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Matt, Mechanical Engineer
Category: Jaguar
Satisfied Customers: 21977
Experience:  BEng hons Mech engineering, in auto industry 22 years
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fires but will not start,

Customer Question

fires but will not start, is over fueling, if injectors disconnect engine runs until fuel in manifold used. Has has fuel regulator,ecu, crank sensor, new plugs. Car first started stalling at round abouts hard to restart unless left for 10 mins any ideas? Has had coolant temp sensor and 1 lambda sensor both coolant and temp readings on data logger reading ambient temperature before cranking , only DTC 1621. Oil in sump contaminated with petrol drain and refilled with fresh oil

Submitted: 4 years ago.
Category: Jaguar
Expert:  Matt replied 4 years ago.

These cars are very prone to issues with the throttle position sensor becoming 'noisy' and giving misleading information to the ECU

as it can create an overfuelling scenario by fooling the ECU into thinking you're always trying to accelerate

the only issue its very awkward to get to as its under the manifold
however its not too expensive and you can measure its resistance before replacing it to confirm if its the issue

as the resistance should increase smoothly and slowly as you open the throttle and sudden changes then its needs replacing
Customer: replied 4 years ago.

data logger shows a steady increase/decrease when pedal used and no sign when I use the oscilloscope

Expert:  Matt replied 4 years ago.

if thats OK ( data logger - nice work)
I'd try replacing the coolant temperature sensor as this has strong effect on the fuelling and its cheap enough part to try
Customer: replied 4 years ago.

this was the first item I replaced, both air and coolant ream ambient before trying to crank, have also tried with maf disconnected, & lambda disconnected

Expert:  Matt replied 4 years ago.
Hello Graham

OK next thing to check is if the injectors are being cycled corrrectly

So I'd check the condition of the injector plugs / connections 1st and then confirm th eoperation with a 'noid' light
Customer: replied 4 years ago.

Hi Matt have done this and all seem too be switching with a noid light



Expert:  Matt replied 4 years ago.
Hello Graham

if yours has the start-up injector on the end of the inlet manifold its possible that something in this system is fautly and leaving that injector on all the time

if it hasn't then it must a fault with the MAF meter as this is the main load input sensor

so try unplugging it as the car shoudl fire up without it
Customer: replied 4 years ago.

It has not got a start up injector and have tried with and with out MAF, this is what lead me to ecu but have also changed


Expert:  Matt replied 4 years ago.
Hello Graham

OK one other thing to try is the state of the wiring loom earths on the engine

check that the large engein earth strap is tight and the connections are clean but more importantly also check that the small ring terminal earths on the engine are also tight and clean as these are the ECU injector and igniton driver earths and could be providing an offset by adding a resistance into the circuit
Customer: replied 4 years ago.

Ok I have done a visual on all earth points I could find and checked their security will now do a volts drop test on all earth points then get back to you


Expert:  Matt replied 4 years ago.