Running Lean At Startup

[SvtCobra.5000]

My car is running really lean at startup it doesn't matter if its cold or warm. CEL comes on & throws these two codes P0171 & P0174 my wideband is maxed out at 22.4 & it idles high, it takes about a minute for the idle to level out and than the a/f ratio slowly lowers to 14.7-15.3 I get a steady 12.5 at wot so I don't think its a fuel pump/injector issue (could be wrong) it takes 2-3 turns of the key to prime the system before it will start. I've cleaned the maf, checked for vac leaks, checked fuel pressure. Not sure what to do next:shrug:

 

[mwolson]

Most likely: vacuum leak.

Less likely: exhaust leaks on both sides, bad O2 sensors on both sides, etc.

Here's a cheap, easy way to do a smoke test that will tell you very quickly where the problem is: https://www.youtube.com/watch?v=zMok2y05jNE

 

[SvtCobra.5000]

Thanks for the link i'll give that a try, but if its a vacuum leak wouldn't it run lean all the time or at least while idling? Or does the fact that the a/f ratio eventually lowers mean that the pcm is adding fuel to reach stoich level essentially "hiding" the vac leak.

 

[mwolson]

You got it.

When it is in closed loop peration, the PCM watches the O2 sensors and adds or subtracts fuel until it sees stoich, so it is constantly trying to compensate for the leak. Once that compensation hits a pre-defined threshold (25% in a stock tune), it sets a DTC.

When it is in open loop operation, typically during startup and at WOT, it runs out of tables, rather than looking at the O2 sensors. So it assumes the correct amount of air is coming in. A vacuum leak allows extra air air to come in during open loop startup, causing it to run lean.

However, on startup the pcm normally runs the engine lean for a couple of minutes in order to heat up the cats faster. That is when the SAI runs, too.

 

[tt54l32v]

Yes it is prob supposed to do that. Plus your wideband will have to warm up to read correctly.