Rough idle when outside temperatures are high

KCARZ3

MEMBER
2010 FORD F-150
5.4L
V8
4WD
AUTOMATIC
188,000 MILES

When I start my truck in the morning and the outside temperatures are lower I do not notice any rough idle all the way to work. At the end of the day after the truck has been sitting in the parking lot all day in the Florida heat I get a rough idle as soon as the truck starts. Rough idle continues the rest of the day. Plugs were changed and no difference. Code read bad throttle body and that was changed with no difference. Any help would be appreciated.

5 Replies

JACOBANDNICKOLAS

MECHANIC
109,896 POSTS

Hi,

A few things can cause this. First, check to see if there are any engine vacuum leaks. Here is a link that shows how it's done:

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Next, if you have a live data scanner, check the engine coolant temperature sensor reading (ECT) and let me know what the short term fuel trims (STFT) are both in the morning and then again when you leave work.

I'll watch for your reply.

Joe

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Friday, May 22nd, 2020 AT 7:06 PM

BMDOUBLE

MECHANIC
1,139 POSTS

I've had this issue with these trucks in the past with fuel injectors that bleed down when they were warm intermittently. The easiest way that I've found to diagnose them is to remove the two eight millimeter bolts that holds down the fuel rail, spray the base of the injectors with penetrant, wiggle the injectors side to side to break the seal, lift up one side of the rail slowly until all of the injectors are off of the intake then suspend the rail with a bungee cord and look for fuel dripping. The engine should be warmed up before trying this and use a flashlight because sometimes the mist is really hard to see. It's a quick and easy test, and if you have any of the dripping you can replace them while they are out.

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Friday, May 22nd, 2020 AT 7:23 PM

KCARZ3

MEMBER
2 POSTS

Update: So I had a moaning sound coming from the engine compartment. Diagnosed it to be a belt pulley. Took the belt off and found it was the idler pulley. Picked up a new idler pulley and belt. Replaced both and checked all over for any cracked or broken hoses causing a vacuum leak. Did not find anything. Local weekend weather was a little cooler and cloudy all weekend and did not notice any rough idle. Today it was 93 degrees and good and hot in the parking lot at work. Drove it home and no rough idle or shaking. Wondering if I have had a slightly loose belt the entire time I have owned it for 4 years causing possibly a vibration that made it feel like rough idle.

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Tuesday, May 26th, 2020 AT 5:12 PM

JACOBANDNICKOLAS