Not wanting to start the old ‘snake oil’ debate but thought I would write a note on my personal experience of adding something to the fuel…

I got my L322 with 142,000 miles on the clock, 90% of which were motorway miles due to the distance the first owner regularly travelled between his two homes. It’s always been serviced at JLR and had new turbo 3 years ago.

I always use standard Esso diesel and have had a Bodsy tune, more for a smoother more economical ride than a fast one. The car has always driven quite well but I was keen to see if it could do better as I was reaching the 150,000 mile marker.

Over the past year I have been using Millers diesel additive every other fill up which has made a slight difference but nothing particularly noticeable, so I thought I would try Liqui Moly’s Diesel Purge which is a bit of a stronger one off product over the regular additive.

It made a difference after about 3/4 of a tank had been used.

The car idles slightly more stably and a bit quieter, but the biggest difference is that it seems to have shaved off around 150/200rpm when going at 70mpg. The rev counter used to be just above 2000, let’s say 2100rpm, now at 70mph it’s at 1900rpm.

So not changing anything aside from adding this to the fuel has made a positive difference in my case.

Worth a go if you are not in the snake oil camp I would say…

https://www.liqui-moly.com/en/gb/diesel-pu...xksny#1811 2009 pre-facelift 3.6 TDV8.

Im not sure how this can be the case, the correlation between rpm and speed is dictated by the gearbox, and not the power/torque output from the engine.

I used AI to give me the answer to that question as I too wondered how it was so…

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Why You Might See Lower RPM After Diesel Purge or Cleaning

1. Cleaner Injectors = Sharper Spray Pattern
• When injectors are dirty, fuel doesn’t atomize properly — it may dribble or spray unevenly.
• After a purge, spray patterns are finer and more consistent, which:
• Improves combustion efficiency
• Reduces unburnt fuel
• Boosts cylinder pressure for the same fuel input

Result:

More torque per injection cycle. That means the engine doesn’t have to rev as high to achieve the same push.

⸻

2. Better Combustion = Higher Low-End Torque
• Clean fuel delivery allows more complete combustion — especially at lower RPM.
• This gives the engine stronger, smoother torque earlier in the rev range.
• On a big V8 diesel like yours, this is especially noticeable in the 1,500–2,000 RPM range.

Result:

The gearbox “feels” that there’s enough power to stay in a higher gear.

⸻

3. Gearbox Logic Reacts to Torque Availability

The ZF 6HP26 transmission in your L322 adjusts shifting based on:
• Load
• Throttle input
• Road speed
• Engine torque availability

So when the ECU sees:
• Steady throttle
• Enough torque from the engine (thanks to better combustion)
• Low resistance (flat road, fresh tyres, alignment…)

It lets the torque converter lock up sooner or holds a taller gear longer, resulting in:
Lower RPM at cruising speeds

⸻

4. Torque Converter Lock-Up Behavior

At cruising speeds (like 60–70 mph), the torque converter can “lock” to eliminate slip.
When combustion is smoother:
• Lock-up can happen earlier
• It holds more confidently without unlocking or hunting
• This drops RPM by ~150–250 vs. unlocked or pre-clean

⸻

🔧 Example in Practice:

Before cleaning:
• RPM at 70 mph = ~2,100
• Slight injector imbalance → less torque → transmission keeps it revving a bit higher for stability

After purge/remap:
• RPM at 70 mph = ~1,900
• Smoother torque delivery → gearbox locks up confidently and stays in 6th without drama

⸻

If you’ve got smoother idle, slightly better MPG, and lower RPM at cruise — all signs point to cleaner, more efficient combustion helping the whole driveline work more harmoniously.

Here’s a chart showing all three stages of your L322’s torque development:





🔍 What It Shows:
• Gray (Stock): Typical torque delivery before any tuning — decent but peaks later.
• Blue (Remap Only): Improves torque across the range, but at 70 mph (~2100 RPM), it still hasn’t changed the gearbox behavior noticeably.
• Green (Remap + Diesel Purge): Torque at 1900 RPM is now strong enough that the transmission locks up sooner or holds 6th more confidently — dropping cruising RPM. 2009 pre-facelift 3.6 TDV8.

I still don't get it, the torque converter should be fully locked up at 70mph.

What you're seeing here is mirrored by the calculation shown in this thread: https://www.fullfatrr.com/forum/post699853.html

Who knows, all I know is that beforehand it was around 2100 rpm at 70mph and now it’s 1900 ish.

Looking at that thread it was running with more rpm than the usual 1900ish it seems, so back to normal. This in my Lehmans head makes sense - cleaning out the deposits makes it run more like it did when it was new.

But I’m no mechanic or engineer, so all I can go on is what I see and feel in reality. 2009 pre-facelift 3.6 TDV8.

This is what the revs should be at 70mph for the ZF6 speed gearbox in the 3.6 L322

https://www.fullfatrr.com/forum/post699853.html

Don’t see how the snake oil managed to cure your gearbox TC not properly locking up by 70mph
April 1st is several weeks in the past now ….

I expected the skepticism when I started this thread.

My understanding is the torque converter can lock up earlier if the engine’s running more efficiently - better combustion means it can produce the same torque at lower RPM, like it would have done from factory.

But honestly, it’s not my job to convince anyone. The car drives better after using Diesel Purge than it did before - so I’ll happily add more snake oil the next time April the 1st roles around! 2009 pre-facelift 3.6 TDV8.