No power up long grades

Car - 1985 XJ6 135K miles - I have owned for the last 25 years.

About 5 years ago (last 3 I did not drive it due to other projects and 20 year old tires) I noticed it has trouble maintaining speed up long grades. You have to go 80mph at the start of the climb and hope it does not drop below 50mph by the time you get to the top. It accelerates great on on-ramps and zooms as best as these cars can up to 70 mph. Does great on level roads and keeps up with traffic and passes cars with ease. When I take it up a nearby two mile incline over the pass (i.e. over the the Appalachian Trail in Northern VA - USA) - the speed decays steadily and it will not accelerate regardless of how much pedal you give it or if you kick down to 2nd. Just stays flat - revs do not go much above 2500. No misfires just flat. It used to excel in hill climbing with the torquey XK motor. Car starts and idles great. Something is wrong.

Replaced - plugs, wires, cap, rotor, fuel lines, injectors seals, cleaned injectors, new fuel pump, new fuel pressure regulator, cleaned crankcase vent screen, new air cleaner, flushed trans fluid, new trans filter, new tires, checked timing, drained all old gas - new 93 oct. fuel.

Not sure what it could be - I started thinking maybe the cat-converter is getting clogged? Vacuum advance not working (I did test that with a vacuum pump to see if weights move) But not convinced on any of this stuff. Have not done a compression check. Maybe the engine is getting old. Maybe the air-flow sensor need some work?

What do you think?

You forgot to list the fuel filter. There’s an inline filter as well. If you have power but it goes down after a few seconds it’s either fuel delivery or a clogged exhaust. It’s not going to be the engine itself and unlikely to be injection/AFM related. Does the cat rattle? Do you have lots of fuel returning into the tank if you look all the way down the filler neck?

Yes - replaced the fuel filter - forgot to add that. Power if fine until I climb. I hit the grocery store - drive around town - no issues. Just the climbs. Really odd.

Quite simply really, you have enough capacity for everything but long climbs. These need lots of power for a long time, so something exceeds that capacity. Either it is the exhaust flow or the fuel delivery.

While it happens, try to switch tanks. Add that to my tips; I meant to say in tank filter not inline, duh. Try that, and look in the tank while the engine idles, and then report back. Good luck!

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Questions arising, Ian…

…like how steep is the hill. As car slows down and rpms drop, power drops off and if the hill is steep enough the car cannot accelerate even in 2nd - there is simply not enough power. Max torque is somewhere in the middle 3000s - at 2500 there is not all that much power and torque; certainly not enough to prevent speed bleed-off if the hill is steep enough. The rule of thumb is ‘that if the car won’t accelerate; you are in a too high gear’…

You may of course have a power issue, but if ‘x’ horses are required to climb a hill, you need enough rpms to produce that power. You may have a gear shift problem - have you tried manual gear selection to maintain rpms?

That vacuum advance does not work is ‘imperfect’, it means that timing is not to spec in all situations, and loss of power to be expected - even if at high throttle settings at high load there is not much vacuum. A compression test may clarify matters - and you could crudely test possible exhaust clogging with a vacuum test…?

xj6 85 Sov Europe (UK/NZ)

I think you’re right. My wife’s old Merc was fine around town because she drove like it had a brick under the go pedal but when i tried to give it an Italian tune up or belt it over some hills it would bog down. Just needed a new cat.

I think it would have to be a really steep hill and he would have to be fast enough that the transmission doesn’t shift down…
Vacuum advance gives economy at part load, no extra power.
Fuel delivery or cat.

Certainly, David, but incorrect timing means loss of both and no gain for either - basic function of vacuum regulation is to avoid pinking as load varies…

xj6 85 Sov Europe (UK/NZ)

The vacuum advance fails if the mechanism seizes or more often if the diaphragm ruptures, in both cases theres 0 vacuum advance just like at full throttle.
The other advance will still work.

Plus, the timing is not at all suspect since he says that he HAS power in the city when he just accelerates. While it will make sense to repair the distributor it has nothing to do with the uphill issue.

Further testing - switched tanks - no change. Fuel returns to sending tank as expected. No issues there. I think fueling is fine - plus no misfires or stumbles.

I don’t hear any rattle - on the Cat. Converter. How does one test a cat? Substitute pipe? Would the temp on a clogged cat be excessively high? The front cat does have a nice bluing to it from the heat.

There’s a few things on the forum.

I had similar loss of power problems with two of my former Series III XJ6s that were due to clogged catalytic converters. The link that David provided includes posts from me in 2016 where I describe how I tested the catalytic converters with my shop vacuum after I removed them from the car and proved that there were flow restrictions in the catalyst matrix. In both cases a visual inspection of the removed catalytic converters revealed matrix blockage and the performance in both cars improved dramatically once I removed and replaced the old blocked catalytic converters.


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I suspect it just is what it is. Heavy car. Limited engine size. Limited HP Plus a high ratio rear. 2.88!!!

Just watched the old Lemans ford vs Ferrari movie. seveloping the GT40, the eary cars just could not get the needed speed of 220 mh!. Driver/engineer proclaimed the 289 was not big enough. Shelby and crew found a way to squeeze in th 427 NASCAR engine Wowee, exclaims ken as he tested it 7 grand and mre and 220 was there.

Cubic inches trump most of the stimes.

Testing the cats separately requires their removal, Ian - and specific data on expected flow resistance. The cats run very hot, they are meant to - temp is not really a good indicator of function…

A general test on clogged exhaust ‘in-car’; connect a vacuum gauge to manifold - run the engine with a steady throttle at various rpms. If initial vacuum measured gradually drops; it’s a sign of clogging exhaust - including cats of course.

The cats are an emission control devices - they are of no benefit to the engine. You could bypass the cats and try the hill run without them - though the work involved may not be worth it…?

xj6 85 Sov Europe (UK/NZ)

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I agree with you on ‘European’ logic where dist receives manifold vacuum, David. I have too little experience with ‘US’ ‘ported’ vacuum, and less with ‘retard’ ditto - except noticing the different centrifugal settings in the initial advance set-up with the different source. Ie, how ‘ported’ vacuum varies with throttle settings/loads/rpms. Centrifugal advance is of course tied to drop off in rpms…

That said, given his symptoms are not ‘perceived’ rather than ‘real’ - ign vacuum timing advance may certainly not be the root of them, though timing itself is not immaterial…

xj6 85 Sov Europe (UK/NZ)

Thanks @davidsxj6, @Paul_M_Novak David and Paul - based on the link sent - I have to say I am in agreement - bad cat. And engine is just a big air pump - without the exit the pump can only go so high in RPM. It has to be it.

@Cadjag - the Jag XK engine has gobs of torque - that being said it has had no problem climbing the mountain pass in the past. Yes a bigger displacement engine would have a greater effect, but I have what I have and will not swap it out. For the bigger displacement solution I go to my 1984 V12 Series 3 XJ - 5.3 liters and 300HP - Euro Spec with no cats and only the little mufflers at the back. It goes. :grinning: :grinning:

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My only experience with blocked cats was on a BMW 740i which had all its power in first and second gear but would simply bind up around 100-120 kph. The ECU would then eventually detect misfires and shut off the cylinders on the bank (the XJ6 is not as smart). It really went well until it saw some load for more than a few seconds.
The cats were ceramic and I could shake out bits of the matrix once I had removed them from the car.

For Carl 4.2L is little, for Europeans it is a lot! Lucky him.

I hope the cat (or its removal) solves it, good luck!