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GM Bob, ASE Certified Technician
Category: Chevy
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Experience:  30 plus years. experience as GM technician, ASE Master Technician and GM Certified.
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I have a 2008 chevy tahoe flex 5.3 keep getting p0300 and

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i have a 2008 chevy tahoe flex 5.3 keep getting p0300 and p0174, after engine replacement and possibly codes were thrown prior to replacement
JA: Just to clarify, what's the exact code? And have you tried to reset it?
Customer: p0300 and p0174
JA: Are you fixing your Tahoe yourself? What have you tried so far?
Customer: i am, and i have replaced the 2 front oxygen sensors, maf sensor, spark plugs when swapping motor, crankshaft sensor, (relearned with scan tool), with the scan tool i monitored the cylinders and getting 1,3,2,6,8 missing but keeps setting p0300
JA: Anything else you want the mechanic to know before I connect you?
Customer: i tried swapping coils around, injectors i also checked fuel pressure running i have 48psi, with key on i was getting 55psi, now when i cycle the key there is no psi i also dont hear the pump cycle until i start the vehicle. then the psi goes up to 68psi when i shut the car off, but during running its48-50 psi, and its a flex fuel tahoe, thanks

good evening my name is ***** ***** I will assist

it sounds like you have a fuel pressure issue. low fuel pressure will set both codes.

the fuel pressure is not steady as you see. it should be 50 running. you are at the low end of the chart. the fact that it varies all over the place makes it a suspect.

I would replace the pump and re test


please dont forget to rate me as it is the only way I get paid for my efforts


please dont forget to rate me as it is the only way I get paid for my efforts


Customer: replied 12 months ago.
Hey bud, i replaced the fuel pump, and got the gauge not to drop anymore, but still getting the random misfires and trac light blinking. not sure whats going on anymore suggestions?

ok, did you test the fuel pressure before replacing the pump???

is it the 300 code??? did you clear it after you replaced the pump?? the trac is a separate issue


Customer: replied 12 months ago.
i checked fuel pressure prior, it is at the same psi, it just doesn't drop anymore, when i shut the key off. and yes i also cleared trouble codes. when i go into vin mode on my snap on scanner i monitored all the cylinders and have each cylinder misfire at different times, cylinder #3#1and #8 misfire the worse but all except #4 misfire. at idle i have no misfire registered on the scanner when monitoring the waves

ok, was the engine new or used?? if used, how long did it sit before you bought it???


Customer: replied 12 months ago.
i was a remain, from a vendor we always use. I have a small auto repair shop.

ok, thanks.

I was concerned about valve train issues

what about the coils???

Customer: replied 12 months ago.
i swapped coils, i also use the #4 injectors to see if that change #8 and the others that were misfiring

ok, does it feel like a ignition miss fire??? can you feel it driving???

my other concern is the injectors. since you have multiple miss fires, it could be injectors. have you run cleaner in the injector rail and injectors to verify???


Customer: replied 12 months ago.
at idle it has a little sputter, you can hardly notice unless you are looking for it, at running speed freeway or in town no noticeable misfire at all. and no i did not run anything through the fuel rail, but i did swap #4 which cylinder never showed a misfire, to the other cylinders to test. and after swapping around #4 still did not misfire
Customer: replied 12 months ago.
i also like to mention, i had one of the vendor inspectors come out to look at the engine, and we boro-scoped the reluctor to see if there was any damage or bent teeth, and none were found

ok, if that were the case, it would have coded for that

does it still have the 174 lean code???

also, did you do a back pressure test on the cats??? I had one a long time ago that had missfires and it was the cats were blocked.

are the O2 sensors reading correctly???


Customer: replied 12 months ago.
i havent had the 174 come back yet. and the bank 1-2 2-1 o2 sensors are flexing like crazy but the rears are normal. the readings for the top ones go from 785-85 then jump back and forth. the rears stay at 780 or so. i did remove the front o2 sensor and had a steady ready of 485 in that range

the rears should be stable.

back pressure test???

Customer: replied 12 months ago.
no sir, i have not done a back pressure test yet. I will try that and get back to you, thanks
Customer: replied 12 months ago.
i will check both banks


Customer: replied 12 months ago.
did the back pressure check, and i have -1 on 2500 rpm's holding steady and +2 on the snap shoots right back down to -1

the +2 is high. I am concerned that the cats are the issue

Customer: replied 12 months ago.
its only on the snap it spikes to +2. on a 2500 rpm its where it should be. its just the cat is a very expensive guess.

yep, I agree. I am here and not there so I cannot see it or touch it. I am going to opt out and re list to see if anyone else has any ideas.


Customer: replied 12 months ago.
ok thanks

Hello my name is Bob.

Have you tried removing both front O2 sensors? Run the engine see if the misfire goes away.

Customer: replied 12 months ago.
Hey Bob, yes sir I just did this a couple hours ago and still get misfire reading on my scan tool

Did you check for spark?

Do you hear the injectors clicking?

Have you done a compression test?

Have you done a leak down test?

sorry for so many questions.

Customer: replied 12 months ago.
No problem, but yes i have performed all these test, the motor is a remain just put in, and i still did the compression check and have great compression on all cylinders

Are they all misfiring the same?

Or is there some misfiring more?

Customer: replied 12 months ago.
they are not all the same, I have never had #4 misfire, but all the other ones do, #1#3#7#6#8 misfire the most at 2500rpms. At idle no misfire detected from scan tool.
Customer: replied 12 months ago.
just to mention when i drive on the freeway there is no feeling of a misfire, but the scan tool picks it up and every so often the trac light and check engine light will flash, and eventually will set a p0300 or even p0308

You have done a crankshaft sensor relearn?

I have had this before on a van had to reprogram the ECM. Than do a relearn.

Showed misfire at highway speed but felt nothing.

Customer: replied 12 months ago.
i did the relearn already with my scanner that was one of my first things when i put the crankshaft position sensor on.
Customer: replied 12 months ago.
i haven't had the ecm flashed just relearned the crank sensor. i had to swap this motor 2 times, the original problem was a dead hole on #7 this was the original motor with 160k on it. The remain we received from our vendor did not throw these codes but after 3k miles we had a lifter tapping and had them send us out a warranty motor. Now after installing this one we are getting the random

Only time you feel anything is at idle and that is hard to feel.

Nothing felt at when driving.

But the scan tool is picking up misfire and setting code.

Is this correct?

Customer: replied 12 months ago.
i dont really feel a misfire at idle but it will hiccup a little, when driving i feel nothing like a misfire but the scan tool picks up multiple misfires

If the crank sensor and reluctor teeth are ok.

Did the block where the crank sensor goes in look ok? Any type of machine work at the hole?

I was thinking the sensor maybe to close to the crankshaft teeth? Any marks on the sensor?

Customer: replied 12 months ago.
not sure i will have to look. im out of options at this point. i think i covered everything.

I would see if a reprogram would help.

I have seen this on the V6 engine GM had us shim the crank sensor

Customer: replied 12 months ago.
ok thanks for the advise, i will let you know the results

Thank you.

GM Bob, ASE Certified Technician
Category: Chevy
Satisfied Customers: 1133
Experience: 30 plus years. experience as GM technician, ASE Master Technician and GM Certified.
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