Is there really a problem with the Innova's diesel engine?

Our tech guru has the answer
by Ferman Lao | Nov 30, 2011

Hi, Ferman!

I am new to Top Gear and I have been reading your columns which I find very informative and educational. I would like to ask you regarding my Toyota Innova G diesel with automatic transmission.

I recently purchased a brand-new 2011 Innova AT diesel variant. From the very first time I drove it I was impressed by its performance. It's a smooth ride. Engine performance and response are very good for a diesel engine. It has a somewhat bare interior but it suits my taste. Overall it's a good buy with my hard-earned money.

But recently, as I was browsing the Internet, I chanced upon a forum where the thread topic is about problems encountered by owners of Innovas, particularly the ones with D4-D engines. Most of their Innovas were 2005 to 2009 models. The most common problems cited were hard starting, choking and stalling. Most of them had to bring their Innova to the casa to have it repaired. At first the common reason given out by service advisers was that the fuel filters were clogged. And the culprit was dirty fuel. Repairs were done but after a few months the problem occurs again. Some had to have their fuel pump replaced. On the extreme side, one unit had to have the CRDI system replaced. There was one who only had 2,000 kilometers on the odometer when this problem occurred. It would be of no concern to me if this was an isolated case but a lot of Innova owners had gone through this. Most of them have been saying that the D4-D engine has a design flaw and that the fuel pump is flawed. It's okay if this happens during the warranty period. But what if it happens after? These parts are expensive. Toyota never admitted a design flaw nor did it announce a recall. All they blame is dirty fuel.

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From the posts at the forum others say that the problem has been resolved in the newer models and that the defective parts have been upgraded by Toyota. Some say that the problem still persists. Now I am pondering whether I made the wrong decision in buying the Innova or not. I chose the Innova diesel because of fuel-consumption concerns as my old 1998 Revo was a guzzler.

Have you encountered or heard of this? If you did, is it true that these problems have been resolved or do they still exist?

Hope to hear from you. Thanks in advance.


Hi, David!

To better understand the problem, we would have to look at how the modern EFI diesel engine works. Much like its gasoline equivalent, it relies on the engine's myriad of sensors to tell the engine control unit (ECU) what's happening and what the corresponding action should be.

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The electronics for the most part are practically identical to the petrol engine counterpart save some.

Most petrol and diesel engines will normally have a crank sensor and a camshaft sensor. These tell the ECU where the pistons are, and when and how often the fuel is going to need to be injected into the engine. The amount of fuel required will be based on a few more sensors: the throttle position sensor and either the mass air flow meter or manifold absolute pressure sensor or sometimes both on some cars.

Other sensors like the coolant temperature sensor also have a hand in determining how much fuel is required.

One of the more critical sensors in diesel engines, which at the moment are not common yet in petrol engines, would be the fuel-rail pressure sensor. This monitors the fuel pressure and is used by the ECU to control the fuel pump to maintain a certain level of pressure required for proper operation.

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When this sensor gets clogged with foreign matter such as contaminants from fuel, it will read a lower fuel pressure. What happens next is that the ECU increases the fuel pressure to maintain what it thinks should be the proper pressure.

Should the fuel pressure read excessively low (for whatever reason) and the check-engine light activates, the engine will be operating at a greatly reduced performance to prevent anything else from failing.

Most of the problems encountered will generally be caused by either the rail-pressure sensor or fuel pump failing over time because of the contaminants that accelerate the wear of the moving components.

I don't have much first-hand information on the issues that the Toyota owners encountered when the D4-D system was first introduced here. Like you, I've only heard about the issues thru other parties and what they were told that was the cause of the problem. What would be interesting to know though is how many units were actually affected by the problem from the total number of vehicles sold at the time, and the quality of fuel at the time compared to the fuel quality that's available now.

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On that note, there doesn't seem to be much of those stories circulating now.

Best regards,
Ferman Lao

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