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Heavy Isuzu 4x4s now come with a choice of engines

The Australian 4WD truck market leader has extended the appeal of its 8-9-tonnes-payload capacity 4×4 truck models with the addition of two new diesel engine variants. A new, powerful four complements a variant of the proved six that’s purpose-designed for off-road operation.

Isuzu Trucks Australia gave its market-leading F Series mechanical and cosmetic changes in 2016. 

What was immediately apparent was a refreshed look for the F Series, with a new grille design, badging and nomenclature, as well as updated seat trim and steering wheel design. 

The mechanical changes were headed by a new four-cylinder, turbocharged diesel and a significantly revised six-cylinder engine, aimed mainly at vocational applications. 

The 5.2-litre four-cylinder retained the same bore and stroke dimensions as its predecessor, but the 2016 4HK1-TC engine had new cylinder head, camshaft, block, main-bearing ladder frame, combustion zone components, higher-pressure injection system and injectors, and a two-stage turbocharger.

Two-stage turbocharging improved torque at low engine speeds, to the extent that the 240hp four had more torque than the six formerly used in 12-tonnes-GVM F Series trucks.

The 4HK1-TC had two output levels: 154kW (210hp) and 726Nm, and 177kW (240hp) and 765Nm.

Increased performance from this engine allowed Isuzu to increase its application in the F Series, so the 4HK1-TC powered  FSS 4×4, 11-tonnes-GVM models.

The 13.9-tonnes-GVM FTS  retained its six, but with significant differences.


Task-specific emissions strategy

On the 4HK1-TC engine cooled exhaust gas recirculation (EGR) and diesel particulate ‘diffusion’ – filtration – (DPD) were used for emissions control to Japanese ‘post new long term’ (PNLT) standards that are expected to be accepted as part of ADR80/04 (Euro VI) when it’s eventually adopted in Australia.

Unlike most other engine makers Isuzu was confident it wouldn’t need selective catalytic reduction (SCR) with urea (AdBlue) injection on the 4HK1-TC to meet ADR80/04 requirements.

Part of this emissions package was a DPD unit with its own ‘regeneration’ injector. This a departure from existing Isuzu practice that sometimes used post-combustion injection in each engine cylinder to increase exhaust gas temperature, to burn off soot and particulate matter from the DPD.

On-highway trucks normally don’t require post-combustion injection, because exhaust gas temperatures are high enough to clean the DPD continuously, but slow-running 4WD truck engines sometimes need to be actively regenerated.

Active regeneration requires the driver to park the vehicle and let the engine’s electronic control unit run through a regeneration cycle.

Another issue with post-combustion injection is the possibility that some unburnt fuel can find its way past the piston rings and into the engine sump. Part of Isuzu’s existing DPD-equipped engine maintenance is regular checking for a rising dipstick oil level.

Although the 4HK1-TC retained a DPD unit it  had its own regeneration injector, so post-combustion injection was carried out in the DPD, not in the engine cylinders. Sump oil fuel dilution from DPD regeneration was no longer a major concern, but active regeneration was needed in some low-speed and stop-start truck applications.

Two-stage turbocharging and improved DPD should make the new 4HK1-TC a very popular engine, but there are some 4WD applications where even this combination won’t be optimum.

Isuzu was hoping that a revised version of the proved six-cylinder diesel 6HK1 would be the answer for these vocations and this was the new powerplant for the FTS, introduced in 2016. This development used new injectors and combustion zone components, had no DPD and, hence, no need for DPD regeneration.

The 6HK1-TCC 191kW (260hp)/761Nm Euro V engine met current emissions regulations by using cooled EGR in conjunction with a diesel oxidation catalyst (DOC). It was the only medium-duty diesel engine in the market not to have DPD or SCR, but as Isuzu pointed out, it was not a Euro VI engine like the 4HK1-TC.

Transmisson offerings remained similar: six-speed manual with two-speed transfer box (2.09:1 reduction) in the FSS and either a six-speed manual or Allison LCT2500 automatic – now a six-speed – combined with a two-speed transfer box (1.91:1 reduction) in the FTS.

Sadly, the Non-DPD Euro V engine was discontinued in 2018 and, like all Isuzu trucks, the 4×4 range had DPD across all models.


Other changes in a nutshell

The 4HK1-TC-powered FSS had a ‘start assist’ function, to allow easier, faster clutch engagement.

A revised multi-information display, with a larger screen and incorporating satellite navigation, was standard across the entire F Series range. Off-road models got the big screen but not navigation, which seemed odd for vehicles that are likely to get lost!

Given that daily checks aren’t always done daily, the addition of a low-coolant-level sensor to all models was timely.

A fuel-line cooler was introduced with the new 4HK1-TC and 6HK1 DOC engines.

An upgraded anti-lock braking system (ABS) module and ABS 8 software was fitted to FSS and FTS models.

The rear seats in crew-cab models were given additional padding in the seat bases and realignment of the back rests, for greater support.

A much-needed passenger ISRI 6860 air suspension seat was standard in the FTS 4×4 single and crew cab models.

A 90-amp alternator was fitted to all new F Series models.

There’s an extension of oil change intervals from 15,000 km to 20,000 km for those models powered by the new 4HK1-TC or 6HKI DOC engines. The entire F Series range now has 20,000 km service intervals.


Previous models – FTS800 Auto

Isuzu’s 4×4 F-Series is the preferred vehicle of choice by many rural fire authorities and is also popular with councils and those who have to work off the beaten track.

The original FTS 800 had conventional leaf springs and a part-time 4WD system and so wasn’t driver and crew friendly, but progressive upgrades saw the 2012 model with taper-leaf front springs and full-time 4WD. Ride quality was still firm, but not as jarring as before and driveability was eased by a full-time 4×4 system.

Other improvements to the FTS 800 included driver airbag and seatbelt pre-tensioner, an ISRI 6860 driver’s seat and cruise control.

Like the manual-transmission model the FTS 800 Auto was available with a short-sleeper cab or a crew cab. The automatic transmission was an Allison LCT 2500, five-ratio box that linked electronically to a proved Isuzu 7.8-litre turbo-diesel, with outputs of 176kW (235hp) and 706Nm.

The standard manual transmission was an Isuzu six-speed manual (five ratios synchronised) with a tall overdrive top gear (0.72:1) and a constant-mesh first gear with 6.6:1 gearing. The manual-box FTS 800 had excellent crawl capability, thanks to a low-range reduction of 1.9:1 and final drive ratios of 5.57:1. Overall crawl gearing was 70:1, allowing walking pace progress up and down steep grades.

To show off its  automatic-transmission 4WD truck Isuzu put on a drive day at Victoria’s Australian Automotive Research Centre, near Anglesea in Victoria. The weather was drizzly rain that turned clay slopes into skating rinks.

The test truck was a single-cab tray model, loaded to 12.9 tonnes.

When venturing off-road in the FTS 800 the truck needed no 4WD selection, but the driver could lock the centre differential for additional traction and then choose low range for stump-pulling toque and crawl capability. The standard rear axle differential was a self-locking ‘No-Spin’ that required no driver intervention.

The manual box FTS 800s we’ve driven in the past were easy enough to operate, with the only possible difficulty for novice drivers being the need to double declutch and match revs to pick up first gear on the move.

The automatic box shifted superbly on this test, but there’s no such thing as a free lunch: in exchange for hands-free and foot-free shifting the automatic transmission sacrificed some low-speed capability.

With a torque converter stall ratio of approximately 1.8-2:1 the auto model had overall gearing reduction that was almost identical to the manual’s, so uphill climbing ability was about the same. We found it possible to idle the loaded auto FTS 800 up slopes that were too steep to stand on, but it was a different story when descending those grades.

Torque converter stall doesn’t apply when running downhill, so the effective gear reduction changed to only half that of the manual vehicle’s and so it tried to run away on downgrades that would see the manual creeping down in low gear. In the auto it was necessary to use the service brakes to control speed.

OTA reckons that operators who work in very steep conditions – like the Victorian High Country – would be best served by the manual-transmission FTS, but those who don’t need powerful downhill gearing have the choice of both boxes.

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