Rough Idle No Check Engine Light

[Gary Sigmund]

Just had the OEM 5.0 Liter motor in 98 Ford Explorer replaced with a younger 80,000 mile from the junk yard. Experiencing a Rough Idle condition after engine has warmed up and stopped at traffic light - can’t get a fault generated. idle is intermittent as it is fine for 10-15 seconds then goes rough with a kean conditiin according to the mechanic. Already replaced EGR components and cleaned Mass Air Flow Sensor. Any ideas? Has anyone experienced this? Frustrating w/o Check Engine Light and a Fault Code.

 

[Blue m&m]

Vacuum issue?

 

[Gary Sigmund]

Just had the OEM 5.0 Liter motor in 98 Ford Explorer replaced with a younger 80,000 mile from the junk yard. Experiencing a Rough Idle condition after engine has warmed up and stopped at traffic light - can’t get a fault generated. idle is intermittent as it is fine for 10-15 seconds then goes rough with a kean conditiin according to the mechanic. Already replaced EGR components and cleaned Mass Air Flow Sensor. Any ideas? Has anyone experienced this? Frustrating w/o Check Engine Light and a Fault Code.

Vacuum issue?

thanks m&m! Checked for hoses being disconnected throughout - no signs of any unattached hoses - in fact i disconnected vacuum line frim EGR valve and plugged it with a screw to try and force an EGR valve fault which eventually did set the check engine light and EGR fault; and when i restored the vacuum connection and cleared the fault the check engine light no longer set.

what is the best way to detect or expose a vacuum leak and its location?

 

[Blue m&m]

Smoke it.

I can’t remember if the gen 2’s have the PCT valve on the left bank, but if they do, it might not hurt to check around there.

I had a cracked hose and it ran like absolute crap at idol. Never threw a COL during that adventure.

 

[Gary Sigmund]

thanks Blue M&M!
maybe i can create a smoke machine and insert it - where is the best port of entry

i am also wondering if there may be a kink in the PCV valve hose or if i could have a bad coolant temperature sensor and maybe a fuel pressure regulator? not sure where to find those locations. - the idle breakdown is of type “lean condition” ~~ Siggie~~ Gary R. Sugmund

 

[EB4X]

Drench your throttle body with carb / throttle body cleaner..... eliminate that possibility.......

 

[J_C]

If it regularly alternates between smooth and rough, it could be the A/C compressor cycling on in conjunction with a failing IAC valve, which you could test by keeping the HVAC shut off and/or cleaning the IAC.

 

[gmanpaint]

Does it run fine after it has cooled off, and sat for a while?