Faulty speedo, wild oil consumption and paranoia

[Jameel]

So I've had my '83SC since April 14, 2016. In that amount of time I've used many quarts of oil. 14.5 in fact. I've driven the car 4940 miles. I've enjoyed every minute of them. Well, I should say that some family members drive the car too (that landed my older brother in a speeding ticket class, for which I will relish ribbing him forever for) but 95% of those miles are mine. Doing the math, I've got 340 miles per quart avg. since I took ownership. Always Castrol GTX 20w-50.

I've checked mileage and oil consumption since I bought the car. The worse "mileage" I've got was right when I bought the car. Drove it home about 400 miles and it drank up a quart in the first 218 miles. That gave me heartburn the whole way home. I don't think the PO drove the car so much (but his records show good maintenance) so about 1000 miles into my ownership it just kept getting better. The next time I put oil in I got 423 miles per quart. Then it went back down as I drove the car on shorter, 20 mile or so drives, around 300 miles/qt.

Went up to Road America this summer for the vintage races (about 4.5 hour drive) and put oil in before leaving. After driving up to Elkhart and then down to Milwaukee (300+ miles) it needed oil again. So I put in 1.5 quarts (always to halfway between the dots) of a different oil since the station didn't have Castrol. I used Valvoline VR-1 Racing 20w-50. Next time I added oil after the VR-1, I did the math. The VR-1 yielded 686 miles per quart. I was shocked when I checked this. To test my theory I put Castrol back in twice since then (4.5 qts) and my consumption has dropped back to around 280 or less.

So I've got two possible variables. First, the PO sent along a gallon jug of Castrol when I bought the car. I would fill the car from this jug and monitor the level from the little window on the side of the jug. Not exactly sure how accurate these are. So those early numbers might be off. Second, my odometer. For that past 1000 miles or so I've noticed the speedo won't move until I get to 30-35mph. It stays stuck at launch, then jumps to 30 or so when I get to that speed. If I take off, then tap once on the glass (I thought that only worked in movies!) the needle will come to life. My concern here is that I don't always tap the glass, or simply forget. I'm wondering if the odometer is somehow related to this, such that it perhaps doesn't come to life either until I hit a certain speed. I'll admit I don't really know the internals of the mechanism. So if my odo is registering less than my actual mileage, then perhaps my oil consumption is much less than I've calculated.

The car does not smoke, save for the occasional puff at startup. It doens't not leave a smoke trail at heavy throttle. I have not examined the plugs for oil. But the PO had the plugs and wires changed right before I bought the car, and the mechanic (a big Porsche dealership) didn't note anything. In other words, there are no obvious signs of oil consumption that I've seen. I get dollar-size drip on the garage floor if the car sits for several days. If I drive it every day, pretty much no drips in the garage.

I'm actually not paranoid, just curious more or less. I bought the car figuring I'd be doing a top end at some point, so news of that won't be a huge shock or anything.

So, aside from switching over to VR-1 when I do an oil change next, you all have any thoughts?

[Trackrash]

I think you are looking at a rebuild in your future.

IMO Castrol GTX is crap. I just changed to it for the first time since 1972 when I thought is was crap. Granted my motor is on the verge of needing a rebuild, but since I changed to GTX my plugs are looking much oilier than when I used VR-1. And they are fouling more often.

I would change to VR-1 and do a compression and a leakdown test.

Is there any history on the motor? Any rebuild in it's past? Receipts?

[jwakil]

Maybe you have an oil leak while driving? That kind of oil consumption should be causing all kinds of smoke and smells if you were burning in the cylinders.

[Jameel]

Originally Posted by Trackrash

I think you are looking at a rebuild in your future.

IMO Castrol GTX is crap. I just changed to it for the first time since 1972 when I thought is was crap. Granted my motor is on the verge of needing a rebuild, but since I changed to GTX my plugs are looking much oilier than when I used VR-1. And they are fouling more often.

I would change to VR-1 and do a compression and a leakdown test.

Is there any history on the motor? Any rebuild in it's past? Receipts?

Crap. Ok. Well, after the VR-1 experience I'm convinced. No history of a rebuild. PO fixed a bunch of oil leaks, thermostat, new lines, etc. in the last 10 years. So I don't think its leaking oil while driving. It's a little wet looking here and there underneath, but nothing that would suggest a lot of oil getting blow towards the back.

[rwest]

Wow Jameel,

You're basically "changing" the oil every 4,000 miles just by dripping, burning or blow by.

My engine is pretty wet on the underside and some small drips and my guess is I'm only putting in a few quarts during that mileage period.

Does the tail pipe smell like oil?

Best,
Rutager

[fanaudical]

That oil consumption does seem high.

If you're getting drips on the floor, then you do have a leak. (No drips when driven frequently just means the oil is getting blown around.)

I doubt that adding 1 qt of VR-1 to a full load of Castrol improved your oil consumption. Am guessing that driving habits during that time had a bigger influence.

I would swap to another oil (I use VR-1 without issue), drive it for a few hundred miles, and then do a compression and leak-down test. Please post results; we're all curious how this comes out.

[manbridge 74]

Take your oil tank cap off while engine is running. Do it cold, warm and normal operating temp. See a lot of vapor coming out?

Pull plugs and look for oil spotting or worse on center electrode.

Compression and leak down if possible.

If above is good and it runs well try a diesel oil like rotella or Delvac 15w40 as they have more detergent than regular oil and might (a big might here) help free up oil control rings. Won't help with valve guide wear. You can buy a lot of oil vs cost of rebuild.

[DanielDudley]

Generally speaking, 911 engines use oil through the exhaust guides. A half decent cat converter will help to mask that, but if you do the math, a quart of oil in 500 miles is not a whole lot per actual combustion stroke. Seriously, it is about one one millionth of a quart assuming 2000 rpms at 60 miles per hour. That's 120,000 revolutions per hour for approximately 8.4 hours, or 1,008,000 revolutions per quart. So no radical smoke.

However, it isn't the greatest for your valves, because the wear is enough to keep them from seating perfectly, so they rattle a little bit while seating. Even so, maybe the next time you add oil, you should add a quart of ATF instead, and just drive it around the block gently for a few minutes. Then change the oil to something like Valvoline VR1 20/50, or Rotella T6 20/50, and run the sucker. Nobody on this forum really recommends the T6, but it is a decent oil with the correct additives, and it has a lot of detergent, not that you probably need it. Run that for a while, then change over to VR1, and drive it like you stole it for 500 miles, just to see if your rings won't free up a bit. Not that this is likely, but it is possible.

Then do a compression and leak down test. BTW, do you know when the valves were last adjusted ? It won't change your consumption, but it is a good thing to know, and it could effect your leak down test.

I'm not a doctor, and I don't play one on TV, but I have found that simple things like changing your oil and running your car can affect oil consumption. Castrol, in spite of its reputation, is not a really great oil, and can gum up your engine. Get it out of there.

Doubtless others more qualified will come in and give you some decent advice, but a little detergent oil, and a few quick changes has helped more than a couple of engines in my past.

[porsche930dude]

Your spedo probrobly has something to do with it but its still excessive. Id try a heavy dose of marvel mystery oil. It worked wonders on my old 350 chevy went from a quart every 250 to almost nothing.

[Reiver]

If your speedo isn't working properly it is hard to get a proper figure and my speedo and odometer are off 6 mph over or 60 miles every 1000 clocked.
I'd change to the VR1...you need a good flat tappet oil anyways (see some of the oil threads about zinc/phos etc.)...and do a route drive of a known distance at normal throttle/rpm and see what you find out.
300 mpq is definitely rebuild time for the top end (you didn't say what the overall mileage was) and in the 600's is rebuild time for me too altho some keep at it.
In my old 930-16 (83 SC US model) I'd get in the 600's mpq and only see a puff of smoke on start up and hard deceleration. Adding that much oil is a pita.

[zedsn]

I own a 89 944 turbo and used to use 20/50 castrol but switched a couple of years ago to VR1 and my oil consumption did go down some. Won't go down like you want yours to but I did notice a difference in adding intervals in my turbo.

[Jameel]

Wow, thanks everybody. Oil change scheduled for next week. I'll do my assignments and report back! I may even do a leakdown.

[Jameel]

Update on my oil consumption. A couple weeks ago I did an oil change and filled her up with VR-1, and added some Seafoam at the same time. No metal on the either drain plugs, and I dropped the sump plate to inspect as well (first time for me.) It was clean, and no sludge or anything in the bottom of the case when wiping my finger around in there. I've since driven the car 607 miles and I'm happy to report am improvement in oil consumption. Of course being a cooler time of year, I'm not sure if this has affected this. I put 1.3 quarts in since the change, and that comes out to a bit over 400 miles to the quart. Most of that way steady highway cruising with some spirited back roads driving. Not quite as good as I was getting when I got that 686 mile/qt. back in July. My math must have been wrong. But its certainly better than the upper 200's that have been the average.

I also had a Porsche mechanic do a leakdown and compression test earlier this week. #3 was at 30% leakage, another at 16%, and the rest below 10%. Compression was 185-165 across the board. We think the leakage on #3 is getting past the rings since we could hear the hiss when removing the oil filler cap, but not through the airbox (filter removed) or exhaust. We couldn't really hear any audible leakage on the other cylinders.

So for now I'm going to simply drive the car and enjoy it while saving up for the inevitable rebuild. I think if we'd heard leakage coming from a valve on #3 I'd be a bit more paranoid, but with worn rings it's not so much of a worry, right?

[zedsn]

Since you are planning on doing a rebuild sometime in the future you might want to consider adding engine restore to the crankcase if you wish to aleviate some oil consumption. I use it in my TR4 with good results.

[nickyclyde]

No all you need to do to know if your odometer is acting up in concert with your speedo is drive 1/2 mile under 30 mph and see if the odometer reading goes up 5/10's

[universeman]

Or use a GPS-based speedo app for your phone, phone in the car, go for a drive, compare the two.

[Walt Fricke]

The speedo and the ODO get their initial signals from the same place: 8 magnets attached to the ring gear in the transmission. When the car moves, these magnets ground the odo signal circuit 8 times per right rear wheel revolution. There is no way that this can cause a sticky gauge.

For the odo, these pulses either turn a motor which turns the odo gear cluster (which is how the old cable odos did it - mechanical rotation of a counter system)or do something else like pulse a solenoid every so many pulses and cause a gear to move a tooth for every X rotations or fractions of a rotation. Again, things don't stick.

The speedometer needle is different - these pulses are translated in an electromagnetic force which causes the needle to move against the pressure of its spiral spring. Something in that system could get stuck. Send it to a speedometer shop - they can fix it.

Checking the odo is easy as pie, though - just use the mile markers on a highway.

[Jameel]

Thanks for reminding me Universeman, I did that with the app and it checked out.

And Walt, fantastic post. I love learning this sort of thing.

[DRACO A5OG]

This is what Master Walt is talking about:



Fixing Speedo will do nothing but mask the problem if a magnet or two fell off.

[Drisump]

If you have added "1.3" quarts I'm thinking that you are constantly on or near the max level on the dipstick. Most of the gurus on this board will tell you to use mid point on the stick as max and add a full quart when you're at min. Many people experience a radical decerease in consumption when this is followed. My 85 does 2500 miles between quarts when this procedure is followed but it did about half of that when I was "topping up" to the max line. Cheers