P0300 and fuel pressure

[mm66]

I've got a friend with a 1999 Tahoe with a 5.7 equipped with the stock SFI system. About two weeks ago, the vehicle began to misfire and illuminated the CEL. It is throwing a p0300 Random Misfire code. We hooked it up to the computer with my scan tool and saw a lot of misfires. The misfire count seems to be increasing faster on the rear cylinders (5,7,6,8) and is not isolated to one bank. All cylinders were showing some misfires.

The idle is very erratic and the truck has a definite hesitation while driving. The exhaust smells very strongly of unburned fuel and in fact appears to spray fuel out of the tailpipe when accelerating after being stopped.

We went through the entire ignition system to see if that was the problem. Coil seems to be fine, as do all the plugs and wires. Wires, distributor cap, and plugs have recently been replaced (Approximately 1k miles ago). With the ignition seeming to be ok, we went ahead and moved on to fuel delivery.

We hooked a fuel pressure tester to the fuel line and immediately thought we had found the problem. Upon the key being turned on and engine still being off, the fuel pump will pressurize to 55-60 psi, but that pressure bleeds off completely within 15 seconds. When running, fuel pressure seems steady around 55 psi. I think that's within specs (53-60 psi right?), but everything I read states that the fuel pressure should hold constant after the key is turned off.

With this new data, we replaced the fuel pressure regulator. Unfortunately, this has not changed anything. Fuel pressure still bleeds off immediately after the key is turned off and the engine is still running rough. Worried that one or several of the injectors might be stuck, I pulled all of them out of their ports. Not a single one leaks a drop of fuel when the key is turned on.

At this point, I'm stumped. Everything I read points towards this being a fuel delivery problem, more specifically, the pressure regulator. Is the SFI system shot? Is my friend at a point where an MFI conversion is justified?

Thank you for your time.

 

[lesterl]

You dont have an SFI system, you have a CSFI system. Was one side of the Manifold washed (clean), these engines are notorious for bad CSFI units, there is, however, a MPFI upgrade for it that is drop in plug and play...... which I just saw in your comments as being an option. Pressure regulators do go bad, but usually get one side washed of the manifold.

 

[mm66]

Yeah, sorry. I thought I had the acronym correct.

Anyway, the manifold seems just as dirty on both sides. The driver's side of the manifold may be slightly cleaner. I've linked a picture of what the manifold looked like when I pulled the upper off.



I replaced the regulator already, but it didn't make any difference. I'm just having a hard time understanding why the fuel is leaking down so quickly.

I'm perfectly willing to do the MPFI conversion if that is what's needed.

 

[lesterl]

Back 1/2 of the manifold looks a tad washed, but I would OHM out the CTS near the water neck. Also cleaning the MAF sensor might be in order as well. Make shure there arent any vac leaks that would fool the PCM.

Does it act up when the engine is cold/ just fired up?

 

[mm66]

Agreed, the back half does look a little cleaner than the front. Those are also the cylinders with the highest misfire count.

It seems to make no difference whether the engine is warm or cold. As soon as you start the engine, it's running rough. If the vehicle is driven and the engine allowed to warm up, it's still running about the same.

I'll have some free time tomorrow evening or Sunday and will check the CTS and for vacuum leaks.

Thanks for your time.

 

[lesterl]

If you could swap some injectors around I would swap some from the front to the back and see if your missfire moves, then you would know you have an injector issue.

Another thing would be, have you done a intake cleaning on it? Some have (on the 4.3l version) pulled the injector assembly off and soaked the poppets. Has there been any Fuel Injector cleaner run through the gas tank?

Might get some GM top end cleaner to do the intake, and get some stout stuff for the gas.

 

[lesterl]

Search FJ10566 on www.amazon.com, the injector assembly is under $300 for the MPFI unit.

 

[mm66]

I pulled the CTS and checked the readings by putting it in some near boiling water (Didn't feel like putting the whole upper back together just to warm up the engine). Resistance values seemed ok.

I'm going to go ahead and clean the injectors by hand and then run a can of cleaner through after I'm done. I should be able to get at it this Thursday. I'll let you know how it goes.

Thanks for all of your help

 

[lesterl]

If it actually runs bad cold, I would have tested at room temperature......

Lettuce know what you find....

 

[shortbox88]

having issue with a 1999 Chevy Tahoe Lt 5.7.... it will just cut off like u turn the key off.

i've been told it was the crank position sensor,, so i replaced it, not the issue.
i had the started tested it was bad, replaced it , was under warranty.
replaced the fuel filter.
replaced the ignition control module , that was tested and bad, ....
still having these intermittent cut out issues. anyone got any ideas. ?
not wanting to just toss more $$$$$$'s at it .. no codes .
thanks
email is [email protected].

 

[lesterl]

Ign Coil? Will it restart when it dies or does it have to cool off?

Have you tested fuel pressure when it cuts off? How about spark? Will it fire right up if you put a little gas into the intake? MAF sensor test?

 

[shortbox88]

tahoe dieing out

not tested fuel pressure yet. that will be the next item..... if we let it set for a bit . then it will start..... replaced, fuel filter , ignition control module, that was bad. replaced starter, that had a bad spot in it .. crank position sensor ,

IT JUST DIES , WITHOUT ANY HESITATION. JUST AS THOU U TURNED THE KEY OFF....

 

[lesterl]

IGN COIL can die like that also, when it dies next time, take a can of carb cleaner and spray some down the intake while cranking to see if it will fire on that, if it does you have eliminated SPARK, and have found a fueling issue. If it wont fire on the carb cleaner then you have eliminated fueling and identified an ignition issue.....

 

[shortbox88]

@ lester

didn't know that it would possibly fire up using carb cleaner.. it is a throttle body....

 

[lesterl]

Carb cleaner is combustible, I fired up my 89 S10 on carb cleaner when I put it back together and the pump was bad, idled it for a few min that way just spurting away...

Oh, and it will work on newer stuff also..

 

[marinekilz]

ignition switch or maybe relay for the fuel pump

 

[clkelley]

To the original poster, with a P0300, I would also check the wiring leading to the O2 sensor at the rear of the catalytic converter on the passenger side, that code is most commonly associated with that O2 sensor and can cause the same symptoms you are reporting. If the wiring is good, then have the catalytic converter itself flow tested to make sure it isn't clogged. Then if that is ok, replace the O2 sensor itself.

To the second person with a problem. I would have to bet that you have a short near the fuel pump itself. check all the wiring in the harness leading to the pump and make sure you do not have broken wiring and or a dead short somewhere.