how a lean condition kills engine - po171

[96 SSEi]

hi all, I have been getting a po171 that comes and goes and cant find leak with propane test
I read here that a lean condition is bad for engine and am interested to know how the damage happens and why.......thanks

[MKMike]

Damage is due to detonation , here's a rather detailed explanation http://www.progl.com/General/detonation.htm

It's a mistake to assume that a P0171 code absolutely indicates a vacuum leak because there are a number of other possible causes of the code.
Even a vacuum leak may not be external if the PCV valve is sticking open.

http://www.obd-codes.com/p0171 enumerates most of the causes.

[redzmonte]

Could be plugged injector or low fuel pressure (regulator, filer, pump) but most likely a vac leak or exhaust leak. I found mine with a Smoke machine. Check vac harness around throttle body and LIM under snout.

Any mods? Anything get changed or replaced recently?

[96 SSEi]

thanks for replies - no mods so I will try the carb test and if no go, a smoke test,,,,,thanks

[RJolly87]

It would probably be better to watch live data on a scanner. As mentioned, you could be looking for a vacuum leak that isn't even there.

No point in smoking the system until you know for sure it is a vacuum leak.

[redfury]

I chased around a vacuum leak for both banks on my Astro for years, finally broke down and replaced the LIM gaskets and my fuel trims were even leaner! The motor eventually ate bearings and I junked the thing in favor of my Bonneville. Fuel flow/pressure can be culprits as well, but you really want to look closely at every source of vacuum. Could be Lower intake manifolds, a plugged injector, a leak in the exhaust. Usually it's a source of unmetered air entering the combustion process that sets it off.

Take a water bottle and poke a hole with a needle in the end, run the motor and spray it along with seams of the intake manifold, around the throttle, around anything that could be sucking air that's not coming through the air filter box.

[96 SSEi]

It would probably be better to watch live data on a scanner. As mentioned, you could be looking for a vacuum leak that isn't even there.

No point in smoking the system until you know for sure it is a vacuum leak.

thanks, I have done that and st fuel trims are about 16 at idle and go to 0 when i accelerate so its looking like a vacuum leak.

[l67se]

Had the same code in my 2000 SSEi. Mass air flow sensor was the culprit.
Replaced the MAF and the code disappeared.

[Jfridge92]

Be sure to check around the injectors where they mate into the heads. The o-rings can flatten and cause a vacuum leak. It took me forever to track those down on mine. Also, the egr pipe going from the egr valve to the LIM likes to crack. It's pretty hard to get to it with the throttle body on, so it can be difficult to test with propane/carb cleaner (or anything else you're using to check for vac leaks).