P0300, 305, 171 - My Year Chasing This Problem
Posted: Sat Apr 26, 2014 1:19 pm
I'm new to the group. Reading one of your threads helped me get to the bottom of the problem I've had with my 96 Olds 88 LS, so I've joined to share this success story that might help others (and to gloat a little).
My 88 has 214 kmiles. I've had a problem with idling and hot starts for the past year. Part of my commute includes sitting in stop-and-go traffic for 30 min. Sometimes I would pull into the nearby Walmart to stay out of the traffic until it starts moving again. When idling in traffic, the coolant temp rises to 200 deg F. During the past year after 20 min. of idling, the engine began to stutter and the various codes would set. I kept a pocket reader in the car to reset these on the run. I found that leaving traffic and driving on the service road would cause gradual improvement, and it would eventually stop missing altogether as coolant temps fell back to 180 deg F. Alternately, if I had shopped at Walmart for 30 minutes, the engine would be reluctant to start and would exhibit the same behavior as if I had been idling in traffic
So I had a temperature related ignition (P0300, 305) problem or fuel delivery problem (P0171 - left side lean). The sequence of part replacements I made over the previous year was: fuel filter, fuel pressure regulator, coil in 2-5 position, plugs, plug wires, plugs again, crank position sensor, and ignition module. The problem recurred each time.
Finally I read one of this site's threads discussing similar symptoms and found that fuel pressure was probably the issue. I rented a fuel pressure gauge kit at Autozone. Fuel pressure spec on my car is 40 to 47 psig. My pre-start and post start pressure was only 25 psig.
Problem explanation - Here's what I believe was happening. The fuel rail receives supply at cylinder 6. Fuel flows from there past cylinders 4, 2, 1, 3, and then finally 5 where it is letdown by the fuel pressure regulator and returned to the tank. Low fuel pressure output by the fuel pump caused the fuel pressure regulator to remain closed, so fuel flow in the rail tapered off as it flowed past each successive injector. Fuel flow in the rail at injector 6 was the sum of all injectors. Fuel flow at the end of the rail at injector 5 was only that needed by injector 5. Essentially, fuel was dead-headed at the end of the rail. With low/no flow at the end of the rail, fuel at that section tended to heat up, and this effect is made worse by low fuel use (idling) and increased temperature (200 vs 180 deg F). Eventually, the fuel vaporized at injector 5 and starved that cylinder. It misfired and the oxygen sensor saw the resultant high O2 and set either P0305 or P0171 codes. Alternately, shutting down the engine would allow vapor locking of the entire fuel rail since all of the fuel had been pre-warmed and then was kept bottled up in the rail above a hot engine while I shopped. This explains why I could not start the engine at all on two occasions.
I dropped the fuel tank this weekend. The exact OEM pump was not available. The Delphi replacement part offered by GM and O'Reilly didn't fit the assembly as well as I would have liked plus the original power supply plug would have been replaced by cutting it and butt-splicing a new and different plug. I opted for a better fitting aftermarket pump from O'Reilly and installed it with a new strainer. Pressure is now up to 38 psig. I assume this pressure is the result of the pressure regulator now doing its job, but it's still a little low compared to the spec. The regulator was replaced earlier and was from O'Reilly also.
I'll have a good test on Monday when I idle in traffic, but the initial test drive shows some improvement in throttle response, especially at low speed.
Hope this helps the next guy. It's been a year of reading posts and swapping every part I imagined could be heat affected. The camshaft position sensor was the real disappointment and made me realize that going after the ignition was probably barking up the wrong tree. A good mechanic might have found the fuel pressure issue easily, but might also have done the parts swap fiasco just like I did. At least I avoided a labor charge and have a lot of new installed parts to show for all my weekends chasing this problem.
My 88 has 214 kmiles. I've had a problem with idling and hot starts for the past year. Part of my commute includes sitting in stop-and-go traffic for 30 min. Sometimes I would pull into the nearby Walmart to stay out of the traffic until it starts moving again. When idling in traffic, the coolant temp rises to 200 deg F. During the past year after 20 min. of idling, the engine began to stutter and the various codes would set. I kept a pocket reader in the car to reset these on the run. I found that leaving traffic and driving on the service road would cause gradual improvement, and it would eventually stop missing altogether as coolant temps fell back to 180 deg F. Alternately, if I had shopped at Walmart for 30 minutes, the engine would be reluctant to start and would exhibit the same behavior as if I had been idling in traffic
So I had a temperature related ignition (P0300, 305) problem or fuel delivery problem (P0171 - left side lean). The sequence of part replacements I made over the previous year was: fuel filter, fuel pressure regulator, coil in 2-5 position, plugs, plug wires, plugs again, crank position sensor, and ignition module. The problem recurred each time.
Finally I read one of this site's threads discussing similar symptoms and found that fuel pressure was probably the issue. I rented a fuel pressure gauge kit at Autozone. Fuel pressure spec on my car is 40 to 47 psig. My pre-start and post start pressure was only 25 psig.
Problem explanation - Here's what I believe was happening. The fuel rail receives supply at cylinder 6. Fuel flows from there past cylinders 4, 2, 1, 3, and then finally 5 where it is letdown by the fuel pressure regulator and returned to the tank. Low fuel pressure output by the fuel pump caused the fuel pressure regulator to remain closed, so fuel flow in the rail tapered off as it flowed past each successive injector. Fuel flow in the rail at injector 6 was the sum of all injectors. Fuel flow at the end of the rail at injector 5 was only that needed by injector 5. Essentially, fuel was dead-headed at the end of the rail. With low/no flow at the end of the rail, fuel at that section tended to heat up, and this effect is made worse by low fuel use (idling) and increased temperature (200 vs 180 deg F). Eventually, the fuel vaporized at injector 5 and starved that cylinder. It misfired and the oxygen sensor saw the resultant high O2 and set either P0305 or P0171 codes. Alternately, shutting down the engine would allow vapor locking of the entire fuel rail since all of the fuel had been pre-warmed and then was kept bottled up in the rail above a hot engine while I shopped. This explains why I could not start the engine at all on two occasions.
I dropped the fuel tank this weekend. The exact OEM pump was not available. The Delphi replacement part offered by GM and O'Reilly didn't fit the assembly as well as I would have liked plus the original power supply plug would have been replaced by cutting it and butt-splicing a new and different plug. I opted for a better fitting aftermarket pump from O'Reilly and installed it with a new strainer. Pressure is now up to 38 psig. I assume this pressure is the result of the pressure regulator now doing its job, but it's still a little low compared to the spec. The regulator was replaced earlier and was from O'Reilly also.
I'll have a good test on Monday when I idle in traffic, but the initial test drive shows some improvement in throttle response, especially at low speed.
Hope this helps the next guy. It's been a year of reading posts and swapping every part I imagined could be heat affected. The camshaft position sensor was the real disappointment and made me realize that going after the ignition was probably barking up the wrong tree. A good mechanic might have found the fuel pressure issue easily, but might also have done the parts swap fiasco just like I did. At least I avoided a labor charge and have a lot of new installed parts to show for all my weekends chasing this problem.
