Bonneville 97 SE code p0171B without check engine light
 

I have 1997 Pontiac Bonneville SE and I have a code p0171B without a check engine light. No problem starting the car but may get rough ride when the car is put into gear and the gas pedel is pressed. This does not happen on the first start but it may happen on a second start or sunsequent start after a short rest period.

The MAP was recently replaced.

Read this: https://www.gmforum.com/obd-ii-trouble-shooting-153/p0171-284036/

Did you clear codes after changing the MAF? If so, sounds like you have a vacuum leak.

The code was cleared, and no problems were encountered for several months.

My last response seems to be incomplete.

The code were cleared after the MAP was replaced but after several month of no problems, the problem is back. It is getting more of a problem.

The problems (very rough ride) occurs only after the car seems to be warmed up. The first start is fine; it is only after the second or subsequent start (after a short time rest period) does the problem begin to happen. Only after the car is put in gear, the bucking action takes place. If I leave the car running for several minutes, the problem seems to disappear and I can drive off peacefully.

Those aren't symptoms of the P0171. LB1 or lean bank 1 typically shows no symptoms beyond the code. Which is kinda what usually makes it tough to diagnose.

I'd be checking plugs, wires, coils, ICM, fuel pressure etc.

I had brought the car to a GM dealer and they stated that the fuel pressure was half of what it should be. This was for all situations, start, idle and in gear. The dealers stated that this is not possible. If the fuel pressure was this low, the car would not start, much less even go. They concluded that since the car starts and was running, the pressure must be right. They also stated that this had nothing to do with the problem I was experiencing. They could not find the problem.

there is a rubber boot with two hard plastic lines coming out of the throttle body, one goes to the evap the other goes to the fuel pressure regultator. check those for hardness/dryrot/cracks. if the fpr isnt getting vac signal the pressure wont be right. alot of people have had to replace that boot, gm lists it as the line set for the evap now if you go to try to buy it.

Factory spec for fuel pressure is 58-62 with key on/off. I have the GM FSM's (manuals) that the dealership has and there is no running/idle/driving specs to be found. I have looked extensively and searched as it's on the pc.

Your engine will start and run with fuel pressure at 20-25psi (I learned this during an attempted upgrade fule pump swap). The car can run this way..but will have running issues. I didn't try to drive the car this way as I knew it was running lean.

Another car I tested, we knew the fuel pump was on it's way and tested the pressure with key on/off as the FSM states. Pressures looked good. We started the car and once in gear and moving the pressure dropped significantly. Literally had to tell the guy to stop using the throttle too much because driving..... we were at 10psi. I didn't think it possible for the engine to run at that pressure, but it was still running..somehow.

I'd consider a second opinion. Based on them telling you it was half of what it should be, but that's not possible. How can that be explained, as a bad pressure gauge? Doesn't make much sense to me.

The indicated that the fuel pump was okay. Besides, everything was running well for months until just a week ago when the problem started. Once again, if the car is allowed to remain out of gear for 5 minutes, it runs well (only after restarting the car after a short rest period).

It's cool, I was trying to help out.

If the dealer who tells you the pressure is low and things don't make sense, is good enough. I'm happy.

Me, I'm not really a fan of the orangy yellow glow of a check engine light or a car that runs poorly. If you would like to figure it out, please start with what i posted above and holler when you are ready for help.