Jag X300 stumbling when pulling away

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Freebird
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Jag X300 stumbling when pulling away

#1 Post by Freebird » Fri Aug 17, 2018 10:40 am

Hi All

A few moths ago my Jag (Auto) would "stumble" when pulling away on gas and on petrol. By that I mean it would idle fine but as I applied the throttle it would die and often stall. If I hit the throttle hard it would hesitate and then take off, but this is often not a safe approach!

I thought I had cured this by removing the throttle position sensor and giving it a good clean - it was then fine for a few months, but now it's doing it again but this time (it seems) only on gas. If the car is stationary or moving slowly when it changes to gas it will stall too - I have to leave it on petrol far longer than normal and change to gas manually once moving at a reasonable speed.

I doesn't do it once it is thoroughly hot though - and I mean 10 miles hot. I have pulled over while is has been playing up and the vapouriser is hot to the touch. My thinking it that it's still the throttle position sensor and it's more sensitive on gas than petrol - just wanted other peoples thoughts before I swap it out.

Cheers

Glen.

Gilbertd
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Re: Jag X300 stumbling when pulling away

#2 Post by Gilbertd » Fri Aug 17, 2018 11:13 am

Could be TPS or could be the IAC valve. On a lot of modern engines the IAC valve opens slightly as soon as the throttle starts to open rather than relying on the airflow through the throttle butterfly. Give it a good clean, it could be that it is sticky when cold but frees off as it gets hot.
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Freebird
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Re: Jag X300 stumbling when pulling away

#3 Post by Freebird » Mon Aug 20, 2018 10:54 am

Thanks Gilbert

I need a deep 5.5mm socket to get the idle control valve off, so an waiting for one to arrive. I will let you know how I get on.

Glen

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Re: Jag X300 stumbling when pulling away

#4 Post by LPGC » Mon Aug 20, 2018 11:57 am

What LPG system is on it?
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Freebird
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Re: Jag X300 stumbling when pulling away

#5 Post by Freebird » Mon Aug 27, 2018 10:40 am

Hi Simon

It's a Stefenelli system, you may recall (unlikely though) we had a long post about it a couple of years or so ago when you correctly diagnosed duff lambda sensors?

I have cleaned the crud out of the IAC now and it made no difference. It also struggles to start from cold sometimes (not every time) now, dying a few times before eventually starting - on petrol of course.

Cheers

Glen

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Re: Jag X300 stumbling when pulling away

#6 Post by LPGC » Mon Aug 27, 2018 1:10 pm

I don't remember the thread, will have a look for it but you might have a link?

Stefanelli would be a sequential system, usually fitted with cheapo injectors that wear out pretty quick, a common problem is the reducers that fail in a way which sees pressure output drastically reduced. Problems I might anticipate finding given your description of symptoms would be failing injectors or reducer.. If pressure is way low then when you touch throttle to pull away the injectors might even not be able to flow enough gas for those conditions leading to hesitation and/or the not great pressure compensation might have thrown fuel trims.

Edit - Found the thread http://www.lpgforum.co.uk/viewtopic.php ... li#p109735

Regards,

Simon
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Freebird
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Re: Jag X300 stumbling when pulling away

#7 Post by Freebird » Thu Nov 08, 2018 1:27 pm

Hi All, I thought I would let you know that I cured my problem in case it's of use to anyone else.

I fitted a thoroughly cleaned second hand throttle body complete with TPS and IAC valve and it made no difference, I then fitted a second hand MAF and that too made no difference. The previous owner had already fitted a new coolant temperature sensor so it was only a couple of years old, however I still had the original one, so I fitted that and all my problems went away.

A bit annoying that I spent quite a bit of time and money chasing the problem around when I already had the solution in stock which took all of 3 minutes to fit, but you live and learn...

Cheers

Glen.

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Re: Jag X300 stumbling when pulling away

#8 Post by LPGC » Thu Nov 08, 2018 9:39 pm

Hehe ahh well it's sorted. The duff coolant temp sensor probably made the petrol ECU see the hot engine as cold and add some cold running mixture enrichment.
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