Hi i noticed that sometimes when i'm on the highway with my '95 xjs with aj16 engine , and i'm driving say 120 km/h, i get very short kind of hiccups, like if there's a misfire of several cilinders or there's no gasoline for a tenth of a second. it also feels like a rough sudden gear shift, from 4 to 5, but there is no 5... it is something i can't explain, and it worries me a bit. i hope someone has an idea?

thanx, pim

 

The 'could be' list might get a little long.

A failing ignition coil is my first thought.

Cheers
DD

 

I've moved your question from General Tech Help to XJS forum. This is the place to post technical questions about your model.

Graham

 

I’m with DD. Probably sign of failing coil, but it’s too early to say.

Id immediately ask you:
a. When was the last time you cleaned your throttle body
b. How old are your current coils?

After that, probably several more questions. Understand that these cars are reluctant to give out engine codes, so if you’re waiting to see one as proof of a problem, it might, or might not come.

 

Well, if it’s a coil, I would be almost completely certain that the misfire must manifest itself at idle.

 

You would think that, wouldn't you. Coil issues are odd. That coil might just be at the slightest point of failure.

It could also be a spark plug that is going bad.

Could also be oil in one (or more) of your spark plug wells.

Could be the TPS going bad.

Could also be the Crankshaft Position Sensor dying.

As you can see, the list of possibilities is long.

 

correct- but as far as the car not “throwing” a code via OBD2, I have found that my 95 mainly fails to “throw the light on”—- meaning the CEL does not come on, but the codes are in fact stored and readable upon connecting the OBD2 reading device.

 

I had a hiccup in my '95 XJR6 the other day at 75mph in the fast lane.
Pulled into the slow lane and the engine just stopped.
Luckily I managed to coast into a layby and then waited several hours to get trailered home.
Haven't figured out what it is yet (ticking over at 1500rpm so could be any of the suggestions above) but if yours is starting to hiccup then I would start trying to sort it soonest in case it dies on you at speed.
No error codes stored on mine.

 

hi vee, the car looks like it has been maintained pretty well, but i wouldn't really know since i just got it.
i would also think its a failing spark plug or coil. its just that the 'shock' i feel when it happens is rather noticeable, more than i would expect from one cilinder not contributing.
i never cleaned the throttle body.

thanx, pim

 

Sometimes we describe things differently. If this is a “blackout” situation that happens at high speed, where it’s like the car basically shuts down for a few seconds, then your TPS is gone.

 

Hi no its a very short hesitation, of 0,1 seconds or so, not a blackout.

happens maybe 4 times in an hour

thanx, pim

 

These things can be hard to nail down. Sometimes you hope a problem gets worse so it can be more easily identified.

Assuming (perhaps incorrectly) that no codes are stored to identify a specific cylinder misfire or other clue, I'd replace the spark plugs. Easy, cheap, can't hurt, might help.

If no joy I'd install one new coil and drive the car, swapping coils one cylinder at a time. It only take a couple minutes to change. Take a good look at the coil wiring as well.

Replacing the Crank Position Sensor can almost be considered routine maintenance on these engines. I wouldn't hesitate to replace it on general principles.

Beyond these steps I'd be tempted to wait for further developments before spending more money.

Speaking of coils, here's and old trick:

Remove the coil cover. Note that the top of each coil has a translucent potting material. Run the engine in a dark garage. Are any of the coils giving a light show/arcing/sparking in the translucent material? If so, the coil is failing. Note that lack of this light show does not prove the coil is good.

Some replacement coils have black potting material so this 'test' can't be conducted

Cheers
DD

 

I once had a hiccup every 32 seconds at idle. It was a failing intake manifold gasket, that was apparently difficult for a few shops to find.

Do you have the ability to connect to your OBD2 port and get the long term fuel trim numbers? (LTFT)

 

Hi Vee; i will do that next time i'm going to drive the car. weather is bad right here now in the netherlands, so i rather not take it out.
keep you posted!
thanx, pim

 

All right, so i replaced all coils; found a good deal on eBay, for new lucas coils. put in new spark plugs, but unfortunately, no difference.
But i've tested some more to see when the problem happens;

When i'm driving the car, using the cruise control, and i am on a flat road, the engine hesitates. so if the engine is giving minimum power it really is shaking as if several spark plugs don't work. when im asking just a tad more power, so if i accelerate, there is no hesitation, also not when i'm slowing down and release the gas pedal. when using the cruise control on a road that goes up or down, there's no hiccups. idle is perfect, and very fast acceleration also no problem. only when im driven at constant speed.

Oh and Vee, i've checked the LFTF bank 1 and 2, they both read -89,84 %. these don't change so im hesitating if the app is reading it ok
thanx, Pim

 

today i measured again; and the long term fuel trims were at -100%, constantly withou moving.

Just a nice shot

This is a screenshot of the car scanner app
The short term fuel trims were between +2 an +7% or somerhing like that.

when i unplugged the second sensor on the intake manifold; the one thats closest to the driver; there was NO difference at all in driving; and the problem persisted. LTFT syayed at -100%. Which one is the sensor that reads the LTFT?

thanx, pim

 

Hi everybody.. I have taken out my 95 xjs out of the garage; and started driving it again; its lovely. except for the hickups... since my last post i have cleaned the throttle, replaced the cable to all the coils, even have taken out the throttle sensor, and fiddled with it. no change. the car still hicks when i'm driving 1800-2000 rpm. for the rest it is very smooth. idle is very stable, the engine runs very strong and good. i do think the hickups are getting worse. I hope some new member (or old) recognises it and can help me... its very frustrating to have a nice car that does not drive as it supposed to do at the most used speed...

thanx, pim

 

Have you checked the LTFTs now that you have it back on the road? Still at 100%?
When you had the car in the garage did you have the battery disconnected?
I found that leaving the battery disconnected for a long period (about a month in my case) made some definite changes to mine.
Also, have you tried a new CPS as Doug suggested a while ago?

 

Hi pete, thanx for your reply... the battery has been disconnected for half a year so that was not the answer. I did clean the tps but i think its a very expensive piece to replace just to check whether its bad. the lftf issue is something i don't really understand. what are those, i can read them in my carscanner app but from previous answers i understood that the obdb readings from this car cannot really be trusted. but still, i would like to understand those readings. what are they and what should they read`/
thanx, pim

 

pete, i misread; the CPS has not been changed. but i am buying one now.
thanx for the reminder!