1999 Suburban, chasing a cyl 3 misfire

The vehicle has 210000 on the clock. 5.7L vortec
I've been getting a code p0303 for about a month. It consistently throws the code when going uphill with the engine lugging in 3rd or 4th. Once it kicks down, the miss stops.

I swapped the 3 and 5 plug wires and plugs, but the code stayed with cyl 3.

I changed the distributor cap.

I've idled the engine and sprayed brake cleaner, propane, and hose water all around the intake manifold and head to block seam, with no change to the idle. I haven't tried a smoke test.

Today I bought the BAFX bluetooth thing, and DashCommand. Screenshots are below. Some of the fuel trims seem way out of whack. They also vary widely. The screenshots were taken over about a 20 minute period.

I have a distributor ready to go in, because either a worn gear, or wobbly bushings seem to be a common problem. But I have no evidence that's the problem.

Not familiar with that scan tool however you can’t always trust scan data. Can it graph? I’d like to see fuel trims AND O2 data.

Also freeze frame data for the code.

Is there a lack of power?
What happens to o2 data at WOT?

Check for weak spark?

Cylinder specific misfire under load is typically a secondary ignition problem... I would triple check plugs and wires and maybe even swap the cap once more before moving away from secondary ignition. try swapping plugs and wires from 4 and 6. since they are very close in the firing order pattern. Computer maybe picking up the misfire on the wrong cylinder.

Well, today I replaced the distributor. Still had a cyl 3 misfire.
Smoke tested by blowing cigar smoke into the brake booster hose. No leaks showed up.
Took the advice of the poster above and swapped the 2 plug and wire with the 4 plug and wire. Also swapped the 6 and 8 plugs and wires. Drove it up the my test hill, and still had the cyl 3 misfire.

O2 voltages are fine at 0.5v.

Earlier I replaced the crank position sensor.

I replaced the spider injector with the retrofit borg warner injectors about 6 years ago. I guess it's possible it could have gone out again.

Sticking exhaust valves are the only other thing I can think of.

Can you do a Relative Compression Test? Or, disable fuel and LISTEN for an abnormal/rhythmic engine cranking cadence.

will the injector wires reach to swap them round?

Thinking it's time for a compression test, or a relative compression test like Chad suggested

I also agree that a compression test is long overdue. Remember the basics- suck, squeeze, bang, blow. If compression is bad, follow up with your leak down test to find cylinder contribution.

So I finally did a compression test on #3 and it was a little over 150 psi.

Is it possible for one "nozzle" of the spider injector to go bad? I replaced the injector and poppets (not sure if that's what they're called) about 4 years ago. It's possible it could have gone bad.

James5 wrote: So I finally did a compression test on #3 and it was a little over 150 psi.

Is it possible for one "nozzle" of the spider injector to go bad? I replaced the injector and poppets (not sure if that's what they're called) about 4 years ago. It's possible it could have gone bad.

Very possible. You can catch this with a secondary ignition trace, OR an injector balance test.

Because you're not seeing a P0200, I'd suspect the injectors are electrically fine. And, because you don't state that the engine misses at idle, I also doubt you'd see a missing pintle hump on an injector primary waveform.

Trying to figure out if this misfire is due to a vacuum leak.
It misfires up a hill in 3rd or 4th gear at between 1500 and 2000 rpm with steady throttle.
Is the engine vacuum high or low during this condition compared to coasting and accelerating?
If it's high, then I might need to look harder for a vacuum leak. If vacuum is low compared to other driving conditions when it doesn't misfire, then it's probably not a vacuum leak.
I'm going to reset the codes and log data driving up my test hill tomorrow.

The freeze frame for when the code is set is below:
Fuelsys1 - CL (closed loop?)
Fuelsys2- CL
Load_Pct % - 21.6
coolant temp - 198
Short term fuel trim 1 - 17.2%
Long term fuel trim 1 - 12.5%
Short term fuel trim 2 - 21.1%
Long term fuel trim 2 - 12.5%
MAP (kPa) - 23.3
RPM - 1847
VSS (km/h) - 52
MAF (g/s) - 6.601
TP% - 30.2

those fuel trims indicate you have a rich condition...

CL is Closed Loop... the 02's are working.

What do the ST and LT fuel trims do when you raise the RPM from idle to 3k.