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AJ16 revs not wanting to drop easily on closed throttle
So, with the first full tank that I've put in my AJ16 convertible for 18 months (Disgraceful, I know!!), I drove off to the beach today on the hottest day in the UK this year. A refreshing(!) swim in water that hasn't warmed up yet this year, and then back to watch England win at football this afternoon! So a great day? Well, almost...
The car ran really well, as usual, apart from one thing which I've been noticing for some time. The throttle doesn't seem to close off immediately I take my foot off the pedal. It seems to almost hold the revs and only very gradually drop them over quite a long distance of freewheeling. The same seems to happen if I coast down a steep hill without pressing the accelerator. The increased speed seems to open the throttle and only when the incline finishes does it gradually start to close the throttle. If I flick the car into neutral when the problem happens, the revs will drop off to normal idle speed but not with the immediate urgency you'd expect.
It's almost as if the throttle return spring is very weak and almost (but not quite) able to keep the throttle open with road speed. I know that AJ16 XJSs had a recall on this issue when new and a revised throttle spring was fitted. I've even read of specialists who have put an extra turn in the spring to give it more tension and a quicker close response.
Of course, it might not be the spring, it might be something like the butterfly plate not wanting to close easily or the TPS getting confused?
So, before I start pulling the throttle parts apart to have a look, I wondered if anyone had had a similar symptom on an AJ16 engine and what was the cause? Without wishing to be rude, please don't tell me about similar symptoms on a V12. The throttle capstan, TPS, MAF etc are so different on a V12 that I feel it won't really help me.
I’d certainly attempt to clean the butterfly before replacing the spring. Cleaning it will only cost you no more than two hours of your time and, at worst, a can of carb cleaner.
I’d certainly attempt to clean the butterfly before replacing the spring. Cleaning it will only cost you no more than two hours of your time and, at worst, a can of carb cleaner.
Somewhere in the Jaguar literature, can’t recall where, it indicates that the AJ16 oil filters should always be changed from the top. This forces you to remove the air meter and elbow hose to throttle body and allows you wipe any excess oil regularly, thus preventing any type of build up to ever happen or oil dripping to the TPS.
Tks, It's a good discipline, even though you definitely don't have to remove the elbow etc to change the oil filter. I always change mine from the top (including last month!) and it's quite accessible in situ.
However, I like that idea and will probably do that annually now.
Interesting, I guess that makes sense...however, as ptjs1 says, you definitely don't need to disconnect it to have easy access to the oil filter. I've added an oil catch can, and it seems to have helped. I still see some evidence of oil dripping into the intake elbow, but I believe my butterfly is still clean.
I wonder of wiping the throttle body from the outside, while still connected to the car would really help?
Oh yes, it’s definitely do-able. Without removing the Elbow, But I can’t recall where, but the recommendation somewhere is that all this gets undone and wiped at the time the oil filter is swapped. Maybe a TSB? But I definitely have seen that recommendation. I can confirm that I do it once a year or sometimes maybe 18 months, as my car is not a daily, and there is ALWAYS oil residue that has “breathed” in that direction just from weekend driving ….Over time, this accumulates, gums up the butterfly, and if build up enough, will drip down to the TPS.
Rather frustratingly, the investigation continues!
I popped over to the car yesterday and started by checking free movement on the capstan. All turns smoothly, the capstan seems to return acceptably under spring pressure. I then checked the movement of the accelerator cable and that the capstan returned when the pedal was released. Again, all seemed ok.
So, I removed the elbow and rubber bellows collar to the throttle body (3 x 6mm nuts holding clamps & a 10mm nut holding the elbow to inner wing) and then examined the collar and inside of the throttle body expecting to see a lot of oil and grime buildup.
Except I didn't! The base of the throttle body and butterfly were nigh-on spotless. And the bellows collar had the merest misting of oil in the bottom folds.
A quick clean and I put it all back together and reflected. A colleague told me he had a similar symptom, except that his idle was always high and was only eventually cured through a reset of the adaptations.
I don't have a high idle at rest. I just have a throttle that feels as if its sluggish to close when the pedal is released. Anyway, I'll take it out again when it's stopped raining and see if the minor cleaning has improved anything.
All I can think of, after reading what you have checked etc, is the "Idle Stepper Motor" is gummed up. They were a PITA on the AJ6, and JLM12185 is the Part Number I have found.
The daughters 3.6 was sticky, I had no time back then, so she took it to the Indy. He removed, cleaned the gunk, reset it, all done, about 1 hour from memory.
Both my AJ16 (X300) cars had them, but did not play up, so I never messed with them.
Nothing else rings bells, apart from ALL that you have covered, but her 3.6 had exactly what you describe.
I have 4 or 5 old throttle assys from the AJ16 that were discarded on the shop floor when the tiny bolts were broken trying to disassemble and clean the throttle body.
I learned to use a butane 'micro-torch' to loosen the thread locking compound before trying to remove those fasteners.
Some of the throttles I was able to get the screws out and recondition the assys. They were later sold on eBay as cleaned throttles.
Even though the throttle spring will close the throttle plate with the engine OFF, the air load on the plate seems to 'hang' the throttle open at times with the engine idling.
The slightly stronger spring mentioned in the TSB will help if the plate and bore are clean and free from deposits.
Tks very much for that, both the suggestion about using heat to free the loctite, and also the suggestion about the air load hanging the throttle. That's a really interesting point!
I discovered the air across the plate when I would check the throttle ENGINE OFF and it would slam shut just fine.
Start the engine and the plate seems to want to stay partway open as if it is 'hanging' a little bit.
Jab the throttle and the plate falls closed and the the engine idles normally at times.
After finding so many horror stories about the two small bolts holding the stepper motor shearing off when you try to remove them, I decided to leave that for now. Instead I decided I'd change the return spring as per the original Jaguar TSB. As you may know, there are 3 springs - 2 springs as part of the pedestal (1 above and 1 under the mount plate) and then the throttle plate return spring mounted on the throttle body below the pedestal. The 2 springs on the pedestal aren't identified and sold separately. So I purchased the spring referenced in the TSB and set to to dismantle things.
First disconnect the throttle cable (easy)
Then remove the 3 pedestal retaining screws ((tight but easy)
The pedestal can then be easily removed.
Then of course, I had to succumb to the usual Jaguar ownership obsession and decided I would have to polish the alloy pedestal! So, there was a 24-hour gap!
I then removed the throttle spring, cleaned up the area and carefully reinstalled the new spring, noting the instruction to pre-tension it one full turn anti-clockwise.
For good measure, I also then added an additional pre-tension turn to the lower of the 2 pedestal springs.
I then reinserted the pedestal, connected the accelerator cable and stood back and admired my work, came home and had a beer!
I'm now just about to go back to the car, start it up and see if it's made any difference whatsoever! Fingers crossed!
Well, my plans slightly changed! I didn't take the car out yesterday. I decided to go for broke and just do a 280 mile return trip to the national Jaguar Summer festival today and I've just got home. 1500 Jaguars in one place! A great day out, with many significant cars on display, including Tom Walkinshaws XJS racer. But more importantly, how did my car run?!
Annoyingly, I don't think it's completely cured it! It's definitely a bit better, but I'm not sure it's got that instantaneous throttle close effect that I'm expecting. Or maybe I'm so confused, I've forgotten what normal feels like! I tried flicking it into Neutral when doing 70mph and dropping the throttle. The revs drop from about 2400rpm to about 700rpm but take about 2-3 seconds. I don't know if that's about right or whether it should drop quicker? Any comparisons from other AJ16 owners very welcome please!
I spoke to a few of the Jaguar specialists that I know who were at the show. Even though my car can idle acceptably at 650-700rpm, most suggested that I take the stepper motor out (recognising that those 2 little bolts are apparently very likely to shear!) and also suggested that I might need to get the adaptations reset using a proper diagnostics tool.
The good news is that, according to the trip computer (which has had an average optimistic variance of 2.63% over 19 years) the car delivered 31.1mpg over the 280 miles.(UK figures). That will be the best consumption that the car has ever delivered, and included motorway hold-ups, torrential rain and the roof down whenever it wasn't raining. So, even though the throttle MIGHT not be closing properly, it's clearly performing ok!
So, I now have to decide if I pull that stepper motor (with the potentially inevitable bolt drilling!). Hmmm...
06-13-2021, 02:13 PM
AJ16 revs not wanting to drop easily on closed throttle
