Fires on two injectors at idling. Misfires under pressure. Air bubbles in delivery line from fuel tank. Fuel tank disconnected. Run car out of a can of diesel, air bubbles stil appear. Idling seems to inprove but when reving engine, misfire is stil present.
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code p 1399 refers to random misfire ===causes --fuel /ignition--MAP sensor---IAC valve ( idle air control valve )
misfires are more from faulty injectors and low fuel pressure than from electronic problems
unless you have been burning lots of oil , the ports will be clean so have the injectors serviced
codes p 0300 through refer to misfires on each cylinder with the end number ( 301 is no 1 cylinder , 302 is no 2 cylinder ans so on)=== causes --engine mechanical fault , ---wiring---ignition/ fuel ---injector/s---ECT/ MAF sensor/s ---ECM
most misfires are from injector faults like dirty/blocked tips, low fuel pressure , no pulse or problem pulse from ECM rather than plugs and wires and coils
codes p0171 refers to system to lean bank 1===causes ---intake exhaust leak---air system---maf/vaf sensors ---fuel pressure / pump---injectors ---heated oxygen sensor ( HO2S)
code p 0172 is system to rich-====causes ---intake blocked ---EVAP cannister purge valve ---fuel pressure --EGR system---injector/s ---HO2S
couple these two codes with the misfire codes , I suggest you look at the injectors, fuel pressure (too low) from EVAP problem not venting tank, fuel pressure regulator, EGR system
many people mistakingly believe that misfires are the results of ignition problems only. There are several other variables that have to be considered . They are fuel pressure and injector performance. If the fuel pressure is too low then the injectors squirt instead of atomising an the resultant air/ fuel mixture will not combust properly resulting in misfiring. Plugs used in the engines manufactured in the last 12 years will generally out last the engine . Quality HT leads if separated correctly will last many years . But injectors which are subject to electronic problems and dirty fuel (gum and tars not removed by filters) will be the culprit in most engine misfires. So have a pressure test done on the fuel manifold to check the fuel pressure regulator, pump and filter. have the injectors cleaned and regularly maintained .
I'm with you leaning on a maf/sensor, also be sure to clean out the Idle Air control...usually hand in hand...another issue could be "bad gas" did your GMC sit for any period of time? Try adding two bottles of gas tank injector cleaner...I prefer the STP super concentrated in the black bottle...Walmart is definitely the cheapest for these bottles by 2 or 3 bucks...Hope this helps.
Could be a number of issues. Fuel pressure or regulator issue, but...
If you have a coolant sensor problem, as in reading wrong, and it tells your computer it is colder than it actually is,,,you will get more fuel delivery.
First, take a quick sniff of the exhaust when the engine is idling. If you smell gas there, check your spark plugs and wires to eliminate misfires, then check for leaky fuel injectors (leaking excess fuel into a cylinder).
If the exhaust smells normal, you'll need to inspect the fuel delivery system. The most probable place for a leak would be the fuel lines under the car; it's old enough that a line could be rusted through and have a pinhole leak somewhere. If you suspect a spot but want to verify it by turning on the engine to pressurize the line, be sure to wear safety goggles. You don't want gasoline sprayed in your eyes.
this is one of those situations that a professional diag would of saved you alot of money. Its possible you just got bad gas and your injectors are partially clogged. An 02 sensor would not give multible misfires. EGR would only misfire at idle if stuck partially open since its suppose to be open cruising. Fuel filter would give you a starvation issue but if the misfires happens at idle then the filter most likley is ok.
Data from a professional data scanner will store the conditions when the misfire occurs. Also for a chevy each cylinder will have a misfire counter and one can see the misfire happening as it happens. If its idling and misfiring and gets better as you rev it up most likely you have clogged injectors or a intake vacuum leak. You can test for a intake leak by spraying the intake area with water(idle will faulter if it draws water in), or use brake cleaner (idle will raise when fluid gets drawn in).
If no leaks are found I would do an injection service or at least add two bottles of techron to a full tank, drve till half a tank then top off again.
Also if the catalytic converter is clogged the backpressure could cause all sorts of misfire issues. Removing the two upfront 02 sensors and driving will indicate the exhaust is clogged or not. We put a pressure sensor in place of the 02 sensor. If the pressure is above 5psi this would indicate some blockage. It should be below 3psi at 2000 rpm. Also if you have K/N filter the oil from it will coat the maf sensor wires in time,, (always). You can remove the sensor and spray the wires with brake cleaner to clean. DO NOT touch them. They can be brittle. Look at the wires for color. They should be shiny silver. If its coated the PCM will not see the air volume or accurately see the air temperature. Without this information the PCM will not add enough fuel which will create a misfire.