CarTalk.com Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Rough Idle on startup....02 sensor?

Vehicle: 1994 Nissan pickup (V6)

Lately I’ve been getting a rough idle on startup (cold start). Runs fine for first 2 seconds, then gets rough. If I press on the gas, it gets rougher. Some black smoke out of the exhaust. To me, this means I’m getting too much gas.



After 20 seconds, it evens out and idles fine, and the exhaust clears up. If I start it again (warm) it’s fine. I haven’t noticed a drop in fuel economy, and it’s not sluggish when driving.



I’m thinking O2 sensor, mass air flow sensor, or maybe a faulty injector, but I’m no mechanic. Should I be worried about this? I’m about to start a cross country drive in 10 days.

It’s NOT the oxygen sensor…Try cleaning the MAF…

This could be caused by a loss of fuel pressure; maybe a leaking injector or fuel pressure regulator.

It could also be caused by a loss of residual fuel pressure. This is maintained by the check valve in the fuel pump when the engine is at rest and if that’s the problem this means a pump replacement. The rough running is caused by air being burped out of the fuel lines and this is not something rare.
A fuel pressure test should show if this is the case.