Smoky startup

I have a 92 Sentra with a 1.5 carb engine running on Compressed Natural Gas. Some days on cold startups i get a bit of smoke or something that looks like smoke. But this goes away shortly afterwards unless its a damp wet rainy day in which case i get something like steam all the time.

Today was a bit chilly but not damp and i got some smoky vapour. Also there’s plenty of condensation on the tip of the exhaust which is water (i.e. not oily). My coolant level remains the same. Doesn’t overheat when running AC on full load. But oil level does fall a little bit over time. Most mechanics here are saying CNG driven cars consume a bit more oil than petrol driven cars. CNG run cars have a little advanced timing.

I’m stumped and wondering should i be worried now or later or hopefully never at all?

Where Are You Checking The Coolant Level?

The exhaust vapor could be normal or abnormal. It could be coolant or engine oil coming from the pipe caused by a defect.

Are you checking the coolant by looking at the resevoir? Does your car have a radiator cap? Have you checked it there with the engine cold?

It could be oil burning caused by worn or aged valve stem seals. That oil that is being consumed is going somewhere. Does it puff smoke after it’s warmed up when you accelerate following a period of idling?

Also, I’d check the PCV valve and PCV system for proper operation. Why not just replace the PCV valve if it’s been a while?


Thank you so much. I check both the coolant reservoir and the radiator level when cold. Both level and normal. There’s also no frothy/milky/oily deposit on the radiator cap or the oil cap. All clean.
Car sometimes puffs a bit of smoke but mostly on cold damp days. Dry warm days there’s nothing, just condensation on the exhaust tip. But the smoke is usually very very temporary. Valve seals might be possible and from what i know they shouldn’t be difficult or expensive to replace. Or are they?
And can you telm me where the PCV valve is usually located on these cars?

Black, white or blue smoke? I suspect it is now fall and the morning temperatures are low enough to cause the condensation. My vote is it is normal operation. Especially if you do a lot of short trips.

Where is your muffler located? The reason I ask is that I had the same situation with my 1965 Rambler Classic and have the same problem with my 1978 Oldsmobile Cutlass. On both cars, the muffler is at the rear of the car at the end of the exhaust system and is mounted transversely to the car. My guess is that the exhaust cools more before it reaches the muffler, and the moisture condenses at the muffler and then turns to steam. At any rate, the Rambler did this the entire 8 years I owned the car, and I’ve had the Oldsmobile 31 years, so I guess it isn’t a problem.

White smoke i think. and yeah, my trips are usually short in distance but kinda long in terms of sitting in traffic. i take the car out on weekends and let her rip.

yep, it’s on the end but mounted legthwise as the car. i get some smoke initially and later it’s just water vapor on the exhaust tip. checked again today, and it’s just clean water. i’m planning for a more powerful engine but that’s a plan for a lot later when i can get some money. for now, i want this to run good.

White smoke, not steam, but smoke that smells like oil most likely are valve stem seals that are bad.

That is an indication that everything is normal. It may well be nothing more than just a little condensation in the system and you only see the smoke (actually vapor) on cold days. It will go away next summer.

thanks. i sure hope so. you’ve been of much help.