Diagnosing Rough Idle Hell

[DylChrFla]

So I was just thinking about this today. About 15 years ago I had a rough idle, did my standard diagnosis of the time by hitting the forums and, rather than diagnose, I swaptroniked most my sensors with crap Autozone/Oreilys/no name sensors. Dummy me threw the OEM sensors away thinking "I had brand new ones...why would I need these old things?" Well, the idle issue turned out to be the fuel pump all along, but after the new pump was installed, the idle never was where I remembered. I had never really noticed idle driving this truck, but after my parts bukakke, it surged, was rough at times...it never stalled, just noticeable and 'off'. Over the last 4 years, I've corrected my error by replacing all the AutoZone sensors with AC Delco or GM Genuine (not Gold or Silver) sensors, and the idle is perfect. Ditching platinum plugs, no name cap, rotor and wires for Delco OEM items helped a bunch. So, just a thought and a warning not to ditch those OEM sensors. They may be made in the same factory in China or Mexico, but I would be willing to bet that GM/Delco lot tests or at least provides access to factory test fixtures to qualify a part as GM genuine, gold or silver.

I try to stick with AC Delco for all of my replacement parts, though I might have a couple masterpro sensors. Luckily, I’ve saved everything I’ve ever replaced on the the truck’s engine and it has come in handy a few times. I also noticed my voltage gauge is reading consistently lower since installing the fresh ICM.

 

[DylChrFla]

The link is a video file of my engine idle at operating temperature. If you listen closely, you can hear the rhythmic “miss”. In colder weather, you can see a small puff of smoke when this happens. Please help if you have any idea what causes this.

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[thinger2]

Simultaneously the most glorious and disgusting phrase I've ever heard as a parts man.

Im in my mid fifties.
I had to look up *******.
I thought it was some kind of chinese theater.
Turns out it is some kind of theater just not what I thought it was.
No wonder everybody is still wearing a mask at Oreillys

 

[DylChrFla]

As dumb as it sounds I think my issue was due to an incorrect thermostat. The popping is likely preignition caused by a lack of coolant flow restriction or possibly my computer didn’t play well with the cooler than stock temp. It had a 180° thermostat in the truck from a previous owner. I decided to change it to a 195° last night and the idle has been noticeably smoother. I’ll keep paying attention to it and hopefully it truly did solve my issue.

 

[Schurkey]

Anything is possible...but it seems unlikely.

 

[xYARBY]

I had a 93 blazer that did this and turns out it was a cracked vacuum hose fitting. Not saying it’s the problem but something to check

 

[thegawd]

I dont recall reading it in the thread but the vacume brake booster could also be the source of a vacuum leak. the line, the grommet or the booster itself.

the longer the breaks are held the worse it gets as the computer compensates to the vacume loss. then you let off the brakes and the truck falls on its face and may stall.

this can be seen in the short term or long term fuel trims on a scanner as well.

Al

 

[DylChrFla]

The whole thermostat thing didn’t actually fix my problem. It only prolonged the warming up period and kept running smooth for longer.

 

[Erik the Awful]

Have you tried looking for a vacuum leak, to include spraying WD-40 around the intake gaskets, etc.? Also including a thorough visual inspection?

When your engine is cold, the computer runs it a bit rich, which has a side effect of compensating for a vacuum leak. Once the engine is warmed, the computer leans the mixture out some, and a vacuum leak can then cause lean misfires.

 

[tsr2185]

The whole thermostat thing didn’t actually fix my problem. It only prolonged the warming up period and kept running smooth for longer.

Keep us updated. Im having the same exact issue you are having and cant seem to figure it out, with pretty much the same exact new parts and troubleshooting you did. About to order a ALDL cable to data log as Shurkey is notorious for advising people to do so lol.