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Renix MAP sensor question

tjmotter

NAXJA Forum User
NAXJA Member
Location
Texas
I posted a while ago about a high idle issue I was seeing on my '90 XJ. As background, if I simply turned the key on and cranked, I was pretty much guaranteed to see idle sitting at 2K+. If I turned the key to on for 15-20 seconds before cranking, idle would settle nicely at 800. The issue is that over the course of a drive, the idle would start to rise (as high as 3,000RPM in one case). If I shut it down for a minute and fired it up again, it was back to normal.



I finally had a chance to take a closer look at it today and while looking for vacuum leaks found that if I moved the vacuum line going to the map sensor it would bring the idle down. I initially thought I had found a vacuum leak but I was wrong. It seems that with the vacuum on the MAP sensor, I get high idle. If I pull the vacuum line off the MAP sensor, the idle drops (and stays) at 500 rpm. This makes no sense to me. With the vacuum line pulled off I am flooding the intake with air so I expected the idle to rise, not fall.


Anyone have any theories about what is going on here?
 
IAC? I would think if you removed the MAP and idle changed that the sensor is not reading right.
 
I keep the XJ at my condo in a different city. I will check the TPS readings when I get back there next week.


thx

Todd
 
Ok, I think I have it figured out. Intake bolts had backed off a little and once the head/intake/header heated up it would warp just enough to allow a small bit of air to leak in. I got off track with the MAP sensor test and realize now that disconnecting the vacuum on the MAP caused the computer to start dumping fuel into the intake which would bring the mixture back in line and bring the revs down.


I started it this afternoon and started taking measurements off the TPS. As usual, the idle started to rise as the engine warmed up. Measurements looked good so I decided to double check the intake bolts. Once tightened, I fired it up again and let it run for 20 minutes. Rev's sat happily at 800 the whole time. Thanks to the member who posted on seeing high rev's and finding an intake issue. Got me thinking and seems to have solved my issues.
 
I had the same issue, the cheap plastic tube was badly cracked. I swapped it with rubber tube. Problem solved.
 
Just a last update to close this thread out. It has been 3 months now since I tightened up the Intake bolts and the XJ continues to purr like a kitten. It starts immediately and quickly settles in at a rock solid 800RPM in Park (with the AC turned off). Turning the AC to max brings the RPM's down to about 500 but it idles beautifully with no vibration nor bucking. Placing it in gear with the AC running on max doesn't seem to affect the RPM's at all. Even with 200,000 miles, the XJ has great power, is getting great gas mileage and doesn't burn/leak any discernible oil between changes.



Feels almost new. The only "nit" I can point to is that when I set the automatic transmission to "Power" mode, the light in the switch gets pretty hot. I guess I can live with that :)
 
Just a last update to close this thread out. It has been 3 months now since I tightened up the Intake bolts and the XJ continues to purr like a kitten. It starts immediately and quickly settles in at a rock solid 800RPM in Park (with the AC turned off). Turning the AC to max brings the RPM's down to about 500 but it idles beautifully with no vibration nor bucking. Placing it in gear with the AC running on max doesn't seem to affect the RPM's at all. Even with 200,000 miles, the XJ has great power, is getting great gas mileage and doesn't burn/leak any discernible oil between changes.



Feels almost new. The only "nit" I can point to is that when I set the automatic transmission to "Power" mode, the light in the switch gets pretty hot. I guess I can live with that :)

Ya done good.
 
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