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99 Suburban rough idle

I have a 1999 Suburban K1500 with the Vortec 5.7. Over the last week or so, I've noticed the idle seems a bit rough and around 500 RPM's +/-. I mainly notice the roughness when it's in gear. If I put the truck in Park, the idle comes up to around 800-900 RPM's and feels much nicer/smoother.

At this point, it's not where I am worried the truck will die, but that doesn't seem far off. When I go to take off, it almost feels as if flooded a bit.

What might cause this?

I do have a check engine light on, which has been on/off for a month or so. I have a code reader and believe the code is 420 and 430. P0420 Catalyst System Efficiency Below Threshold (Bank 1) & P0430 Catalyst System Efficiency Below Threshold (Bank 2)

I replaced the donut gaskets on both sides (previous owner said they go out about every 6 months or so) and then ended up losing a nut on the driver side flange. I noticed it just the other day, because the exhaust got louder. I haven't been able to find the correct nut to replace it with. Parts store was no help, so I need to crawl back under, pull one of the other nuts off and take it in. I'm not sure if my O2 sensors are bad, the donut gaskets still leaking or the cats are both bad. I am also wondering if this could cause my current issue, which is new.

Thanks for any ideas on where to start looking...

Reply to
MASTER Brian
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Sounds like an O2 sensor issue to me. A dealer could tell you better. They can check the air/fuel mixture in real-time at cold start and idle to see if it's running too rich or too lean.

Dave

MASTER Brian wrote:

Reply to
DirtyDave931

Well, if you think possibly O2, I may just replace both of those and see. At probably $10 x2, I'm sure that's much cheaper than the dealer would want. If that doesn't cut it, then I'll take it in.

Reply to
MASTER Brian

Where do you find O2 sensors for 10 bucks? The cheapest I've found for my

99, 5.3L is 35 dollars.

MASTER Brian wrote:

Reply to
George

I was just guessing. That's about all I've ever paid it seems for one. Maybe they are more on my '99.

$35 does change things a bit. Especially considering how many there are on this thing.

Reply to
MASTER Brian

Reply to
George

fix the exhaust leak first. It can throw off the o2's

Reply to
ShoeSalesman

Yeah, I plan to get it fixed. I thought I had it fixed, the light came on again almost immediately. I decided it was either a bad sensor or cat, so I left it be, until I noticed the exhaust leak.

George, yes that does add up. I didn't realize they were that expensive. I actually thought I had asked >> I have a 1999 Suburban K1500 with the Vortec 5.7. Over the last week or

Reply to
MASTER Brian

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