Hello all,

I am currently trying to diagnose a 1996 Century Custom with the 3100 motor.

Basically, the car has trouble starting and keeping a solid idle. Sometimes it may idle ok, after 2-3 attempts. Better after engine is slightly warmed up.

I started off by reading the trouble codes and came up with these...
P0122
P0131
P0171


I started off by replacing the TPS and there wasn't a significant change.

Idle is MUCH smoother (not perfect by any means) with the TPS UNPLUGGED.
Sometimes better if I unplug it and lug it back in while the vehicle is running.

Maintenance history is spotty, at best. I can provide more details if any one responds with specific questions.

Charging system has not been Load tested yet and I have not done any electrical troubleshooting yet.

Looking for any advice on this issue.

Thanks!

That P0171 (fuel trim lean) plus the rough idle tells me you probably have a vacuum leak. Inspect all vacuum hoses and connections carefully. Spray the hoses and connections with carb cleaner while the engine is running. A change in RPM will let you know you found a leak.

you could have a wiring issue with the o2 and tps. or the o2 is bad. do the codes come back if you clear them now? i think i would clean the IAC cone and seat in the throttle body too.

This would not make sense for the codes you have but a MAF is always something to check when your car does not idle properly, especially when it'* running hot. Many mistake this for a fuel delivery problem when it is actually an air delivery problem, if your car starts fine from a cold start after it has been sitting for a while a bad Mass Air Flow sensor could very well be the case.

Thanks for the info so far.

Are there any known common areas for Vacuum leaks for this vehicle?

What would be the best way / best tool to do a "smoke test" on these lines? Not sure of the correct terminology for this procedure, but basically blow smoke into the vacuum system to ID a leak?

smoke test machines are very expensive, if you test for a vacuum leak do it roy'* way

Test the Mass Airflow Sensor with the engine running, tap the side of it with a screwdriver, and see if the engine stumbles - or, as just happened to my 2 month old MAF, the engine runs better. Also, try pulling the MAF out and giving it a good cleaning with Mass Airflow Sensor cleaner (ONLY). It'* not uncommon for a flaky sensor to cause the ECM to set codes for several other sensors. LOL

One major thing to check on the 3100s is the rubber tee that is at the front of the throttle body. That'* a common failpoint with the 3100s, and I've seen it on four different cars with that engine.

Some overlooked vacuum leak points: power brake booster/check valve, fuel pressure regulator connection, vacuum reservoir tank, heater controls (listen for hissing in the cabin near the steering column - very easy to overlook!).