I have a '98 Ranger 4.0 SOHC. While driving about 20mph the other night, I experienced sudden stuttering and jerking, and a light knock only present while driving (which I suspect is due to one or more bad plugs and/or wires). The truck continues to run rough, especially while acceleraring, and it is difficult to surpass 65mph while the AC is running. While the engine is running, you can hear a significant vacuum leak, so after performing a smoke test I found that I have large leak at the black cap in the attached photo. The smoke seemed to come out from around the cap, as though it isn't in all the way. What is this part? Is this likely the source of my problems?

Attached Images

14910895135331973658960.jpg (90.8 KB, 24 views)

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1998 Ranger XLT 4x4 4.0L V6

PS: Reset ECM and still no CEL present

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1998 Ranger XLT 4x4 4.0L V6

I believe that is your IAC Valve (Idle Air Control). Can't be sure. It have electical plug? The knock sound is likely a "ping" from a lean condition when running. While the engine is running and at operating temp(no A/C), unplug the IAC and see if idle drops down significantly or even stalls out.

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1995 Ranger XLT 3.0L OBDII
Trans: A4LD (E) 2WD
Rear: 7.5 / 3.45 NL LHD
Tires:GY P235 75/R15 (orig 225 70/R14)
Long Bed, 20 Gal Tank
Up to 229,900 miles

Yes it is the IAC

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2002 XLT Extended Cab 4.0