Hi guys,

Trying to find a definitive answer regarding a smooth idle in Park.

Ive never really had a “very" smooth idle in Park since my 12 plus year of ownership (1996 Jaguar XJR).

When I first start my car from cold the idle is perfect for the first 15 seconds or so and then progressively gets more lumpy as the IACV starts closing.

Its a little lumpy when in Park but very smooth when in Drive.

Ive read many posts that reckon its due to the Injectors "pulse width" difference from when in (P) versus (D). And that it's just the nature of the beast with higher flow rate injectors at a lower pulse width.

Had my injectors cleaned before when chasing down a intermittent start & stall issue which then turned out to be a faulty IACV / Stepper Motor.

Couldn't really see any problems with the injectors when tested for spray pattern & volume (cc/min)?

2 of the 6 injectors were ever so slightly less in volumetric output though - not sure if that will be the problem. Havent yet tested it at perfect idle conditions (650 rpms / Park). Could be that one or more are not in spec when at the lower rpms & load?

My idle is now;

• 650 rpms in Park
• 580 rpms in Drive (which seems spot on).

So I guess my question is am I just chasing my own tail and or is there anyone that had / have a perfect idle in (P) combined with 630 - 650 rpms in Park? Is it even achievable?

I shouldn't have any vacuum leaks as Ive done a pressurised smoke test recently after cleaning my TB and replacing the faulty IACV. Also done a fuel trim reset / flash via IDS / VCM (TPS & O2 adaptations) afterwards as well as replaced my O2 sensors (NGK) recently.

Any feedback would be greatly appreciated.

Regards.

 

When was the last time you replaced your IACV? Did you replace the gasket? Was the o-ring present? (They are typically not)

Have you considered cleaning out the EGR? Could be that too.

I wouldn't worry too much about it, but those are the two parts that I would look at. After that it's kind of a rabbit hole.

 

Quote:

Couldn't really see any problems with the injectors when tested for spray pattern & volume (cc/min)?

I had the very same problem. I sent my injectors to https://www.jaguarfuelinjectorservice.com/

After cleaning, all the injectors were technically within spec but two were right on the cusp while four were solidly in the middle. David sourced two replacements for me. I recall that it [at least] improved the idle. Perhaps it fixed it. It was quite a number of years ago and my memory ain't what it used to be. I might have posted the results here but you'd have to go back at least 10-12 years in the archives.

Cheers
DD

 

Vee:

Thanks.

Yes I have installed the IACV with O ring & Gasket

Have read before that you had an issue with this and affected your idle .., my idle (in Park only) had the same issue before and after the IACV replacement.

Luckily no EGR down here - one less (potential) problem ..,

Dont know if the Normally Aspirated AJ16’s will have the same problem(s) with regards to the idle (Park vs Drive) injector pulse width as the Supercharged Injectors are almost double in volume / flow rate. Hence might be more finicky at lower pulse widths?

AJ16 Naturally Aspirated: D3156KA - 220 cc/min @ 43.5 psi

AJ16 Supercharged: D3165KA 380 / 388 cc/min @ 43.5 psi


Doug:

Thanks have read your previous posts on this topic .., think you mentioned that it did improve it although just temporarily - if memory serves me correctly.

I probably will have to pull mine again to get properly tested under various rpm’s - especially around the 680 rpm’s and see if there are any discrepancies just to make 100% sure!

Have been in contact with David and he also referred to the notion of the higher flow rate ( > 380 cc/min) and that that could lead to a “rougher” idle at lower pulse width’s as is the case with the idle in P.

Like Ive mentioned before Ive never had the same smooth idle in both P & D and would just like to find out if its even possible? My idle isn't terrible but would like to get it as smooth as possible if possible :-)

Regards.

 

I’ve had various Jags as daily drivers for 25 years now. All of them had idle quality issues of one sort or another.

Some guys persevere longer than others. Sometimes with success. Other times not.

Personally, when it comes to seeking a consistently smooth idle, I threw in the towel years ago.

Cheers
DD

 

Well i have had issues for a while.
tried this, tried that, nothing worked.
had been told to get a throttle reset done. Did not have the ability to do it, noone with equipment.
finally got it done and she is purring like a sweet little kitty now.
My iacv was installed previously with a gasket and a metal spring washer, currently it is installed with a o-ring and no gasket as the old gasket was ****.
i may replace it but the o-ring keeps it sealed well.

where are you located?

 

Thanks Doug,


Will probably roll the dice once more with doing some more in-depth injectors testing .., see what happens?

If nothing I will live with it as is ..,

My idle in P, N & D are basically spot on with accordance to the Jaguar Repair Manual.

Interested to know if the theory of the higher flow rate's re the injectors from the Supercharged AJ16’s vs the Naturally Aspirated AJ16 cars will have a noticeable difference at Idle in Park (no load)? meaning that the Supercharged versions are more prone for the slightly lumpy idle when in P but smooths out when in D?


Spud Maat,

Thanks for the feedback.

See you from Sydney so even further down south than me .., I am from Cape Town (South Africa).

Yes I luckily got the old IDS 118.5 & VCM1 diagnostics so could do the IACV & O2 adaptations myself.

Unfortunately my idle still the same as before but what made quite a difference was that my LTFT was sort of “stuck” for ages - before doing the adaptations. After the adaptations were done the LTFT changed and actually started making sense again .., somehow my ECU got locked with regards to Long Term Fuel Trims (LTFT). I will now probably do it every year or two and or whenever working on the TPS / Throttle Body.

Glad to hear all good with your car (idle).

 

Have you tried smoke testing the exhaust manifolds for leaks yet?

Larry Louton

 

they smooth out in drive. if rpms aren’t surging there’s nothing more to it…

 

Fuel injector flow rates :

AJ16 Naturally Aspirated: D3156KA - 220 cc/min @ 43.5 psi / color band X

AJ16 Supercharged: D3165KA 380 / 388 cc/min @ 43.5 psi / color band Y

that is a huge difference that makes idle pulse width that much narrower and subject to injector drag error

But injector drag may slow the opening and not be at open when suddenly losing the pulse , this would imduse low idle

Probably a underfueling misfire not on all cylinders

 

Thanks Guys,
Thanks guys.

Larry:

Havent tried a smoke test on the exhaust side per se but have had headers made & installed by a well known exhaust racing shop and twice tested by exhaust specialists shops. Both times no leaks. Also cant hear any with hose to ear test?

Recently done a thorough pressurised smoke test (inlet side) and showed up with some vacuum leaks .., actually had one big leak (couldn't immediately identify) and one small (where reinforced inlet hose connects to TB).

I first picked up the leak(s) through my entry level Autel Scanner by observing the live Fuel Trims suddenly changing when on a “spirited” test drive to see if all still good under the hood.

Fuel Trims (STFT & LTFT) were all of a sudden changing .., LTFT went from (0) to (- 9.4) and STFT went way negative (-3) to (-23) when under boost but settled down to around 0 when at idle.

Just recently replaced IACV, thoroughly cleaned TB, tightened return springs and new coolant hoses etc - all good until the spirited “Sport” drive.

The pressurised smoke test showed a massive leak around the underside of the Supercharger & TB area .., turned out to be at the bottom end of the smaller Crankcase Ventilation Hose where it attaches to the nipple of the SC Bypass Valve Body (see pictures).

My original Engine Breather Hose still looked good so I didn't see a reason to replace initially .., but then developed a major tear at the nipple end and had to be disassembled & replaced for the second time ;-(

I do see that Al_roethlisberger covers this topic in great detail in previous posts and might be worth reading.

https://www.jaguarforums.com/forum/x...essary-180030/ - "Is crankcase ventilation hose to throttle body necessary?"

https://www.jaguarforums.com/forum/x...oes-go-185405/ "XJR6 - Crankcase ventilation hose - where does it go?”

Attached 2 photos of a where the smaller Breather Hose must go from the Cam Cover to the SC Bypass Valve Body.

Please note the second picture is wrong. See the RED ARROWS for correct position (not my own pictures / taken by another forum member).

Apparently some members have found disconnecting (or even plugging) the smaller Breather Hose connected to the SC Bypass Valve to have no significant effect on drivability etc.? also installing aftermarket Oil Catch Cans.

As for my experience I only really noticed it with my Short Term Fuel Trims (STFT) going negative under boost and LTFT going and then staying fixed at negative - Believing me to think I am loosing performance BOOST / pressure / vacuum escaping when under boost. I stand to be CORRECTED if wrong and that the Engine Breather Hose has nothing to do with my Fuel Trims when under boost / forced induction .., please do enlighten me if wrong!?

Refitted a new Breather Hose and Fuel Trims definitely improved. LTFT almost at 0 again. Maf readings from 7/8 g/s to 5 g/s (at idle and operating temps). The difference in Maf readings before and after might be due to the small vacuum leak in front of the TB thats now also been rectified.

Idle might have ever so slightly improved? could be related or not? ..,

All I know is that it's not a 5 min job if needs doing again (hoses are not that readily available for the XJR’s and very pricey if not in the UK & USA).

Could maybe even cap (reinforce) the small hose end that goes over the nipple (attached to the Bypass Valve Body) for extra protection as Al_roethlisberger suggested. Unfortunately, I havent due to only reading the related thread(s) afterwards. I just renewed the hose.

The funny thing though is that with all my trials & tribulations (vacuum leaks fixed, faulty parts & sensors changed, tune ups done, IDS Adaptations & Orientations etc, etc.) my idle in Park have never really been that smooth even before, during or after the afore mentioned as in the case with Idle in Drive thats always been relatively smooth .., granted my rpms @ idle are on factory specs now, no more surging, hanging or high idle etc., as with Fuel Trims and general running (power & drivability) also being much improved.


Xalty,

Noted your feedback and will keep in mind.

Does correlate with Doug and some other members ..,

Still considering a last "in depth" injector testing.


Parker7,

The injector drag & under - fuelling misfire sounds plausible ..,

Regards.