Car starts & runs ok when cold , starts to run poorly as it warms up. I have replaced the coils, fuel pressure reg,plugs, wires,&fuel purge valve

possible causes for code P0300
you've already change coil and plug....

P0300 could be bad #3 plug or coil... switch those also and see if code change..

if it persist try the next thing..

I would try trading fuel injector 4 to 5 and putting 5 into 4 and see which code you get.....

• Faulty spark plug (s)
• Faulty ignition coil (s)
• Clogged or faulty fuel injector (s)
• Intake air leak
• Fuel injectors harness is open or shorted
• Fuel Injectors circuit poor electrical connection
• Ignition coils harness is open or shorted
• Ignition coils circuit poor electrical connection
• Insufficient cylinders compression
• Incorrect fuel pressure

code P0305 is simmilar....

• Faulty spark plug 5
• Clogged or faulty fuel injector 5
• Faulty ignition coil 5
• Fuel injector 5 harness is open or shorted
• Fuel injector 5 circuit poor electrical connection
• Ignition coil 5 harness is open or shorted
• Ignition coil 5 circuit poor electrical connection
• Insufficient cylinder 5 compression
• Incorrect fuel pressure
• Intake air leak

Possible causes - Faulty spark plug 5
- Clogged or faulty fuel injector 5
- Faulty ignition coil 5
- Fuel injector 5 harness is open or shorted
- Fuel injector 5 circuit poor electrical connection
- Ignition coil 5 harness is open or shorted
- Ignition coil 5 circuit poor electrical connection
- Insufficient cylinder 5 compression
- Incorrect fuel pressure
- Intake air leak

The P0305 code means that the cylinder 5 is misfiring or is randomly misfiring. Start by checking for intake leaks an if no leaks are found the next step is to replace the spark plugs on cylinder 5. If the problem persist more tests needs to be done to diagnose problem, see "Possible Causes



When is the code detected? Engine Cylinder 5 is Misfiring.


When a misfire occurs, engine speed will fluctuate. If the engine speed fluctuates enough to cause the Crankshaft Position (CKP) sensor signal to vary, the Engine Control Module (ECM) can determine that a misfire is occurring.

check for a faulty fuel pressure regulator that is allowing the fuel to drain back and the hot fuel to boil off causing a vapour lock in the system

Is the check engine light on ?
Has anyone checked the fuel pressure cold and hot ?

replace fuel filter, clean fuel injectors.could have bad fuel, do a code scan before buying new parts.faulty egr valve or faulty engine coolant temperature sensor cause engine run poorly.replace spark plugs and wires because if spark plug wires is bad or spark plug and ignition coil is bad that will cause rough idling misfiring and stalling you have dead cylinder not firing properly..

No on the pressure regulator. You've changed more than enough friend. Except for ONE thing. Everything is connected to the computer. You said yourself it was running rich. That should have been the only clue you needed. When the engine is cold, the temperature sensor will place the engine into cold start mode, kinda like a choke. After warmed up, the sensor tells the computer to stop cold start mode and run at regular fuel delivery. Makes sense, right. You turn the choke off after warm-up. Now, IF the temp sensor is faulty, it can't tell the computer to stop cold start mode and keeps dumping excess fuel in when it isn't required. Therefore, the temp sensor would naturally be the culprit. Brilliant deduction my dear Watson. Go for it Brother. Later.

I know this is an old post, but I found the solution for my 01' Yukon P0171 and P0174. I have changed all the vacuum line and the PCV, throtle body gasket, MAF, plugs wires, fuel pressure reg. All was changed and the codes always came back.

I finally noticed the purge valve on the manifold was constantly purging. I replaced the purge valve abnd code has not been back. It was $40.

a faulty coolant temperature sensor could be at fault. if its telling the computer the engine is hot when its actually cold, it can cause performance problems. when the sensor tells the computer its cold, it richens the fuel mixture. once the car warms up, it leans the mixture out. if the mixture is too lean at startup, it will run poorly

it starts from cold because the auto choke is on ,allowing a richer mixture ,try removing the sender unit in the tank and seeing if the connection from the pump to the outlet connection is ok or leaking causing a loss of fuel pressure

have you check your coil? when coil get hot they will break done .i had a car that did the same thing . and it was the coil . check your fuel fulter ok . have a good day.