98 Ranger rough idle/ surging throttle
Ok need some mechanical advise... I have a 98 Ranger 4L sohc that has an irregular idle. At a stop light the rpms seem to surge up/down (500-700) and engine shakes until brake is released, it hesitates and lacks power to start then evens out at higher rpms normally. Once it's cruising (say 35mph) rpms low and throttle steady, the surging / pulsing/ loss of power problem continues. Feathering the throttle at speed evens it out for the most part. I've had this problem intermittently for the past few years it's mainly just annoying to deal with and bad on gas mileage 16+or- I've replaced the iac sensor and tps with no change and now the pcv which was pretty dirty but still functioning and that has made a slight difference but problem persists. I've heard vacuum leaks can cause these symptoms (upper and lower intake gasket leaks most common) but have yet to find it leaking (starter fluid and propane tests). If you know a better test let me know. About to replace spark plugs and wires but doubt it will fix problem. Any help on the subject would be appreciated getting annoyed with throwing money at this truck with no change.