Vortec 350 missing

[Alex V.]

And not found under hood after brief search... bummer.

Trying to fix a consistent misfire in a stock '97 K1500 on #5 cylinder. 182,000 miles, shows a hair over 180 PSI compression on that cylinder when the others in that bank are exactly 195. Plugs and wires were done recently, and the fouling on that plug was dusty, soft, white buildup - swapping in a new plug didn't do anything for it, even temporarily. I pulled the plenum off and performed the injector balance test via my Snap/on "brick" graphing scanner and a fuel pressure gauge - #5 behaves just like all the others. I cleaned the intake, externally cleaned the injectors, and replaced the distributor cap & rotor which were absolutely toast, but the miss is still there.

Initially the owner said it needed intake gaskets - that it was burning antifreeze on no.5. But by my troubleshooting and the fact that there are no coolant passages anywhere near #5's port from intake to head, I suspect the white buildup was just ash and such from that cylinder not firing at all, and lower compression was due to that cylinder being cool and having junk built up from no combustion taking place. The tip of that injector has a dark redish-brown tarnish while all the others were bright and clean - suspected the injector was bad as soon as I noticed that but it worked in the test so I'm puzzled. Is it likely the injector worked during the test but not under running conditions? The owner is open to replacing the injector spider assy. if need be but neither of us want to throw parts at it. Only code stored when I brought it in was 0300 Random misfire. Any help?

[Alex V.]

Woops, wrong forum. Can this be moved to engine & drivetrain?

[truckster]

I went through two distributor caps before I eliminated the misfire in my 5.7 Vortec.

(And my initial response was, "Did you look under the hood?" You beat me to it.)

[Alex V.]

Ha, gotcha.

[jjzepplin]

Chased down a couple busted valve springs on engines with these symptoms.

[Alex V.]

Pulling the valve cover and verifying proper mechanical function was one of the next things on my list, so ditto jjzepplin.

[truckster]

Inspect your new cap really, really carefully. Because of the way the paths cross, these caps are way more susceptible to misfire than the older style caps. What I found in my brand-new cap was a hairline crack that allowed the spark to jump to an adjacent cylinder.

[B. W.]

Stock 5.7 vortecs were notorious for bad distributor caps. We replaced so many I can't even begin to count. But it seems the manufacturers corrected the problem & I've not seen a bad one in a few years. Vortecs are also known for their bad intake gaskets. Done more of these than I care to remember. The best gasket set to fix the problem is Felpro MS98000T. I've never had to replace one of these. If you still have the stock CSFI injection system the poppet valves are known to leak.
If I had to guess I would say intake gaskets because it is setting a random misfire code, not a specific cylinder & you tested the injectors & tuned it up.
How do you know #5 is the miss?
Is it using any coolant?
Does the miss intensify after gassing on it (passing someone) then backing off the throttle? ( high vacuum will suck coolant in & cause a miss)

If everything checks good, & you still have a random, intermittent miss then check/adjust the cam angle. 5.7 Vortecs have a crank sensor so you can twist the distributor & it does not change timing BUT it does change whether the rotor lines up with the proper cylinder inside the distributor cap. Too far out of adjustment will cause it to fire two cylinders. It is most noticeable under a load when more KV is needed.

[Alex V.]

Turned out to be the intake gaskets, after all. After watching the misfire go away completely above 1000 RPM on my scan tool and finding that plugging the PCV with my hand would also stop the miss, I was sure enough that I tore it down - found the bottom half of 5/7's gasket leaking along pretty much the whole length, and some questionable spots on other cylinders, too. Purred like a kitten when I took it back to him.