Troubleshooting: BT-50 & ZD30 Bosch Common Rail Fuel Systems Hard To Start

The Bosch common rail injectors, or common rail fuel systems, are very robust in their design and mode of operation. This means when a DENSO system starts to fail, you get a significant of signs, such as engine starting to run rough, blow smoke, and rattling engine noises. The Bosch system doesn’t do any of that, so the first problematic sign you have is when the fuel system is almost completely cactus and is struggling for rail pressure.

The Problem

Generally speaking, you’ll find that there’s a high pressure leakage from the high pressure side of the fuel flow circuit to the low pressure of the leak-off part of the fuel pressure circuit. So an easy way to tell if you have a drama with a Bosch equipped fuel circuit system, such as BT-50, Rangers, and ZD30 Patrols, is to first check the leak-offs out of the injectors. The leak-off comes out of the top + center of the injectors so just pull them out for a 30 second test. You can use tubes but it may get a little messy.

The Solution

A good injector will have approximately 10ccs of leak-off per minute, a bad injector will have around 300 – 400ccs. If you’ve got a friend that can watch under the hood, you can try to start the car, and you’ll generally see 1 or 2 cylinders spewing fuel out on the other two, in virtually negligible volumes.

The Alternative Problem

If they’re okay, the other common problem you see is a leaking pressure from the relief valves in the common rail. As a safety feature, the engines have a pressure relief valve which is meant to open at 2,000 bar. But we’ve been getting a number of phone calls for these that are leaking. There will be leak-off lines coming off the end of the rail.

The Alternative Solution

The pressure relief valve looks like a 7 or 8 millimeter hex-head bolt. If you go to the end of it, two black tubes should be coming out for your leak-off, so just pull them off and there should no fuel flow out from the rail at all. If you have fuel flow, then you know you’re losing rail pressure. Replacement pressure relief valves are about $180, so it’s a fairly cheap option rather than replacing the rail. If the injectors have excessive leak back, they will require replacement.

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