08 Tahoe 258000 miles hard starting and wet spark plugs

[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

 

[LordWayback]

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

Did you change the fuel pressure regulator when you did the injectors? Sometimes the mesh screen gets clogged and causes a lean condition, fuel pressure does not equal flow another lean condition can be caused at the fuel filter/pump
Because it’s a flex fuel I would try to go back to normal gas if you have been running E-85 or similar majority ethanol based fuel another place to check that often gets overlooked is the throttle body in 2008 they had the electronic throttle body’s and if the servo in them goes funky or they get dirty or they have too much play on the servo they’ll misfire because the truck can’t control the airflow

 

[LordWayback]

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

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)

 

[j91z28d1]

sounds like that ethanol percentage reset thing.

do you have a scanner you can see o2 sensors voltage and fuel trims?

 

[LordWayback]

sounds like that ethanol percentage reset thing.

do you have a scanner you can see o2 sensors voltage and fuel trims?

There are scanners that do but not mine fuel trims even in the newer ones won’t tell you flow rate they just graph the pressure and how consistent it is I would bark down the throttle body tree if I was you seeing the fuel pressure not being ultra low and knowing of good plugs and probably injectors the throttle body itself and throttle body position sensor itself are well known issues in 2007-2013 GM trucks/suvs ? If you look at the throttle body there’s no visible linkages because it’s all electronic inside the throttle body if these parts wear out or malfunction it’ll cause your exact symptoms with no difference on the live data p0300 can be caused by just about anything with a engine

 

[j91z28d1]

There are scanners that do but not mine fuel trims even in the newer ones won’t tell you flow rate they just graph the pressure and how consistent it is I would bark down the throttle body tree if I was you seeing the fuel pressure not being ultra low and knowing of good plugs and probably injectors the throttle body itself and throttle body position sensor itself are well known issues in 2007-2013 GM trucks/suvs ? If you look at the throttle body there’s no visible linkages because it’s all electronic inside the throttle body if these parts wear out or malfunction it’ll cause your exact symptoms with no difference on the live data p0300 can be caused by just about anything with a engine

by fuel trims I mean short term and long term fuel trims. the ecm looks at the o2 sensor in the exhaust, and depending on how rich or lean it is, it will 'trim" the amount of fuel. by looking at the trims you can see if the ecm is adding fuel or removing fuel compared to what the base calibration is. my guess from his wet fouled plugs it's running really rich. the trims would tell you if it's because the ecm is actively adding the fuel, if the trims are normal and it's super rich that leads you in a different direction. most likely the o2s are lying to the ecm.

oh and the ethanol percentage thing. these flex trucks did away with the actual sensor in the fuel system and it uses a bunch of math to decide the % of e. E needing way more fuel, if that math gets off and it thinks the e % is high but you're running normal gas, it runs super rich. it takes a certain level scanner to reset the math if it's off. I don't remember the details, but a search should bring up a bunch of threads. it's pretty common

 

[LordWayback]

by fuel trims I mean short term and long term fuel trims. the ecm looks at the o2 sensor in the exhaust, and depending on how rich or lean it is, it will 'trim" the amount of fuel. by looking at the trims you can see if the ecm is adding fuel or removing fuel compared to what the base calibration is. my guess from his wet fouled plugs it's running really rich. the trims would tell you if it's because the ecm is actively adding the fuel, if the trims are normal and it's super rich that leads you in a different direction. most likely the o2s are lying to the ecm.

oh and the ethanol percentage thing. these flex trucks did away with the actual sensor in the fuel system and it uses a bunch of math to decide the % of e. E needing way more fuel, if that math gets off and it thinks the e % is high but you're running normal gas, it runs super rich. it takes a certain level scanner to reset the math if it's off. I don't remember the details, but a search should bring up a bunch of threads. it's pretty common

If it works it works

 

[blackelky]

258000? Non afm?

 

[Fless]

Well, hopefully the OP will come back and see some of these comments and suggestions. Hopefully not a hit and run.

I would say to verify the fuel trims and the MAF reading, as well as the alcohol content with a decent scanner, and see if that shows anything. Eric O does that here:

 

[petethepug]

To get it running like it was prior to changing parts, you’ll need a hand with somebody who has a bi directional scanner and knowledge of the LS motor.

You’re throwing parts at it. An LS pro will hook up to it and ask questions like…

* Is it running e85 or pump gas?
* When’s your last tune up?
* Tell me anything you done to the truck in last 6 Mo’s.
* Let’s verify your new plugs are compatible and gap’d etc..