I started a thread in the XJ40 forum, but probably should have come here for input. My son purchased a 1992 XJ6 (148,000 mies)that was not running but in beautiful condition with the understanding I would help him get it back on the road.
I had rebuilt XX120 and 140 motors back in the stone age before he was born,(even owned one of the prototype 120's) so I thought it would be a piece of cake. Ha!
History was description of "white smoke" from exhaust upon start up for a couple of days, then dying upon entrance to freeway, never to start again. Owner swore no overheating occurred, so we hoped we would get lucky.
Compression tests showed #2,3,4,6 cylinders at 170 psi, #5 at 90,#1 at 190.
No visible water in oil, no visible oil in water, all levels normal. Thought engine should run (rough) or at least attempt to start, but it would not.
Did some diagnosing, coil resistance was out of whack according to Haynes manual ,so was replaced.
Plugs OK, looked like good spark, fuel to injector rail.
Checked sensors for resistance per the manual, only coolant temp sensor out of range, but still no start.
Decided to R&R head gasket. Head is now off, all valves appear good, am going to replace valve seals while off and correct valve lash to bring back to specs.(most were at .012-.013, but a couple valve pairs are .015 and .016). Cylinder bores show little visible wear, no ridges at top of cylinders. Engine appears well taken care of, no sludge, water passages perfectly clean. Exhaust ports and manifolds are covered in soot, as are tailpipes.
Camshafts appeared in proper position with #1 at tdc.
Upper chain guide on passenger side is quite worn, almost worn through at rear edge. Can this be replaced without pulling block?
Any other ideas on things to check while i'm this far in?
Hoping to get this back on the road on a shoestring budget, but don't want to overlook anything.
This motor is similar, but a little more complicated than the old 3.4 liter motors with SU carbs.
Thanks in advance for any input.

 

I had head gasket problems with my 90 xj6, number 5 was the problem cylinder,cause of the problem was plugged up cats,both the ones in the manifolds (rear worse) and the ones in the exhaust pipes. You should check them before reassembling the upper end

 

Thanks for the "heads up".What can cause the cats to plug up in the first place?
Ecu problem or something affecting injectors??I appreciate the help.