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Mat sensor???

B

B.RAIDER

NAXJA Forum User
Location
Central Texas
88 XJ 4.0 Auto 4X4
Trying to run down a high idle issue.
My MAT sensor is dead ...no ohms even when put in boiling water to test.
Will that cause my high idle problem?

No vacuum leaks ...verified by smoke test.
TPS is within specs.
Coolant sensor is within specs.
Map sensor is good.
Haven't done the O2 sensor yet.
 
Last edited:
88xjlover said:
What year/engine, etc.
Click to expand...

88 4.0 in his original post.

I would pull the IAC and clean it and the throttle body, make sure the throttle plate isn't loose, and use throttle body safe aerosol cleaner. Also, be careful with the IAC, it can break.
 
joe_peters said:
88 4.0 in his original post.

I would pull the IAC and clean it and the throttle body, make sure the throttle plate isn't loose, and use throttle body safe aerosol cleaner. Also, be careful with the IAC, it can break.
Click to expand...
Did all that.....my mechanic even had one of those IAC dodads that plugged into the IAC to run the idle up and down and it showed to be working properly.
 
88xjlover said:
Test your O2 sensor. They're pretty cheep so you could throw one in.
Does your exhaust manifold have a leak or a crack in it?
You mentioned your MAT sensor being dead so I'd start there then check the o2.
Click to expand...
Can't get the MAT sensor...just found out a little bit ago from the dealer....none of the other auto parts places can get it either. JY will be my only option there.
No manifold leak either (none that we could tell anyways), mechanic sprayed it with something in a aerosol can to see if injectors were leaking and checked the manifold also.
The last thing i have not checked is the O2 sensor. That's next.
They list for $50 bucks here....is there any generic O2 sensors that would work?
 
Last edited:
If the MAT sensor is showing no resistance (shorted) then the ECU will think the incoming air is extremely hot and try to lean out the fuel mixture--high idle.

Here is a link:

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

This may just be one of those parts that is salvage yard/substitute only. I'm wondering if the 91-95 wrangler will work?
 
Yes the MAT will cause all kinds of problems. The basic operating principle of fuel-injection is that the computer determines how much air is entering the system and then controls fuel delivery to match (driver only controls air delivery through throttle body, computer does the rest). In order to determine proper amount of fuel, the computer first needs to know how much air is going into the combustion chamber--it already knows volume (cylinder volume is fixed) but it also needs to know the density of the air. In order to get that number it looks at air pressure and air temperature, which together yield density. The MAT tells the computer the temperature of the air in the manifold. If the MAT is giving wrong data then the computer is unable to accurately calculate the air density and will absolutely positively dump incorrect quantity of fuel.

One other thing is that the MAT is in the plenum so it will get coated with blowby or any unfiltered air that makes it into the manifold (ie vacuum leak) and the likelihood of a bad MAT gets higher every year.
 
ehall said:
Yes the MAT will cause all kinds of problems. The basic operating principle of fuel-injection is that the computer determines how much air is entering the system and then controls fuel delivery to match (driver only controls air delivery through throttle body, computer does the rest). In order to determine proper amount of fuel, the computer first needs to know how much air is going into the combustion chamber--it already knows volume (cylinder volume is fixed) but it also needs to know the density of the air. In order to get that number it looks at air pressure and air temperature, which together yield density. The MAT tells the computer the temperature of the air in the manifold. If the MAT is giving wrong data then the computer is unable to accurately calculate the air density and will absolutely positively dump incorrect quantity of fuel.

One other thing is that the MAT is in the plenum so it will get coated with blowby or any unfiltered air that makes it into the manifold (ie vacuum leak) and the likelihood of a bad MAT gets higher every year.
Click to expand...
Out side of finding a good one at the JY, what other choice does one have?
The Dealer I talked with said that they are obsolete and they can no longer get them and there appears to be no aftermarket subsitute for them.
I'm sure there are other sensors that might work from other engines...but the have to have the same specs or close to it for them to work....so I'm told.
 
B.RAIDER said:
Out side of finding a good one at the JY, what other choice does one have?
The Dealer I talked with said that they are obsolete and they can no longer get them and there appears to be no aftermarket subsitute for them.
I'm sure there are other sensors that might work from other engines...but the have to have the same specs or close to it for them to work....so I'm told.
Click to expand...

Agreed. There are a number of other vehicles that use a MAT, and once you can find one with the same resistance values at temps, moding it in shouldn't be all a big thing. Now, to find the right substitute.
 
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