Rough idle on Vortec 5.7

[Pinger]

If I remember - I'll check my idle speed across drive, neutral and park when on gasoline tomorrow.

I remembered - because I left a note to myself on the dash last night! Anyway, on gasoline or LPG it behaves the same - it drops 100 rpm (but no more that that) when going into drive and regains it when neutral is selected. A deviation of 300 rpm doesn't seem right.

NB: This is something I've been playing with on mine (for reasons too tedious to elaborate on) and because my supposedly tamper proof butterfly stop screw has been, well, tampered with. Might be worth checking if yours has.

 

[Erik the Awful]

When a vehicle runs rough after it warms up, the first thing I check for is always vacuum leaks. Remember, your mixture is richened when the engine's cold, covering a leak. Once it's warm the mixture leans out and any vacuum leaks are exposed.

 

[HotWheelsBurban]

When a vehicle runs rough after it warms up, the first thing I check for is always vacuum leaks. Remember, your mixture is richened when the engine's cold, covering a leak. Once it's warm the mixture leans out and any vacuum leaks are exposed.

Thanks all, I have another service day scheduled for this weekend. Will post any updates!

 

[HotWheelsBurban]

It did pretty well today, going across Houston to my two Eastside stores and back home. Only had a few times, sitting at lights, that there was some misfires that became a rough idle. And only one time where it got very rough. All these times were after being at cruising speed(35-45, some higher speed going up bridges) for a bit. So I'm thinking along with y'all that there is a vacuum leak, and maybe as it gets warmer with higher rpms, that there's misfires in the cap like Vortecs are known for on those crab style caps. I figure most everything is original ( the radiator cap is even still original!).
So after 25 years, I think it's due for a cap and rotor.....

 

[socal k1500]

It did pretty well today, going across Houston to my two Eastside stores and back home. Only had a few times, sitting at lights, that there was some misfires that became a rough idle. And only one time where it got very rough. All these times were after being at cruising speed(35-45, some higher speed going up bridges) for a bit. So I'm thinking along with y'all that there is a vacuum leak, and maybe as it gets warmer with higher rpms, that there's misfires in the cap like Vortecs are known for on those crab style caps. I figure most everything is original ( the radiator cap is even still original!).
So after 25 years, I think it's due for a cap and rotor.....

Did it end up being a vaccum leak? im having this exact issue right now.

 

[HotWheelsBurban]

Did it end up being a vaccum leak? im having this exact issue right now.

Haven't messed with it too much on that end. Replaced the cap and rotor and they definitely helped. Still has a little bit of misfires at idle, but it's probably plug wires. I'll look into it more when I get finished with the brakes; the truck has been sitting since before Christmas, waiting on my off days to coincide with no rain. I'm off tomorrow so I'll play with it then....

 

[97Kharon]

Hello there, wondering if you solved your issue? I'm having similar issues with my 1997 Suburban 5.7 Vortec I've replaced cap and rotor yet still am idling rough gotten to the point my "service engine soon" light is flashing (P0300). After doing a bit of research I've learned it could be anything from a vaccum leak to a bad sensor. Waiting for semi-warmer weather to investigate praying I can fix the issue before it gets worse.

 

[Schurkey]

A flashing MIL means the catalyst is in danger of damage.

DON'T drive the vehicle until this is fixed.

You are likely looking at misfire rather than "sensor problems" although sensor issues may cause misfire. You replaced cap and rotor--did you examine/test the plug wires? Are the spark plugs usable? Does the engine have appropriate cranking compression pressure in all eight cylinders? Has the cam sensor/crank sensor alignment gone goofy due to timing chain or distributor-gear wear? What is the fuel pressure? How old is the fuel filter? Do all the rocker arms move when the engine is cranked, and do they move about the same amount?

Connect a scan tool, find out what the computer wants to tell you. Verify every sensor, and the computer outputs. Check the misfire counts per cylinder to give you a heads-up on which cylinder(s) may be misbehaving.

 

[HotWheelsBurban]

Hello there, wondering if you solved your issue? I'm having similar issues with my 1997 Suburban 5.7 Vortec I've replaced cap and rotor yet still am idling rough gotten to the point my "service engine soon" light is flashing (P0300). After doing a bit of research I've learned it could be anything from a vaccum leak to a bad sensor. Waiting for semi-warmer weather to investigate praying I can fix the issue before it gets worse.

Yes and no, I think I'll have to replace the plug wires, be sure they're not touching or close to each other, and check for vacuum leaks, to completely take care of it. Haven't been dealing with it since we haven't been driving this truck since December 23; and right now the brakes are a more important issue to fix. I got an unplanned day off today but others in the family have plans for the day..... So I guess I'll have to wait until Saturday to work on it.

 

[97Kharon]

Well Suburban down...not sure how or what happened but all my transmission fluid is no longer in the transmission.

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