Vortec 350 Cylinder 4 Misfire

[Ty1er]

I have a 97 K1500 with the 5.7 vortec, over the course of a month or two I noticed my mpg steadily declining but no performance issues...I replaced the plugs and wires, cap/rotor, took the K&N that the PO off and put factory air box back, replaced MAF, and fuel filter all over the course of a week. Still no improvement on mpg, now I have a misfire on #4, less than 100 miles on plugs and it was fouled so I replaced it. Next day, same thing. Its intermittent and only cuts up at lower rpms(under 2k) and under acceleration, to the point it almost wants to stall. Compression on passenger bank cylinders were all good (138-140 psi), there is a P0300 misfire code, and 02 heater bank 1&2. Fuel pressure at the Schrader valve is 61psi but drops rapidly

 

[b454rat]

I have a misfire on my Yukon on #4 as well. Same codes too. It misses at idle, but smooths out as RPMs increase. Has power as well. I was at a loss at what it could be, but posted here, and was told it's an intake gasket leak. I'm gonna take it somewhere to get it fixed, I don't have time anymore to fix a sammich...

 

[east302]

Fuel pressure at the Schrader valve is 61psi but drops rapidly

You can separately valve off the fuel supply and return lines (at the fuel filter and adjacent return line union) and retest to determine if the loss is through the pump, return/regulator or injector(s).

But combined with the #4 misfire it implies that the injector on that cylinder is leaking.

Did other cylinders show misfire counts?

 

[Schurkey]

I have a 97 K1500 with the 5.7 vortec,

I have a misfire on #4, less than 100 miles on plugs and it was fouled so I replaced it. Next day, same thing. Its intermittent and only cuts up at lower rpms(under 2k) and under acceleration, to the point it almost wants to stall.

One misfiring cylinder won't make the engine "almost stall".

Connect a scan tool, check misfire counts for EVERY cylinder.

While the tool is connected, verify each sensor, and the computer outputs.

Compression on passenger bank cylinders were all good (138-140 psi),

What is your altitude? 140 psi cranking compression is lower than I'd prefer...but as they're wonderfully even, I guess your results are OK.

there is a P0300 misfire code,

Is your scan tool capable of displaying individual cylinder misfire codes? Cheap tools can LIE about misfire codes. The "real" code might be P0304, but the tool displays P0300 instead.

and 02 heater bank 1&2.

Usually due to ancient O2 sensors. Look for obvious faults in the wire harness, and if it looks OK, you probably ought to replace ALL the O2 sensors.

Fuel pressure at the Schrader valve is 61psi but drops rapidly

Yeah, you need to diagnose that and get it fixed.

 

[Ty1er]

You can separately valve off the fuel supply and return lines (at the fuel filter and adjacent return line union) and retest to determine if the loss is through the pump, return/regulator or injector(s).

But combined with the #4 misfire it implies that the injector on that cylinder is leaking.

You can separately valve off the fuel supply and return lines (at the fuel filter and adjacent return line union) and retest to determine if the loss is through the pump, return/regulator or injector(s).

But combined with the #4 misfire it implies that the injector on that cylinder is leaking.

Did other cylinders show misfire counts?

I'll retest fuel like you mentioned. No other misfire counts on other cylinders

 

[Ty1er]

What is your altitude?

1990 ft
What is ideal psi?

 

[Ty1er]

I have a misfire on my Yukon on #4 as well. Same codes too. It misses at idle, but smooths out as RPMs increase. Has power as well. I was at a loss at what it could be, but posted here, and was told it's an intake gasket leak. I'm gonna take it somewhere to get it fixed, I don't have time anymore to fix a sammich...

Keep me updated if that solves it for you. Mine doesn't miss as bad at idle as it does under pressure

 

[Supercharged111]

1990 ft
What is ideal psi?

Often times the regulator will fail and dump fuel down the intake. When that happens, it can dump extra fuel down a cylinder or 2. The regulator lives in the middle but on the opposite side of cyl 4. I'd spend more time characterizing with the scan tool before popping the plenum off. I think these trucks with crappy 4x resolution don't always accurately report which cylinder is to blame. I'm surprised it didn't just spit out a P0300 like Schurkey said. You could have a junk regulator AND junk poppets, among other things. For the O2 sensors, check to see if the ground fell off the housing. I had one do that and the sensor was less than a year old so I hose clamped it back to the housing and voila the heater code went away.

 

[Schurkey]

1990 ft
What is ideal psi?

I didn't know there was anywhere in ND that high up.

I get really suspicious when I see a stock engine with less than 150 psi cranking compression; and I'd expect a TBI to be more than that at my altitude--but you're a bit higher than I am; so there goes ~5psi right there.

140 isn't looking so terrible, especially since it's so even.

 

[Ty1er]

Thanks for all the input, sorry I've been a no show until now. Since my last visit here I have replaced the distributor(one of the screws that hold the rotor down had broken off), fuel pump, and just this afternoon the fuel injector assembly. After the injectors I was quite hopeful, she was running great for the first twenty miles. Then it was limping and backfiring worse than it ever has. While I was crawling around it I did notice that the pass side exhaust is broken at the flange, it is a true dual system w/o cats and 12" cherry bombs so the noise wasn't all that different lol. I wouldn't think that exhaust could be solely to blame tho.