Hi I have an intermittent issue with my 2003 2.5 S type. On some cold mornings, the car stumbles at idle, and along with that, i get loss of brake boost from the servo. it seems to cure itself after 30 seconds to a couple of minutes. once warm, it never happens. On one occasion, it put out a few DTC codes for cylinder misfires..
I can't figure out what could be causing it....all pipes and connections seem secure.
Vacuum pipes don't have a leak and then fix themselves!. Diaphragms don't get a split and then fix themselves Surely??. I don't want to splash out on a servo, unnecessarily, as they are very expensive. I cant hear any vacuum leaks or hissing when it happens.
Any Ideas Guys?? ('n Gals)!
Cheers Andy

 

It sounds like you have a small vacuum leak that closes when the material expands with heat. The misfire when cold would be a confirmation of this. Small vacuum leaks tend to not give you auditory confirmation of their existence, so searching for a hissing sound is not likely to help...

 

My '02 V6 occasionally does something similar on cold mornings. I think it's a combination of 2 minor faults.

1) The power brake system has a check valve to maintain boost even when the muscle force (manifold vacuum) is low or not present. Typically there's enough residual vacuum available to give you 2 or 3 boosted brake applications with the engine off. The general idea is to maintain boosted operation even if the engine stalled while driving. In theory, the system can retain the vacuum for an unlimited time, but in practice it's more like several days of inactivity. On a cold start in the morning, if you step on the brakes before turning the key (I always do this), that consumes one application's worth of residual vacuum. If this reserve vacuum supply is already a little low before engine start, you may lose some (or all) boost until the engine can replenish it.

2) If the engine is running a little rough while cold (for whatever reason), the computer holds the throttle valve open a little more than usual to prevent stalling. This decreases available manifold vacuum until the engine warms up enough to run better. On my car, the rough idle always clears up within about 20 seconds at the most. Sometimes I momentarily give it a little extra gas and that quickly smooths things out. The roughness is very mild and always clears up, plus I'm lazy, so I've never investigated further. The engine (360K miles) is old like me, so I can't begrudge it briefly acting cranky first thing on a cold morning.

So in my over-inflated opinion, your scenario is likely a combination of two minor faults. If either fault was present without the other, there would be no effect on the boosted brake operation.

For the engine side of the equation, use a scanner to make sure fuel trims are within reason. If you had a major vacuum leak, the fuel trims will be way off.

 

Hi KRYour 'over inflated' opinion seems sensible to me! Mines got only 140k miles, but is the same age, and rubber parts do suffer and age... I may look at removing and re-sealing vacuum pipes with silicon sealant...thanks

 

It's also possible the booster itself has a small leak and is causing both problems. Fuel trims should help make sense of the situation.

 

Noooo! Not silicone sealant. It cannot cope with the heat or oily fumes. If you must use a sealant, use a flexible one that is petrol and oil resistant.

However…have you replaced the seals on the variable inlet flaps? I can’t remember what Jaguar call them, but they are a known problem.

 

IMT O-rings may need replacing

 

Yes, that’s it “IMT”! I couldn’t remember what they were called. Duh!🤪

 

Hi guys just a little update for you all ..
thanks for all the guidance and advice the inlet manifold actuators ( IMT ) seals were pretty good , but swapping out the two small o-rings on the vacuum pipes (both at the back of the inlet manifold and underneath the throttle body) has definitely improved the situation.
only needed to buy 1 kit from jaguar parts because one of the plastic cages broke.


you could save yourself an awful lot of money by just buying two 2.4 mm CS x 8.3mm ID o-rings , as long as you can get the plastic cages out without breaking them.
Definitely ticks over better when cold.
I did it with the manifold off as it's a lot easier (ultimately it's only held on with half a dozen bolts and two brackets...)
Thanks guys👍