Firstly, thanks for this source of valuable information. Secondly, I have very little experience with posting. Once again the xj6 (the ANTI CHRIST) is giving me problems. This time the power drops above 1500 rpms below 1500 it runs fine. My alternator needs to be replaced. Is this causing the power loss? I am not a mechanic but I am retired with plenty of time to fix the beast.

 

Cap,

Can you give us a wee bit more info?

Can you rev the engine above 1500 when in Park or neutral?

Is the power loss absolutely rev limited or speed/load limited? Ie can you drive at higher rpm in say 2nd gear?

What are the symptoms of the power loss? Does the engine stall? Do you hear it misfiring and shaking?

Do you have a check engine or other light (transmission) on?

Do you have access to an OBD scanner? The data from such a device can be very helpful in diagnosing problems. If you don't have one, you should be able to use one for free at a local auto parts store.

You say that the alternator needs replacing, what led you to that conclusion. A defective alternator could show some of your symptoms but at some point the car would shut down completely. Have you tried running the car with all the electrical accessories on? If you have a bad alternator, it won't be long before the car dies.

 

Don't spend much time revving it above 1500 in Park or Neutral: I happened upon a (for me) little-known tidbit last week while reading through some TSB's....seems revving above 2K in P or N will damage the transmission!

 

Allan tks for you reply
The check enginge light is on and the engine misses and backfires when in park and neutral above 1500 rpms. The codes from the scan are:
P0138 - HO2S sensor circuit low voltage cyls. 123 downstream
P0301 - Misfire detected cyl. 1
P0302 - Misfire detected cyl. 2
P1138 - HO2S sense circuit lack of "swing" cyl. 123 downstream (2) sense circuit indicates rich combustion
P1313 - catalyst damage misfire detected cyls,123
P1316 - misfire excess emission
This morning I looked at the ignition coils. NO. 2 cyl. coil's boot is snapped off. that is an easy fix. Do you think there might be another problem causing all these codes?
The alternator gauge has been acting up recently. Climbing to 14 when the engine heats up and then dropping to 11 when I turn on the heater, wipers, radio. lights ect. Since the loss of power it has pegged itself to 11. Do you think replacing the coil and alternator will solve these problems. I'm seriously considering an exorcism as my next step. hehe

 

I would first change spark plugs with champions. Then get denso inhibition coils. Get the paper gasket for.them. then while.you do that. Change valve cover gasket. Spark plug tube seals. And cam cover bolt bushing.

 

Bob,

Well that list of DTC codes certainly would explain the loss of power

I would definitely pull your spark plugs and look at their condition. Then you could check the resistance of the coils, you're looking for a value of 0.75ohms. You could swap coils and see if the misfire moves to those cylinders. You should also look at your bank1 downstream O2 sensor and check that the voltage is changing rapidly from about 0-1.2v

Hopefully the misfiring has not caused excess fuel to pass into the cat where it could burn off and unfortunately melt the cat. This would cause the P1313 code and one of the classic symptoms of a melted cat is loss of power

You can check the cat a few different ways. One way is to measure the exhaust temp before and after the cat, you would expect to see the output temp be a good bit higher than the input, probably 80-100F or so. Another is to do a back pressure test by fitting a pressure gauge in place of the upstream O2 sensor. You would expect to see a pressure of no more than say 1psi at idle much more than that and you have a blockage somewhere downstream (cat, muffler, collapsed pipe)

If you don't feel confident tackling these issues or don't have the tools, any decent muffler shop will be able to check out the exhaust system and O2 sensors. I urge you to check into this asap as it is a serious condition and driving the car like this could lead to some very expensive additional repairs!!

For now I would concentrate on these issues before turning your attention to the alternator although that issue will need to be addressed in time as well.

If the cat needs to be replaced, you must find the underlying cause of the problem or it will just happen again. (I doubt that the underlying cause would be the alternator)

 

Allan
Thanks so much for the info. It'll be a couple of days before I can get this done. I'm a shade tree mechanic and the weather has been rough here in PA. Let's pray the cat isn't fried. You will hear from me as soon as I'm done. And again thanks for your expertise.