Hello All,
My daughter's 2004 Tahoe, 4.8L, 178K miles, was idling and running very rough and stalling often. She was told that she needed to replace the coils, plugs & wires which she did, but it did not help.
I towed it home to try to fix it. It was showing a P0300 code: random misfire.
Compression test showed 2 cylinders with 160psi, 2 with 190 & the rest 200-215. All the plugs were white except #2 which was all black (it also was on the highest rated compression @ 215psi). All the coils were tested for spark with the respective plug wire attached to a spark testor set for a .040 gap plug (the testor's free air gap was over 1" at that setting). All the new coils & wires proved fine. The fuel pressure tested at 55psi.
This vehicle has no external fuel filter, no external fuel pressure regulator, no idle control valve and no egr valve, and no mechanical linkage to the throttle body.
I cleaned the MAF sensors which appeared clean, and the throttle body, which was quite dirty. I put it all back together to check results. The idle was markedly improved, but it still had a miss. I swapped the #2 spark plug, which was still black, with another plug, and took a short test drive which showed no tendency to stall. I checked the #2 plug again & it was well on the way to becoming all black, then I cleared the P0300 code.
In hindsight, I should have run a cylinder balance check, but I did not. Instead, I assummed #2 injector was hanging up & running rich, so I ran a can of SeaFoam with 4oz of Lucas fuel injector cleaner directly into the fuel rail with 50psi pushing it. I figured since this was 5psi lower than what the internal regulator was putting out, none of the cleaner would go back into the fuel tank. This is where I made mistake #2: To be absolutely sure that all the cleaner was going through the injectors, I decided to pull the fuel pump relay, plus the 20A fuel pump fuse, and gently crimp the rubber hose coming from the tank. I crimped the hose, exposed the fuses & relay, got distracted, and did not remove either. With the engine at running temperature, I ran the cleaner though the injectors.
The miss was unchanged and the P0300 was back. So, I pulled the 2 fuel rails off with the injectors, filled my injector cleaner tool with gasoline, set it to 60psi and looked to see if #2 injector was leaking with the engine turned off. The injector was dry, as were the other 7. I then ran the rest of the gas through by cranking the enging while observing the spray pattern of all 8 injectors. They all appeared to be dispersing in an even pattern.
Running out of ideas, I decided to measure the output of each injector over the same given amount of time. I removed all the plugs, disconnected the coils, released the clamped fuel input from the gas tank, and went to reinstall the pump's fuse & solenoid. This is where I realised the pump was running against the clamped hose all the while I had the engine running. Upset with myself, I continued on with my attempt to measure/compare each fuel injector output, but now the fuel injectors would not spay ... the fuel pump had quit working somewhere in the cleaning process.
Thinking that, in working against a blocked hose, the pump either overheated and burned out, or it blew the 20A fuse. The fuse was fine. Just in case, I swapped the relay with another identical one, but the pump still did not come on.
I do not have manuals to tell which relay pin goes where, but the relay has a diagram on it's side showing it's 4 pins: #30 and # 87 are the poles of its switch, so one of them must go to the pump & the other to the 12V bus. The other 2 pins (86 & 85) are shown as the 2 leads to the solenoid coil, so one of them must go to ground & the other is activated by the computer to turn on the pump.
#30 has 12V with swith on or off, Pin 87 had no voltage, but it showed less than 1 ohm resistance to ground, so it had to be the pin also going to the fuel pump. I immediately applied 12V through a fused meter to it and the pump turned on and drew 7.5 amps. Therefore, the problem has to be with the circuit that is activating the pump.
I checked out pin #85 & it was less than 2 ohms, so I guessed it was the relay's coil side to ground, which would make pin 86 as the one with the voltage from the controler. With the car swith off, it showed 0 volts and infinity in ohms. When I turned the switch on, it showed 12V. If this were true then the relay would have to swithed on the pump, so both relays that I swapped must have been both bad.
I tested both relays, as per its diagram, and both had coils that activated when 12V was suppied at pin 86 and pin 85 was grounded, and both lit up my test light when it was connected to pin 87 with 12V applied to its mated pin #30 and then the coil activated. Confused as to why a pump that was proven to be good, would not work with a proven relay, and with the relay's pins proven to have the required voltage and grounding, I put my test light on pin 86 (the activating source which prior had 12V in run position and 0 Volts in off) now did not indicate anything with my light testor.
I figured that since my meter showed proper voltage, but the light testor would not come on, that there was not enough current available to register the light. Must be a poor connection. But when I put my meter back on pin 86, it now showed no voltage, on or off, and has stayed that way.
The conclusion I now draw is that I have a "on and off" connection for the voltage source which activates the fuel pump relay. Is it just a coincidence that the problem occured right after blocking the output of the pump? I do not know what if anything else is associated with the relay/fuel pump and the computer controler somewhere upstream of the relay, without the schematic details. Is this problem independent of the misfire condition or related? I got hungry, ran out of daylight and called it a day. I am a shade tree mechanic outside of my comfort zone. Any advise would be most welcome. Richard
My daughter's 2004 Tahoe, 4.8L, 178K miles, was idling and running very rough and stalling often. She was told that she needed to replace the coils, plugs & wires which she did, but it did not help.
I towed it home to try to fix it. It was showing a P0300 code: random misfire.
Compression test showed 2 cylinders with 160psi, 2 with 190 & the rest 200-215. All the plugs were white except #2 which was all black (it also was on the highest rated compression @ 215psi). All the coils were tested for spark with the respective plug wire attached to a spark testor set for a .040 gap plug (the testor's free air gap was over 1" at that setting). All the new coils & wires proved fine. The fuel pressure tested at 55psi.
This vehicle has no external fuel filter, no external fuel pressure regulator, no idle control valve and no egr valve, and no mechanical linkage to the throttle body.
I cleaned the MAF sensors which appeared clean, and the throttle body, which was quite dirty. I put it all back together to check results. The idle was markedly improved, but it still had a miss. I swapped the #2 spark plug, which was still black, with another plug, and took a short test drive which showed no tendency to stall. I checked the #2 plug again & it was well on the way to becoming all black, then I cleared the P0300 code.
In hindsight, I should have run a cylinder balance check, but I did not. Instead, I assummed #2 injector was hanging up & running rich, so I ran a can of SeaFoam with 4oz of Lucas fuel injector cleaner directly into the fuel rail with 50psi pushing it. I figured since this was 5psi lower than what the internal regulator was putting out, none of the cleaner would go back into the fuel tank. This is where I made mistake #2: To be absolutely sure that all the cleaner was going through the injectors, I decided to pull the fuel pump relay, plus the 20A fuel pump fuse, and gently crimp the rubber hose coming from the tank. I crimped the hose, exposed the fuses & relay, got distracted, and did not remove either. With the engine at running temperature, I ran the cleaner though the injectors.
The miss was unchanged and the P0300 was back. So, I pulled the 2 fuel rails off with the injectors, filled my injector cleaner tool with gasoline, set it to 60psi and looked to see if #2 injector was leaking with the engine turned off. The injector was dry, as were the other 7. I then ran the rest of the gas through by cranking the enging while observing the spray pattern of all 8 injectors. They all appeared to be dispersing in an even pattern.
Running out of ideas, I decided to measure the output of each injector over the same given amount of time. I removed all the plugs, disconnected the coils, released the clamped fuel input from the gas tank, and went to reinstall the pump's fuse & solenoid. This is where I realised the pump was running against the clamped hose all the while I had the engine running. Upset with myself, I continued on with my attempt to measure/compare each fuel injector output, but now the fuel injectors would not spay ... the fuel pump had quit working somewhere in the cleaning process.
Thinking that, in working against a blocked hose, the pump either overheated and burned out, or it blew the 20A fuse. The fuse was fine. Just in case, I swapped the relay with another identical one, but the pump still did not come on.
I do not have manuals to tell which relay pin goes where, but the relay has a diagram on it's side showing it's 4 pins: #30 and # 87 are the poles of its switch, so one of them must go to the pump & the other to the 12V bus. The other 2 pins (86 & 85) are shown as the 2 leads to the solenoid coil, so one of them must go to ground & the other is activated by the computer to turn on the pump.
#30 has 12V with swith on or off, Pin 87 had no voltage, but it showed less than 1 ohm resistance to ground, so it had to be the pin also going to the fuel pump. I immediately applied 12V through a fused meter to it and the pump turned on and drew 7.5 amps. Therefore, the problem has to be with the circuit that is activating the pump.
I checked out pin #85 & it was less than 2 ohms, so I guessed it was the relay's coil side to ground, which would make pin 86 as the one with the voltage from the controler. With the car swith off, it showed 0 volts and infinity in ohms. When I turned the switch on, it showed 12V. If this were true then the relay would have to swithed on the pump, so both relays that I swapped must have been both bad.
I tested both relays, as per its diagram, and both had coils that activated when 12V was suppied at pin 86 and pin 85 was grounded, and both lit up my test light when it was connected to pin 87 with 12V applied to its mated pin #30 and then the coil activated. Confused as to why a pump that was proven to be good, would not work with a proven relay, and with the relay's pins proven to have the required voltage and grounding, I put my test light on pin 86 (the activating source which prior had 12V in run position and 0 Volts in off) now did not indicate anything with my light testor.
I figured that since my meter showed proper voltage, but the light testor would not come on, that there was not enough current available to register the light. Must be a poor connection. But when I put my meter back on pin 86, it now showed no voltage, on or off, and has stayed that way.
The conclusion I now draw is that I have a "on and off" connection for the voltage source which activates the fuel pump relay. Is it just a coincidence that the problem occured right after blocking the output of the pump? I do not know what if anything else is associated with the relay/fuel pump and the computer controler somewhere upstream of the relay, without the schematic details. Is this problem independent of the misfire condition or related? I got hungry, ran out of daylight and called it a day. I am a shade tree mechanic outside of my comfort zone. Any advise would be most welcome. Richard
