cold_beer839

My '93 GTS 5-spd burns oil badly when I decelerate and downshift to a stop.

I know they all burn oil, but mine really pumps out the smoke when I decelerate. That is the only time it creates any visible smoke, never under hard acceleration or engine revving.

I was told that there may be a vacuum line that is closing shut when the intake manifold vacuum pressure gets high (as it does we you let off the gas and decelerate in gear). We know these lines get very soft with age and strong vacuum can pull them shut.

I have the 928 Specialist SharkVent system on the car and that works fine, it catches a lot of oil and puts it back in the oil pan, plus the 2 crank case vent lines that used to route behind the MAF are no longer routed that way, so I know that is not the source of the oil.

My question is where would it be coming from?

I plan on an intake R&R within the next few weeks and want to have ideas on where I should be looking for this possible oil sucking line while the intake is off. If not a soft vacuum line, then what oil passage in the motor would act this way under these conditions?

dr bob

David, my experience is oldschool on this, not particular to the 928. When I'd see a lot of smoke on trailing throttle, it was almost always worn valve guides and seals. On your 928 with all the crankcase venting and the recovery can, the only other option is high crankcase pressure mixed with the low chamber and intake pressures. That would cause oil to be left on the cylinder walls on decel.

When you have the intake off, look for the coked oil deposits on the intake valve stems. This is a telltale of oil getting sucked past the intake valve guides. Obviously, combustion chamber oil would not coke the intakes unless the valves leaked a lot.

Also, take a look at the plugs after you drive a bit. Try to do a coast/trailing throttle run right to where you check the plugs. If they are uneven colored or splashed you can narrow it down of maybe a few cylinders.

No matter what, if you are sure you aren't pushing oil up the intake anymore, you will have the heads off for guides. You'll see telltale scratches in the cylinders from any broken rings.

Just curious what your warm compression numbers are like. That will generally clue you to ring and piston problems, especially if you do a wet test after the dry.


That's my two sense.

stealth

Hoping I am not singing the same song, just ordered my Sharkvent which happened to come to market during my intake refresh. Nasty black inside that intake but intake valves look clean.

I don't get smoke on decel, though-- but I do think engine braking is a large part of what "queues it up" for the huge cloud on any downshift to 2nd that makes the PSD go to work (for example). Guess it collects in the TB or plenum waiting for that rush of air.

At last check, I had 185-195 compression on all cylinders, which also makes me hopeful that the breather system is my culprit.

IcemanG17

I don't get any smoke....but the widows intake port was quite a it more black-carbon then sharkys was...the good news is the valves were spotless on both....I'm buring about 1/4 qt per 20 minute track session.....it doesn't appear to be leaking, so it must be pumped into the intake, which can't be good...

stealth

Yeah, I have no leaks, and there was nothing in the engine valley except the gas I didn't quite catch when disconnecting the fuel rails.

I actually burn LESS at the track than on the street. Mine's one of the worst, about 300-325 miles/quart on the street but at the track or at...ummm...higher speed roadtrips it makes it to nearly 500.

cold_beer839

I have 44k miles on my engine and the PO took excellent care of the car, so hopefully it is not a valve guide issue.

Jadz928

Excuse my ignorance, but are there oring-type rubber seals in the valve guides?

Rob Edwards

S4+: In the valve guides? No. Above/on the valve guides, yes:

RKD in OKC

Luxury. We dreamed of getting 300-325 miles/quart. My 46K GTS was getting 200 miles/quart.

The previous owner hadn't put many miles on it so I presumed the rings were probably a little gooked up and sticky. I did a fresh oil change and added a quart of Marvel Mystery oil to help clean up the rings. Understanding that it might take a while to free up the rings if stuck I add another quart of MMO as the oil level dropped. It didn't seem to make much difference as it continued to use oil at the rate of a quart/ 200 miles.

After adding about 3 quarts of oil to keep it topped up I tried some Rislone Compression Repair. Soon after I did a 400 mile highway drive and the GTS didn't use a drop of oil! It did start using oil again, presumably after the additive heat cycled and got dirty. But not as bad as it was. It is now using a quart of oil about every 600 miles and has continued that consumption rate for the last 3 top up quarts.

I just had an intake refresh done and at 52000 miles. While off I inspected the inside of the intake and intake ports, looks clean and has very little oil film. Not really what I was expecting. Looking oil consumption is rings or valve guides. I've always understood to tell tale sign of valve guides was oil smoke on startup when they are cold. Mine does not do that. And no noticeable smoke any time.

I have watched the oil level religiously. It doesn't seem to use much doing highway driving with long periods at constant rpms, especially high speed It also doesn't seem to use any oil autcrossing where there is lots of full throttle, on-off throttle, and engine braking downshifts, the type of driving that made my 90GT suck oil down the intake like crazy. Seems to use oil most in stop and go traffic with lots idling at stop lights.

andy-gts

my gts has consumed about a quart every 2200 miles ....so change with the sc set up yet!
I feel lucky.
andy

GregBBRD

Change your GTS oil to Torco...it cuts down the burning issue by a huge percentage...also have the correct additive to protect your camshafts.

cold_beer839

What weight of Torco?

I'm running Mobil-1 15w50 synthetic now.

RKD in OKC

Is Torco the guys that make the concentrated race gas or the guys that sell the muffler bearings and blinker fluid?

cold_beer839

My guess is these guys: http://www.torcousa.com/

GB, what weight of Torco oil do you recommend?

RKD in OKC

Ouch, 1 liter is $17.95!