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1999 2.5 driving me mad.. intermittent bucking

Tiffx19

NAXJA Forum User
Hi Folks, 1st post here,from Scotland.. I own a 1999 model XJ 2.5 which I've done a fair bit to,to get it on the road. Now it's got an MOT and is road legal, it's just about un-useable.


It starts instantly and perfectly every time, idle is smooth and constant and sounds sweet. Once out on the road, it starts bucking and kangarooing at around 1800-2800 rpm. It's quite violent and is just like switching the ignition on and off. I've scanned it and it has no codes at all, pending or stored. Live data all looks good, O2 is switching properly, map sensor data checks out, tps data is correct.. all good. I checked the timing yesterday and thought I'd cracked it as it was out, but having very carefully set up the distributor using the factory manual it drives even better than it did when it isn't playing up.. This is the worst bit. I can do 50 miles and it's perfect, then it'll start..it's very intermittent. It really feels like an ignition cut but everything I've looked at is fine. Its never actually broken down or failed to start, gauges show what they should, even when jerking badly.. I'm lost. Bear in mind I'm in very rural Scotland so can't just pop down to NAPA etc!


I've gone through grounds
New battery
new cap and rotor
new crank sensor




Thanks in advance for any ideas.
 
Welcome to the boards!

Couple quick thoughts:

1. Maybe the fuel filter is clogging up? Should be relatively easy to get/replace, and if you don't know how old the one in the truck now is, it's probably a good idea to do it anyway.

2. Since you say it feels like the ignition cutting out, maybe the ignition switch itself is acting intermittently? I'm not exactly sure how to check this, other than to have someone riding second seat with a voltmeter monitoring a 12V line that's only on when the ignition is, to see if it starts dropping out when the bucking happens. If so, it might be time for a new ignition switch.
 
Thanks Rob!


Fuel filter is something I thought of, but reckoned it'd give problems on max throttle rather than light throttle at cruise. ? It pulls well when under load uphill etc.


I also thought a faulty ignition feed would drive the dash crazy and give other symptoms-nothing flickers when it's happening and even on live readings the RPM doesn't fluctuate.



I ran it for about 30 miles last night and it only happened 3 or 4 times and it's completely random.
 
I would hook up a fuel pressure gauge and watch it while it is acting up.

You could have a random breakdown between spark plug wires. Mist them with water and see if it goes crazy. I have seen something similar with a cracked spark plug insulator. I have also see a coil go randomly bad and cause all sorts of missing and issues.

Maybe start it and get a hair dryer and selectively heat up different sensors and components.

This made me thing of what my wife told me one day when I said the same thing about something driving me mad... don't worry it won't take much gas since it is such a short trip. ;-)
 
My 4.0L XJ was doing exactly this a couple of years ago. Good O2 waves on the scanner.
After months of very much picking and prodding - it turned out to be a dodgy repair by a
previous mechanic along the O2 Sensor Wire Harness near to the #1 exhaust port tube.
I had to open the split-loom cover to find it. The previous repair utilized crimp connectors and house wires (yes, Romex!).

After using proper wires and connection repair method (twist, solder, and a double layer
of heat-shrink tubing) - all has been fine to this day.
 
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