Rough Idle - Even After Warmup

[Zig10]

Got to drive the Expy quite a bit while we were on vacation a couple of weeks ago, and found that it seems to be idling a bit rough and maybe even a little lower than I remember. No CEL showing at all. It's my wife's daily driver, and she doesn't pay enough attention to notice it anyway, but it's driving me nuts.

It settles in around 500-550 RPM when stopped at a light, and honestly feels like it wants to stumble from time to time but never really does. This is running 91 octane. It only has 23k miles on it, and seems to run just fine otherwise. It certainly yanked our boat around for 1200+ miles on the vacation, and that was largely in 90+ degree heat and heavy traffic.

I'd seen in another thread where someone mentioned changing out the plugs, but never saw a resolution on it. Maybe I've got some fouling since my wife usually only drives around 5-10 miles daily and it's not really heating up all the way and running a bit rich. I would think that if it had some carbon buildup the highway towing that we just did would've dislodged that.

Any input would be greatly appreciated.

 

[Nate2020KR]

You could pull a spark plug and check out how they are wearing. Or just preemptively change them. That's what I did. I think mine idles nicer and doesn't have the hesitation and seems to run in a lower RPM a little, I did a closer gap since I'm tuned.

 

[black_limited]

Got to drive the Expy quite a bit while we were on vacation a couple of weeks ago, and found that it seems to be idling a bit rough and maybe even a little lower than I remember. No CEL showing at all. It's my wife's daily driver, and she doesn't pay enough attention to notice it anyway, but it's driving me nuts.

It settles in around 500-550 RPM when stopped at a light, and honestly feels like it wants to stumble from time to time but never really does. This is running 91 octane. It only has 23k miles on it, and seems to run just fine otherwise. It certainly yanked our boat around for 1200+ miles on the vacation, and that was largely in 90+ degree heat and heavy traffic.

I'd seen in another thread where someone mentioned changing out the plugs, but never saw a resolution on it. Maybe I've got some fouling since my wife usually only drives around 5-10 miles daily and it's not really heating up all the way and running a bit rich. I would think that if it had some carbon buildup the highway towing that we just did would've dislodged that.

Any input would be greatly appreciated.

When you say “rough” does that mean you can see the rpm needle dancing up and down on the cluster? If you can see it moving up and down, I would definitely pull the plugs and replace them with motorcraft and close the gap a bit. I run SP 542 (think they are called gt plugs now) on my 2016 because I had an idle where I could see the needle drop and the truck would vibrate slightly and then the needle would dance around a bit. Changed the plugs and gapped at .030” and everything went away. Also, even though it’s a 4th gen, I would highly suggest an oil catch can (jlt passenger side would be just fine). If your MAP sensor above the manifold has any type of buildup on it you will be able to tell nc the truck will start running weird.

 

[Zig10]

Thank you both for the replies. I'm probably going to start by pulling the plugs and just seeing what condition they're in, and at a minimum regap them. I was thinking of giving the TB a good cleaning, and checking the MAP and MAF sensors as well.

The tack does indeed bounce around a bit at idle, roughly between 500 and 550, and it doesn't like to stabilize.

Thanks for the input.

 

[black_limited]

Thank you both for the replies. I'm probably going to start by pulling the plugs and just seeing what condition they're in, and at a minimum regap them. I was thinking of giving the TB a good cleaning, and checking the MAP and MAF sensors as well.

The tack does indeed bounce around a bit at idle, roughly between 500 and 550, and it doesn't like to stabilize.

Thanks for the input.

Just so you know, your truck does not have MAF sensors. The 3 sensors are a manifold absolute pressure (map), and 2 intake air temp (iat) sensors.

 

[Schini]

I recently had a similar issue - ended up being the ignition coils and plugs - only 50k miles on a platinum. Only replace with OEM.

 

[TheDoug]

I changed my plugs at a similar mileage because I run a 93 tune on it. I did notice smoother idle after change. (I wouldn't call the idle before rough though) I put ngk plugs in... there is a post here somewhere that recommended the best ones for tuned vehicles and thats what I used.

Mine idles at 625

 

[Zig10]

Just so you know, your truck does not have MAF sensors. The 3 sensors are a manifold absolute pressure (map), and 2 intake air temp (iat) sensors.

Ha! Thanks for the heads-up. Saved me some aggravation there!

 

[duneslider]

You could always reset the ecu first and see if the relearn process on the TB fixes it. Just unhook the neg and use piece of wire between the neg cable and pos for 20-30 seconds to clear everything out. Hook it back up. Press the start button without foot on pedal to let the ecu turn on and sweep the cluster (just takes a few seconds). Then fire it up and let it idle until it gets to full operation temp. (10-15 minutes). This may let it relearn the throttle body settings to idle smoother.