Heckler1963

Hi guys I need help with something. I have a 2000 XJ 184.00 miles and I'm having the rough idle on restart problem. I installed new manifold heat shield, heat wrapped fuel rail, heat wrapped fuel line and heat wrapped all six injectors, installed new 4 hole injectors,new coil pack and new spark plugs. Installed new MAP sensor, idle air control valve, and throttle position sensor. Checked fuel pressure, gauge was jumping around so i installed new fuel pump now fuel pressure is at 49 psi steady. Did compression check all cylinders were 148-149. Did a leak down test all good. Checked for vacuum leaks none found. Still have problem. I'm the original owner and never had this problem till this year. Anyone have any ideas? Really could use some help. This problem is driving me nuts!

tjwalker

So to confirm, couple of questions.

You have a "rough idle" only on restart of a HOT engine? It does not idle rough on a COLD start?

No other driveability issues and no check engine light?

The one engine management sensor that comes to mind as something to check would be your "coolant temp sensor". Located on your thermostat housing. If that sensor goes south, your computer gets incorrect input data from this sensor and it may for instance, think that your coolant is cold rather than warm, and that can lead to a richer than normal fuel mixture and that could lead to a rough idle when hot.

When the CTS is cold, internal resistance is high and the computer monitors high signal voltage which means a cold engine. As the sensor warms, the internal resistance is low and the computer monitors a low signal voltage which it interprets as a warm engine. The sensor can be tested. The manual will recommend removing the sensor, beaker of water, calibrated thermometer and measure resistance. I have had fairly good luck though just measuring the resistance without removing it and comparing the recommended resistances to outside air. Measure cold and hot and should have a huge swing in resistance. Any manual should have the chart of recommended resistance values.

Getting this on a scan tool and looking at live data would be helpful here.

Heckler1963

Thanks I will check it out. I was just driving stopped for lunch came out started truck ran rough started to drive and my check engine light came on. Went to Autozone to check code and it came up as P0305 misfire on cylinder 5.

MWMahoneyJr

I just did the whole resistance measuring thing - it was awesome. Get a $5 meter at harbor freight and go for it.

Stick the probes into the plug that feeds the sensor to make sure you are getting the 5 volt feed.



Coolant Temp. Sensor Plug Volts<br/>
Good, the sensor is being fed the right amount.

Now let's check and see what the sensor reads. Switch your multimeter to Ohms and check the sensor value (Why Ohms? I don't know. Google it.)




Coolant Temp. Sensor Ohms Check. Ambient Air temp is approximetely 36 degs.

Look at this Coolant Temperature Sensor values chart: http://www.fixjeeps.com/coolant-temperature-sensor.html

It was 36 degrees in the warehouse yesterday when I did this, so based on the chart, my Ohms should be up above 29,000.
My reading is much lower - 16.43 This translates to up near 60 degrees - this sensor is no good. I'm ordering a new one (and the air intake temperature sensor) from Crown today.

Oswalla

I have had something similar on my '90, except mine clears up after a minute or so of bringing up the rpms and holding it there. It acts just like a choked/flooded carbed rig. No success in figuring it out as it happens only 1 or twice every few months, and when I get to my tools it is fine.

tjwalker

Did you have the same symptoms BOTH before and after all of the work you did?

What brand spark plugs did you install? Gap em to .035? I've found the 00-01 to be a bit more finicky on plug brands than other years.

For a single cylinder misfire on an 2000 XJ, first things to check are the spark plug, connection to the coil rail, and fuel injector. (Swap plug first and then fuel injector to see if the misfire follows) You already performed a compression test which showed good numbers.