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-   -   Lack of engine braking - Is a throttle body controller the remedy? (https://www.team-bhp.com/forum/technical-stuff/239055-lack-engine-braking-throttle-body-controller-remedy.html)

I have visited this thread but was unable to find an answer -> https://www.team-bhp.com/forum/techn...nnova-d4d.html
Hence the new thread.

Up until recently this year, I was used to driving a diesel engine powered car (Tavera's 4JA1-L to be precise) and hence enjoyed engine braking. When I had to go down a ramp, I would simply slot in 1st or 2nd depending on the ramp length, with the only input being the steering. The car would glide down in a controlled speed on it's own.

But now that I own a diesel car with a throttle by wire system, things aren't as sweet. The engine in question is the from the same manufacturer - Isuzu 4JK1-TCX. Even in 1st gear, the tachometer doesn't seem to stop rising and eventually having to use the brakes.


Question: Is anyone aware if a plug n play throttle body controller can help here? or Are the gear ratios to blame(The D'Max seems to have a poorly positioned 1st gear)?


Can anyone point me as to how to get back engine braking into my life?

In my opinion, the engine braking aspect of a vehicle, depends upon the reciprocating masses i.e. the weight of the Pistons, Crank, Gearing etc.

At low speeds, to control speed, Best solution is to apply brakes and control the speed. At low speeds(since you mentioned 1st gear, I am assuming it is low speed),it is very easy to control speed through brakes and with flooring full clutch. 90% of the drivers use this method to control speed at low speeds while on ramp downwards.

Engine braking is more efficient and effective only at high speed (5th or 6th gear) for manual transmission cars.

Additional accessory just to control speed is overkill and may add more complexities, in my opinion.

I am not much of a car technical man but IMO, as long as the crankshaft is directly connected to the power train, you must have engine braking while in gear. However, how much depends on gear ratio of the 1st or 2nd gear and not all vehicles have the same ratio. This applies to FWD vehicles. 4x4 may be different?.

Throttle has nothing much to do in events of coasting as you described. Of course, a lot also depends on the weight of the vehicle too.

My throttle body is controlled by wire. They call it e-gas. There is a sensor at the pedal and this sends signals to the throttle body. This has nothing to do with your engine speeds while driving downhill.

Nothing wrong with your car. You dont drive down steep hills on first gear. It is more logical to drive downhill on second gear and using brakes as needed.

Your revs going up just shows that your speed is too high when you start the descent. Be a little easier on your accelerator and you should be fine.

For braking you need some sort of resistance. The engine might have reciprocating parts but it is designed to operate with as less friction as possible. So when the engine is being rotated by the drive train when decelerating, the mass itself would offer little resistance. It is the vacuum created in the intake manifold that provides some sort of resistance since the engine would be somewhat choking due to lack of air. That is why petrol engines offer very good engine braking and diesels with a throttle also provide the same. Throttle body in diesel engines have been introduced due to emission regulations lately and this is closed during deceleration and when the engine is shut off to prevent runaway.

Yes , If this is a relatively new car then nothing to fault the engine for.

Engine will have engine braking - Inlet air throttle or VGT will be present, along with ECU stopping fuel when there is no load means vehicle will have good engine braking - BUT - the gear ratios have to be proper.

Imagine two vehicles with CR diesel engines with VGT, one goes down a trail with no feet on the pedals (1st gear foot off) and another needs constant application of the brake pedal - the answer is gearing usually.

The latter engine seems to have a lower compression ratio, which is the main deciding factor (apart from gearing) in engine braking. Hence the lower engine braking.

I dont think a Throttle Body Controller will help, as without any Throttle input, the ECU sees it at 0degrees anyway. Someone who reads maps inside the ECU will be able to tell you weather fueling has a role to play when coasting. If not, there's nothing much you can do.

The old 540's with Peguot engines used to have excellent engine braking, not so with the newer Thar. CR played a major role there.

I think this sums up my situation almost perfectly. From this statement, I would say the 1st gear of the D'Max (which is possibly more suited to the 3.0L engine sold in South Africa & Australia) is a probable cause of this.


Intriguing point Dhanush. The Tavera's 4JA1-L has a compression ratio of 18.5:1. Whereas the D'Max's 4JK1-TCX has a compression ratio of 18.1:1. This makes sense as the latter would pull less of a vacuum compared to the former.
I didn't know that a difference of 0.4 could have such a big impact (along with the gearing in this case).

I read somewhere, the Indian version has a compression ratio of 16.xx hence the comment.

I got the values from Wikipedia -> https://en.wikipedia.org/wiki/List_of_Isuzu_engines

If the Indian version is indeed 16.xx, then what I have faced practically makes even more sense.

An ECU tune ought to help figure out if there's any more engine braking to be gained.

Perhaps the valves that let in air are kept lifted when you get off the throttle so as to give the illusion of a "smooth" drive. Perhaps the air fuel ratio is kept rich when you get off the throttle. Many modern cars raise the idle rpms when in gear as compared to when in neutral this could also cause a variance in engine braking levels. Could be something to do with the exhaust gas recirculator and how aggressively it functions.

There could be several reasons for lack of engine braking and I feel you ought to be able to get what you want with a good ECU tune. Contact the folks at Tune-O-Tronics. They ought to be able to help you out.

Engine braking depends on the cubic capacity, compression ratio, gearing and weight of the vehicle.
Since we’re comparing two turbo diesels of similar capacity here, we can move on to -
Compression ratio 18.5(Tavera) Vs 16.5(guesstimate for Dmax as I could only find figures for the new 1.9L)
Gearing- 4.12 with a 3.909 final drive with the Tavera=16.1 gearing.
4.942 with a 3.727 final drive = 18.41 gearing
Weight-Tavera kerb wt not given so derived from gross wt of 2550 - (10pax X 72)
- 44kg fuel= 1786kg.
No idea how much allowance given for luggage so calculating for the 10pax version.
Dmax-2059kg

Other factors-
Internal friction reduction in the Dmax due to a more modern driveline is somewhat negated by substantially larger wheels and tires.
The Dmax even has a lower idle speed then the Tavera!

So it’s the lower weight and higher compression ratio of the Tavera Vs the higher gearing of the Dmax.
I’ve got a feeling I’m back to square one!:Frustrati

Till I thought of another factor- Emissions.

Stricter norms in force for the Dmax. Possibly the ECU won’t let the catalytic converter drop too much out of its temperature range so injects a little more diesel in the overrun to maintain it leading to reduced engine braking.

I think emissions is a strong factor along with the weirdly positioned 1st gear.

I believe the compression ratio of 2.5L Isuzu engine in the D'Max can be found on Wikipedia. It would be nice to see what kind of numbers you get if use 18.1:1 for the D'Max