Hey all,
Wanted to ask you all about two minor annoyances I've had with my 99 XJR. Car runs great, good power, low oil consumption, no overheating, long term fuel trims under 5% etc.

1. When car is warm, when coming to a stop at a traffic light and especially when turning into a parking lot at idle speed, the idle will dip to about 400rpm, which results in a very minor shudder. Never stalls, recovers quickly, etc, but a bit annoying. My idle speed at hot idle is about 550rpm, which seems a touch low. I could probably very slightly adjust the tps sensor, but i had the same idle with a different throttle body so seems like not the issue. Throttle butterfly and TB are clean. Any thoughts about whether this is normal or should I be chasing something?

2. The past 2 weeks or so, at cold start, the car occasionally takes a few extra cranks and then sort of stumbles into life. It doesn't just jump up to 1200rpm as you might expect. When hot or even slightly warm, no issues at all. Fuel pumps are new, confirmed fuel pressure at rail. Has never failed to start on the first turn of the key, but not quite as sprightly as you would expect. Starter is newish and battery is 12.6v. I've heard the coolant temp sender could cause this, so I ordered one, but I'm not confident thats the issue.

Any input on either issue would be great.

Thanks all.

 

Vacuum leak check the throttle body and it's rubber
Check the intake and the breather hose may have tiny crack check the filter box too I had same exact problem 2 years ago it was the breather hose and the maf seal itself was broken however it came back again but honestly I'm just not in the mood to pull the intake and fix it since the coolant hose need replacement too you know our jaaag can't stay quiet

 

I agree that it sounds like symptoms of a small vacuum leak. A large leak will affect fuel trims and throw a check engine light, but a small one may not.

There's a small vacuum hose at the rear right corner of the supercharger. It's easy to see, but I fought symptoms like this for a while before I noticed that the hose had been pulled off - probably by me accidentally when I was working on something else. Anyway, reconnecting it cleared up the issue. Just a thought. If you have access to a smoker, that will reveal a vacuum leak pretty quickly.

 

You should be able to watch fuel trims as it happens and see if they flick around. It does sound likely to be a vacuum leak and they'll show it if so.

You might also be able to see that MAF doesn't change or even decreases despite trims showing more air (from the leak).

 

So my ltft are 3.1 for bank one and 4.4 for bank 2, so might be a very small leak, but those numbers seem normal to me for a car of this age. Will investigate.

Thanks

 

Maybe just re-clean the throttle bore and plate; clean the MAF; run a can of good fuel system cleaner through a tank of gas . . .

Plugs?

 

plugs have about 8k on them, but yea I can do all that.

 

smoke tested the intake via the MAF hole. no leaks at all on two rounds. That sucker is sealed up pretty good.

Any other ideas?

 

Brake servo leak

 

Interesting thought. What's the best way to test this?

Thanks

 

In my case there was hissing inside the car I could hear
But once I brake the hissing gone and the rpm slightly gone up it was the servo however it coup be the TPS too