Good day. I own a 2005 Jaguar Vanden Plas and hopefully can get an answer before I pull out my hair!!!

When in fourth and fifth gear (1500-1800 rpm's) the car will begin engine surging. The RPM hand goes up and down with the car feeling like it's missing. I have changed the coils, spark plugs, breather, throttle, MOF senor, fuel injectors...etc. To no avail, nothing changed. Took the car to Aamco to have the trans checked and all came back well. I have been chasing this for two years now, and I am DONE!!! Help Please!!!!

 

Så.. does this rpm fluktuation appear att this specific rpm always, or only when you hit fourth gear?

I am aiming at rpm specific or a load specific problem.. since the rpm do fluctuate means that the engine is loosing ability to keep rpm/load steady.. it overshoots and let go.. drop to low.. and rews up again...

A few things on a modern car communicate with the engine and have some influence, among the main ones are aircondition and the gearbox and beleive it or not, the powersteering.. some cars has a sensor that sense stearing load and if faulty it can get the engine rpm confused.. Aircond.. well the engine needs to know the load in order to compensate.. also in order to comand ac to ease off if needed.. geearbox.. well.. that relationship is not really likely to cause issues without a bunch of usable faultcodes..

You dont mention anyhting abbout what faultcodes this throws at you if it has, you should present them here togeather with maybe some more specifics.. ounder what conditions this happen and when it does not happen... im sure someone will figure out what is going on...

If no faultcodes... i would bet on an vacuum or an airleak somewhere... a rough way to check this out is to carefully troubleshoot with startinggas.. just spray it on the pipes, tubes and connections.. or certain areas... if you have a leak somewhere.. the rpm will go nuts.. repeat to verify... and then take it from there..

/E

 

Thanks for replying. The fluctuation usually starts in fourth gear and into fifth. The needle goes up and down, up and down. I tried the water test looking for a vacuum leak to no avail. The code I got in the past was a faulty MOF sensor which caused misfires. The last code was a small evap leak which was said to be around the gas cap. I didn’t think that would be causing the problem.

The needle doesn’t drop when fluctuating, is seems to be in a consistent rhythm and if I really get down on the gas, the car will choke down.

 

You mentioned the Ac... back in the summer the car would really act up when the AC was running. So much to the point where I would have to turn it off to drive it. Didn’t consider he power steering.

 

Possibly the same torque converter issue that affects some S-Types.

 

Just in case its not the converter.. here is some "shoot from the hip" suggestions;


-What is the state of your battery? if it is in poor/old condition, it might be the electrical load management system that gets confused..


-The Variable Valve Timing system uses information about engine oil temperature, meaning the viscosity of the oil, in order to operate in anexact manner. If that temp sensor is erratic it might mess with valve timing.. especially when load conditions of the engine changes at higher gears..


-Was the air condition system, when it was acting up,repaired tinkered with in any way?


/E