Strange over revving

Hi, just added a 1987 Daimler 3.6 XJ40 to the family table. The old girl needs some work which will begin soon. One item has me a little perplexed. Looking for some experienced advice.

When the car is stared, it doesnt idle all that well, but I suspect that is due to bad fuel and lost ECU learned behaviour since the battery was disconnected. As it warms up some it settles down to an almost acceptable idle.

Then if it is revved at all (ie, drive it), the revs jump to 2000 and will not go down again. If I get under the bonnet and disconnect what I think is the accelerator cable, the revs return to normal, but of course you cant drive far like that.

Checking the cable play, restrictions etc back to the pedal, it all seems fine. cant see any reason that the pedal/cable arrangement is holding up the revs, but it certainly seems to be the case.

Any ideas/suggestions most welcome. Thanks


First thing I would do is remove the throttle body and clean it and make sure that the butterfly is set to .002” gap.
You will probably find that the TB is gummed up with carbon which isn’t allowing the butterfly to close completely.


Thanks. will do. I am snot sure this explains why releasing that cable drops the revs but will certainly try it. Thanks again

I was wondering if you have studied the parts diagrams within the jaguar landrover classic parts site (or elswhere). I took a look at the throttle linkage one as well as the accelerator pedal/cable one and they are quite detailed. They might alert you to something you missed in your checks after you disconnected the cable.

There also are a few other threads on this site that discuss similar, but maybe not exactly the same problem. These might be useful readings.

Thankyou Roger. Any and all ways of looking at a problem are most useful and welcome. Before I pull it to bits, I will do as you suggest. cheers

I would take a look at the idle air control valve and the housing it screws in to, there is a gasket for the housing and if that is broken air gets in, also check the iacv is screwed in tightly and the connector is pushed down properly/clipped in

well I’ll be buggered!! Never seen this before… ever… even on the oldest dirtiest barn finds… Pulled the air filter out… box was filthy… maybe rats?? Anyway only the orange outer rubber seal was intact… guess where the rest is… inside the throttle body… inside the manifold … and maybe even inside the combustion chamber… surprised it ran it all!! At least we know whats wrong with it… Phew!!

Wow! I’ve never heard of that either. Seems like it deserves a spot on my car-idles-oddly troubleshooting checklist. I guess the “is the air filter dirty?” item could be augmented by adding “or has it been partially digested by rats?”