Stumbling idle...help please

[mgreen84]

So I have been battling a stumbling and general bad running condition at idle, I have replaced all obvious tune up parts, plugs, wires, cap, button, cleaned the TB, and replaced the IAC. I was beginning to think that it was the fuel pump since it whines pretty good, but the pressure is fine....... I was just outside fiddling with it and noticed that when I unplugged the Manifold Temp sensor (at least I believe thats what it is... it is on the manifold just behind the TB and has two wires on it) anyways it stops stumbling and runs fine when the MAT is unplugged?.... what are the symptoms of a bad MAT? and could this have been my problem all along? it is a :
1987 4.0 AW4 Stock motor and ugly as sin!

 

[birchlakeXJ]

Test it.

Disconnect connector from MAT

Measure theresistance of the sensor with a high input impedance digital volt-ohmeter. The resistance should be less than 1000 ohms with the engine warm Refer to the resistance chart in the manual and replace the sensor if it is not within the range of resistance specified.

212F - 185 ohms
160F - 450 ohms
100F - 1,600 ohms
70F - 3,400 ohms
40F - 7,500 ohms
20F - 13,500 ohms
0F - 25,000 ohms
-40F - 100,700 ohms

Here's a link to testing sensors on your RENIX vintage XJ.

http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm

 

[mgreen84]

thanks....

 

[SoCal88XJ]

Bad idle is often a result of a vacuum leak. Check all your vacuum lines, especially the ones the run through the firewall and under the dash. I see more XJ's with leaks there.

 

[birchlakeXJ]

disregard this post.......fat fingered the quote button while trying to edit.