G60 fluctuating idle

[Andy T 0]

Seems to of developed an intermittent dodgy idle, but not in the way i'd of expected.

 

When coming to a stop, the idle falls smoothly and rests at 900rpm, no probs.

 

But then after a few secs it sometimes starts to fall to about 500rpm, then upto 1100rpm then recovers, and the process repeats itself until I drive off again.

 

I'd of thought that if it were a problem with the ISV, it would be the opposite way round, and would eventually settle down to a steady idle.

 

Could this be an electrical problem/hall sender/ECU ???

 

I've also noticed on the motorway at a certain speed, the car will loose power very slightly then return, the a few seconds later it will do the same gain, repeated. No jerks or stutters, and it wouldnt be noticed if you were constantly changing speed etc.

 

Could the two be related?

[Fat Tony 0]

Sounds like a vacuum leak of some sort. When we where at the rolling road day they pulled a pipe out of the throttle body (Fuel pressure regulator or something) and the car kinda stutters and then gets the revs back up. So if you have a leak same thing I guess.

 

Do you have a boost gauge? If not buy a cheap one stick it on and see how much VAC you have at idle.

 

I don’t know much about engines so probably not the best person but the symptoms seem to match

[Andy T 0]

cheers, I will check all the vac pipes for leaks/missin connections.

 

Standard airbox is removed so that is one vac pipe that is missing for a start!

[g60bv 0]

check the mfa tube isn't split, this is quite common cos it black and nilon and rubs against other tubes, wears through and you loose vac and boost, it is located off the brake vacume line , follow it round to the right where it gets lost in amongst other tubes and then goes through the bulk head to the dash, if not check all the vacume tubes as these do wear and spilt . oh and the isv rubber tube also wears, good luck, my c had a irratic idle turned out to be a weak mixture.