Oil level gauge weirdness

Jerome74911S · 10-21-2010, 04:02 PM

The oil level gauge reads either all the way up when hot (while stopped), or all the way down (while driving). There is no middle ground. This is new - what happened?

RSTarga · 10-21-2010, 04:11 PM

ground wire? Faulty sensor

Drisump · Drisump's Garage

You know that it is only accurate at idle, level ground, and at running temperature right? My car usually is accurate to the dipstick when those conditions are met, reads low while driving. Cheers

dshepp806 · dshepp806's Garage

I'm a lucky one, in that my gauge will track (quite closely) what's happening...as drisump noted, the gauge is only to be read in the "oil checking" conditions. In my case, when the oil is at temp, I will read a mid mark reading on the stick and see a mid mark indication on the oil level indicator. This is NOT the case for many owners, but the purists know that the stick IS the reference, aside from whatever that oil gauge would show.

Should there be disparity, then (of course), the oil level sender could be the culprit, as it may be sticking and delivering the wrong resistances (subsequently, voltages) to the instrumentation,..(as noted by RSTarga)

How's the stick readings (meter aside)?

Best,

Doyle

Walt Fricke · 10-21-2010, 10:27 PM

If the sensor wire has no path to ground, the gauge will peg. You can see this if you pull the gauge out of the dash and disconnect the sensor wire.

When the sensor float is low (as in low oil level in tank) the wiper on the resistance winding is at its lowest point - the least resistance/fewest windings between the wiper (which is what the sensor wire connects to) and ground. The most wear on the resistance windings comes somewhere in the middle. If the resistance wire has worn through there, you will get normal readings until the oil level rises. Which it does as you drive the car. Then, suddenly, the resistance goes infinite as the slider passes the break. And the needle will, all of a sudden, peg.

Jerome74911S · 10-22-2010, 11:59 AM

The engine was just put back into the car. Unfortunately, the idle will not go below ~1500 rpm when hot. It seems not to be a vacuum leak, but I have not had time to work on the car to track down and fix the problem. So, at 1500 rpm, hot, the dipstick shows nothing while the oil level gauge pegs to the top. Initially I put 8 litres of oil in the car, so the dipstick should show something at least. Therefore, I think both the dipstick (due to too fast idle) and the gauge are wrong. Could this be?

Jack Olsen · 10-22-2010, 12:16 PM

My advice would be to ignore the dash gauge for now. Trust the dipstick. (And fix the idle problem)

If you're a quart low, or even three, it's not a big deal so long as you're just idling the car. How much did you add when the motor went in?

Wil Ferch · 01:43 PM

Hint ..... the gauge will *not* read correctly at a high-idle speed. Someone here mentioned 1500 RPM? Will read "low" just like when you're driving.

The drill is...warm ( 190 degF or so, say 8-9 o'clock on the gauge), level ground, and something like 800-900 RPM.

Walt Fricke · 10-22-2010, 01:27 PM

1) never distrust the dipstick - it show where the oil level is at that moment. You need to interpret that to decide if you have too little or too much oil, but that's a different matter.

2) looks like your oil level sender is broken. Get a new one. It is a bit of a pain to get out and back in, but doable. Be sure to get a new gasket to go with it - they can be fragile. If the pegged needle bothers you (it did me, for some reason), disconnect the +12V from the back of the gauge. That way it will stay at the bottom of its scale.

There is a slight possibility that either the sender wire terminal at the sender, or the separate ground wire terminal at the sender or where it goes to ground, have dirty connections which are affected by temperature, movement, whatnot. So you could clean those up to see if they, rather than the sender resistance wire windings, are the problem. If you are handy with an ohmeter, you could use that to do some connection checking.

Jerome74911S · 10-22-2010, 02:38 PM

Quote:

Originally Posted by Jack Olsen

My advice would be to ignore the dash gauge for now. Trust the dipstick. (And fix the idle problem)

If you're a quart low, or even three, it's not a big deal so long as you're just idling the car. How much did you add when the motor went in?

I put 8 litres of oil in the car when we put the engine back in. The original owner's manual for this car calls for a fill of 10. Should I dump in the entire 10 litres and expect the amount to be correct - dipstick or not?

Jack Olsen · 10-22-2010, 02:43 PM

You're fine until you fix the idle. If you have a front cooler, some of the oil was in the lines even after you drained the tank and the crankcase.

Drisump · Drisump's Garage

My car (a 3.2,) won't register an 8 quart fill on the dip. Unfortunately, idle at 1500 isn't going to allow you to trust the dip stick, but I assume Jack knows what the capacity of your car is. However.... fix the idle so you can "trust the dip stick". Cheers

Jerome74911S · 10-23-2010, 05:32 PM

I'll dump in the 10 litres that the manual calls for. The weather should be good tomorrow in between the constant rain we have been having, so I want to drive the car - I'll have to find time to work on the hot idle speed later. Don't know why there should be a disaster with this approach, although I won't do it on a prolonged basis.

Wil Ferch · 10-23-2010, 06:20 PM

Don't just "dump in" 10 qts. The disaster might be that you overfill by maybe 1 qt.

Dump in 8......wait....take a reading......dump in 1 more....wait....take a reading.....and then only add 1 more if it's necessary.

Target filling halfway between "full and "low" marks on the dipstick. Total oil capacity for cars with front oil coolers is 13 qts.... changeout volume is 10 qts so 3 qts always stays behind. For future....what you totally take out for oil change should be dumped from your receiving oil pan to empty gallon milk jugs. If you end up with 2 full milk jugs and one half-full.....ONLY THEN "dump in 10 qts" at once for refill...as now you'll KNOW what just came out !!

Jerome74911S · 10-23-2010, 06:36 PM

Yes sir. The car has no front oil cooler, and since the engine was tipped this way and that on an engine stand while out of the car, legitimately it was empty of oil. Therefore, why would it be wrong to install 9 litres, or even 10? I know all about the correct methods of measuring oil level and where it should be on the dip stick, but if in this case Porsche says 10 litres did they get it wrong?

Wil Ferch · 07:00 PM

I'm not saying "Porsche got it wrong"..just as much as saying eating an entire pizza is not "wrong", but if you do...then eat it by the slice !

I'm just sayin' you can easily dump 80-90% of capacity fast....but do the last 10-20% slowly and with caution as temps and such change with a running engine.

OK...try this then for an answer--->

Cars without front oil cooler at all have a total capacity of closer to 10 qts. System capacity. So...not 13 like the cars with front oil coolers. And how this distributes itself is not entirely intuitive...as the RR fender mounted oil thernostat needs to open before the entire 13 qts is in "communication" with the entire, onboard oil capacity.

Then there's this little nuance---> going back to the 13 qt cars. 10 qts (+/-) seems to be the change out volume. If the car was recently run, 6-7 qts will come from the dry-sump oil tank, and 3-4 from the engine block. If the car sat overnite from the last run...and you do the oil change 12-24 hours later...the numbers are reversed. About 6-7 will come from the engine block drain plug...and 3-4 from the dry-sump oil tank.

Just sayin don't go hog wild and dump 100% "spec value" all at once ....without coming up to the last percentages slowly. Just good practice. Hope you don't overfill. Have seen a number of Porsche shop work orders with the notation... "customer overfilled engine on oil change". Can do a real mess including excess finding its way into the intake tract.

Caveat Emptor.

Jerome74911S · 10-23-2010, 07:19 PM

Thanks for your further prudent remarks, Will. Obviously it is good advice and should be followed.

I guess I was just raising a theoretical issue in reference to Porsche's specs. Actually, the engine was overfilled by the PO and I did get oil in the intake and it was a mess, since the drain hole in the airbox was wide open and the oil went all over the place. That is fixed, but I sure don't want oil in the intake again.

I'll add oil gradually - actually I would have anyway.