P0171

'95 Riviera periodically/intermittently throws a P0171 Code. Sometimes several hundred miles without throwing it; sometimes 2 times in 10 miles. Have checked for vacuum leaks & bad electrical/ground connections to no avail. Sometimes accompanied by a P1257 code (this code only happens when car is not entirely warmed up, within the 1st 5 minutes of driving). Any ideas? Thanks.

Terry, ever had the injectors cleaned? Other than a vacuum leak, that's the only other thing I can think of that would cause a lean condition unless the O2 sensor is FUBAR. What does your vacuum/boost gauge say you are pulling in Park, at idle, fully warm?

Larry, replaced injectors about 500 miles ago. Greatly improved mileage & performance, but no help on those 2 codes. Vacuum is 20-30 inches under those conditions. O2 Sensor & MAF Sensor replaced a few months back due to codes; no codes now. Fuel pressure is spec.

I have to tell you I just don't get it. Regarding your P1257 code, I just don't know how the computer knows there is an over boost condition if it has no way of measuring what the boost actually is. If there is no MAP sensor on the Series 1 engines, I am unclear how the computer knows to display a code. BTW, that is an OBD2 engine code, both the 171 and 1257 code, yet the 95 is technically OBD1. You may have the OBD2 data connection, but the system is OBD1. Some describe it as OBD 1.5.

I wonder if changing the computer might eventually be the thing that fixes the problem.

Yeah, I don't get it either. Yes, it's 1.5; OBD1 with OBDII connector. My Scanner is an Actron CP9185. I've tried 2 other computers; one known good one from another car & one reman. from Cordone. Used original PROM. Same results on all 3. The P1257 comes up as a P0257 on the Scanner, does not exist except as a ..1257.

tloukaitis wrote:

Yeah, I don't get it either.  Yes, it's 1.5; OBD1 with OBDII connector.  My Scanner is an Actron CP9185.  I've tried 2 other computers; one known good one from another car & one reman. from Cordone.  Used original PROM.  Same results on all 3.  The P1257 comes up as a P0257 on the Scanner, does not exist except as a ..1257.

My computer code book lists P0257 as injection pump fuel metering control B, range or performance problem (cam, rotor, injector).

That doesn't really help, but there is a P0257 OBD2 code. My 98 FSM doesn't list that code. I'm not really sure you can assume the code is 1257 considering your boost gauge does not show overboost.

I think the main problem here is the fact that you have OBD 1.5. It is kind of limited.

Here's what it actually says: "P0257 - Turbo Boost Sensor Signal Circuit Problem."

OK, then there is a boost sensor? Where is that?

There is a Boost Control Solenoid & a Boost Control Actuator. The Solenoid is simply either open or closed, activated by the PCM. The Actuator operates the Boost Bypass Valve, & is controlled by vacuum from the Solenoid & also vacuum from the engine. So, there are 2 vacuum lines going into it, with different signals, depending upon what the PCM is telling the Solenoid; one from the top from the engine, one from the bottom from the Solenoid. So, the PCM is telling the Bypass valve when to open & close, thereby supplying more boost or less.

So all the PCM knows is whether the bypass is commanded open or closed? It doesn't really know the amount of boost. For some reason, the PCM thinks there is over boost even though there isn't. That is the heart of the problem. Figure out why the PCM thinks there is over boost. What signals it is getting, or not getting that leads it to set a code. There really isn't an engine performance problem, I mean the car drives fine, right?

The PCM commands the bypass (by commanding the Solenoid off or on) to open or close, or move one direction or the other. What it uses to determine these commands is a mystery to me. The GM repair manual says it uses "various other information". In fact, I think it gets some info. from the trans., air temp., etc. Downshifting eliminates the problem, as well as the car warming up. The car runs excellent, other than setting off these 2 codes. I've checked air temp. sensor readings, water temp. readings, trans. shift points, etc. All seems normal with no change between codes being thrown or not.

Without knowing what exactly the PCM is using to determine overboost, I don't think you'll ever know why it is setting the code. Wish I had another idea, but I think we have hit a wall here.

Could it be a bad PROM in the PCM?

Hard to say, maybe. I see you are also talking to John Csordas over on the BPG forum. John is the director of the N.E GS/GN club of which I belong. I've never met anyone better than John, he is an ACE mechanic. I'd pick his brain some more.