Hi All,

I am trying to solve the problem of a sudden loss in gas mileage on my 1990 Cherokee 4.0, 5 spd, 4WD. The problem occured shortly after replacing a defective idle air control valve (stuck open = 2500 RPM idle). Prior to the change, I had been averaging 16.1 MPG on over 4000 miles. After replacing the valve, it has dropped to 13.5 MPG (~4000 miles).

My driving habits, mixture of city/highway miles, tires, and vehicle weight have not changed. I have noticed strong fuel smells (burning rich) when backing up, and black soot in the tailpipe and spark plugs. Ther are no fuel leaks that can be seen. The air filter is clean, and I have replaced the O2 sensor with no improvement.

I am getting ready to go on a long Summer trip in the Jeep next week and could really use those extra 2.5 MPG. Any suggestions?

Thank you,
David

 

If I had replaced the Idle Air Control and O2 sensor and still had all the symptoms of running rich (which you do) then I'd check the Idle Air Control for proper operation.

Here's how I would check it:
http://www.2carpros.com/how_does_it_..._iac_works.htm

If the Idle Air Control works properly and you still have all the symptoms you described I would suspect the control module, and suspect the old carboned-up idle air control was actually helping things!

 

Thank you for the suggestions and the link.

The first IAC was working good until it stuck open. I had to drive it for a week with a high idle until I got to the junk yard for a replacement. When I put it in, the idle went back to normal, so I think it is working properly.

My Jeep starts fine and idles cold or warm at 750 RPM. The IAC acts like a second throttle controlled by the ECM. Three things needed for basic fuel metering: RPM, manifold pressure, and air temperature. Regardless of where the IAC is, the MAP sensor should be telling the control module the manifold pressure to use for the fuel metering.

I'll check the IAC as suggested in the article and see if it helps.

Other possibilities?