p0300 multiple misfire

[jsfrmsj]

Hello All, I've been trying to figure out a multiple misfire code and rough idle that recently popped up. The truck was fine one day and broke the next day. No other codes are popping up. I can hear slight popping in the exhaust. It idles rough and is down on power but it still drives.

15k miles ago the intake manifold gasket was replaced with Fel-pro, at that time I changed distributor with a reputable billet n brass one. Fuel filter is changed regularly. No exhaust leaks. Fuel pump is about a year old.

I've replaced the coil with a delphi unit, NGK platinum spark plugs, and msd 8.5 wires last week, still no change. Unplugged the MAF and it still runs rough. I did a compression test and all cylinders are within range.

I did a fuel pressure test and got 55 psi at idle and it holds pressure with the key off.

I'm leaning towards it being the spider injectors but I want to be sure before I spend the money.

Here is Freeze Frame from my obdII scanner. Can anybody tell me what this may be pointing too?

SHRTFTB1S1 is 0.8
SHRTFTB2S2 is 0.99%


View Freeze Frame
DTCFRZF() P0300
FUELSYS1() OL
FUELSYS2() OL
LOAD_PCT(%) 3.1
ECT(¡£C) 51
SHRTFT1(%) -0.8
LONGFT1(%) 0.8
SHRTFT2(%) -4.7
LONGFT2(%) 7.0
MAP(kPa) 30.0
RPM(rpm) 731
VSS(km/h) 0
MAF(g/s) 7.82
TP(%) 0.0


Since DTCs cleared
MIL() ON
MIS() OK
FUEL() OK
CCM() OK
CAT() INC
HCAT() N/A
EVAP() INC
AIR() INC
O2S() INC
HTR() INC
EGR() OK

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These are the plugs in order as if you were looking at the engine with the hood open

 

[jsfrmsj]

So I've learned that the short-term fuel trims from sensors 1 are the only ones that matter when looking to see if the engine is running lean or rich.

As you can see from above the short-term fuel trim bank 1 sensor 2 (SHRTB1S2) is NA. Short-term fuel trim from bank 2 sensor 2 (SHRTB2S2) is 99.2% indicating that 99.2% fuel efficiency.

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[jsfrmsj]

FYI

Fuel trims can help you zero in on the problem, especially when there are no other trouble codes present. Knowing whether a vehicle is running too rich or too lean will help narrow down your diagnosis. Fuel trims that differ greatly from one cylinder bank to the other will also point you in the right direction. Always evaluate fuel trims at idle and at 2500 RPM.

Running too rich – High negative fuel trim corrections can be caused by MAF sensor problems, high fuel pressure, leaking fuel pressure regulator diaphragm, faulty evaporative emissions components, leaking injectors, defective O2 sensors, exhaust leaks/pinholes before the O2 sensor, coolant temp sensor problems, and base engine issues such as low compression and incorrect camshaft timing.

Running too lean – High positive fuel corrections can be traced to MAF and O2 sensor faults, vacuum leaks from intake gaskets/hoses, unmetered air (intake snorkel leak), clogged or dirty fuel injectors, fuel delivery issues, and exhaust restrictions such as a clogged catalytic converter.

Short Term Fuel Trim

Short Term Fuel Trim (STFT1 and STFT2) is the computer’s immediate response to adjust the air/fuel ratio. In positive corrections, fuel is added to adjust for a lean condition, while negative corrections respond to a rich condition. STFT corrections represent the current engine run cycle and react very quickly to O2 sensor input. If you were to create a large vacuum leak at Idle by disconnecting the PCV hose, the computer would immediately add positive fuel trim to balance the mixture. Short Term Fuel Trim is not stored in Keep Alive Memory (KAM) after shut down and automatically resets to 0 for the next start/run cycle.

Long Term Fuel Trim

Unlike STFT, Long Term Fuel Trim (LTFT1 and LTFT2) is learned over time while in closed loop operation. It is stored in the KAM and also used for open loop fuel calculations (like start up and wide

open throttle). LTFT is a coarser adjustment and also works to keep STFT within specification.

 

[Stepside_fever95]

Could a bad fuel pressure regulator cause low fuel pressure? I am also at 55 psi, 2 year old Delphi pump and a year old fuel filter.

Sent from my SM-S727VL using Tapatalk

 

[jsfrmsj]

if the fuel pressure doesnt hold when you turn the key off its probably a bad fuel pressure regulator or leaking injectors

 

[jsfrmsj]

well, I replaced the spider injectors with brand new delphi mpfi upgrade injectors, still runs like ****.

 

[jsfrmsj]

Update: went to start the truck today after sitting for a few weeks, on initial startup the truck backfired, puff of smoke came out of the hood.

The only thing that looked wrong was the plastic piece on the intake manifold where the PCV valve plugs into the intake. pushed that back down and the truck started fine. I'm guessing it backfired through the intake manifold.

The truck seems to be idling better, I'm able to drive it without it surging and almost stalling at stop lights, has power again but still got the p0300 code with slight knock and rough idle. I'm thinking this may be temperature related? The weather here has warmed up significantly.

Rechecked the fuel pressure and got 48psi key on ignition off. Started it up and got 50 psi, 55 psi when I give it gas.

the weird thing is when I turn the truck off, key out the fuel pressure slowly creeps up to 60psi?

 

[jsfrmsj]

So I was messing with the truck today, cleaned the MAF and tightened up a ground that the mechanic must have missed. Nothing changed BUT when I plugged in the scanner I got a p0304 (misfire cylinder 4) and p0011 (Camshaft position timing over advanced).

Cleared the codes, drove around and got p0305, cleared it again and got p0302 this time.

I'm going to try and replace the cap + rotor and cam position sensor next weekend.

 

[JoeySS]

Let us know how it goes. I'm having a similar p0300 with backfire at idle. I'll be replacing cap/rotor, upper & lower intake gasket, spider and regulator.

 

[jsfrmsj]

SOLVED

Cap was fudged, new cap and rotor, runs like a champ now!