JayPoorJay

Good morning all...

I'm Barney Rubble... Feel pretty confident dealing with regular and larger mechanical types of issues,,, but fine tuning,,, I'm a cave man. I'm hoping you all can help me begin to understand the more complex inner workings of this fine machine.

Since getting her going and on the road more, she pulses at idle. There is a video, I don't know if the audio will help, but in unison with the needle, is a low regular pulse. Basically, the idle pulsing up and down in regular intervals at normal operating temperature. This is after a 25min drive to work.

This doesn't happen as noticably (or at all I think) when cold or at start up. I notice it more after up to temperature.

I have no idea. I have and will continue to check for vac leaks. But other than that, I'm not sure where to begin. I have had idle problems. Doesn't seem major but it's noticable.

Maybe it's related to this? Sometimes when I am at a light, say, behind a car,,, and the car ahead of me moves forward as if to take off, and I let off the clutch and press the accellerator a bit to start forward,,, and THEN the person ahead of me changes there mind and stops,,, and I have to RE-DEPRESS the clutch and let off the gas.... Engine will respond by reving UP a bit,,, up to about 1000 or 1100, maybe 1200 for a few seconds,,, and then settle back to the 700 750 rpm range. I thought it had to do with me MISsetting the TPS (not fully into the "click" at resting throttle) when I cleaned TBody and under the intakes, but I am still unsure.

I need help. And would be grateful for any pointers, suggestions and advise.

Any ideas? See short vid.

bureau13

Oddly enough, my 931 oscillates about that much and at about that frequency sometimes, but not at idle...at steady-state cruise. I actually can't remember if the O2 sensor is used at idle or not, but it's a pretty easy test to unplug it and see if the problem goes away.

I haven't done that test on my 931 either but I should...

928NOOBIE

I had same thing with my car and it was a vacuum leak. The vacuum connections to the brake booster were not air tight.

I wound up building a solution using PVC and very little rubber hose and it did the trick...idle is sitting on the low side just above 400 rpm and steady as a rock. I had everything else air tight...the brake booster vacuum connection for years had been leaking on my car.

There is a special rigid hose that Porsche used to prevent collapse to hold vacuum but these are petrified now and I couldn't make it seal airtight any longer where it connected so I made my own solution.

She's getting a new ball joint but will be happy to post pics. I left 2 inches of flex in the rubber tube connecting to the booster only to minimize any loss of vacuum due to rubber hose collapsing....which was what was happening when I replaced the rigid one with another one...I couldn't find anything rigid enough...I left that 2 inches to allow for flex for engine movement because the part that connects to the car has no flex....it was simple to make and made an amazing difference.

JayPoorJay

Thank you Noobie and Bureau...

Know I will be trying both of these fixes as soon as I get out (unless I get sneaky) of work... Lol. I find myself sneaking out of work to go tinker on my car... Is that normal 928 pathology?

bureau13

Sadly, yes, it is

928NOOBIE

Its when I get most of the work I do on mine done . I don't want to waste perfectly good weekends

JayPoorJay

Bureau... Do you mean unplug AT the passengers side foot well connection? If so,,, that would be too easy. But what then? Replace? Is there a dummy light on the dash for a toast 02 sensor?

bureau13

Yep, exactly. If that fixes the problem it's pretty easy and cheap to replace. I did mine (on the 928) simply because it was likely original and I wanted to rule it out as a possible cause of future running issues.

JayPoorJay

So,,,,

At normal operating temperature after a 20 or 30 like drive, I disconnected the 02 at the plug in the footwell and that had a pretty dramatic effect on idle speed and steadiness ---- for the positive.

Rpm's went from what looks like 750 - 800 plugged,,, the first (200rpm) line doesn't exist on the tac,,,, to just over 600 unplugged,,, and much steadied,,, no more pulse and this very very slight girggle I am/do get at idle disappeared...

So now what?

I haven't driven her without 02 sensor plugged. Wondering if I even should. Could that destroy the CAT if she runs super rich,,, or something?

JayPoorJay

Is there an 02sensor warning light?

bureau13

I don't think there is a warning light?

I also don't think disconnecting it will have an bearing on the cat life, not sure what other possible impacts. Might be time to get a new one (sensor) though...

928NOOBIE

How old is your O2 sensor? I'd still look at the brake booster vacuum setup...my symptoms were exactly as yours and that was with a brand new O2 sensor...

JayPoorJay

Hey NooBie, Bureau...!

I will look into both, no doubt....

I just got the car, have no idea how old many many things are... But,,, if I decide to just start replacing things cuz I don't know the age,,, I'll be living under a bridge. Gotta be careful about getting into that pattern... $$$$$$ don't grow on trees...

I will certainly be replacing the 02sensor... I'm just a bit worried about driving without it. What are the consequences??? Really. Real terms.

I just removed the RH front hub. Don't even want to make public what I found there. Jeeeeeze Louise! Lol...

JayPoorJay

A good day...

So I got a bit more into it and turned out the 7mm fuel air mixture screw. It was set at 3 full turns out from bottomed out.... First, with a telescope type magnet I checked to see if it was steel for the FEAR of dropping it into and under the intake. It's brass, it seems. So I turned it out all the way and back down a half a thread, then clamped down on it with a super long pair of needle nose vice grips to pull it from the throttle body so as not to drop it into the abyss. Super gummed up. Cleaned it and sprayed what I think is the best gum cutter, berkebile 2+2, and reinstalled. Sprayed the gum cutter into the hole with her running, turning the throttle up by hand... She didn't like that much.

I didn't find any vac leaks. But ones never knows. I'll always be looking.

I re-threaded the air mixture screw... Turned it all the way down and back out 1 and a half turns. Brought the idle down to a fairly steady 650. But something still isn't right and I know I will have to go for a good drive to see about the setting of the screw and if idle really settles at 650. I'll bring the 7mm socket and extension with me a while. Pulsing seemed to stop, at least lessen.

The way it seems, if I get such a reaction disconnecting the 02 sensor, it's still working....??? So I will be leaving that connected for now...

On the single wire from the 02 sensor, is there a right reading I should be looking for to check the sensor function? Oscillating over and above .45 milli volts or so? I better start getting used to using a damn VMeter... Electronics are German, to me, haha

I took off, re-greased/re-packed and reset, both front bearings today. I will be getting new bearings soon but both looked damn good. The passengers side I noticed had a lot of play which is why I checked. I was SHOCKED to see that someone had to take a drill to the front RH spindle to remove the outer bearing cuff in the past. I'm guessing that's what happened... It looks like they carefully drilled under the outer lower cuff of the bearing, then filed down their work on the spindle and just went with what was left. The Spindle was pretty narred - but cleaned up well... Can't go there now. Just can't. There were no signs of excessive wear on the spindle, bearings or races. So, I Reset the bearings and kept it moving.

Well see how the idle goes.

bureau13

I really don't think there is going to be any sort of damage from disconnecting the O2 sensor. If I recall correctly, it's only really used for cruise or light throttle applications. When you push it, I think the car runs open loop (no O2 sensor feedback). When you disconnect it, the car is just forced to run open-loop all the time.

That being said, the more I think about it, the more I am not sure why the O2 sensor would cause the idle symptoms you observed. It occurred to me that it was related because the frequency of oscillation seemed pretty close to the oscillation of the O2 sensor readout during closed-loop operation...but if the O2 sensor output looks like that, that would be normal...still shouldn't cause your idle to follow suit. So they may be related, without the problem actually being the sensor. Not sure. All that being said, a known newish O2 sensor is not a bad thing to have, so I would still get a new one I think.