85 Carrera Warm Idle Surge and 02 Sensor
 

Greetings - this is my first posting here, but I have made liberal use of the wealth of information that is available on this site. Thanks to everybody who contributes knowledge and experience to help the rest of us avoid mistakes and wasted time as we play with our cars.

In October is purchased my second Porsche - a 1985 Carrera, with 75K miles. After a lot of looking, I was lucky to find a super clean and well cared for car. My intention is for it to be an "almost" daily driver rather than to keep it locked away in the garage, only to be driven on dry sunny days. Living in Seattle, that approach would limit my time behind the wheel to less than half the year. And, I have learned that the car will not melt if it gets a little rain on it.

I am really having fun doing some projects to get everything dialed in - repacked front wheel bearings, all new front brakes, completely rebuild everything in the shifter/linkage, new clutch cable and helper spring, new motor and tranny mounts, soon will be replacing tanny oil, and a few other things.

The car runs great - mostly, which is the reason for this post. With the weather warming up, the engine is reaching higher temperatures than it had during the winter, and now I am experiencing an idle surge condition (700 - 1200 RPM swing) when the motor is warmed up. Prior to yesterday, the surge would come and go, and wasn't that big on an issue, except I could feel it when trolling in parking lots at idle speeds with the clutch engaged.

Shortly after I bought the car, while snooping around in that super cool engine compartment, I noticed the O2 sensor was disconnected. I reconnected it, and I didn't notice anything different, good or bad, so I left it attached. Given how tight that plug is, it doesn't seem likely that it just fell off, so I'm assuming that somebody disconnected it intentionally. But, since it seemed to be running fine, I just left it connected.

Yesterday, the surging became more pronounced, and wouldn't go away as long as the engine was warm. Then I read some posts here about this situation, and decided to disconnect the O2 sensor to see what happened.

With the sensor disconnected, the surge is completely gone, and the idle is super smooth. The parking lot "lurching" is gone too. It also seems like I have more power and smooth running at speed. So, I'm thinking my sensor is ready for the trash bin and I am have one waiting for me at Carquest will call today, along with some new 02 sensor relays (just in case).

Sorry for the ramble, but now for my questions.

1) Is it too simple to assume that the O2 sensor is the culprit and replacing will correct the situation?

2) I have no records to indicate the sensor has been replaced, so is it just a good idea to replace it now considering the mileage and nearly complete lack of being driven for the last several years?

3) What are the implications of driving with the sensor disconnected? It seems to run okay from a subjective driveability and performance perspective.

4) Are there any modifications that the previous owner may have done that would involve disconnecting the sensor?

5) Assuming I replace the sensor, should I also replace the relay at the same time since I will have a new one?

6) When a new sensor is installed, should I expect anything funky as all the electronics and controllers "relearn" what the engine is doing?

Any other tips or advice are greatly appreciated.

Thanks in advance!
Craig

Idle Hunt can be attributed a vacuum leak, dirty/weak ICV and/or incorrect base idle.

A simple test, when she hunts, take a screw driver and tap the ICV, if she clears up then it needs cleaning.

For vacuum leak, while car is on, remove oil cap, idle should drop, if not you have a vacuum leak.

To check base idle, jumper B & C connector at the engine bay, if your baby is not chipped it should idle at 780RPMs if chipped 880RPMs. Adjust if needed.

A point of clarification - I just realized the O2 sensor has 2 leads. The one that was disconnected is the fat rubber plug. The other lead with 2 wires and the hard plastic plug is still attached. I haven't messed with that.

Not sure if it makes a difference, but thought I should be accurate.

BTW - is there a way to edit a post you've made?

Thanks

Yup, lower right....gui that says edit. Also on the 02 sensor you have may not be the right one. Check your manual if it uses a 2 or 3 wire 02.

the fat rubber plug is the one that matters, thats what sends the signal to the DME
the other connector with two white wires is the heater circuit that preheats sensor

Old sensor out, new one in
 

Well, I replaced the O2 sensor, connected all the plugs, and things seem pretty good. Was cold when I first started after the change, and that felt/sounded good. Driving during warming up was good, short freeway run was good, and no surge anywhere along the way. I hope this continues when I get it really warmed up.

In case anybody is interested, I will attach a photo of the old sensor - pretty significant residue build up on the tip. Does this look like a sensor that needed to be replaced?

Thankshttp://forums.pelicanparts.com/uploa...1362879790.jpg

Doesn't look healthy. 02 sensors are a wear item and need to be replaced a lot more than speed or reference sensors. They are cheap.....when in doubt, replace.

FWIW My '87 was having the same idle surge when warm. As part of the winter project list I cleaned the ICV and installed a new O2 sensor. The old O2 sensor looked the same as yours. With no proof of when it was last changed, I replaced it.
Idle surge is gone.

Ken

cool, another satisfied customer :D

O2 sensor is a 60K mile service item. My car definitely responded well to a new sensor when I recently did the 60K service.

Here's a thread on my recent O2 sensor problem: http://forums.pelicanparts.com/porsche-911-technical-forum/733292-3-2-oxygen-sensor-issue.html

Oh, and try disconnecting your O2 sensor, the rubber boot lower end, while it's idling warm. You should notice a slight idle change--the car runs better/smoother when the sensor is connected and functioning properly.

A bit bummed
 

Update on my situation.

Today I got the engine temp up again, and the surging idle when warm returned. Bummer.

I have cleaned my idle control valve before I installed the new O2 sensor, so I don't think that's the problem unless my cleaning was not sufficient. Are there any tricks to this?

I believe that valve is functioning based on the sound and vibration when the key is on.

I'm assuming that if I had vacuum leaks the issue wouldn't kick in at the same temperature pretty consistently. I've had the cold idle adjusted before - is there some sort of warm idle adjustment?

Again, if I disconnect the big rubber plug on the sensor, the idle is really smooth and the car seems to drive very well. Is there harm in driving with it disconnected?

Any suggestions would be appreciated.

Thanks for the help!

I don't think there is any harm in running with the o2 sensor disconnected. You will simply be running in "open loop". The only problem you might run into would be fouling the catalytic converter from a slightly richer fuel mixture? I plan on doing a CAT delete on my car and deleting the 02 sensor. I believe the European cars could be purchased without the catalytic converter or O2 sensor.

It sounds like a vacuum leak. Check all hose clamps, hoses, intake manifold bolts, etc. Chasing vac leaks is time consuming. You might get lucky with a can of starting fluid, but not guaranteed that will work.

Idle control valve. Almost 100% sure. Might be wiring to it, Ingo pointed out in the past, with the ignition on, car not started, you should be able to feel the ISV buzzing.

It is currently stuck a little on the side of open, good for a cold car, but to much air when it wars up. It can't close to decrease the amount of air coming in.

This could also be caused by an air leak, and teh ISV can not compensate, even fully closed air could be in.

DOes anyone have a photo of the ICV location on an 86 3.2?

Appreciate the help
 

Thanks for all the ideas.

About the idle control valve - it does buzz when the key is on, so is there another way to test if it's working properly? Does it make sense to clean again?

About vacuum leaks - wouldn't those be present when the engine is cold? And wouldn't those persist when then I disconnect the O2 sensor and the idle becomes smooth and constant?

It's funny how smooth everything is up to the point when the temperature gauge reaching the first hash mark above the "block" at the bottom of the range. When the weather was colder, I could drive all day/weekend with no issues, but the needle would stay just below that hash mark. Now, with warmer outside temps, the needle reaches the hash mark, or goes slightly above, and the surging starts.

I clearly do not understand all the intricacies of how everything works in the idle control circuits, so hearing ideas from those more knowledgeable is very helpful.

Thanks for your patience as I try to figure this out,
Craig

Craig,

No, the issue is "false air" throwing off the mixture. False air is unmetered air that leaks into the motor due to a vacuum leak. If these leak(s) is (are) cumulatively large enough, then the O2 sensor will sense a lean condition, and richen the mixture. The unmetered air then allows more lean condition to occur, and the rich/lean adjustment is what is causing the idle to surge. Not the ICV. You can test the ICV by removing it after the engine is warm. Move the vane to a closed position, and then plug it back in and turn on the key. The servos will move the vane to a half open position. If it's dirty, then clean it out while it's off of the engine, with some electronic parts cleaner.

Not sure if what Rusnak mentions is causing the blipping idle, but I have first hand experience of the ISV stuck almost fully open. When the car is cool, the extra air did not cause the rpm to go above the max rpm at idle. If the DME reads the idle switch closed, and idle goes above a set point, which happened to me when the car warmed up, possibly 1200 rpm, it will cut either spark or injectors, never figured out which.
If your ISV is working properly, I believe air is coming in, and increasing the idle. When your engine is cold, it requires more air to run, so the idle does not go above the set point. As it warms up, the extra air causes a higher idle, the ISV is fully closed, and the idle still goes above the set point. A self blipping idle happens.

To test this theory, manually hold the throttle position switch closed, open the throttle and see how much rpm you get before it cuts out, and starts blipping. Just a theory, can't test right now..

Thanks for the additional info guys.

I will do some hunting for vacuum leaks, and maybe tighten intake bolts/nuts.

Are the idle control valve and the ISV the same unit? I am familiar with the control valve, but not the ISV.

You can spray starting fluid on a COLD engine around the manifold bases. This will tell you if there are vacuum leaks there. The idle will rise significantly due to fuel being sucked into the engine.

For the rest, use a strong small flashlight, and a mirror. Check the tightness of the hose clamps, and run your hand around all of the hoses to check for cracks. Check the little hose that connects the ambient air valve with the back of the throttle body. For the crankcase venting hoses, you have to remove the air box and the air flow meter. Don't forget the brake booster hose that runs forward to the trunk. The cruise control hoses, etc all need to be checked as well. The little leaks all add up to one huge leak.