the same year. It all has over 30,000 miles on it, runs great.

Until a few days ago. Filled up the tank and about 30 minutes into the drive I notice when I let off throttle (still in gear) I hear popping and and what sounds like small backfires. I stop the car and restart and it's idling very rough and slow. So I adjust the idle screw a little. It helps a little. Car is parked for a couple hours, take it out and it runs fine. Then next time I drive it idles slow again and does the popping and rough running. Once I get to 3000 rpm though it wakes up and runs pretty good. But then back at idle its rough and popping. It's random which is the weird thing. I put an AFR meter up the tail pipe and got 22.9. But it smelled really gassy, like it was rich, so that's another mystery. Seems like a fuel delivery problem. I have not put my meters on the fuel system yet, was hoping someone had an idea before I start running the numbers. I got the system setup using the Dummies thread when I rebuilt it, so everything was tested then and running fine. I also am running an adjustable WUR. And my green wire was new when I rebuilt the CIS. Idle also drops when I remove the oil fill cap, so vacuum seems good.

Here's a video of the idle. Notice the the popping at the end. I don't have a pop-off valve installed and my air box was brand new when I rebuilt it (with the metal spider inside going to the runners) https://youtu.be/bNC_QZU6dKk

Do you have a multimeter that is capable of reading duty cycle? If so, red to the green/white wire on the 3-pin test port and black to ground. That would give you an idea if the ECU thinks the engine is lean or rich and which way it’s trying to compensate.

Then disconnect the O2 sensor and see if it stabilizes.

You’ll need to verify warm control pressure but you knew that.

Check the plug wires and the coil wire are snug. Pop the dizzy cap and have a look.

I guess your engine suddenly leans out, thats where the poppig and backfires comes from.
Check your dome light, if not ok, check its fuse in the front cause the mixture control is connected to that fuse as well.

And check the OXS relay below your passenger seat - in many cases thats the guilty one.
(see link in my sig)

I’m with Andrew on the OXS relay.

There should be 3 states.

Cold start:
O2 sensor input is ignored.
FV duty cycle is fixed at 65% until engine is warm.
WUR is in cold control pressure state. CCP is lower than WCP and AAR is open to allow extra fuel and air to enrich the mixture.

Warm idle:
O2 sensor is considered by ECU. If O2 reads lean, ECU increases FV duty cycle to compensate. If O2 reads rich, ECU decreases FV duty cycle. If O2 sensor is disconnected the duty cycle is fixed at 50%.
WUR is in WCP state.
AAR is closed.

Warm, TPS switch tripped:
O2 sensor input is ignored, duty cycle is fixed.
WUR in WCP state.
AAR is closed.

My interpretation of your symptoms is cold start and “above 3k rpm” is fine because duty cycle is fixed.

The question is what’s going on in warm idle state. Some people say that inserting the AFR in the tailpipe isn’t a reliable reading. Let’s say your 22 AFR is right or at least directionally correct. Exhaust is really lean. In that case, the FV duty cycle should be near 100% to compensate.

Try swapping the OXS relay. If no joy, try disconnecting the O2 sensor to fix the duty cycle. Then measure the AFR again. If it’s still lean then that needs further investigation.

Also need to determine if the issue is really random or not. Check that the fuse to the interior light circuit/ECU isn’t loose causing intermittent power loss.

The Duty cycle is the result an interval of "pass through to ground" via a mosfet inside of the ECU and the ECU (and btw the frequency valve itself) needs power via ... the OXS relay. So ... if the the relay is flawed, at the end the frequency valve permanently gets no current – no matter if cold start or whatever rpm ... so the engine permanently leans out. :-)

It's a bipolar transistor.

You may have an intermittent Lambda ECU (under passenger seat). Just unplug it when then engine is running good.
If it runs poorly like you have been experiencing, then the Lambda ECU is bad or its power source is intermittent.

Can be very problematic!

I was quite shure that’s a mosfet due to the huge current at 12-14v as the FV comes with 2 ohms.


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That's only 6-7 amps. For most bipolar darlington transistors, it's an easy design to implement.

You are right. Just checked a newer ECU of the same generation used for a GOLF or similair.
Here the duty cycle signal is passed by a BDX53A transistor which also handles a current of 8a, but here (independant of that) a 10 ohm resitor is installed after the FW incomming connection. Will have a look at the orig '82 SC ECU – where I know that there was no resistor with that value installed in front.


Yes, thats exactly what happend to me a very long time ago as some of the connections in the front fuse box where poor. So no fully 12v reached the ECU which resulted in such droputs. Maybe he has luck and its the guilty one.

Im with Andrew and Tim on posts 3 & 4 above. Mine did the same thing and was behaving so randomly that it sent me on the typical route checking vacuum leaks, checking fuel pressures, electrical troubleshooting only to find the damn OXS relay wire at the fuse box (and the fuse) were causing intermittent contact. Easy thing to check at first.

I was speaking to symptoms. Right now we don't have a lot of empirical data. Just a bunch of symptoms. So I was suggesting he do some things so we know instead of making educated guesses.

My car did the same with popping and slight back fire. I changed the octane from 93 to 91 and this caused my car to run lean. I went back to Shell gas 93 octane and no issues.

Is the FV working ?

I have been so busy today I didn't get a chance to look into this further. Hoping to get to it tomorrow so I can report back. Great info, you guys rock! I'll do these tests and see where it goes.

Looks like I got this fixed. Ordered up a new Bosch relay after testing my original one (it failed a continuity test) Plugged relay in just now, fired it up, perfect idle, afr 14.9. Revved the engine, perfect throttle response. Will take it out for a spin in a bit to test further.

Excellent. Glad it was an easy fix!

Thanks everyone, and especially Tim and Andrew. Just got back from a test drive and the 83 is running like a champ. Thanks for narrowing this down so quickly guys. I bought two relays, will keep one in the glovebox from now on.

Excellent Pelican result.

John

Perfect Jameel, nice to here you fixed it [emoji3]


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