03 LQ4 Dies
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Teching In
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03 LQ4 Dies I have a 2003 GMC truck with the LQ4. Been a great truck but it died on me exiting the interstate yesterday. It would start and stay running if you kept it at 2000 rpm or above but die as soon as you let off the throttle.
Codes were p0101 and p0300
I replaced the MAF which didn’t help.
I then took off the catalytic converters as I read clogs could cause it. One was partially clogged but the other fine.
It is still hard to start and will not stay running at idle, only when giving partial throttle.
I tried pulling the battery for a while. No change.
Unhooked MAF. No change.
unhooked upstream 02. No change.
The truck then set A p1514 and went into reduced engine power mode. When that happened you could no longer give it throttle but it idled perfectly and stayed running.
I completely removed the connector from the throttle body and it started and idled fine.
Thinking it must be the throttle body I got one from Napa but after installing it had the same issue and also set a p1516, unplugging it would let the truck idle.
I have ordered an OEM throttle body but it hasn't come in yet.
Does anyone have any ideas?
Strangely when checking on the p0300 I found misfires on 1, 2, and 3. Moved coils and wires but the same cylinders still misfire. 1, 2, 3 only. Unplugging the throttle body stops the misfires as well as correcting the idle. That doesn't make any sense to me.
Codes were p0101 and p0300
I replaced the MAF which didn’t help.
I then took off the catalytic converters as I read clogs could cause it. One was partially clogged but the other fine.
It is still hard to start and will not stay running at idle, only when giving partial throttle.
I tried pulling the battery for a while. No change.
Unhooked MAF. No change.
unhooked upstream 02. No change.
The truck then set A p1514 and went into reduced engine power mode. When that happened you could no longer give it throttle but it idled perfectly and stayed running.
I completely removed the connector from the throttle body and it started and idled fine.
Thinking it must be the throttle body I got one from Napa but after installing it had the same issue and also set a p1516, unplugging it would let the truck idle.
I have ordered an OEM throttle body but it hasn't come in yet.
Does anyone have any ideas?
Strangely when checking on the p0300 I found misfires on 1, 2, and 3. Moved coils and wires but the same cylinders still misfire. 1, 2, 3 only. Unplugging the throttle body stops the misfires as well as correcting the idle. That doesn't make any sense to me.
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from my personal experience both. The wires like to break 2-3 inches from the connector in the bend on the harness that goes to the TB.
the grounds normally get corroded or oil soaked. I'm pretty sure there is a TSB about both of those.
the grounds normally get corroded or oil soaked. I'm pretty sure there is a TSB about both of those.
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Thanks for the suggestions. I went over the wires and grounds. They were all ok. I did some more pid monitoring and noticed the map sensor was reporting 66kpa at idle which seemed high. I unplugged the map sensor and the truck would then run and idle(unplugged it logged 10kpa ) but poorly. No misfires but the short term fuel trim was all over the place. It surges and barely idles.
I also noticed the TPS reading stayed at 11.9degrees regardless of throttle position the entire time.
Still waiting on the new throttle body with tps to come in now. Does that sound like it could cause it and the map reading be unrelated?
Thanks in advance. Sorry if I used in wrong terminology. I'm new to this.
I also noticed the TPS reading stayed at 11.9degrees regardless of throttle position the entire time.
Still waiting on the new throttle body with tps to come in now. Does that sound like it could cause it and the map reading be unrelated?
Thanks in advance. Sorry if I used in wrong terminology. I'm new to this.
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Thanks for the suggestions. I went over the wires and grounds. They were all ok. I did some more pid monitoring and noticed the map sensor was reporting 66kpa at idle which seemed high. I unplugged the map sensor and the truck would then run and idle(unplugged it logged 10kpa ) but poorly. No misfires but the short term fuel trim was all over the place. It surges and barely idles.
I also noticed the TPS reading stayed at 11.9degrees regardless of throttle position the entire time.
Still waiting on the new throttle body with tps to come in now. Does that sound like it could cause it and the map reading be unrelated?
Thanks in advance. Sorry if I used in wrong terminology. I'm new to this.
I also noticed the TPS reading stayed at 11.9degrees regardless of throttle position the entire time.
Still waiting on the new throttle body with tps to come in now. Does that sound like it could cause it and the map reading be unrelated?
Thanks in advance. Sorry if I used in wrong terminology. I'm new to this.
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Thread Starter
Teching In
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Verified the fuel pressure was 45 with key on and 55 while running.
Put in the new Throttle body with TPS and it is correctly indicating throttle position now but the truck still will not idle and has to be revved up to keep it running. It has also developed a tapping/knock in the 1/3 cylinder portion of the motor. p0300 is the only code now.
Maybe it was mechanical the whole time. Thanks for the help, guess I'll start pulling stuff apart now.
Put in the new Throttle body with TPS and it is correctly indicating throttle position now but the truck still will not idle and has to be revved up to keep it running. It has also developed a tapping/knock in the 1/3 cylinder portion of the motor. p0300 is the only code now.
Maybe it was mechanical the whole time. Thanks for the help, guess I'll start pulling stuff apart now.
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Has it been sitting for a long time by now? What's the oil pressure?
I'm pretty sure that fuel pressure is too low. It should be somewhere around 52-55 psi? I don't have the specification in front of me. What happens to the fuel pressure as you rev the engine? Does it fall or hold? You can disconnect the return hose (return system, right?), temporarily cap the line from the intake and prime the pump. Pressure should max out what fuel pump is able to deliver statically. I think when I was chasing something on my 99 1500 it maxed to like 70 psi IIRC.
But I think low fuel delivery pressure should throw a lean code, usually on both banks. Is it going to closed loop?
I'm pretty sure that fuel pressure is too low. It should be somewhere around 52-55 psi? I don't have the specification in front of me. What happens to the fuel pressure as you rev the engine? Does it fall or hold? You can disconnect the return hose (return system, right?), temporarily cap the line from the intake and prime the pump. Pressure should max out what fuel pump is able to deliver statically. I think when I was chasing something on my 99 1500 it maxed to like 70 psi IIRC.
But I think low fuel delivery pressure should throw a lean code, usually on both banks. Is it going to closed loop?
Last edited by strutaeng; Dec 29, 2022 at 08:44 PM.
Joined: Apr 2006
Posts: 3,891
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From: Grand Rapids, Michigan
Fuel pressure should be 58 PSI. Even key on, engine off, I believe. My '06 Silverado has a problem with a miss as well, and I did the same thing, coils swapped, tune up, no change. I am leaning toward clogged injectors in my case. In yours, you might have an injector stuck open. Leave the fuel pressure gauge on, start it, then shut it off. The fuel pressure should drop slightly and hold for a few minutes at least. A 5 psi drop after say, 5 minutes, is acceptable, but any more than that you probably have a leaking injector. You may have more than one issue as well. I'd get a cheap stethoscope ( or just use a long screwdriver ) and listen around the engine to pinpoint the knocking sound. From the symptoms you describe, I'd guess it might be a broken valve spring.