08 Tahoe 258000 miles hard starting and wet spark plugs

[Jkiel84]

Yes I’ll get the trims and o2 voltage all the 02 sensors are brand new Bosch ones cause I thought it was the cats cause I got the code for the cats performing under threshold so I changed them and the cats looked good I also did a compression test lowest compression was 165 and the highest was 180 I did the pressure test to see if the head gasket was leaking and that was normal but I get a code now for the fuel pump command module that comes and goes I changed the plugs now it’ll start right up and all I get is a p0300 code I’m at a lose I’ll get the o2 sensor voltage and fuel trims and it’s never had e85 in it I’m the second owner and my boss was the first other thing it ran fine one day then the next it wouldn’t run at all unless I pushed on the gas pedal

 

[Fless]

I can't see the PID descriptions, but if the g/s reading is for the MAF, and the truck is at idle, the MAF is over-reporting. Warmed up and at idle that value should be roughly equal to the engine displacement in liters (e.g., 5.3).

 

[Jkiel84]

This are the trims and o2 sensor values but it doesn’t seem like the trims are doing anything shouldn’t they fluctuate a little

 

[Fless]

This are the trims and o2 sensor values but it doesn’t seem like the trims are doing anything shouldn’t they fluctuate a little

The LTFTs should have values in them unless they were recently reset. The STFTs would need to have the engine running for a bit and would start reporting once it warms up enough to go into Closed Loop. Something does seem to be wrong, with nothing being reported on either LT or ST.

 

[j91z28d1]

somewhere on the scanner it should say if it's in closed loop. from those pics like said above.. the ecm isn't doing anything to correct any. and they always do something while in closed.

 

[Jkiel84]

I can't see the PID descriptions, but if the g/s reading is for the MAF, and the truck is at idle, the MAF is over-reporting. Warmed up and at idle that value should be roughly equal to the engine displacement in liters (e.g., 5.3).

It wasn’t quite warmed up yet it’s misfiring on four cylinders I’ll show you the misfire chart

I can't see the PID descriptions, but if the g/s reading is for the MAF, and the truck is at idle, the MAF is over-reporting. Warmed up and at idle that value should be roughly equal to the engine displacement in liters (e.g., 5.3).

it wasn’t quite warmed up it misfiring on 5 cylinders

 

[Jkiel84]

The LTFTs should have values in them unless they were recently reset. The STFTs would need to have the engine running for a bit and would start reporting once it warms up enough to go into Closed Loop. Something does seem to be wrong, with nothing being reported on either LT or ST.

It was close to warmed up I didn’t see if it was in close loop but I get codes saying all my o2 sensors have a problem but there brand new but it shows the o2 sensors are working

 

[Jkiel84]

Yes I’ll get the trims and o2 voltage all the 02 sensors are brand new Bosch ones cause I thought it was the cats cause I got the code for the cats performing under threshold so I changed them and the cats looked good I also did a compression test lowest compression was 165 and the highest was 180 I did the pressure test to see if the head gasket was leaking and that was normal but I get a code now for the fuel pump command module that comes and goes I changed the plugs now it’ll start right up and all I get is a p0300 code I’m at a lose I’ll get the o2 sensor voltage and fuel trims and it’s never had e85 in it I’m the second owner and my boss was the first other thing it ran fine one day then the next it wouldn’t run at all unless I pushed on the gas pedal

 

[Jkiel84]

I r

If it’s been “barely running” the throttle body servo might be stuck barely open and is stopping it from running as it should (within reason)

an it everyday up until this point without a problem

 

[j91z28d1]

this is just a general rule but when looking at live misfire data, it will be counting up as it's missing. if it's just a value like 5 to 20 and not moving, work on the high and currently moving one first. I'm guessing you have a isses in cyl 8.


I'd have to look up the codes and what the ecm does when those specific o2 sensor codes are thrown, but it very well could stay in open loop if both front o2 sensors codes are set. might have to clear the codes before it will go into close loop. but look over the freeze frame data first if it has it that will be lost when cleared.


my 2 cents on o2 sensors is I'm not a fan of Bosch. but I'd hate to say change them if newish. Just keep in the back of your mind they could be fouled out like the spark plugs were. the obd codes usually have a : and then 2 more numbers that are more specific as to why it set the code if your scanner can see them. it could be as simple as the heater circuit fuse is popped and the got cold and fouled.

 

[j91z28d1]

I can't see the PID descriptions, but if the g/s reading is for the MAF, and the truck is at idle, the MAF is over-reporting. Warmed up and at idle that value should be roughly equal to the engine displacement in liters (e.g., 5.3).

that's so good to remember and I always forget the g/s should be closer to liters.

 

[Fless]

The O2 sensor readings are very near what they would be if they're not warmed up. You might want to check the O2 heater fuses and make sure the sensors are getting heated up so they can work.

And clean that MAF or replace it with OE.

 

[Jkiel84]

this is just a general rule but when looking at live misfire data, it will be counting up as it's missing. if it's just a value like 5 to 20 and not moving, work on the high and currently moving one first. I'm guessing you have a isses in cyl 8.


I'd have to look up the codes and what the ecm does when those specific o2 sensor codes are thrown, but it very well could stay in open loop if both front o2 sensors codes are set. might have to clear the codes before it will go into close loop. but look over the freeze frame data first if it has it that will be lost when cleared.


my 2 cents on o2 sensors is I'm not a fan of Bosch. but I'd hate to say change them if newish. Just keep in the back of your mind they could be fouled out like the spark plugs were. the obd codes usually have a : and then 2 more numbers that are more specific as to why it set the code if your scanner can see them. it could be as simple as the heater circuit fuse is popped and the got cold and fouled.

It did have more to I’ll hook it back up and let you know what I says it was something with the heater circuit if I remember right but I’ll get what the reader says also it take a while to start like if I don’t put my foot on the gas it won’t start

 

[Jkiel84]

The O2 sensor readings are very near what they would be if they're not warmed up. You might want to check the O2 heater fuses and make sure the sensors are getting heated up so they can work.

And clean that MAF or replace it with OE.

I’ll check the fuses I haven’t checked that yet thanks everyone for chiming in it helps even if I haven’t fixed it yet but I will fix it and hopefully on my own without taking it to a shop it was runny decent to then I had those lean codes and my mpg dropped from 15 a gal to 12 real fast so I changed a few parts like the o2 sensors fuel injectors and coils and wires and plugs

 

[Jkiel84]

It did have more to I’ll hook it back up and let you know what I says it was something with the heater circuit if I remember right but I’ll get what the reader says also it take a while to start like if I don’t put my foot on the gas it won’t start

And those o2 sensors are new new like I put them in when I did the cats a few weeks ago when this problem started it gave me the cat below threshold code and I know all the parts are original so I figured couldn’t hurt to change them since the truck was acting up anyway so I put new cats new o2 new plugs new wires changed the oil and still ran the same takes awhile to start and will only start if I put my foot on the gas then it’ll act like it wants to die unless I keep it running then after a min or so it’ll stay running but then the check engine light flashes I’m assuming cause it’s misfiring

 

[Jkiel84]

My 08 Tahoe 5.3 flex fuel all of a sudden on day just had a p0300 random multiple misfire so I changed all the plugs ac Delco iridium’s mind just bought all new coil packs also ac delco it ran fine then the p0300 code checked the plugs they were all black and wet with fuel so I changed all the injectors since they were still oem from the factory still no change it starts really hard then when it starts it’ll idle up and down like it wants to die it also has brand new Bosch o2 sensors and new cats before the misfire thing happened it ran good not great but good was getting 15 miles to the gallon that’s why I say good not great running and that’s highway and then the next day I came out and it would barely stay running fuel pressure is normal with just key on it was 55psi and with it running it was 60psi I know there’s some knowledgeable people here need some help thanks for reading. All cylinders a between 170 and 180 one cylinder is at 160 don’t know if that helps and all coil packs are firing brand new spark plugs just looked in the cylinder with a scope and they all look good gonna do a pressure test to make sure the head gasket isnt leaking but truck wasn’t overheating and never lost coolant need some help buddies

So just a update I ran a injector test and my cylinder 8 injector isn’t firing could this be problem to everything

 

[Jonmurphy]

My 08 Tahoe 5.3 flex fuel all of a sudden on day just had a p0300 random multiple misfire so I changed all the plugs ac Delco iridium’s mind just bought all new coil packs also ac delco it ran fine then the p0300 code checked the plugs they were all black and wet with fuel so I changed all the injectors since they were still oem from the factory still no change it starts really hard then when it starts it’ll idle up and down like it wants to die it also has brand new Bosch o2 sensors and new cats before the misfire thing happened it ran good not great but good was getting 15 miles to the gallon that’s why I say good not great running and that’s highway and then the next day I came out and it would barely stay running fuel pressure is normal with just key on it was 55psi and with it running it was 60psi I know there’s some knowledgeable people here need some help thanks for reading. All cylinders a between 170 and 180 one cylinder is at 160 don’t know if that helps and all coil packs are firing brand new spark plugs just looked in the cylinder with a scope and they all look good gonna do a pressure test to make sure the head gasket isnt leaking but truck wasn’t overheating and never lost coolant need some help buddies

Hi ,Bosch sells heap crappy parts face it..

 

[j91z28d1]

So just a update I ran a injector test and my cylinder 8 injector isn’t firing could this be problem to everything

it's a place to start forsure.

check for spark, sounds like you have fuel in that cyl if it's wet.

oh and check the spark plug is snug. I have seen a loose spark plug missfire.

 

[petethepug]

Where did you purchase the new o2 sensors? I've seen another owner chase issues down a rabbit hole and it came back to non OEM o2 sensors that were prolly counterfeit.

 

[Jkiel84]

it's a place to start forsure.

check for spark, sounds like you have fuel in that cyl if it's wet.

oh and check the spark plug is snug. I have seen a loose spark plug missfire.

I ran a injector test off my scan tool the other 7 make noise that one don’t but it’s doing the same thing it did before I changed injectors so I’m gonna see if it’s getting power if not hopefully that’s my problem thanks for helping