Hi there, I wonder if anyone could help me with an ongoing problem I am experiencing with starting my '99 X308?

Since purchasing my XJ8 a few months ago it has been plagued by a curious intermittent fault: the engine always starts first time, whether hot or cold, but then sometimes - and generally when the engine is warm - it's as if the 'automatic choke' immediately cuts in and the engine then maintains a somewhat lumpy fast idle (1500 rpm) for the next couple of minutes. During this time there is strong smell of petrol from the exhausts and, more critically, the engine cannot be revved. It's as if the throttle pedal has somehow become detached from the engine: depressing the pedal produces absolutely no response at all. After approximately two minutes (sometimes less) the tickover suddenly drops back to a normal c.700 rpm, the throttle pedal 'comes alive' once more and the car can be driven away.

As I say, this is an intermittent fault and sometimes I can go several days without the problem recurring. I find it usually happens upon restarting with a warm engine, though occasionally I have also experienced the same symptoms starting from cold. I took the car to my a local garage and they put the car on their diagnostic fault code reader - but no stored faults found.

Some kind of intermittent overfuelling issue, obviously, but what might be causing it? And the throttle pedal going dead / coming alive when the overfuelling cuts out again is unlike anything I have ever experienced before. Any ideas what might be causing it? All suggestions / advice appreciated. Thanks.

 

Hi XJ-Driver,

The first suspects that come to mind are the Engine Coolant Temperature Sensor (ECT or CTS) and the Intake Air Temperature Sensor (IATS). The Engine Control Module (ECM) relies on the signals from those sensors to determine cold-start fuel enrichment. If one or both sensors are giving false "cold" signals even after the engine is warm, the ECM will overly enrich the fuel mixture and choke the engine.

A lazy O2 sensor might also be a possibility.

How long has it been since the car has had a good tuneup (plugs, filters, fluids, etc.)?

Also, since we're a friendly group, please visit the https://www.jaguarforums.com/forum/n...-intro-must-5/ and post an introduction so we can give you a proper introduction and learn something about you and your Jag.

Cheers,

Don

 

Thanks Don. That gives me something to think about... I had already wondered about a faulty temperature sensor causing the fuel overenrichment, but would that knock out the throttle pedal too? Engine has been serviced, including plugs, in the last 12 months - and very few miles since.

BTW - I just posted a quick hello on the intros page. Cheers, J.

 

Monitor fuel trims and see what it thinks it's doing - might be an air leak. That mimics pressing the accel pedal.

OBD will also let you look at the ECT & IAT in case one is bad.

 

I have seen water in fuel cause all kinds of weird symptoms.

Eliminate this possibility by adding two red bottles of STP gas treatment or dry-gas / sea foam...............etc.

Wally C
03 XJ8

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I too would focus on temp signal circuit. And yes it sounds like the engine is in limp home mode when doing this. This takes away throttle connection except for 75-80% throttle opening. Ie if you floor it it will go. This is to keep you from getting smashed on the freeway if in limp home while youre driving. Most owners are unaware because they have never read the owners manual. But then again maybe jaguar didnt elaborate on limp home in the manual. also clean the throttle body, and dirty sticking throttle and sticking dirty idle air solenoid and passage can all create this is also.

 

Thanks for all the advice and suggestions - gives me some idea of what I should be looking at. Cheers!

 

XJ....I had the same issues with my 01' XJR. The "over-fueling" was the result of a bad MAF preset that was stored in my ECU that occurred in conjunction with a fuel pressure issue. The ECU was calling for maximum enrichment at idle. After cleaning the MAF Sensor and checking the continuity of the 5 terminals between the MAF-Ground-Fuel Tank Pressure Sensor and the ECU (which didn't reveal a fault) I got my hands on a Diagnostic Tool that recognized the Jaguar Codes (Matco Windows Based System) and cleared the fault codes that failed to clear by disconnecting the battery or clearing with a generic OBD reader. They were mostly "U" Codes that are specific Jaguar Codes. Once cleared, the car started right away and has been performing better than it has for the last year. 600 miles without any issues, and I have picked up 2.5 MPG. The ECU was reading a value from the MAF that was 10X the proper idle value. Once cleared, the MAF control signal reverted to the factory setting.

 

TPS can cause the same assumptions ,