High dwell on 911 SC ‘81 lambda (US)

[wosm]

Hi, I need your help on following issue. My dwell angle stays at 88 degree when engine is warm. It doesn’t matter if I disconnect the O2 sensor, thermoswitch (45 degree C) or temp switch (15 degree C). It still stays at 88 degree dwell. The dwell only decrease to ~70 degree if I use a 1.5V battery connected to the O2 sensor wire.

What can cause this?? I already read al lot of forums and did lots of testing , but still investigating how to continue

The porsche backfires when cold/warming up having slight throtle, stumbles when cruising and also backfires a litte bit when WOT if really cold wheater. Apparently, it is running lean in closed loop if I understand correctly?

[Bob Kontak]

What voltage does the oxygen sensor deliver at idle when warm?

Given the Lambda box responds to the battery juice maybe it's missing input from O2 sensor?

Not saying it's THE problem as my 81 will run ok with O2 disconnected. Just a variable that can be crossed off the list if working.

Also, for grins, pull the "OXY" fuse and see if the running degrades. You can tell at idle. Engine will kind of chug. Blipping throttle will deliver intake backfires and sluggish response. Just a check to see if Lambda box is powered

[wosm]

Hi Bob, sorry for late reply. I drove the Porsche again today. I removed the oxygen relay after the engine was warmed up. No difference noticable and idle rpm stays at the same level. My O2 sensor is new by the way and voltage measurements are OK in its range, but disconnected as already told.

I asked some help to have a look at the ignition and its timing. I have hheard the mechanical parts are sensitive and can fail over time when the car is standing still for a long period. Is there a correlation between bad ignition(timing) and the continues 88degree dwell (cold and warm engine) I measure on the frequency valve?

[AndrewCologne]

If no sensor is attached normally the ECU switches from 65% DutyCycle to 50% Dutyccle when temperature is above 15°C.

So a test would be: disconnect the wire from the lambda sensor in the rear left engine compartment. Then lead the wire from the 15°C Switch on the right chain housing to ground. Then the DutyCycle should rise to static 65% DutyCycle or 58% dwell. If not then the ECU might be flawed.
If this works, then reconnect the Lambda sensor wire and do the test again with leading the 15°C switch wire to ground. If now this wont work and it doesnt rise to 65% duty cycle, then it could be that the sensor wiring from the sensor towards the ECU under the passenger seat is flawed/broken or touches ground.

Do these tests and report.

My O2 sensor is new by the way and voltage measurements are OK

If your ECU only provides a static duty cycle that high as you reported above, I guess you're not able to properly test your lambda sensor.
You only can test a narrowband lambda sensor if working in a stoichiometric exhaust flow where this makes the sensor output bouncing between 0.1 and 0.9v.

[mysocal911]

Originally Posted by AndrewCologne

If no sensor is attached normally the ECU switches from 65% DutyCycle to 50% Dutyccle when temperature is above 15°C.

Then lead the wire from the 15°C Switch on the right chain housing to ground. Then the DutyCycle should rise to static 65% DutyCycle or 58% dwell. If not then the ECU might be flawed.

It's simple as that! That's the pin 7 input to the ECU. Make sure it's not ground at the ECU.

[AndrewCologne]

Thats why he should do the 2 tests on the 15 °C switch which directly shows if something is ground where it shout not or if the ECU is not properly working.

Or ... he runs a wrong ECU i.E. built for a '80 model where the cold Dutycycle is significant higher.

[wosm]

Gents, thanks for your thoughts on this issue. I already did all this tests to exclude these diffrent rootcauses. I checked the colors of the wires and its pins (12 pin connector), checked conductivity on all lambda wires, removing cable from 15degree temp sensor and also ground it, disconnect thermotime switch, bridge it, etc. etc. The only way to get the needle moved on the dwell meter (from 88 to ~70 degrees) is by using a 1.5V battery on the O2 sensor. Also turning the air and fuel mixture screw doesn’t change the dwell reading. The ECU box is matching on my 1981 SC.

Car run great and drove ~5000 miles after full revision , but was parked in my garage during wintertime. Suddenly after a few months (end of winter) this issue popped up when I wanted to drive it again

[AndrewCologne]

These ECUs normally last forever.
But maybe a high voltage spike went through it due to a non properly working alternator Regulator.

Do you have a chance to get an ECU from someone else for testing?

[wosm]

I need to make some calls to find a spare ECU to check. It was also on my actionlist, but it rarely seem to fail.

What I can remember is that I cranked the car with cable removed from ignition coil that goes to distributor cap. I grounded the loose end of this cable. I did this because my car was standing still in my garage during winter and getting oil around my engine. However, when I took the car out again from the garage the issue was not present. I drove it for one hour without any problems. The second drive a few months later the issue of backfire started for the first time!

I also discovered my heater blower is not working / rotating anymore. I am curious if I accidentally did something wrong. Can or could a voltage peak reach this blower and/or my lambda ECU?? I can’t find any schematics showing the connections in between (if exist).

[AndrewCologne]

Can or could a voltage peak reach this blower and/or my lambda ECU??

Unfortunately it "could" be. Depends on the Voltage spike or what ever blasts the ECU.
An ECU with disconnected Sensor and a grounded wire from the 15°C switch should always output a 65% duty cycle signal. Thats what makes me wondering in your case. But ... first do test with a spare ECU before beeing concerned regarding the ECU.

[mysocal911]

Originally Posted by wosm

Gents, thanks for your thoughts on this issue. I already did all this tests to exclude these diffrent rootcauses.

Have you checked that none of the enrichment inputs (some pins from pin 9-14) haven't been grounded.
Also, use a voltmeter to check that pin2 (O2) reads ~.50 volts w/o O2 sensor. Hopefully, no water damage to the ECU.

[Flat6pac]

Defective relay at the lambda box will mimic a bad pressure warmup regulator.
Bruce

[Funracer]

wosm were you ever able to get your issue resolved? I am having same issue cannot adjust FV on 83 lambda
Thanks

[AndrewCologne]

Are you having "exactly" the same issue? What are your specific symptoms?

[Funracer]

Originally Posted by AndrewCologne

If no sensor is attached normally the ECU switches from 65% DutyCycle to 50% Dutyccle when temperature is above 15°C.

So a test would be: disconnect the wire from the lambda sensor in the rear left engine compartment. Then lead the wire from the 15°C Switch on the right chain housing to ground. Then the DutyCycle should rise to static 65% DutyCycle or 58% dwell. If not then the ECU might be flawed.
If this works, then reconnect the Lambda sensor wire and do the test again with leading the 15°C switch wire to ground. If now this wont work and it doesnt rise to 65% duty cycle, then it could be that the sensor wiring from the sensor towards the ECU under the passenger seat is flawed/broken or touches ground.

I do not wish to highjack wosm thread so if I am doing so incorrectly let me know and I will start a different one.

Took me awhile to get back to your post. My FV doing very similar things. I tested my 15C switch a few weeks ago and thought it was good but I followed your test suggestions for wosm and got the following indications on warm engine idling at 1000 RPM:

If the O2 and 15C switch are connected as normal, my FV is 80%. If I then disconnect O2 nothing changes at all. If I then ground the 15C switch the FV goes to 65%. If I plug the O2 sensor back in with 15C grounded, FV stays at 65%.

In other words, disconnecting the O2 changes nothing.
Grounding the 15C switch will always drop the FV to 65%, no matter if O2 connected or not.

Conclusions: From what I have read the FV is supposed to go to 65% when O2 is disconnected. This is not happening on my car unless the 15C switch is grounded.

Not sure where to go from here

[mysocal911]

Again:

Originally Posted by mysocal911

Have you checked that none of the enrichment inputs (some pins from pin 9-14) haven't been grounded.
Also, use a voltmeter to check that pin2 (O2) reads ~.50 volts w/o O2 sensor. Hopefully, no water damage to the ECU.

It's that simple!!!!!

[Funracer]

Originally Posted by mysocal911

Again:



It's that simple!!!!!

Lots of things are simple once you know how to do it.

I guess you are saying one of the inputs to the 14 pin connector is shorted to ground.
I will watch some Youtube videos and try to figure out how to check for grounded inputs.

Thanks

[mysocal911]

Originally Posted by Funracer

Lots of things are simple once you know how to do it.

I guess you are saying one of the inputs to the 14 pin connector is shorted to ground.
I will watch some Youtube videos and try to figure out how to check for grounded inputs.

Thanks

You can just look at the connector and see what wires go to pins 9-14. Typically there's just two.
Just count from pins 4 & 5 which are the main ECU grounds.

[Funracer]

Originally Posted by mysocal911

Have you checked that none of the enrichment inputs (some pins from pin 9-14) haven't been grounded.
Also, use a voltmeter to check that pin2 (O2) reads ~.50 volts w/o O2 sensor. Hopefully, no water damage to the ECU.

This pin2?




This O2 pin reads .5 VDC

[Bob Kontak]

Originally Posted by Funracer

This pin2?

Pretty sure the tests mysocal911 is referring to are done on the Lambda ECU connection under the passenger seat.