P0300
Thread Starter
Melting Slicks
Joined: Mar 2018
Posts: 2,502
Likes: 187
From: California
P0300
This p0300 code came on so I replaced the plugs wires, plugs, and coils packs all 8 of them. Took it out for a test well lights flashing p0300 is back, read it could be a injectors,egr pipes are clogged, mass air flow sensor is failed. The car does seem to run better than before. Still rough at idle a little and take off it stutters. But runs noticeably stronger then when the issue first started yesterday. I put on 8 Dura last coils. 8 ac delco plug. And gold Duralast wires.
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Joined: Sep 2013
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From: Cape Coral, Florida
Speedy,
A P0300 is a random misfire but unfortunately most DIY’ers go for the plugs and wires...MANY MANY things can cause this DTC to set... yes, it could be ignition related but could be a vacuum leak, faulty injectors, fuel delivery, low compression, bad cam/crank sensor, bad fuel (also running on less than almost an 1/4 tank), bad MAF, bad TPS sensor and there are maybe a few I probably missed...to be honest with you and to save yourself some a lot of money and headache I’d take the car to a good diagnostic shop...you can change parts but if you don’t check all the electrical inputs to these sensors you’ve accomplished nothing...without a scan tool (and a DSO is even better) and some basic diagnostic knowledge you’ll just be chasing your tail...probably 99.9 percent of our C5 owners on this forum don’t have the tooling to accurately troubleshoot these problems and if they did we wouldn’t have this forum !!...if you’d like just give me the direction you’d like to go and if you have a 12v test light and a DVOM I can give you some help on what we can check !!
A P0300 is a random misfire but unfortunately most DIY’ers go for the plugs and wires...MANY MANY things can cause this DTC to set... yes, it could be ignition related but could be a vacuum leak, faulty injectors, fuel delivery, low compression, bad cam/crank sensor, bad fuel (also running on less than almost an 1/4 tank), bad MAF, bad TPS sensor and there are maybe a few I probably missed...to be honest with you and to save yourself some a lot of money and headache I’d take the car to a good diagnostic shop...you can change parts but if you don’t check all the electrical inputs to these sensors you’ve accomplished nothing...without a scan tool (and a DSO is even better) and some basic diagnostic knowledge you’ll just be chasing your tail...probably 99.9 percent of our C5 owners on this forum don’t have the tooling to accurately troubleshoot these problems and if they did we wouldn’t have this forum !!...if you’d like just give me the direction you’d like to go and if you have a 12v test light and a DVOM I can give you some help on what we can check !!
Last edited by C5 Diag; Jul 2, 2018 at 12:26 PM.
Thread Starter
Melting Slicks
Joined: Mar 2018
Posts: 2,502
Likes: 187
From: California
Thanks rwobs ya after doing some more research I could be something I’m just not cable of doing. I cleaned out the mass air flow threw it back in also car ran worse than ever. So I took all the new parts put the old ones on. Gonna bring her to a shop. Like you said I would be chasing my own tail at this point dumping money where it doesn’t need it. I’ll let ya all know what it turns out to be. I got told by a mechanic shop either the coil packs or plus and wires would clear it up that wasn’t the case. My hands are way to sore to keep up lol. I will start it inside and look for loose connections. I just hope it didn’t bust a spring or something internal went wrong. But like I said the car was running excellent 20 mins prior to shutting down grabbed a coffee. The car was be never raced or beat under my ownership.
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Joined: Sep 2013
Posts: 15,312
Likes: 3,925
From: Cape Coral, Florida
Thanks rwobs ya after doing some more research I could be something I’m just not cable of doing. I cleaned out the mass air flow threw it back in also car ran worse than ever. So I took all the new parts put the old ones on. Gonna bring her to a shop. Like you said I would be chasing my own tail at this point dumping money where it doesn’t need it. I’ll let ya all know what it turns out to be. I got told by a mechanic shop either the coil packs or plus and wires would clear it up that wasn’t the case. My hands are way to sore to keep up lol. I will start it inside and look for loose connections. I just hope it didn’t bust a spring or something internal went wrong. But like I said the car was running excellent 20 mins prior to shutting down grabbed a coffee. The car was be never raced or beat under my ownership.
Good decision !!...if you do have a DVOM and a 12v test light I can have you check a few things electrically if you want. I'd look for a good diagnostic shop or a tech who is at least ASE certified and who has an L1 Advanced Engine Performance Specialist patch on his arm. He'll fix it !!...Oh, here is a video by Paul Danner the diagnostic "Jedi Master" diagnosing a P0300 on a Hummer...it's a 4 part series and as you can see it's a LOT more involved than just replacing parts.
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Joined: Sep 2013
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From: Cape Coral, Florida
What O2 sensor DTC did you get with the P0300 ??...now just because it was an O2 DTC doesn’t always mean it’s a bad O2 sensor...without looking at scan data it’s just a guessing game !!
Thread Starter
Melting Slicks
Joined: Mar 2018
Posts: 2,502
Likes: 187
From: California
Got the car back from this mechanic he says the cam shaft lobes are worn. Ok there’s no ping or ticking from the engine just don’t understand how it would be the cam. I don’t think the car would even run. Got told it had 160 compression then it drops to 90. He never pulled a plug or did a compression test. Also mentioned a failed intake gasket any ways he wanted 2200 for the repair. I told him to get lost. One guy says plug wires and coil packs,the next guy says the cam is worn. Also my oil gauge stopped reading after getting out of his shop great great great.
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Joined: Sep 2013
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From: Cape Coral, Florida
Got the car back from this mechanic he says the cam shaft lobes are worn. Ok there’s no ping or ticking from the engine just don’t understand how it would be the cam. I don’t think the car would even run. Got told it had 160 compression then it drops to 90. He never pulled a plug or did a compression test. Also mentioned a failed intake gasket any ways he wanted 2200 for the repair. I told him to get lost. One guy says plug wires and coil packs,the next guy says the cam is worn. Also my oil gauge stopped reading after getting out of his shop great great great.
Better to find another mechanic and fast !!...he’s the one I wouldn’t let change my oil !!...now without doing a compression test did he know the cylinder was low ??...now I’ll do a “relative compression” test by using a lab scope with an amp clamp around the battery cable (as the starter turns depending on how hard it has to work turning the engine the ammeter will show the starter current draw)...just to compare all the cylinders !!...if one is a little lower I then use a mechanical pressure gauge to measure the true compression...then a leak down test to figure out where the leakage is occurring...intake, exhaust, or rings...what O2 sensor DTC did you see ??
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Thread Starter
Melting Slicks
Joined: Mar 2018
Posts: 2,502
Likes: 187
From: California
I saw one scanner hooked up went to work for a 1 and came back that was his answer. Bad cam lobe. I don’t believe it. He didn’t even charge a fee to diagnose. So I fired up the car it seems to start and run fine until you hit the throttle a little. Oil gauge was reading 75 psi or so on cold start slows to 40psi. Tap the throttle and the car is a little shaken returning to low idle. Then oil gauge goes flat to zero. Is the wiring harness to the coil packs also measure the oil pressure ? I’m thinking either the harness is shot or possible vacccum leak is cause the loss in pressure for the gauge. Dunno if that harness to the coils also powers the oil gauge ? Suggestions I am open to everything.
Moderator, Tech Contributor
Joined: Sep 2013
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From: Cape Coral, Florida
I saw one scanner hooked up went to work for a 1 and came back that was his answer. Bad cam lobe. I don’t believe it. He didn’t even charge a fee to diagnose. So I fired up the car it seems to start and run fine until you hit the throttle a little. Oil gauge was reading 75 psi or so on cold start slows to 40psi. Tap the throttle and the car is a little shaken returning to low idle. Then oil gauge goes flat to zero. Is the wiring harness to the coil packs also measure the oil pressure ? I’m thinking either the harness is shot or possible vacccum leak is cause the loss in pressure for the gauge. Dunno if that harness to the coils also powers the oil gauge ? Suggestions I am open to everything.
so now in addition to the P0300 we have an oil pressure problem ??...how long was the oil pressure at 0 ??...if it were 0 psi that engine would have made quite a racket !!...what is the oil pressure gauge vs the pressure on the DIC look like ??...the power from each bank is powered through 2 separate fuses which power the primary side of the coil and the control wires come from the PCM so no it does not power the oil pressure sender...you would have to find a faulty wire within that harness to find a problem...you wouldn’t change a whole harness !!...the oil pressure sender is a VERY common issue on the C5’s...some can last for 100,000 miles and some for 10,000...it easiest to replace it with the intake manifold removed but can be done with it installed but hope you have a high frustration level...LOL !!...I would get this pressure issue resolved then look at the P0300 misfire !!
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,312
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From: Cape Coral, Florida
WOW !!...we’ll have to get Speedy to look at that !!...a mechanic told him it was worn cam lobes diagnosed with only a scan tool...LOL !!...if I were him and he has a dial indicator I’d measure those cam lobes and if within spec the I’d tell him he’s a J$$KA$$ !!...there are way to many things to set a P0300 !!
Thread Starter
Melting Slicks
Joined: Mar 2018
Posts: 2,502
Likes: 187
From: California
Dealer says it’s impossible you would know it’s the cam if the cam is gone . It would be a really adnormal sounding ping ding, and it prob wouldn’t run. Its at the dealer now check in later. I feel as they have every tool in the bag to produce a accurate diagnosis unlike those other fools who out are to take people on a ride.
Last edited by Speedy007; Jul 3, 2018 at 01:41 PM.
Thread Starter
Melting Slicks
Joined: Mar 2018
Posts: 2,502
Likes: 187
From: California
Hi !!
so now in addition to the P0300 we have an oil pressure problem ??...how long was the oil pressure at 0 ??...if it were 0 psi that engine would have made quite a racket !!...what is the oil pressure gauge vs the pressure on the DIC look like ??...the power from each bank is powered through 2 separate fuses which power the primary side of the coil and the control wires come from the PCM so no it does not power the oil pressure sender...you would have to find a faulty wire within that harness to find a problem...you wouldn’t change a whole harness !!...the oil pressure sender is a VERY common issue on the C5’s...some can last for 100,000 miles and some for 10,000...it easiest to replace it with the intake manifold removed but can be done with it installed but hope you have a high frustration level...LOL !!...I would get this pressure issue resolved then look at the P0300 misfire !!
Last edited by Speedy007; Jul 3, 2018 at 01:58 PM.
Moderator, Tech Contributor
Joined: Sep 2013
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From: Cape Coral, Florida
A vacuum leak is real easy to diagnose if a scan too is used and looking at fuel trims...fuel trims will be way positive and ratchet down as the RPM is increased...car runs crappy at idle with vacuum leak but will run fine at higher RPM’s !!...you can use brake clean or even water squirted around intake manifold or around vacuum lines...I thought you said the car was running rough at idle ??...you can even disconnect the MAF sensor and see how it runs...you mentioned you cleaned it but you I would check it electrically to see if power and grounds were good...just 3 wires to check...other 2 are for the IAT sensor !!
Last edited by C5 Diag; Jul 3, 2018 at 02:02 PM.
Drifting
Joined: Apr 2007
Posts: 1,517
Likes: 159
From: Lafayette Louisiana
I replaced all 4 O2 sensors. Problem resolved.
P0133 Heated Oxygen Sensor (HO2S) Slow Response Bank 1 Sensor 1
P0134 Heated Oxygen Sensor (HO2S) Circuit Insufficient Activity Bank 1 Sensor 1
P300 Random Missfire
Check Engine Light (sometimes flashing)
Thread Starter
Melting Slicks
Joined: Mar 2018
Posts: 2,502
Likes: 187
From: California
A vacuum leak is real easy to diagnose if a scan too is used and looking at fuel trims...fuel trims will be way positive and ratchet down as the RPM is increased...car runs crappy at idle with vacuum leak but will run fine at higher RPM’s !!...you can use brake clean or even water squirted around intake manifold or around vacuum lines !!
Last edited by Speedy007; Jul 3, 2018 at 02:10 PM.
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,312
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From: Cape Coral, Florida
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,312
Likes: 3,925
From: Cape Coral, Florida
O2 sensor codes as follows. Scan showed very little activity on bank 1 sensor 1. Wish I had saved graphic of the scan but did not..
I replaced all 4 O2 sensors. Problem resolved.
P0133 Heated Oxygen Sensor (HO2S) Slow Response Bank 1 Sensor 1
P0134 Heated Oxygen Sensor (HO2S) Circuit Insufficient Activity Bank 1 Sensor 1
P300 Random Missfire
Check Engine Light (sometimes flashing)
I replaced all 4 O2 sensors. Problem resolved.
P0133 Heated Oxygen Sensor (HO2S) Slow Response Bank 1 Sensor 1
P0134 Heated Oxygen Sensor (HO2S) Circuit Insufficient Activity Bank 1 Sensor 1
P300 Random Missfire
Check Engine Light (sometimes flashing)
Drifting
Joined: Apr 2007
Posts: 1,517
Likes: 159
From: Lafayette Louisiana
GREAT !!...was the O2 sensor flatlined low ??...a quick test if it’s low is to spray brake clean into the intake...if it responds and Mv increases the sensor is good and it is really a lean condition...no response means sensor is toast !!...0133 and 0134 together is usually a bad sensor !!
