SC: Occasional climbing idle with O2 sensor

[schoward]

81 USA SC in good working order. In the past, Most mechanics have set my mix rich and left the o2 disconnected and it has run great. Most just seemed to set it that way because they felt 'they run better'. Checking my mixture myself revealed usually around 3.0% CO.

Since my car has been checked for leaks and pressures etc, I wanted to resurrect the o2 sensor system as I saw no reason not to use it and have been getting tired of the poor fuel economy. As I mostly putt around town these days, if I lost a tiny bit of oomph (supposedly), I may be able to live with it in return for better economy and emissions.

I put in a fresh o2 sensor for good measure, achieved a mixture setting of 0.6-0.8% CO using my gunson using the cat test port (o2 unplugged) and got the idle to 950. Runs well and has very even power throughout the range. No hitching feel and idle rhythmically hovers around 950 when warm. I rechecked mix a week or so later and again same reading. Actually pretty pleased with how it runs.

One issue that I have now that only seems to occur with the o2 plugged in: Occasionally (2 or 3 times in an hour long drive around town - sometimes not at all), I will be stopped and idling and idle will kind of suddenly start to rise on its own from 950 to about 2200 and stay there. It kind of seems like it has a mind of its own. Every now and then it comes back to 950 on its own, other times I have to put some load on engine and it will come back. Drive away and it's fine. 98% of the time it idles dead on 950 with a little rhythm. It seems to happen after a prolonged idle (2-3 mins stopped) but most other times it could seemingly idle forever and not do it.

Since I am fairly sure I could recreate it, I am going to get a good dwell meter and put it on the test port to see what is going on when this occurs. In the meantime, any ideas? I have searched but can't find this issue. Most have constant high idle issues. Mine is great other than this occasional occurrence.

Thanks in advance.

[dyount]

Sounds like decel valve

[Bob Kontak]

Originally Posted by schoward

if I lost a tiny bit of oomph (supposedly), I may be able to live with it in return for better economy and emissions.

You will not lose oomph but I am not sure if you will gain oomph.

On the 81,2,3 SC's there is a wide open throttle switch which the earlier SC's do not have. It allows lost o' gas to go in but I do not know the inter-dependencies of the fuel control components.

By that I mean the O2 sensor and frequency valve work in unison to keep the car at stioch when cruising. No O2 sensor and the frequency valve defaults to 50% duty cycle, wasting gas when you are cruising. When you floor it and trip the WOT switch, I am guessing the Lamda box tells the frequency valve to go to the duty cycle that allows a rich mixture, with or without the O2 input. But I am not sure.

Regarding rising idle. I plugged the decel valve as this was happening on really warm days when I was stuck in traffic. Just plug a nail into the little hose. I am not sure that this was the problem though as I had a a vacuum leak at one of the intake runner bases and once I corrected that, I plugged the decel back in and never had the issue again.

Try the starting fluid trick, spraying at the base of the intake runners when idling. Idle will rise. I also had a loose injector that popped up from spring on the metal lines. Those you can visually check.

[matt demaria]

1. The Gunson tester is not accurate. I have found it is usually a point of CO off cal.
2. Check to see if the frequency is vibrating.

[schoward]

Thanks for responses.

I am going to plug the decel line and see if it goes away. Coincidentally, it was plugged for years, but hooked it back up this summer to try it and I actually like what it does. Perhaps the newly connected O2 is sensing false air and adding fuel. I still want put a dwell meter on it also to confirm mixture and any behavior.

Bob - I will recheck my intake and injectors as suggested. My intake and injectors were just checked by a shop as I blew an injector seal in the springtime which gave me instant 'piffing' out the exhaust. Fixed pretty easily by shop. But you never know...

I dont think Ive gained any oomph, but power seems more even as I ascend throughout RPM range. Dont think Ive really lost any either.

Frequency valve is running; I replaced O2 relay under seat for good measure as well which made a very slight difference in running (for the better). Old relay still powered the FV as well but suspect it was getting tired. Like a mini-tune up. FV doing its thing as I pull relay and all goes bad.

Any additional ideas welcome. If I can knock this down, I will be in business.

Thanks!

[Bob Kontak]

Originally Posted by schoward

......My intake and injectors were just checked by a shop as I blew an injector seal in the springtime which gave me instant 'piffing' out the exhaust.

piffing = a weird mini putt putt, right? Had that as well with my intake leak.

Try the decel first.

Take your time with the dwell meter. Read up some. User psalt has some good info in prior CIS posts.

[schoward]

"piffing = a weird mini putt putt, right? Had that as well with my intake leak."

Yep. My shop said 'Thanks for the good description - it helped us find the problem very quickly" And they meant it. It was the only word I could think of.

Thanks for the ideas. Yes, been bookmarking psalt posts all over the place...

[Bob Kontak]

He likes to dial it in just a touch rich, let the O2 sensor bring it to the correct AF ratio, and when you give it the beans you get a little extra juice.

Also, read up on distributor timing. You can cheat a wee bit on SC's with 9.3:1 using higher octane. They are pretty wimpy as far as advance. I am seat-of-the-pants thinking 2 degrees. Nothing silly.