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Drove 50 miles with Oil Cap Off, Now Burning Oil

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Long time listener, first time caller...


I have a 2002 C4S 6 speed that I was driving back from CT to VT on a trip.  I stopped to fuel up and decided to top up the oil (I usually put in a quart over a 5k oil change interval, Joe Gibbs DT40).  Stupidly, I got distracted and didn't put the oil fill cap back on.  I drove a little over 50 miles (almost all highway) before I reached a friend's house and I didn't notice any issues until I got into their neighborhood and I noticed some oil smoke out the back.  It seemed to have consumed about a quart over the 50 miles, and on start up again in my friends driveway, had a huge puff of oil smoke.  We cleaned out the intake through the throttle body as best we could and took the car for a lap around the neighborhood with my friend tailing me.  The smoked seemed to clear up (maybe oil got blown through and stuck in the cats?) but on restarts, the passenger tailpipe (driver's side bank) still seemed to puff smoke and then you could faintly see oil smoke while idling.  Revving seemed to clear it up.  Additional restarts had puffs out of the passenger side and very minor puffs out of the driver's side tailpipe at times.  With out my borescope or manometer we couldn't really troubleshoot further.  I didn't think to take a video, but I do have a picture of what the intake looked like after our initial clean out attempt.


More facts about the car; about 11k miles ago I completed a full rebuild for a cracked cylinder wall with 3.8 Nickies from LN Engineering.  Hoffman Automotive Machine rebuilt both heads.  The engine has been running fine since the rebuild, but the passenger tailpipe (bank 1) has always been darker than the other tail pipe.


really hope it's not bore scoring, I can't imagine it's valve related.  I had the fuel injectors rebuilt and tested when I did the motor and I replaced the AOS.  I had to carry on to VT in my friend's spare car (lucky me he has one!), so it will be a week or two before I can recover it and do more investigation.  Could driving for 50 miles with the oil fill cap off have actually caused any damage or is it just a coincidence?  Is oil stuck in the the cats and will eventually burn off due to some weird pressure from having the cap off?  Occam's razor keeps putting me back to bore scoring with makes me sad.  Any other ideas?


Thanks in advance!


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  • Moderators

Welcome to RennTech :welcomeani:

First, don't panic.  When you leave the oil cap off, the AOS is unable to maintain the correct level of vacuum in the oil sump that helps the very low-tension piston rings in these engines seal. So, no vacuum, no ring seal, and lots of oil gets up into the combustion chambers.  Once the oil cap is back, everything returns to normal and the rings seal back up. 


Just make sure the oil level is correct, and I would add some Techron gas additive to the tank and fill it up.  A couple of nice long drives should get rid of any residual oil 😉

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This moderator's response is 100% spot on.  Your engine couldn't maintain the proper level of crankcase vacuum.


This is why for engines burning oil (not because the oil cap is off) one of the first and easiest things to do is check the crankcase vacuum to see if you have an issue with the AOS, which is tasked with regulating the crankcase vacuum to spec.

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What a relief, thanks for the responses. I'll have my friend top it off and run it for a bit to see if it improves.  I'll be sure to update with result for any future searches. 🙃

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  • 3 weeks later...

OK, so I was finally able to go retrieve my car from my friend's house.  He topped up the oil and was able to drive it around town a few times before I was able to pick it up which cleaned up the exhaust!  I got a bottle of Cataclean and followed the instructions on my way back (about a four hour drive) and everything seems to be fine again.


This was definitely one of those good news, it's not a problem, bad news, I'm an idiot situations.  Thanks again for the quick responses and reassurance! :thumbup:

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