Hey I have a 90 xj I've been having problems with rough idle and horrible gas mileage I did a complete tune up still run rough so checked the tps it was reading a lil low so changed it then put a new crank sensor still nothing changed with it running I unplugged the IAC and nothing changed I've been trough 3 IAC valves now and no luck also doesn't seem to have the power it should any advice or ideas

 

O2 sensor, new injectors. clean throtle body. check for vacuum leaks. start with the cheapest stuff first.

 

How many miles on this thing? What tire size? The IAC will not effect power output in any way, it only handles the idle speed.

 

Make sure your intake manifold bolts aren't loose along with checking for other vac leaks and cleaning the throttle body as suggested.

How old are the plugs, wires, cap and rotor?

 

If you just bought it, change the Oxygen sensor.
Even if you didnt just buy it, change the oxygen sensor... lol

You did adjust the TPS, right?

 

1. Verify your EGR system. It can futz with your idle. More on that below.

2. CLEAN your throttle body. They get carboned up over time and miles. Really important step.

3. For older engines, never hurts to run a compression test on all cylinders. This is a good "snapshot in time" of the internal condition of the engine, which should never be ignored and often is. Even on high mileage engines. The spec compression is 120-150 psi, with no more than a 30 psi variation between cylinders.

4. Test your MAP sensor. It is directly involved with fuel/air ratio. Replace your oxygen sensor if it's older. They are inexpensive.
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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.