1989 Sport 4.0 Auto Trans
Okay so replaced the 02 sensor to possible fix a mileage issue (haven't ran a full tank yet), Anyway every since then it hunts for idle, not drastically but noticeably. I would say 800-700 rpm continuously. Never dies, and once rolling its fine but at idle you can really feel it and see the needle bouncing. IAC has been cleaned as well as the whole throttle body, I used an NTK sensor. Any thoughts would help!
Oh yeah, while changing the 02 Sensor took out all the relays and cleaned the socket and blades and applied di-electric grease before re-installing

 

ALL you did was replace the O2 sensor just before this started happening?

 

Welcome to the site.

 

Yep that is correct, well I've been doing a lot of maintenance to it also ( new vehicle to me) believe it was the factory sensor before I replaced it (127000 actual miles). Also changed the vac line harness and all the lines ( the one next to the air box)

 

Got an exhaust leak at the manifold?

 

possible at the donut gasket, but i ran out of time and had to button things up quick, plan on checking into that in a day or so, i can't hear anything and there isn't a ton of excess heat under the hood, suppose i could put it up in the air cold and run my hand around the header to see. also was thinking about egg valve but not sure if this would cause the fluctuating idle, its not horrible but like i said probably 100-150 rpm,

 

Check the EGR just for giggles.

TESTING THE EGR VALVE FOR 87-90 4.0 ENGINE
Valve Opening Test
1) With engine at normal operating temperature and at idle,
rapidly open and close throttle. Open throttle sufficiently to obtain
at least 1500 RPM. Movement should be noticed in EGR diaphragm.
2) If diaphragm does not move, probable causes are: faulty
vacuum signal to EGR, defective EGR diaphragm or defective
backpressure sensor diaphragm (if equipped), or leaks in vacuum lines
or connections.
Valve Closing Test
1) With engine at normal operating temperature and at idle,
manually depress EGR valve diaphragm. RPM should immediately drop,
indicating that EGR valve is not leaking and had been properly cutting
off exhaust gas flow at idle.
2) If there is no change in RPM and engine is idling
properly, exhaust gases are not reaching combustion chamber. Check for
plugged passage between EGR valve and intake manifold.
3) If engine idles poorly and RPM is not greatly affected by
manually moving diaphragm up, EGR valve is not closing off exhaust gas
flow. Check for carbon between pintle, leaking EGR valve gasket or bad
EGR valve.
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