07-22-2013, 09:55 PM
Cause of Excessive Oil Consumption ...
I searched our forum but didn't find anything relating to this particular discovery, so here's my story. If this is old news, then it's one of those many things everyone else knows about but me.
Every time I revive my car in the Spring, following the usual 3+ months of storage in my unheated garage where part of the time it's below freezing, something leaks, drips or misbehaves. This time was no exception: oil consumption was not at all good. In fact, after a week or so of driving, I determined it was close to the 1.5L/1000Km limit specified in the Workshop Manual, which equates to a mileage of 392mi/qt.
I began monitoring miles and replacement oil quantities, topping up whenever the level was down more than 1/8 the distance between the two dip stick marks (which the manual says represent about 1.6qt, so 1/8 is about 6oz). Interestingly, miles/qt varied considerably throughout this period. In one instance, I drove 150 miles to the track, ran two DE track days, and used only 4 oz. Following the return trip, however, it took 16 oz to top it up. I continued to keep the log for a couple more weeks, at the end of which average oil mileage throughout the log period 398 miles/qt!
What was going on? No smoke in the exhaust, nothing on the garage floor, no oil collecting in the pan beneath the engine, no oil being carried back by the air stream so as to coat the underside (all of which have happened on occasion due to a leak of one fluid or another). So, the oil was getting burned and the cat was taking care of it. Checked plugs, all clean. Checked compression, all good.
What's left? Two remaining possibilities are worn valve seals/guides or a vacuum leak. While mentally tallying up the cost of the former, I checked for the latter using my homemade vac tester (see "Vacuum Leak Tester" in the Testing 1-2-3 topic). Big time leak! Can hear it too, but can't pin point its location. Removed the hose coming from the air oil separator at the throttle body, and plugged both hole and hose. No leak! That suggested the AOS is somehow involved (maybe a cracked body or bad seals?). The prospect of removing the intake manifold (and snapping a bolt or two in the process), so as to get to the AOS, was not appealing. I figured let's try the simplest/cheapest thing first, namely replacing the 4x45 o-ring in the oil filler cap.
I can happily, and somewhat surprisedly, say that was the fix! I've since driven 400+ miles without
burning any oil, let alone nearly a quart as before. The 19 year old o-ring was dried up, and being unpliable (an understatement) wasn't sealing well. I think the variations in mi/qt reported above can be explained by inconsistent tightening of the cap after each of the many oil adds. One thing bothers me: while it was off I ran a swab up the hose part way to the AOS, and it didn't show any sign of oil; you'd think if the intake was sucking oil mist, there'd be some sign.
It's hard to believe a vacuum leak can have such a profound effect on oil consumption, but that's what the result implies. I'd be interested in hearing if there are other interpretations of this result. Meanwhile, replace those petrified o-rings!
I searched our forum but didn't find anything relating to this particular discovery, so here's my story. If this is old news, then it's one of those many things everyone else knows about but me.
Every time I revive my car in the Spring, following the usual 3+ months of storage in my unheated garage where part of the time it's below freezing, something leaks, drips or misbehaves. This time was no exception: oil consumption was not at all good. In fact, after a week or so of driving, I determined it was close to the 1.5L/1000Km limit specified in the Workshop Manual, which equates to a mileage of 392mi/qt.
I began monitoring miles and replacement oil quantities, topping up whenever the level was down more than 1/8 the distance between the two dip stick marks (which the manual says represent about 1.6qt, so 1/8 is about 6oz). Interestingly, miles/qt varied considerably throughout this period. In one instance, I drove 150 miles to the track, ran two DE track days, and used only 4 oz. Following the return trip, however, it took 16 oz to top it up. I continued to keep the log for a couple more weeks, at the end of which average oil mileage throughout the log period 398 miles/qt!
What was going on? No smoke in the exhaust, nothing on the garage floor, no oil collecting in the pan beneath the engine, no oil being carried back by the air stream so as to coat the underside (all of which have happened on occasion due to a leak of one fluid or another). So, the oil was getting burned and the cat was taking care of it. Checked plugs, all clean. Checked compression, all good.
What's left? Two remaining possibilities are worn valve seals/guides or a vacuum leak. While mentally tallying up the cost of the former, I checked for the latter using my homemade vac tester (see "Vacuum Leak Tester" in the Testing 1-2-3 topic). Big time leak! Can hear it too, but can't pin point its location. Removed the hose coming from the air oil separator at the throttle body, and plugged both hole and hose. No leak! That suggested the AOS is somehow involved (maybe a cracked body or bad seals?). The prospect of removing the intake manifold (and snapping a bolt or two in the process), so as to get to the AOS, was not appealing. I figured let's try the simplest/cheapest thing first, namely replacing the 4x45 o-ring in the oil filler cap.
I can happily, and somewhat surprisedly, say that was the fix! I've since driven 400+ miles without
burning any oil, let alone nearly a quart as before. The 19 year old o-ring was dried up, and being unpliable (an understatement) wasn't sealing well. I think the variations in mi/qt reported above can be explained by inconsistent tightening of the cap after each of the many oil adds. One thing bothers me: while it was off I ran a swab up the hose part way to the AOS, and it didn't show any sign of oil; you'd think if the intake was sucking oil mist, there'd be some sign.
It's hard to believe a vacuum leak can have such a profound effect on oil consumption, but that's what the result implies. I'd be interested in hearing if there are other interpretations of this result. Meanwhile, replace those petrified o-rings!
