Surging after O2 sensor reconnected

grahamkissack · 06-08-2010, 08:57 PM

For the first time ever since I've owned the car from 2004, I got around to repairing the O2 sensor connection and plugging the sensor in. I notice the idle is perhaps steadier but the car seems to surge and lag at low speeds (30 mph) and engine rpms at 3000. Any thoughts....perhaps change oxygen sensor? Not so noticeable at highway speed.

Thanks

DRACO A5OG · DRACO A5OG's Garage

do you have a pic of the repair? it is possible the connection mat be bad.

does the idle change if you disconnect it?

psalt · 06-09-2010, 03:26 AM

It is normal for an 80SC to have an oscillating idle. My advice would be to hook up a dwell meter to the test port and let the lambda system tell you what is actually happening. Much better than internet guessing.

Wavey · 04:49 AM

Deleted - I didn't catch it was a CIS car. Duh.

psalt · 06-09-2010, 04:42 AM

Mixture adjustment on an 80SC is done with a CO or a dwell meter. It cannot be done with a voltmeter.

Scott R · Scott R's Garage

On your 80' when you plugged in the o2 sensor the operating mixture range was outside of the expected, and the lambda system is going up and down on the frequency valve to try and correct it.

People tend to adjust the mixture often times to solve other issues, like false air, and the result is a mixture outside of the expected .08 ~ 1.2 limits of the system. So the answer is to rich it up and unplug the o2 sensor.

The other question is why doesn't the system perform as it should? Well then now we need start with control pressures, air leak checks, and injector checks for normal operation.

When I went back to CIS two weekends ago I sealed everything up. New gaskets, boots, vacuum lines, etc. I'm now within the expected CO ranges with the sensor plugged in. I have plenty of power, a nice steady idle and good running temperatures. So it can be done.

Joe Bob · Joe Bob's Garage

Could be a dead sensor.

mca · 06-09-2010, 07:11 AM

Exactly why I unplugged mine years ago. I have a new connection bung and sensor sitting on the shelf though. Let us know what you discover. Considering hooking mine up again in the fall.

fumanchu · 06-09-2010, 07:59 AM

psalt

where is the test port? In the Bosch book it says to tap into the frequency valve wire. I always hesitsted to do that as it's a contortionist act back there + all the platic sheathing would need to be pulled back.

grahamkissack · 06-09-2010, 08:12 AM

Thanks for the input everyone....I plugged the sensor in again because my gas mileage is way down and I was hoping to improve it. I thoroughly cleaned the connections so I'm sure that's not the issue.

The car certainly idles cold better now than it used to but it never surged / lagged like its doing now at low speeds. I have another O2 sensor (used but in good shape) I'll try next.

psalt · 06-09-2010, 09:06 AM

fumanchu,

Remove the wing bolts and plastic cover on the left side of the engine compartment. Look for a loose wire harness with a hinged rubber cap. Pop the cap and you will find three terminals, the green/white wire terminal connects to the coil connection of an analog dwell meter. You must ensure that the connection does no touch the other terminals. CIS lambda will talk to you, not quite OBDII, but it will tell you whether the system is working and give you an accurate proxy to set the mixture. Reading the dwell meter can be a little confusing at first. Cold open loop reading, higher is richer. Closed loop readings, lower is richer. In closed loop, you are reading the correction factor. The duty cycle fluctuations are controled by the ECU pulse timer and average around 10 numbers whether you are rich or lean. You need to remember that when you make an adjustment with the mixture screw, you are only really effecting cold open loop and +35% throttle, because the system is capable of correcting the mixture in closed loop. You can richen up you cold start/running setting and your WOT, and still have stoich for cruise mpg. Try it, much better than blind part swapping or barking "It's the WUR" with the Koolaide drinkers.

fumanchu · 06-09-2010, 10:08 AM

Psalt
Am I setting the dwell to 4 cyl setting? also, can I use a digital dwell or do I need to go analog. I'm guessing does not matter.

How long before the car goes to closed loop when dead cold?

Am I looking for the spec # in the bosch book?... or do you have a diiferent and better # I should be looking for?

sorry off topic but I'm guessing these question can help the post.

Thanks again.

psalt · 06-09-2010, 11:23 AM

fumanchu,

My advice would be to get a manual and try to understand how the system works. Bosch is best, Probst book is good and very practical, Bentley OK, but has some key errors. Unfortunately there is no complete aftermarket description of the Porsche specific changes. Your guess is wrong, you cannot use a digital dwell meter. You can use any scale you like, you have to convert dwell to duty cycle, the 4 cyl scale is 90%dwell=100% duty cycle. The engine should go closed loop after two or three minutes depending on temperature. Open loop default for an 82 is 65% duty cycle. The factory closed loop setting is 45-65% duty cycle, I've found these cars run best set at 30-40% duty cycle. They also have much better cold start and warm up if you disconnect and plug the vacuum retard and reset the idle speed.

1982911SCTarga · 06-09-2010, 11:38 AM

Quote:

Try it, much better than blind part swapping or barking "It's the WUR" with the Koolaide drinkers.

Too funny.

Brian

vash · vash's Garage

Quote:

Originally Posted by Scott R

On your 80' when you plugged in the o2 sensor the operating mixture range was outside of the expected, and the lambda system is going up and down on the frequency valve to try and correct it.

.

i think this is what is happening. mine was doing this mildly. i paid a pcar mech $20 to adjust my timing, mixture and RPM..(one affects the other), he nailed it, car passed smog, car idles like a smooth clock.