Idle Speed Doesn't Drop When Oil Cap Removed, '81 SC, Help Please

[UrQuattro]

Hey all, there are two issues here that I need advice about...

I checked the oil when I got home, and I needed to put some in, but that is beside the point. Every time in the past, there has been a pretty major response to my pulling the cap for the oil off, but today, almost no change at all, and I double, and triple checked it.

Apparently I've developed a vacuum leak. Im pretty surprised because the car seems to run really well, except for the horrible gas mileage (14/16mpg), but the car has always had really bad mileage.

I recently, within the past 1500-2000 miles or so, took the car to a very highly respected and recommended mechanic who is apparently one of the best in the bay area if you want to have the CIS tuned up and looked at. He tuned it up, set the mixture, installed the new warm up regulator that I had brought with me just in case, and he set the timing.

After that, it ran great. There was more power, but the mileage didn't increase, and just as it should be, when I removed the oil cap, the engine idle dropped and it stumbled along until I put the cap back on.

But, today, totally different behavior. I had it smogged today, and the results are in my other thread, but I passed a CA smog test with flying colors.

But I'm really worried about this potential vacuum leak and where it might be coming from, as I had read in a few other threads that you can only get to some/most(?) of the lines by dropping the engine.

I don't have any tools right now because everything is in storage (long story), and I'll hoprfully be able to get them in the next few weeks, but right now, I have no way to work on the car.

Even with tools, I am not familiar enough (I've forgotten so much after
what is essentially a decade of not working on cars, and now when I start back up, it is with an unfamiliar car, and that car is a freaking Porsche 911.) with 911s to know what to do or how to fix the problems correctly.

Theres a good Porsche shop very close by my home that can take care of the car, but i don't know what I should expect to pay. But maybe this is why, even after getting it tuned just right, I'm only getting about 14mpg....

Although that doesn't even make sense because I have once filled up the car and taken the car out for a really hard drive though the Santa cruz mountain road, and I mean I really ran the car. When I was done, I went to the gas station to fill it up again and see what mileage I get when really hammering it. It was 14mpg. Odd... As this is what I tend to average during normal driving over a tank of gas. My normal driving is about 9 miles of highway driving, which I drive at about 75 generally, and there's another mile/mile and a half city driving...

So, I did a test of all highway, it was a 100 mile test, and I averaged about 75-85mph during the drive. I ended up getting just under 20mpg.

I don't understand how that is possible. How can my average mileage be equivelent to the mileage I got when driving as hard as I could on public roads, with the engine at 5000rpm+ most of that trip?

Does anyone have any thoughts about that one?

Thanks in advance...

Michael

[Gogar]

There's a few things you can do without any tools or much effort.

Look here:

CIS Primer for the Porsche 911

There's a section called "vacuum diagrams" where you can see the routing of vacuum lines. Put on a dirty shirt and try to find these items and see if the vacuum is hooked up or not. The picture/diagram for (80-83) lays out as if you were sitting in the back seat, looking straght down at the engine.

If your 81 is a ROW car, it should have a vacuum line to the WUR. If you recently had the WUR replaced, maybe the vacuum line didn't get hooked up. Maybe it's the wrong WUR. etc., etc. In addition, A WUR on a ROW car (or, a USA car, I suppose,) with a misadjusted 'hot' pressure could change fuel economy, for sure.

You can also check for vacuum leaks with a can of carb cleaner, start the engine up and let it get to a reasonable temperature. Spray the carb cleaner on every vacuum line junction. If you get a significant change in the sound or speed of the idle, that means the carb cleaner is getting in and indicates a not so great seal. Spray on every injector, spray at the CIS runner 'boots', spray on the AAR, the decel valve, the WUR, the airbox, whatever.
Also, it's important to know if your car is a ROW car or a USA car, because the CIS system is significantly different. The USA version has more electronics, which means more electronics to possibly fail. Is there a computer box under your passenger seat?

[UrQuattro]

Thanks for the reply...

My car is a full on CA car, so definitely a US car. And yah, I've had a little experience with the computer box under the seat, it's accompanying acceleration enrichment box (does that function electronically or via vacuum), and ESPECIALLY the metal covered relay that goesalong with it. It was the failure of that which actually brought me to ask the board to send me to the Jedi master of CIS in 911s in the sf bay area. I was sent to a shop in Lafayette of which I can't remember the name right now. But I trust this gentleman's work. You gotta see his shop. He used to be a factory tech back in the 356 days. He has an amazing collection of parts.

The WUR I got had the correct part number for my car, as did the extra lambda box that I have for the car just in case. But that wasn't necessary.

He was very specific about having tuned it whilst dead cold. He had me tow it there the day before my appt. so that he could really get it all working right, and get the cold and warm pressures set right. He said my old WUR mostly worked, but that one of the gasses didn't change quite enough for his tastes, so then he installed the new one (it was New in box).

I'll try to take a look tomorrow to see if anything is disconnected, but having the vacuum line diagram will help me trace out as much as I can.

As far as the weirdness with the mileage. I mean, I should be getting at least 17-18mpg based on the mileage I got while cruising on the freeway and while pushing it as hard as I could. It's just weird to get that mileage or less.

Can anyone think of what might impact my mileage so much over a tank of gas when it takes about 6 weeks to go through a tank of gas on average. I dont drive it every day, usually it's 1-2 times/week and then maybe a random weekend fun run.

Is it possible that somehow the multiple starts and stops, up to a week of non-usage, and issues like that might be a factor in making it APPEAR that my mileage is so bad?

I can tell you that it feels like the more times that I drive my car on one tank of gas that it seems to go through more gas. There are some times where it seems like the gas gauge almost changes from where I left it after driving it one week, to starting it up and driving it the next.

As far as gas leaks, I've had that checked by a good shop close to my house. The father of the owner is an ex tech for the Porsche factory way back and has a ton of experience, obviously, and he's trained his son well. I had a leak on the top of the tank where those braided cloth lines attach to that nipple close to the drivers side rear.

Once they replaced those, no more gas smell, and they checked the rest of the car and said that they didn't find any other leaks.

So I just don't get it. It's almost like the injectors are leaking the gas that is in the lines under residual pressure in to the combustion chamber or something like that. But if that is what was happening, I cant even imagine how the car would behave when started. I doubt it would just start up and immediately fall in to a nice 1200rpm idle (although sometimes it idles high, at 1500rpm) for a couple minutes when starting from cold, but then it drops once I'm going and give me a nice idle.

So I'm at a loss...

Michael

[CharlieB]

Assuming a properly working stock CIS system you should be getting mid to high 20's on the highway. My 80SC gets 26-27 mpg on the road at 70 - 80 mph cruising!!

[mca]

Don't have time to read this entire thread right now.

But if your O2 sensor is connected I don't think you will see a drop in RPMs. It will correct the mixture when you remove cap.

Try it without sensor connected.

[81 SC Ray]

Michael,

I feel your pain. I am a daily driver - all in town - short trip and I would love to have your gas mileage. My last tank - 11 miles to the gallon. My car runs like a champ. 1800 rpm on a cold start - settles to 950 rpm over a few minutes. No decel backfires - crisp rpm accel when revved. It shows no sign of running overly lean or rich. I have exhausted my knowledge and need to bring it in - but - it is running so well other than gas mileage....

[Drisump]

I think your fuel economy is on the low side but do you drive it on the mat all the time? On DE and AutoX days my car (a Euro 3.2) gets probably similar numbers to what you're getting. On the highway at probably 65-80 mph it returns about 25-26 mpg. Generating HP costs fuel, every time you demand 200hp from your car, it will demand a corresponding amount of fuel. Sorry, but if she's been recently tuned by a competent Porsche shop, there probably isn't a mechanical problem with the actual running, although frictional loss might be worth investigating. For instance a slight dragging of the parking brake may cause this, but you would think the high spot on the contact area would wear down over a period of time. Cheers

[Bob Kontak]

Hi Michael. You mentioned that you took a 100 mile test run and it returned just under 20 mpg. That was at 75-85 mph. Folks talk of 25+ mpg but I really struggle with buying into that mpg at an 80 mph average. I did a 100 mile trip at 65 to test my 81's mileage and it was 27mpg. That's with the standard SC speedo error of 10% fast. I was actually going 59mph. If you were driving a true 80mph I think that 20mpg would be about right. There is a big difference in hp requirements at 80 vs 60.

Do the carb cleaner test Gogar recommends and I bet you find some leaks. 14mpg is low for normal driving with occasional "peel outs". There are some weird places for air leaks but I don't think any of them require an engine drop. Some painful contortions but not an engine drop. The hidden one is the brake booster. John Walker said he found one cracked and sucking air next to the transmission.

I don't think the O2 sensor will mask the rpm drop. I may be dead wrong but have read that recommendation many times and never until this thread read to unplug the O2 sensor. Try it - can't hurt anything.

[Gogar]

Originally Posted by UrQuattro

My car is a full on CA car, so definitely a US car.

So, knowing that you have a USA car with a lambda computer, your WUR will NOT have a vacuum line, so don't worry if you don't find one there.

[TibetanT]

Your idle numbers seem a little high IMHO.

950 +- 50 is spec I believe.

[UrQuattro]

Thanks for all the suggestions guys. TibetanT, sorry I didn't make it clear, but the 1200-1500rpm idle I was talking about was when it is cold. Once it warms up a bit after a couple minutes, it drops down to right at 900-950.

Bob Kontak- actually, to me, that 100 mile highway trip that resulted in a 20mpg consumption was mind-blowing to me. I had never seen such great mileage. It was illuminating in the fact that I probably SHOULD be getting better mileage because of the results of the other test, which was balls out through the twisties, running the car at full tilt, keeping the engine at a minimum of 4500rpm, but usually higher, and running at WOT every chance I got. And I got 14mpg. And I get 14mpg as an average mileage in my regular driving.

My regular driving is not too aggressive. I could probably be more conservative, but I am not too bad. Definitely not WOT all the time. That's why this just makes more sense.

And thank you for the reassurance on being able to fins the leak...

So yah, maybe I need to have a more thorough bit of testing done. I haven't had the injectors checked specifically for a while now, maybe they ARE leaking a little, or the spray patterns are bad.

But I would think that inefficient combustion would have shown on the smog check. But i had it tested after I did my round trip in to the city and back, so the engine was nice and warm.

I dunno... Argh.

Michael

[mca]

Bob - I never run with the sensor and my rpms always drop when oil cap is removed.

Couple of weeks ago I got the sensor connected and working again. Did my monthly oil check and rpms did not drop. I then unplugged my sensor and tried again - sure enough rpms dropped when I removed the cap.

I only shared this experience because I thought it was important. It makes sense that the sensor would correct the mixture no?