924Board.org

[Geddy T]

I think we dealt with this earlier, but I'm still pretty sure that the O2 sensor doesn't affect anything at idle, but exists to richen the mixture under boost. If it wasn't for the fact that you claim that the condition is corrected with the thing removed, I would think that your problem is elsewhere. I was just playing around with my TB and there is an intake bypass screw facing down that affects mixture at idle. Also, I'm agreeing that you may just have your CO all screwed up. Isn't it the case that the O2 sensor can only enrichen the mixture and not lean it? In that case, what would have to be happening (assuming that all components are working properly) is that the O2 sensor is detecting a lean condition and adding fuel. So then the question becomes why is your engine liking a lean condition at idle? Your idle setting, bypass setting, ignition timing, and CO setting are all related. You shouldn't adjust one very far without adjusting others to match. So look into other things outside of the fuel delivery. A couple of those variables can be measured at a competent shop. The timing can be set correctly and the CO can be measured and adjusted with an exhaust gas analyzer. Then you can deal with your idle settings.

[Rick MacLaren]

Geddy: You said "The timing can be set correctly and the CO can be measured and adjusted with an exhaust gas analyzer. Then you can deal with your idle settings."

Geddy, remember, all this was done, with all the right equipment, and it's a brand new motor with an old oxygen sensor. The claim that it ran well without the oxygen sensor is true, though hardly definitive. It's hard to diagnose definitively here, so the best answer seems to me to be, given the cheap cost, just replace the O2 sensor and wait and see.


[ This Message was edited by: Rick MacLaren on 2002-05-04 11:59 ]

[Geddy T]

Not a bad idea

[Rick MacLaren]

Geddy have you done any tuning to your 931?

I've bought the new 02 sensor and I'm putting it on this weekend. If it fixes the problem, great, else I'll be looking around the system as you've suggested.

[Rick MacLaren]

I'd rather not have any smog crap on my car. Has anyone done a post showing how to remove all the smog crap...I'm still putting the new O2 sensor in however...

[Benski]

Hey Rick,

The O2 sensor isn't just 'smog crap'. It actually makes your car run better =)

You're right - I'd also like to see a write-up of what smog equipment and vacuum accessories can be safely removed on the 931.

[Geddy T]

Rick, I did do quite a bit of tuning just a few days ago. But not because I wanted to and not under normal conditions. I was trying to pick my dead car up in Arizona where I left it. I flew down and brought a new camshaft and followers and alternator and fuel distributor and other things along with me. I threw all the new stuff in, but when I was in there, I noticed that the number one intake valve wasn't coming up all the way--bent valve? Stuck valve? Didn't matter. Bottom line was that it meant I had to try getting it running on 3. The CIS made this a lot harder because the flapper plate was bobbing up and down with the dead cylinder, thus ruining the fuel delivery to all four. But a porsche mechanic and I sat out for a good half hour adjusting the idle screw and bypass screw and CO screw trying to get it running. What he said was that it is usually not a good idea to mess with the idle screw, but not optional in my case because we had to make up for the dead hole. The bypass can definitely make a difference, especially at idle. This is the large, flathead screw facing down behind the TB (clockwise: more bypass, counter: less). You're not idling properly until all of these things are working together. Now, the discouraging news is that the idle screw is probably where it should be and the bypass screw has to be really cranked to make that big of a difference (from what I saw), so you're probably looking at something actually wrong--such as a bad O2 sensor like we suspect. I also replaced my sensor while I was down there just to be sure (and, no, it is not just emissions garbage). Anyway, I got the car running by twisting enough things, but not well enough to get it home, so I'm waiting until I can afford shipping. Then I figure it's rebuild time.
As far as emissions garbage that can be removed... I once removed and rebuilt an engine out of my '84 Honda Prelude and the 924 or even 931 is a dream to work on compared to that. There really isn't that much unnecessary stuff to remove. Most hoses and mess are associated with the turbo and are necessary. There are only really 3 emissions control systems on the car: the EGR (which I wouldn't mess with), the air injection, and the Evaporative Emission Control (EEC). It looks like EEC is not present on some European models, so it must not be necessary. But from the looks of it, I wouldn't touch it. It includes your charcoal canister and control valve and vacuum and fule hose (nice diagram on about page 100 of the Haynes). And air injection is only on the California cars, so you probably don't have that anyway.

I'm seriously not trying for the longest average post award or anything.

[kevrl]

Hmmm - Geddy, you're right about some european models having no EEC, or Air injection, but we have no EGR or O2 sensor either, and our Turbo's fine OK, with LOTS of power, so there must be a way of removing the O2 circuit and still allowing the car to run without problems ?

When mentioning Euro models, I actually only have experience of UK models, but I assume they are the same.

Good luck with the bent valve - howd that happen ?

Kevin

[bho]

Rick, if the new O2 sensor doesn't do the trick, check out your vacuum lines. I had a surging problem at idle on my 81 after I rebuilt the motor recently. This occurred only after the motor was warmed. The problem was tracked to the vacuum line operating the purge valve of the carbon cannister. On the 81 this line runs off a thermo valve at the back of the head. On yours, I believe, it is t'ed of the vacuum retard line to the distributor. (I also replaced my 20year old O2 sensor as part of the trouble shooting before discovering the vacuum leak

[Geddy T]

kevrl, actually, I'm not positive that the valve is bent. It just didn't pop up all of the way and I'm assuming the worst. It could just be stuck in the guide. I had the plunger stuck in my fuel distributor for two days while I was trying to get it running. This meant two days of dumping fuel down the bores and into the oil. It's possible that the oil got too thin and washed the valve guide too much, causing the valve to stick. Either way, I'm pretty sure it's time for at least one new valve and guide.

Also, as for the O2 circuit, what year is your car? It was my understanding that none of the cars had this system until '80 or something like that and then it was put on all of them. Also, it looks like the lamda system (O2 sensor) REPLACES the EGR system, so you'd only have one or the other. Also, like I said, air injection is only California models. So I supose the only thing left to remove is the EEC if you really have that big of a problem with emissions control?

One last thing: I'm looking at the Haynes and in its procedure for adjusting the idle step one is to remove the oxygen sensor. Interesting?....

[John H]

My car like Kevin's doesn't have the o2 sensor, or any of the other stuff you mention - it's a 1981 Euro Spec (ex UK) converted to Carrera GT in the mid 80's with all the factory parts and digital ignition system.

[ This Message was edited by: John H on 2002-05-06 11:28 ]

[kevrl]

Mines an '82 931 (one of the last), it has no EGR or O2 sensor - and it runs great at about 0.9bar boost with no intercooler - and no emmissions kit.

In other words there MUST be a way for all you people in the US to remove your emmissions kit and liberate free HP with no bad effect - if thats legal in your state ? Possibly we use a differant ignition control unit so have a modified advance curve to avoid detonation ? I know that our unit retards ignition when inlet charge temp exceeds 65C to avoid detonation.

Kevin

[larso]

I do not think removing the O2 sensor is going to cause problems since the system goes into a 50/50 cycle. The throttle switches only work if your control unit is connected though, and I am pretty sure Euro models have a different ignition curve and probably a higher rev limit in the ignition box too.

One thing to note is that when you are at full throttle the throttle switch dumps in some extra fuel, so I am guessing the 02 sensor disables in the circuit when this is occurring. It would cause problems if the switch was adding extra fuel and the O2 sensor was trying to take away fuel at the same time.

I don't think the O2 sensor should be used to save your engine. It is there to make minimal adjustments so that when you pass by people hugging trees, they wave at you. I think that if one was to assume the 02 sensor failed and your engine blew up because of it, then there are more serious problems then just the 02 sensor that need to be resolved.

I believe the warm up regulator on the later models of 931s also dumps extra fuel in when it detects more boost. You might be able to tune the regulator to dump in more fuel than stock. The Euro models may do this.


Have you disconnected the 02 sensor and tried driving the car around Rick? Even if the 02 sensor could cause lean running, then just run the car easy around for a drive and see what happens with the idle.

[ This Message was edited by: larso on 2002-05-07 12:19 ]

[John Brown]

Larso has it right.

The 931 has no egr. Only (some - 79??) 924 were so equipped.

The evaporative emissions control system is a pain cause it clutters the place but has no (measureable) effect on power.

The frequency (aka 'chatter') valve defaults to a fixed ratio (same as the924).

The throttle body switch enables and disables the Lamda control at idle and part throttle positions. See the factory turbo suplement for detailed adjustment and diagnosis instructions.

A bad O2 sensor is sufficient to cause idle fluctuations as described.

There is no mixture control on the throttle body of 931 or 924. Only idle air bypass. Adjustment is complicated by the digital ignition idle speed stabiliztion system. See the factory manual for detailed adjustment.

A replacement of ALL vacumm hoses is suggested on these old cars. But I would not expect the idle variations unless the wastegate or boost control is involved.

Do not know the specifics but the boost control box, the ignition box, the timing control box, and the O2 box are interconnected via the wiring harness.

These early systems are reputed to do some amount of 'loping' at idle not matter what you do. In my limited experience that is a true statement. But not, of course, 1000 rpm of 'lope'.

I'd go for the sensor replacement along with a proper system adjustment per the factory manual. There is some info on looking at 924 frequency valve output in the tech section. The 931 operates identically.

[Smoothie]

The mixture isn't set at the throttle body as you said, but it is at the fuel distributor, right? It is on my '82 924T. I've only adjusted mine once - when I first got the car it was putting out black smoke at idle. I disconnected the o2 sensor and set the mixture to a CO gas meter.

Can someone answer this? - Can a way overly rich fuel mixture wash fuel down past the rings and into the oil, diluting it, possibly resulting in a blown engine due to lubrication failure? I have a vague recollection of hearing that a long time ago. Anybody know?