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1999 Suburban, chasing a cyl 3 misfire

  • James5
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13 Sep 2019 20:46 - 13 Sep 2019 20:56 #33588 by James5
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.
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Last edit: 13 Sep 2019 20:56 by James5.

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13 Sep 2019 21:24 #33589 by John Curtis
Replied by John Curtis on topic 1999 Suburban, chasing a cyl 3 misfire
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?

Thinking out loud always helps me in the process.

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13 Sep 2019 22:38 #33590 by ptebo1
Replied by ptebo1 on topic 1999 Suburban, chasing a cyl 3 misfire
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.

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14 Sep 2019 19:05 #33602 by James5
Replied by James5 on topic 1999 Suburban, chasing a cyl 3 misfire
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.

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  • Chad
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14 Sep 2019 19:22 #33604 by Chad
Replied by Chad on topic 1999 Suburban, chasing a cyl 3 misfire
Can you do a Relative Compression Test? Or, disable fuel and LISTEN for an abnormal/rhythmic engine cranking cadence.

"Knowledge is a weapon. Arm yourself, well, before going to do battle."
"Understanding a question is half an answer."

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15 Sep 2019 16:41 #33612 by Crombie
Replied by Crombie on topic 1999 Suburban, chasing a cyl 3 misfire
will the injector wires reach to swap them round?

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  • Noah
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16 Sep 2019 06:38 #33616 by Noah
Replied by Noah on topic 1999 Suburban, chasing a cyl 3 misfire
Thinking it's time for a compression test, or a relative compression test like Chad suggested

"Learn, apply, repeat."

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16 Sep 2019 20:41 #33625 by Alex Ace Auto
Replied by Alex Ace Auto on topic 1999 Suburban, chasing a cyl 3 misfire
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.

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