My 1986 engine runs pretty great, except for a couple of issues:

1) The idle seems high (1,000) in neutral, 800-900 in drive. It does sort of a "rolling" thing; like the speed is following some sinusoidal curve just to the extent of +,- 100 rpms or less. RRRRRrrrrrrrrRRRRRRrrrrrrrRRRRRRrrrrrr.....

2) When cruising, at say 40 mph / 1500 rpm, there is the sensation of the car not keeping an entirely steady pace. It is like it pulls a bit, then backs off, then pulls again..much like the idle issue above. A lack of "steadyness".

Where should I start to look for cures to these problems? I seem to recall that the throttle position sensor (TPS) potentiometer can cause these sorts of manifestations. But it's kind of like a multi-carburetor setup that's not properly balanced.

Thanks for any input,

Lou

 

I had that happen but on mine I felt it only at idle. Very annoying and difficult to track down. In my case new oxygen sensors fixed it but check for vac leaks first.

Good luck

DD

 

Vacuum leaks...hmmm...

Today I looked for the heater valve at the firewall center, found it I think, it has a vacuum hose on it and whatever arm action might be observed is below it. Further investigation with better lighting is required.

However, today the climate control system is actually working, although I am missing the full bore fan speed, which from the FAQ's I have read seems to likely be associated with one of the four relays whose location I do not yet know. If you know where these may be accessed, please let me know.

It is worth noting that about five years ago, a local shop told me the climate control system was FUBAR and needed a new amplifier and their estimate was at minimum $1700 just to "see if that fixes it". I don't go there anymore. They were making a career out of me, with my stupid cooperation.

But I am off track again. I found what appears to be a vacuum nipple missing a hose (?). It is located near the rearmost sparkplug closest to the heater valve. It's a whitish, conical-looking thing. In fact, currently it is resting against the sparkplug rubber socket. Again, I'll have to put more light on the subject and wait til the engine cools before reaching into there.

Any idea of what this may be a part of? If I cannot find a loose hose nearby, maybe I'll just cap it off with a rubber plug and see what happens.

Thanks,

Lou

 

Post a pic of the mystery widget and I'll try to help.

Cheers
DD

 

Here it is, Doug. Just below dead center in the pic, at 6:00 under the enrichment switch.

Is this a disconnected vacuum fitting?

jagvac.jpg?t=1262405623

 

Sorry, Lou, it's not ringing any bells with me. Hopefully someone who still owns an XJS will take a peek at theirs and chime in

Cheers
DD

 

Thanks Doug,

I looked at it more closely today under better light. It's a wire
connector, not a vacuum fitting.

Got some extra-long-nosed pliers on it, and it's definately connected to something near that rear sparkplug.

Fortunately, I didn't break it.

The search for the cause of the "sinusoidal idle" continues....

 

Are your throttle blades set at the correct .002" gap and the linakges synchronized? Can't hurt to check-n-correct this just on GPs. Clean the throttle bodies, too, while you're in there.

Cheers
DD

 

I presume that the 0.002" applies to clearance of butterfly valve to throat at idle? (My Merc outboards spec this adjustment the same way, only at 0.005") I suppose when meeting this spec, the ECU does the rest of the work for the idle speed?

And...how do you synchronize the linkages? Slacken/tighten just doing a visual, or is there a more measurement-based approach?

I've never done either one of these!!

Thanks,


Lou

 

Ok, throttle linkages synced, proper 0.002" gaps, no vacuum leaks, adjusted idle down to where it belongs (750), cold start idle working great.

Surge is a little better and gas mileage has gone up by 3mpg! A tremendous improvement.

But..still the slight pulling and surging at 1500-2000 rpm; it's like the engine speed is wandering. This does not occur at all at increased engine speeds and at partial or full WOT nonexistent.

A local suggests the next thing to replace is the TPS potentiometer. Theory being that it is worn at those areas in it's range that have seen the most use--1500-2000 rpm.

Reasonable course? Or something else first, as this part isn't cheap.

Thanks.

 

Have you recently checked all your spark plugs, it could be that the engine is trying to compensate for a bad cylinder or two, out of your twelve.

Intake manifold leak?? Engine running spray WD40 at the joint, if you notice a change in the engine then you have a leak.

 

Thanks Doug. I have this very issue. Idle pulses about 50 rpm at idle...not like the problem above when running at speed.
I've adjusted the butterflys; .002", set the TPS to .035 VDC at idle; shows linear progression from there, cleaned the throttle bores etc, set the AAV; which seems to be working fine, pulled and capped vacuum tubes one at a time but could not find a vac leak; all to no avail...still pulses at idle. My gas mileage is around 12-14 in town...sucks. (And it runs hot; up to 210F per mechanical gauge with sender in old Smog pump thermal vac switch location...more on this at another time). So MAYBE it is the O2 sensors; doubt they have ever been replaced, 70k miles on her. So if the O2 sensors aren't functioning maybe shes running too lean causing the higher running temps? But should go Open Loop and richer settings if O2s INOP?

 

I can add that the idle pulsing etc, and even momentary drop in idle upon acceleration has been O2 related every time on my car, the later ECUs just need that input or it doesn't run quite as nicely.

First time the shop properly fixed this it felt like the car was remapped for Sport mode. You can buy new ones for a decent price (dont cheap out on the no name brands) or you can soak the ends in gas for a day or so and scrub them clean to break off the soot.

 

Wow, old thread!

As it so happens my present day V12 Jag developed that oscillating/rolling idle a few months ago. It was just the same as years and years ago on my previous V12: a slight but constant 50 rpm up/down cycling of idle speed. The tach really wouldn't so it, but the oil pressure gauge did....and I certainly heard and felt it.

Also, as it so happens again, I recently installed a non-feedback ECU. That is, one that does not have the circuitry to see or use the oxygen sensor readings. The 50 rpm cycling disappeared.


Cheers
DD

 

VancouverXJ6 I wasn't aware you can clean up an old O2 sensor. I've always read not to scrub them..but it certainly is worth a try. If successful...Great. If not then which brand do you recommend?...Bosch I would guess...haven't priced them yet, probably $100 each ball park. Mine is a 90MY, what is yours? Doug, why wouldn't you want an ECU that adjusts A/F ratio automatically? Are you running with an A/F gauge and the ability to manually adjust it from the cabin?

 

Gas or Diesel is a great safe solvent for carbon and grease. Soak and toothbrush seems to work, I would suggest Bosch personally, some things cant be trusted to AC Delco or URO lol...

 

That's about as good as it gets for around-town driving


I don't think it's a matter of the O2 sensors not functioning/being inoperative. I think it's a matter of the sensors being flakey; erratic.

Or.....

The sensors are struggling to compensate for some other fault, trimming the mixture back-n-forth to the maximum extent they can.

I replaced mine on a whim after trying everything else I could think of at the time.

Cheers
DD

 

All the ECUs....feedback type or non-feedback type..... automatically adjust A/F ratio in accordance with the inputs: the pressure sensor, TPS, air and coolant temp sensors, etc. The O2 sensors are an additional input.

It can be reasonably argued that, with everything up-to-snuff, the feedback system provides more precise A/F trimming.

Cheers
DD

 

Odd that the font changed in my last posting!

Cheers
DD