mixture needs adjustment every few months

[code7rpd]

I set my mixture a few months ago on my 81 SC and the car has been running well. I set it to 3.5% CO as suggested by members on the forum. Today, while driving home I lost power and was getting some back fire from the intake after the car had been on the road for about 30 minutes. I got home and checked the mixture to find it was .4 % CO. My question is, why would it change so much out of the blue so soon after I adjusted last?

[john walker's workshop]

a common problem would be no power to the oxygen sensor relay or a bad relay. it gets power from the domelight fuse. check that the domelights work first. the center terminal on the relay needs to be hot after the relay is activated in order for it to power the computer that works the frequency valve. check with a test light at the back of the plug. this system has to function, even if you unplugged the O2 sensor for smoother running. the relay and computer is under the right seat. the relay is in front near the tunnel. unplug it from the plug and see if the idle drops way off. it should. touch the relay several times to the plug and see if it clicks. it can click and still not power the center terminal. you can jump power to the center terminal from one of the other 12V sources in the plug for test purposes.

[code7rpd]

Thanks John - I'll take a look.

[code7rpd]

John,

The dome lights work and the fuse is in good condition. I pulled the relay and the idle did not drop nor does it click when touched. I checked for power on the center plug with the realy in and there is no power. I ran a jumper wire to the center terminal and there was no change in the idle. The car is bucking quite a bit under load also. It put my hand on the frquency vallve to see if it was buzzing and I did not feel anything. What else can I check to narrow down the cause?

[code7rpd]

I also checked for power at the frequency valve and found none. When I jumped power to the center plug if the relay I was getting power to the frequncy valve but I still could not feel the valve buzzing.

[74threeliter]

The same thing happened to me +/-several months ago and after taking my 82' 3.0 CIS to my mechanic and successfully getting my idle back they told me "recharging the frequency valve" was the fix. But guess what happened this morning... yep again (bad idle car shaking back firing three liter) so i dropped it off at TRE Motorsports to see what else could be making this issue arise again in such a short time, stay tuned... keepn fingers crossed



~Alberto

-74 911 Coupe w/3.0 CIS (someday to be 3.6)

[code7rpd]

I ended up replacing the relay as John suggested and the frequencu valve is buzzing again - everything appears to be fine now. Hopefully your issue will get resolved.

[74threeliter]

I received my car back the same day with the shop telling me that a wire had come loose from the fuse relay box and it did work fine the rest of the day, but the next morning it started fine and i thought i had let it warm up enough but when i started driving the "bucking/backfiring" started and the idle dropped about 400 rpm's. i have ordered my OXS relay as well, from pelican, to confirm if the relay i have now is slowly going on the brink (some good days some bad days). I am hoping this will end it but i have suspicions that i may have a short somewhere that resurfaces intermittently and the warm and fuzzy feeling leaves my body. I should get the parts (OXS relay, Fuel pump relay, Fuel filter) by 1-30-2006.

[74threeliter]

Does anybody have any ideas what i may try next?
Replaced oxygen sensor relay (frequency valve buzzes)
Replaced fuel filter
Replaced spark plugs
Cleaned sensor plate & throttle body
Replaced air cleaner
Replaced and inspected vacuum hoses


will be replacing ignition wires (look old and metal sheathing is coming apart)

[74threeliter]

Behavior after replacing all this:
Started fine
let it warm up (10-15mins) setting idle to 950rpms
then as i was about to jump in the idle drops to 350rpms and feels like it wants to cut out.
I take out the air cleaner and reach and lift the sensor plate a bit and it sounds better, i let it go and the idle is bad i cleaned the sensor plate thiniking it was binding (it was very dirty as i also cleaned the oil breather hose)
if i hold the sensor plate up just a bit and give it throttle it sounds good so what i dont understand is why does it behave this way after a bit of warming up? Sometimes it does this after the rpms come down from an initial cold start.

And if i go ahead and readjust the small screw for the sensor plate would i be changing the behavior of my "good" warm/cold starts? i dont have any issues at all with my start it's just after that.

Oxygen sensor is disconnected. 3.0 w/cis. timing is good. also didnt mention i replaced the distribuor cap and rotor. but i think my issue is fuel pressure related??? Any Ideas?

[john walker's workshop]

have you checked the CO yet? timing? distributor mechanical advance is free? intake boots are not swollen/rotted out? sounds like original plug wires should be replaced, regardless. is the freq valve buzzing when the idle drops to 400? was it not buzzing at all before the relay swap? did the relay swap fix anything? did you check the system and control pressures yet.

[74threeliter]

Attempting to answer your questions, thanks in advance for all the advice:

-Have not checked the CO (i believe the gas analyzer is required to properly check this) want to take it in to have this checked

-How do i see if the distributor mechanical advance is free?

-intake boot has got loads of small cracks and small tears around the boot clamps. replacing

-as far as i could tell (given the shaking of the car while on) the frequency valve was buzzing. I would like to know if there is any other way to test this piece, maybe taking the valve to the local shop?
-I really wasnt 100% sure if it was buzzing or not.

-The relay swap did allow me to hear the clicking when testing it with the car on i didnt hear that same click with the old relay.

-I have not checked the system and control pressures yet, which tools are needed?

thanks again JWW

al

[code7rpd]

JW,
I ended up replacing my oxy relay and there was a dramatic improvement. I set the CO to 3.5% and after about two weeks I checked the CO again and found it to be 1.8%. I reset it but what would cause it to drop like that?

[74threeliter]

Well, after much await i have found some interesting things:
1.) The air Intake Boot was replaced.
2.) Spark plug wires replaced
3.) Adjusted fuel mixture to rich setting 2/3 turn (this cured the sputtering)
4.) Leaned out (CCW) idle screw to 950rpms holds steady (smoothed out rich mixture change)
5.) Replaced vacuum hose to warm up regulator
The engine runs quieter but i feel that i have lost some power i am going to check my timing, what is the best (performance wise) setting for my timing? Should i also test for timing advance throughout my rpm range? Does anyone know where i can find this info? I have someone to help me move the rpms up so i can test the timing but i am not sure where the settings should be at say 3k rpms or 4k or 5k or 6k, if anyone has this info readily i would greatly appreciate it.

[74threeliter]

My vacuum is much better now with the replacing of hoses and the air intake boot (cracked around the clip area).
i take off the oil cap and the rpms drop ca. 70rpms