Random Stalling and RPM Drop- P0171 CEL

[aontkos]

I’ve been chasing a random RPM drop at idle for a few years. It’s infrequent enough that it’s hard to chase and I’ve never gotten a CEL until yesterday. It’s a 2000 with a 5VZ-FE and an automatic transmission.

I was going on a camping trip and got stuck in stop and go traffic. RPMs dropped, the truck stalled, and it started back up just fine. The truck ran fine for a while longer and I got a CEL for P0171. It stalled again when I got to my campsite and I was idling scoping out the best spot to setup camp.

I clean the MAF sensor every few years but it may be time for a replacement. Has anyone else had random RPM drops at idle from a bad MAF sensor?

I replaced my IAC with a non-OEM one a few years ago, so I think that may also be the culprit. Fuel filter has been replaced recently, I’m running fresh silicone vacuum lines, PCV is newish, so I’m leaning towards MAF sensor or a crappy IAC...

 

[jgue467]

Don't know about rpm drop due to maf but when mine threw an 0171 the leak was coming from the closest intake 'hose' to the metal intake, see pic.

The hose can be deceiving, looks ok yet pull it apart slightly and there was a deep crack. Luck.

 

[aontkos]

Don't know about rpm drop due to maf but when mine threw an 0171 the leak was coming from the closest intake 'hose' to the metal intake, see pic.

The hose can be deceiving, looks ok yet pull it apart slightly and there was a deep crack. Luck.

Thanks for the insight. I replaced that intake tube for exactly that reason a few years back. I checked over the new one and it seems sound.

 

[LittleCaesar]

My guess, other than a new vacuum leak, is the IAC.

If you have access to live data, a correctly functional MAF should read around 4g/s when fully warmed up.

 

[desertegal7]

Hi, i had the same.. and eventually exhaust valves needed adjustment.
4 month after adjustment it came back but this time couldn't adjust and minimum shim thikness reached. i got new valves to replace bad valves and during the process we discovered a crack in the cilynder head between the exhaust valve seat. new cilynder head installation in progress.

cheers!

 

[desertegal7]

also when things going weird check engine mechanical condition and valve specs / compression before replacing suspected sensors.

to verify wether its an out of specs valve clearance simple put a paper at the exhaust pipe.. it should never suck it in even a little, if it does then definitely valves issue. many videos are available of search for it.

 

[MaineRunna]

Either the engine is running lean (vacuum leak, low fuel pressure, etc) or it thinks it's running lean (bad MAF, O2, etc) Look for a vacuum leak and check O2 and MAF sensor readings