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The Cat's Cat

The Cat's Cat

Deano Paige

Sean wrote:

Howdy all.

I'm a new subscriber and am interested in conversing with anyone who cares to listen or discuss jag problems (my wife sure has no interest in doing so - - she would rather drive a Merc. Sable wagon than my '83 xj6). The jag is in vg condition with 71,000 miles, nice interior and paint, and runs (ran) real nice.

A few days ago my jag developed a problem with losing power. All was well, with good (normal) power and I was running nicely at 75 or so until I hit the normal Saturday morning NY City traffic. By the time things cleared up (1/2 hr or so) the engine seemed to have lost some of its edge. The power loss was real apparent when going up some moderate hills at highway speed. Even with the throttle floored the jag would only do 55 or so. As the road flattened out top speed was better, but acceleration was (and remains) very poor (i.e., 0-60 in 18 sec).

Idle at cold start up is decent, but rough when warm. Gas mileage seems to have dropped, but this may be because I have to really lay on the gas to keep up in traffic. The engine seems a little noisy when accelerating (kind of whiney), but it is tough to tell as the jag has exhaust leak by the first set of mufflers.

I spent some time under the hood (and boot) this evening, and here is what I found:


  • Fuel filter clear
  • Fuel pump volume good (8oz or so in 5 or so secs. cranking)
  • Fuel line from tanks to engine clear
  • Plugs, rotor, dist cap, all look good
  • Timing set +-2 degree from factory spec.
  • No oil in the water and no water in the oil

My initial thoughts is that there is a fuel starvation problem, but I am not sure where to look next. I suppose that it could also be ignition (coil?) but the plugs do not seem to be misfiring.

Any thoughts or assistance would be greatly appreciated.

I'm probably starting to sound like a broken record on this subject, but... next thing to check is the CATALYTIC CONVERTER. If it has not been replaced once in the 71,000 miles, it may well be your problem. The symptoms you describe are one of the major indicators for this condition, I have also seen very rough running and smoke out the tailpipe on startup, smoothing out on warmup, but with that noticable lack of power you describe even though all cylinders seem to be firing.

This condition can also cause overheating of up-stream exhaust components, manifolds, and valves, due to excess backpressure. I have seen the rear exhaust manifold on an XJ-6 virtually crystalize from the heat and cracks form at the upper welds on the converter. The converter can be checked visually by disconnecting the downpipe from the manifold and shining a light down the pipes and into the converter. Should have a noneycomb appearence. If the honeycomb is obscured by grey, black or brown material, it's shot. Backpressure can also be checked with a pressure guage applied at the ports on each manifold. I'll leave it to someone else to recommend what the proper pressure reading is. I've always used the look and see approach. All I know is that the pressure should be very low.

Don't spend the dough for a factory converter -- usually upwards of $400. Weld in replacements are available for <$100 and complete aftermarkets are available for $250 to $270 or so. If you decide to go for OEM ask the list for recommendations on the lowest price available from distributors.

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