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OhTwoBu

rough idle and backfire under load

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OhTwoBu

Good day crew,

I've got an 02 23LSV, 350, which just started having a rough idle and i'm getting what I can describe as a very faint popping under load. Same noise through surf, wakeboarding, and ski speeds.  At idle it's notably shakier than it's ever been, but not awful.  It kind of sounds like a gurgling noise every few seconds when at speed.  There's no loss of power and it starts fine.  Plugs, wires, dizzy cap and rotor were done 2 seasons ago.  I'm suspecting fuel related, maybe a weak fuel pump?  I'll check fuel pressure soon and report back.  Fire away with any ideas!

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JeffK

I'd check the plug wires and make sure one didn't come loose. 

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OhTwoBu

Yeah that’s the first thing I checked, all good there.

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MadMan

Backfiring usually indicate a lean condition/  Possibly bad injector, wire to injector, or check the fuel pressure. 

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JeffK

Any chance you can record it?  I doubt this is your issue, but I had an exhaust manifold come loose and it made it sound like a diesel locomotive.  You could hear ticking and popping.  I didn't have the rough running issues though. 

Oops - I missed the backfire piece since it was in your title but not in the description.  I'd tend to agree with  @MadMan if you are getting a backfire.  Is it backfiring on acceleration, deceleration or both?

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Creekrat

I’m addition to what @MadMan said you may also want to check the timing. 

Edited by Creekrat

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OhTwoBu

Update: I have 35 psi at the fuel rail at key up, idle, and wot. A quick look through the internet and I found an Indmar fuel pressure chart for pre-2004 models and that may be a bit on the low side, but there’s no loss of pressure through the rev range. Pulled my kill switch and did the low-tech compression test by cranking it, and it sounds like I may have a loss of compression on a cylinder. Will do a leak down test and report back.

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OhTwoBu

Caveat: that’s on the hose, not in the water under full load

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OhTwoBu

Compression is:

165, 155, 155, 155, 150, 140, 150, 155

#6 is the 140, but that’s still right at 10% variation. May just need a tuneup.

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justgary

WOT on the hose won't load the engine, so you aren't really getting a good test.  And IIRC, you are looking for more like 42 PSI from the pump.

Your engine's ECM assumes a specific fuel pressure so it can simply time the injector pulses in order to deliver the correct amount of fuel for the current temperature and load conditions.  If the pressure is low, you are running lean.

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OhTwoBu

I agree, bad test, just all I could do yesterday. After pulling the plugs to do compression tests, one of them somehow gapped itself closed. I have no idea how, but the electrodes were all but touching. 8 new ones and it’s idling like the sewing machine it used to be. Will try to find time today to splash it in the lake and try wot pulls and get a proper fuel pressure reading.

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dlb

Looks like you found the culprit.  Now hopefully solved.  If not you need to find out why the gap closed.

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