High idle (1000rpm) when O2 sensor plugged in and hot

[BigBenb84]

When I have my O2 sensor plugged in and the engine is cold it idles normally at 750ish. As it comes up to running temp the idle creeps up to 1000rpm. Also, it losses pep when that sensor is plugged in. Like even in neutral or while driving if I'm at 1500-2000 and blip the throttle it'll struggle for a split second before it reacts. Also, I sense a very slight power pulsating under 2000rpm, like 2 seconds on/off/on. It's very slight but I can tell.

Now, if I unplug the O2 sensor the idle immediately falls to the normal 750ish under running temp. Also, I get all the pep back. No hesitation when giving throttle blips. I know it's running full rich I guess which can't be good for the CAT, carbon?, environment, gas mileage so I would like for it to run nicely with the sensor in.

My troubleshooiing:
-Replaced O2 sensor
-Idle screw is back all the way out, not even touching the stop
-Adjusted throttle linkage so the spool isn't holding it open
-Idle micro switch is tripped, and switch replaced recently
-WOT switch is NOT being tripped
-Idle Speed Motor hums normally, afterall it works fine at holding 750 with the O2 sensor unplugged.

(My half educated guess...I'm a pilot and in the little piston prop planes we have to manually adjust the mixture due to rapid air density changes with altitude, and I know as you lean the engine the RPMs climb due to it running more efficiently. Now since I'm also getting crappy power delivery with the sensor plugged in, maybe it's running too lean causing a uncontrollable high idle??)

[Bitsyncmaster]

Have adjusted the mixture screw with a dwell meter to show you the proper adjustment? That would be the first thing I would do. That may help your increasing idle but I had the same problem. My idle screw was backed all the way out and with a warmed up engine I would idle at 1000 RPM. My problem was the deceleration springs were bad.

[BigBenb84]

Dave, No I haven't messed with the mixture screw but I had a hunch that's where I would wind up. So as I understand it, you can still adjust mixture even with the O2 sensor controlling the show? Like a master adjustment then the O2 does the fine tuning??

I'll have to do a little search to find out how to do that adjustment.

[hmcelraft]

I believe something is missing? What did you do BEFORE this symptom appeared? I don't believe the O2 sensor by itself would not cause the symptoms you describe - by itself.

It sounds like the vacuum lines are connected incorrectly to the thermal vacuum switch and the vacuum advance is coming on as the engines warms up - at "idle". When you disconnect the O2 sensor the lambda sends the fuel to "rich" - a steady 40-50 degrees on the dwell - which would improve engine performance at a lower RPM and too much spark advance. The increase in spark advance seems to be causing the increase in RPM as the engine warms up. Does the distributor vacuum switch work? i.e. come on, when the micro switch goes off? Check with sections D:04 - D:05 in the manual.

[Bitsyncmaster]

Dave, No I haven't messed with the mixture screw but I had a hunch that's where I would wind up. So as I understand it, you can still adjust mixture even with the O2 sensor controlling the show? Like a master adjustment then the O2 does the fine tuning??

I'll have to do a little search to find out how to do that adjustment.

Yes you make the mixture adjustment with the O2 controlling the mixture. That adjustment sets the "center" range of the Lambda control. But more important it properly sets mixture as the engine runs at wide open throttle and when the engine is warming up. The setting is pretty critical (sensitive). You don't get much more than a little more than 1/8 of a turn from dwell reading to go from 10 degrees to 80 degrees.

[David T]

Yes you make the mixture adjustment with the O2 controlling the mixture. That adjustment sets the "center" range of the Lambda control. But more important it properly sets mixture as the engine runs at wide open throttle and when the engine is warming up. The setting is pretty critical (sensitive). You don't get much more than a little more than 1/8 of a turn from dwell reading to go from 10 degrees to 80 degrees.

It does sound like you need to make a mixture adjustment but before you do you must make sure EVERYTHING ELSE is working as it should. That means you must fix any vacuum leaks, worn ignition parts, plugged up air filter, etc. The mixture adjustment is the LAST thing you do once you are sure everything else is in working order.

[Jonathan]

A couple thoughts in addition to what others said (do what they suggested, this is only to be added to your list of checks).


Idle screw is back all the way out, not even touching the stop

So if you're talking about the screw which comes to rest on the base of the pivot arm, there is a portion of the adjustment for that screw that is more than getting the idle micro switch to engage the ISM.

It becomes easy to see if you have the W pipe off, but you don't need to do that just for the heck of it. When that lever arm pivots, it is also rotating the butterfly valves inside the air housing simultaneously. As they open (or rotate), they let air into the engine. If that particular screw setting is too far closed (i.e. without the screw even touching), it can create a lag for when you start to hit the gas/accelerator and when air is actually let into the engine. Like those butterfly valves are past the closed position, if you catch my drift? (ooh, how was that for a retro phrase? lol)

Generally you would set the lower screw to approximately just closed (from the butterfly valves perspective) and then set the engagement with the upper screw after that. This came to mind when you mentioned the hesitation piece.

There are a couple photos of the inside of the air housing and butterfly valves in the thread for "helpful technical photos" or whatever the title of it was.

One other suggestion would be to exercise the electrical connections behind the driver's seat on those ECUs. Take the back plugs off and snug them back on. Off, on, off, on. Look to see there isn't any loose wires coming out the back or excessive amounts of corrosion on the metal tabs of the connectors. Do the same possibly with the bulkhead connectors.

Some of the high idle fixing crusade is best resolved by a multiple prong attack. You might have to confirm a bunch of little aspects before it finally corrects itself and you might not ever know which one it was that did the trick. Good luck. Lots of high idle threads on here. If we had a "trending" counter, I'm sure it would win each year.

[Elvis]

explained 200 times how to test the idle motor.

Just humming does not tell you if its bearings are frozen.
This would explain why idle goes up when warmed up.

[flux3d]

I had a similar (but not the same) problem. My advice is NOT to mess with the mixture until EVERYTHING else has been checked. From bitter experience if you dive in to changing the mixture you might get it running better, but will just be covering up another problem. This was exactly what happened to me. I tried everything. When I started the car I had to keep my foot on the "gas" and raise the revs until the car got warm, or else it would hunt and quickly die. The culprit, the ECU in the end. The idle speed motor buzzed, and seemed ok, but because at least one of the diodes in the in the ECU was fried the "valve" in the idle speed motor was shut all the time, hence starving the engine of the extra oxygen it needed on start-up. Once warm it could cope (thanks to a work around done by the PO to allow more air in using the brass screws in the intake), but only a complete rebuild of the ECU finally fixed it. I changed all the diodes and IC's for new and it worked perfectly.

I'm not saying that your problem is the ECU (although it might be by the sound of it), but beware messing with the mixture screw. Mind you, I did learn a great deal about the engine by sorting out the mess. Depends if you want that type of "forced education".

[Bitsyncmaster]

As long as you have a working dwell (or % duty) reading meter, there is no reason to not check that first. Checking and adjusting mixture cost you nothing and very little time.

If you don't have the meter......don't even think about adjusting that mixture. To many adjust for the best idle and that sets the mixture very rich running open loop.

[BigBenb84]

Wow, love that Dwell meter. I just bought one from eBay and hooked it up. My dwell was sitting at 10 with no fluctation or buzzing of the frequency valve. Forum search told me that meant the ECU thinks it is too rich and is trying to lean it. I leaned it out (counter-clockwise) until frequency started buzzing and the dwell is now alternating 40-50. Also, cold engine holds about 50 as does tripping the WOT switch. I test drove it around, runs great with solid idle.


https://www.youtube.com/watch?v=5y1Zr5pbBzk

[Domi]

Looks like a nice dwell meter with a great range of reading!
Do you have a link on eBay to see it please?

[Bitsyncmaster]

Now you see why I said "don't even try to adjust it without a meter".

I've never seen an oil gauge pressure so jumpy as yours. Probably just the sensor has a dead spot.

[BigBenb84]

Looks like a nice dwell meter with a great range of reading!
Do you have a link on eBay to see it please?

Sure, here is the link to the auction I won: http://www.ebay.com/itm/Sears-Craftsman-Tach-Dwell-Meter-/301468439704?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2047675.l2557&nma=true&si=uRS8ScfaK6uiOeM2X14gp%252FP7dHg%253D&orig_cvip=true&rt=nc


Now you see why I said "don't even try to adjust it without a meter".

I've never seen an oil gauge pressure so jumpy as yours. Probably just the sensor has a dead spot.

The original oil pressure sensor stayed pegged at 80. I read it was a miss-calibrated one the factory was putting in(?). So I got one from Hervey that supposedly is correct. It is aside from the jumpiness, wish it was smooth. 80 when cold, then 40 when hot and idling, back to 80 for anything above 1000rpm.

[Elvis]

Looks good, thumbs up :-)


Question is - why was it too rich ?

Very often the RPM relay is bad - bad solder joints and then the
WUR is not being heated which results in too rich mixture.

Just asking you to check the relay for broken solder joints as it
happens really often !

[Bitsyncmaster]

Looks good, thumbs up :-)


Question is - why was it too rich ?

Very often the RPM relay is bad - bad solder joints and then the
WUR is not being heated which results in too rich mixture.

Just asking you to check the relay for broken solder joints as it
happens really often !

That second output from the RPM relay that drives the WUR also drives the "Lambda" main fuel relay. So if it was bad you would not get any dwell reading. My guess is someone else had adjusted the mixture wrong before this owner got the car.

You still should check that RPM relay for broken solder joints if you don't have mine.

[David T]

That second output from the RPM relay that drives the WUR also drives the "Lambda" main fuel relay. So if it was bad you would not get any dwell reading. My guess is someone else had adjusted the mixture wrong before this owner got the car.

You still should check that RPM relay for broken solder joints if you don't have mine.

One possible explanation for being set over-rich was that it was done to compensate for vacuum leaks and/or a non-functioning frequency valve. Then the leaks were fixed or the valve started to work and the mixture was not readjusted. Very common for the plug and the contacts on the freq valve to get corroded and the valve stops working. Taking the plug off and putting it back on a couple of times often gets it going again. I do not recommend setting the mixture without knowing the condition of all of the other parts. All too often you wind up setting it to compensate for other things that are not right. Adjusting the mixture should be the LAST thing you do.