Motosport

I'm currently trying to track down the cause of my car's excessive drinking problem - as in 13-15 mpg. I have tested various sensors and now the time has come to test the TPS. Using Clarks Garage and other guides I started by testing at the sensor plug and values were all within range. Idle switch showed 0-10 ohms resistance with the throttle closed and went to infinite when opened. It also clicked like its supposed too. Next was the throttle opening circuit test between pin # 2 and 3, and this also came out good, showing 740 ohms resistance and increasing as i opened the throttle until it showed 3,78k ohms at WOT.

Then I moved inside the car to test at the DME and KLR harness plugs and this is where it got interesting - or rather confusing in my case. The idle switch test on the DME plug showed the same values as on the sensor so I quickly moved on to the KLR. And when I tested on the KLR harness plug between pin # 22 and 23 the resistance showed 3,6k ohms at idle/closed throttle and then DECREASING as I opened the throttle until it finally was down to about 700 ohms at WOT. So the resistance levels are working the exact opposite as on the TPS sensor and not increasing with throttle opening like its suppose to according to Clarks and others. I double and then triple checked everything again but same result every time.. I am utterly confused as how this can happen...

The car starts, runs and idles fine and I'm not experiencing any of the symptoms normally associated with a defect TPS. I hope someone can help me out on this one and also that you can make sense of my description, since I am certainly no expert on electronics and english is not my native language..

Alex.

Dave W.

I'd check the basic wear items first. How old is the O2 sensor and spark plugs? They can ruin gas mileage when they wear out and go bad.

Motosport

I am in the process of doing just that and it seems like at least the O2 sensor is bad, since I'm not getting any readings at all from it. I'll check again today just to be sure. Plugs looks fine but I'm probably going to change them anyway since I don't know how old they are.

But I really hope someone can help with this opposite resistance values issue in the TPS, since I'm really confused about it. I have done some searching on the subject but all it shows is that the resistance values should be the same, as in increasing with throttle opening, which of course makes perfect sense. Which is also does when measured at the TPS plug but somehow gets changed on the way to the KLR..

And the funny thing is, that the car runs just fine anyhow but I'm just wondering about the cause and if its has something to do with my poor gas mileage..

So please chime in with any good suggestions or advise on this..

Thanks - Alex.

Motosport

Well I tested the O2 sensor again this time using the correct procedure - that always helps doesn't it...

Not surprisingly its showing quite high values indicating rich condition. With engine warm the voltage was ranging from 0,78 - 0.9 when its suppose to be around 0,5V. Taking the vacuum hose of the FPR to inflict a rich condition, the values jumped up to 1,5 - 1,7V and when removing a vacuum hose on the intake manifold to inflict a lean condition, the voltage only dropped to around 0,5.

So is it the O2 sensor causing this overly rich running condition, or is it just showing that the engine is running rich caused by something else.?

Alex.

Oddjob

Everything stock? Have you checked fuel pressure? AFM?

Do you have driveability problems, stumbling, bad/rough idle, poor throttle response? Black smoke exhaust on acceleration, are the spark plug tips sooty black?

I have had a TPS fail, where is was sending the full load signal to the KLR-DME, even when the trottle was closed. But the car would not run, stalled out from way too much fuel.

Crackership

What readings to you get from the O2 if you shut the car off once the O2 is warm?

Motosport

Yes, maybe some more basic info would help..

The car is an '89 Turbo S, bone stock with 91k miles and in excellent condition, not least mechanically. Its been serviced at porsche shops its entire life, even though the last porsche service was some 18 month ago due to the PO lack of driving it.

I have yet to find any dry rotted hoses, split intake boots, leaking gaskets or any other leaks in my car. So either our climate is more gentle on these cars or the PO has taken very good care of it. The car drives excellent, always starts first time, idles steady at 850 rpms, runs smoothly, no vibrations or misfires, makes good power - for a stock 951S - and holds boost all the way to redline in all gears. The spark plugs looks dark brown but not black and wet, with a little bit of carbon build up at the bottom. It smokes a bit when cold, but not during driving, at least nothing I can see while behind the wheel. There is some black sooth in the tailpipe and when I run my finger inside its very black and dirty.

I tested the DME temp sensor which was fine. AFM was changed recently to a brand new unit and it tested good as well. I haven't tested fuel pressure, mainly because I don't have the tools but also because I have ordered a 3 bar FPR to go with the A-tune I'm waiting to install.

I have just ordered a new O2 sensor to eliminate a problem with the old one, but testing it just made me wonder if the sensor is just measuring a rich condition caused by something else, or itself is causing rich running because of it being faulty.? Anyone knows...??

And then there's the issue with my TPS resistance values showing opposite at the KLR plug as described in my first post. This really has me baffled, even though the car seems to run great anyhow, and I still hope someone can give me an answer on this.. Or at least if that will have an influence on my poor gas mileage..

- Crackership, once again you have good ideas for me to try.. Now you've posted it, I wonder why i didn't think of it myself.. I will try it tomorrow and post back..

Thanks for all your input..

Alex.

Oddjob

O2 sensor will not have a big impact on fuel economy, maybe a few mpg at most. Having run a car w/ a wide band O2 sensor installed to monitor AFRs, and running w/ and w/o the factory O2 sensor - it does not change the AFR that much. Just keeps it slightly closer to a 14.7:1 ratio at idle and during some closed throttle/lift throttle coasting situations.

By the way, what year did european cars start using an O2 sensor?

If there is an actual problem w/ the TPS, the car will not run right. Also, often that the KLR will identify a fault w/ it which causes the KLR to go into "limp home mode" so it will cut the boost.

How are you measuring the mpg and what type of driving? If the car was running so rich as to get 13-15mpg during freeway cruising, you should be seeing some other symptoms, like sooted plugs and smoking black exhaust on accel. and bogging or stumbling. Do you have a heavy right foot and do a lot of city driving?

Motosport

I'm not sure when we got O2 sensors in our cars, I just know mine has one..

I dont think there is a problem with the TPS either, firstly because I'm not experiencing any of the problems normally associated with a bad TPS, and secondly because when I measure resistance levels at the TPS plug they are increasing with throttle openings at they should. Its only when I measure at the KLR plug pins 22 and 23 that the resistance levels are suddenly the opposite way.. very strange..

I drive quite gently with mostly country roads and highway on the menu, and only occasionally going WOT. So gas mileage should be pretty good but I can just barely go 250 miles on a tank. I calculate my mileage on the amount of gas I put in versus the miles driven. And this doesn't even take into account that my speedo is showing 10% to much..

And I'm pretty sure its not leaking gas, since I've had the car sitting in the garage for the last couple of days with the floor being bone dry and not even the slightest smell of gas.

So for now I'm hoping the FPR is the culprit which I'll find out very soon when the new 3 bar unit goes in along with the A-tune. And perhaps installing the new O2 sensor will help as well.. With gas prices of just over $8 a gallon over here I really need to get to the bottom of this issue before I'm confined to only driving my VW diesel DD. What a horror scenario...

Alex.

zerMATT951

There are two sets of pins on the TPS that will give you opposite resistance values, one goes up when opened, the other goes down. The 3rd set is the idle circuit that goes from closed/short to open as soon as the throttle opens. It sounds like you tested one of those functions out at your TPS on 2-3, I think the opposite set is pins 1-2.

schip43

Uh Oh, I hate to throw a monkey wrench into the mix but... the car should be sorted of issues ;just like your describing, before you go modding it!

You throw new performance parts on it, you might fix it, you might not and you've just added a bunch of new variables into the mix!

Just saying.

Crackership

+1

Dave W.

Now that I think about it, the O2 sensor is not the main problem.
The reason a bad O2 sensor can ruin gas mileage is because they read lean when they go bad. This causes the DME to add fuel in an attempt to get a rich or stoich reading.

Fuel leaks can happen. The best time to check for a fuel leak is when the car is running, or just after it's shut down.

Motosport

Yes, I know its best to get this fuel consumption issue solved before adding performance parts and I'll keep working at it. But I figured that if I can eliminate all other causes than the FPR, I'll know if that's the problem once I install the new 3 bar FPR with the A-tune. And if not it will be fairly easy to revert back to stock and keep plugging away at this problem.

- Dave, I see what you're saying and I'll try getting under the car while its running to check the fuel lines front to back. I read somewhere that a pinched return line can cause the fuel pressure to go up, so I can check for that as well while under the car..

Thanks for all your input on this..

Alex.