Bad ECM or ECM relay.
1991 jaguar jx6 will not start
The ecm module is located behind the passenger side kick panel. I don't know of an ecm relay.
behind the radio
i have a 1991 sovereign xj6 and i have the same problem, BUT the gauge would show correct levels when accelerating
Next to the battery.
Behind the glove compartment.
the ECM is located under the hood on the driver side fender it is in a black box i just replaced it
Limp mode is controlled by the ecm (electronic control module). The ecm sets a trouble code (DTC) when there is a problem with a sensor or its connection/wires. Remove the battery cable for a minute or so will reset the ecm code. Check the connections/wires first and post back with the code.
replace the ecm
It is inside the ECM Module
YES it is.