Rough idle on Vortec 5.7

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HotWheelsBurban

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My new-to-me crew cab has been doing something kind of weird sometimes when we are at a long traffic light.
It normally idles around 500 rpm in Drive, 800 in Park. What it is doing,is it starts idling a little rough,and the longer we sit there, the worse it gets ( to where the truck is almost jumpy, like it had too much coffee!) But putting it in Park will kick the idle speed up a bit, and it idles smoothly then.
Even when idling rough, as soon as you accelerate, it runs smoothly. It doesn't do this all the time, seems like it's more likely to when warmed up from driving a few miles. When it doesn't do it, it idles so smooth you can barely feel it.
I have a new fuel filter which I will install in the morning; I'm also going to change the cap, rotor, and plugs ( if the plugs look nasty or worn out). Gotta pull one anyway so I can see what ones to order since there's several possibilities.
This truck is a '97 C3500 CCLB, L31 5.7, with only 123K on it. Pretty sure everything is original under the hood; I plan on doing basic stuff in the next couple of months; just to be sure I get to do the repairs at my convenience and not when the truck wants to pretend it has a blue oval on it....
thanks in advance!
 

stutaeng

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Probably the same thing we recommend to everyone: check for codes, check fuel pressure and hook up a scanner that has live data stream.

On the live data look at everything from MAP, MAF, temperature, O2 sensor waveform, and particularly fuel trims.

Fuel pressure: check with key on, engine off, then at idle in park, and driving around. In your case driving around when it does it. You'll need a long hose to stick to your windshield the guage.

I went through 2 or 3 of the fuel pressure gauges over the years and recently bought the nicer harbor freight one with long hose. It seems nice; hopefully will last longer.

I feel like any DIY that's going to try do basic engine repair or diagnostics and even maintenance should own a fuel pressure gauge.
 
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HotWheelsBurban

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I think (hope) it's probably something simple, like a vacuum line or crud in the gas filter. Since this truck has been sitting so much of the time ( before we bought it anyway). I'm leaning that way. Gonna change it and see what happens....
After spending $700 getting the hydro boost unit replaced,and the tow to the shop, I want to be able to use this one and enjoy the AC. Burb needed time off too.
 

Pinger

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When the problem occurs, will it be running open loop? Could the O2 sensor be cooling enough to lose the signal and going open loop? Can you force it and possibly smell fuel richness at the tail pipe?

In drive or neutral my 5.7 Vortec idles at the same speed (800 rpm on the GM tach, 700 rpm on the - probably more accurate - LPG diagnostics). Either yours is behaving oddly or mine is.
 

HotWheelsBurban

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It doesn't do it until you're driving a bit, so it should be in closed loop. The parts store got a PO151 code out of it, said that's O2 sensors bank 1(left/driver's side) but not which sensor (front/exhaust manifold/upstream) or downstream (in the pipe past the cats). They have changed vendors on their scanners again,and they're not as good IMO as the ones before, which weren't as good as the ones before that. In trying to get a more affordable( and profitable) line, the tool quality and functionality is way down.....
I think old gas has something to do with it too, the more we've driven, it isn't doing this as often. Fuel pump has been replaced recently according to PO.
 

Pinger

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Doubting that it is being fuel starved as it runs fine elsewhere.
If the code relates to the upstream O2 sensor - I'd guess by default it will go open loop. If it's from the downstream one it shouldn't matter - unless the code implies 'implausible reading' or the likes - unlikely I'd think.
So possibly a touch weak due to a vacuum leak or a touch rich (restriction in fuel return line?) that cannot be corrected when in open loop?
If the truck's sat for a while - could some of the various solenoids (eg, EVAP) be a bit sticky?

Still not understanding why the IAC will let it idle at 500 or 800 rpm. Surely it is there to maintain one idle speed (whatever it is) irrespective of load?

Your proposed maintenance plan is a good one. Either the problem disappears as you progress through it or it becomes easier to identify the cause.
 

HotWheelsBurban

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Doubting that it is being fuel starved as it runs fine elsewhere.
If the code relates to the upstream O2 sensor - I'd guess by default it will go open loop. If it's from the downstream one it shouldn't matter - unless the code implies 'implausible reading' or the likes - unlikely I'd think.
So possibly a touch weak due to a vacuum leak or a touch rich (restriction in fuel return line?) that cannot be corrected when in open loop?
If the truck's sat for a while - could some of the various solenoids (eg, EVAP) be a bit sticky?

Still not understanding why the IAC will let it idle at 500 or 800 rpm. Surely it is there to maintain one idle speed (whatever it is) irrespective of load?

Your proposed maintenance plan is a good one. Either the problem disappears as you progress through it or it becomes easier to identify the cause.
That's what I'm hoping. This truck is too good otherwise to give up on.
I think that's what happened with many of the trucks on this forum; they were too troublesome or far gone for their PO 's to deal with any longer. But we few faithful who want to dig a little deeper, can find the gold underneath the dirt!
 

HotWheelsBurban

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And as far as the idle speed being a little higher in Park, every GM vehicle we've had has been set up that way.
 

Pinger

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And as far as the idle speed being a little higher in Park, every GM vehicle we've had has been set up that way.
Maybe, mine is as it is due to its LPG system - I've never looked closely at its behaviour on gasoline as I only use it for cold starting and then only for a couple of minutes at most.

I'd assumed that IAC has sole control of idle speed but possible injector durations also change?
 
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