1988 jag xj6. at idle the rpms go up and down. engine does not settle at one speed at idle. with the car in drive and brakes holding car in place the rpms are at 1200-1300 but are at a constant speed. no raising or lowering in rpm. is the probelm the idle conrol valve?
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IAC - idle air control problem possibly .( You can drive it if you use your left foot to brake but keep the gas pedal down to keep the RPMs up.) Wouldn't be a fuel filter , if fuel filter was clogged partly it would idle ,but would lack power ,low fuel pressure at higher speed's . Do you know what IAC valve is ,an how it works ? Idle Air Control Valve
Or maybe the EGR valve pintle is covered with carbon an sticking open .
Or possibly a dirty or bad MAF sensor - mass air flow sensor - the PCM - engine computer needs info from this sensor so it know's how much fuel to feed the engine .
Hooking up a professional type scan tool to view engine sensor data cut's down on guessing as too what the problem is .
Sounds like you may have a small exhaust leak. This will drive O2 sensors crazy trying to regulate the engine. You end up with an engine endlessly revving up and down.
give it a little time to settle down a bit.sometimes you will never see the light again.
transmissions are bullet proof on these models.it wont hurt to check transmission fluid level while you are at it.
good luck and god bless.
I don't know where you are, but here in Aust. a lot of people swap the entire engine/trans for a Ford or Chev v8 and transmission. Fixing the Jag stuff is just too expensive.
I would check the trans fluid level and overall transmission condition first though.
None of those possible problems have anything to do with idle speed. The first thing might be a very weak throttle return spring. The next would be a sticky throttle cable or foot feed pedal that isn't moving freely. Another possibility is a vacuum leak somewhere. Did changing the spark plug in #1 cylinder help the misfire? If not, you may have a partially plugged injector. Fixing the misfire will only increase idle speed some more. Look for an idle speed motor near the throttle body that is supposed to raise idle speed when the engine is cold and lower it back when the engine warms up. Check to see if it is holding the speed too high. If the throttle position sensor tells the computer where the idle speed motor is set, then it could have an effect on idle speed if the sensor is bad. The position sensor is just a fancy potentiometer. Hope some of this helps.
TRY THIS AND LET ME KNOW IF IT CORRECTS YOUR PROBLEM Here is the most common cause of surges and stalls at stops and low erratic idle speed and rough idle, it is the idle speed control air-bypass valve and throttle valve (IAC for short), they get full of gunk over the miles and cause idle issues (stalls, low idle) like yours, Get a can of intake cleaner from any local parts store, not carb spray, intake cleaner, it is made by a company called CRC, remove the air intake hose to the engine, hold the idle high so the engine won't stall, then spray the can of cleaner into the intake while keeping the engine running, use at least 1/2 the can, shut down the engine and disconnect the battery for 5 minutes, then restart and complete a number of mixed driving cycles, town, freeway, stop and go etc., after a few days the problem will go away as the system will relearn to the clean intake
do a compression test ?? about 180lbs anything less and it could need a valve regrind ,if compression seems ok then plugs and leads and use the bosch plugs with the 3 electrodes (for voltswagon) champion are **** and missfire.if still persists change dist cap and leads
Hi the idler valve has a set up with jag equpment but there is a way round this,ifyou have had O2 sensors changed, is the idler working what are the revs when hot. 650 is to low 850 is ok with a hot engine Mike do you know where the idle valve is and the allen key manual adjustment try a quarter turn but mark it so you can turn it back if need be. let me know how it goes D
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