There are common faults and there’s common sense. Most of you know me from my wealth of knowledge working with diesel systems for a very long time, there is not much I haven’t experienced. Most of which stems back in my apprenticeship days, which was well before we were using computers in cars. Of course, it helps when you have a photographic and a hard drive of a memory like I do.
I recall back to my first encounter of an engine running rough after a workshop had recently replaced the cylinder head and refitted a recently overhauled set of injectors. I would have been in my fourth year of apprenticeship working in the diesel shop. The mechanic was at the front counter having a meltdown with my boss because the set of injectors we’d overhauled a few weeks prior to this engine overheating were no longer operating correctly.
The injectors were not the cause of the overheating. It was a typical case of a poorly maintained vehicle which, due to a cooling system failure, had lost coolant and overheated. The vehicle was an early 2000 model Mitsubishi Triton 4M40T 2.8L fitted with mechanical pintle type indirect injectors. We had not seen the vehicle previously, as most of our clients would drop the fuel system components over the counter for testing and assessment. At the time the injectors were overhauled, we were told they were chasing a black smoke issue.
I stood beside my boss as he copped a spray and calmly responded with – I’ll have our apprentice test and ultrasonic clean the injectors for inspection. My boss already knew what we would see, but we need to be sure we are dealing with the injectors we supplied originally. I tested the injectors on the pop tester, and the spray was ok and pressure was good. After ultrasonic cleaning the injector nozzles whilst still attached to the injector body, we noticed discoloration of the nozzle. This blue/brown color is the result over overheating in which confirmed my boss’s theory – these recently overhauled injectors were installed before the engine overheated and now, they are NON-SERVICEABLE.
Even though they test well on the tester, once they’re torqued into the cylinder head and the engine is warm, the nozzle changes shape. Simple explanation is when a metal object is exposed to extreme heat, its going to change the original shape.
So after 30 years of dealing with the same problem week in week out, I have decided enough is enough and I will try to spread the word via our technical bulletin library and this publication.
TB1283 All diesels engine overheated new head, old injectors installed low power
Models: All diesels, early and late model diesels.
Symptoms: Engine feels sluggish. Low power. No turbo boost. Approximately 40 to 50 percent down on performance. No smoke. In some cases when the engine is at operating temperature, the engine rattles, blows excessive black smoke.
No fault codes.
Failure/Issue: Failed injectors. Original injectors reused after the replacement of head or head gasket due to the engine overheating.
Diagnosis and/or early detection of the fault: In any situation when a diesel engine has experienced overheating, injector replacement is a must.
The extremely high engine temperature causes distortion of the injector nozzle.
When high temperatures are applied to any metal object, it changes shape. For example, an oxygen/acetylene torch temperature of approx. 600 degrees celsius can change the shape of steel and does not return to its original form.
In the case of a diesel engine overheating, although it does not reach the temperature of that received from an oxygen torch, the heat at approx. 120 degrees celsius is enough to create a distortion of an injector nozzle. The injector nozzle which is the end section of the injector (red arrow) that protrudes into the combustion chamber, comprises of two parts. The needle valve, and the nozzle body.
The needle body has multiple holes (five to eight) to enable the fuel to exit the nozzle into the cylinder. The tolerances between the internal surface of the needle body and nozzle needle is very small, around 1-3 µm (0.001-0.003 mm). So, between the needle valve and body, there is not much give for alterations from its original shape.
To assist with understanding how fine these tolerances are, refer to the images of a nozzle in which a human hair follicle was placed into the internal section of the nozzle body. The average hair follicle being roughly 60 to 80 µm. In the image, after the follicle was inserted into the nozzle body, it was followed by the nozzle needle. Once it touched the follicle, resistance was felt, continuing to break the hair follicle.
When an injector nozzle has been exposed to excessive heat, the nozzle formation can alter as much as the thickness of this hair follicle thus causing the internal nozzle needle to grab, limiting its maximum movement causing a reduction in fuel injected. In some cases, the sticking of the nozzle can cause excessive diesel knocking, rattling and a misfire, leading to excessive smoke and may result in a catastrophic failure of the engine.
Special notes: Some people expect to see blue discoloration of the nozzle as a sign of overheating. Often it can be a darker color as shown here:
Overheated nozzle.
