Fuel Injector Heat Soak

Heat soak can be a big problem for your vehicle. Yet some are rarely victimized by it, while other models are more frequent victims. What is it? Why is it? And what can you do about it?

First of all, heat soak is not just an injector problem. It is a more general term for just what it sounds like–too much engine heat for your car’s systems to deal with. This includes heat from too little external cooling air passing through your radiator as well as internal thermal runaway, which is a term borrowed from astrophysics and engineering fields. It refers to a feedback loop where excessive heat produces a cyclical reaction. In electrical automotive applications, high heat produces higher resistance, which in turn produces even more heat, etc.

Batteries can get too hot to crank the engine. Heat that is too high can also affect electrical connections and fuel pressure relays, for example. One highly vulnerable component is the air to water intercooler on a supercharged system. They get really hot and can heat up the entire engine compartment in the wrong set of circumstances. But injectors can be particularly vulnerable because of their location and the fact that they contain unburned fuel that can evaporate under high heat.

Internally, the combustion chamber is the hottest place in your engine. When fuel injectors were moved to an internal combustion chamber location (direct injection) to enhance their overall performance, they also became more vulnerable to heat soak problems. They were made out of far more durable (and expensive) materials. But that still did not reduce the higher heat they had to deal with.

How do injectors and your engine get too hot?

Not counting air-cooled engines, automotive cooling systems are designed to incorporate air flow through the radiator. This cools the liquid coolant, which in turn cools the engine by continually flowing through it and recirculating to be cooled again by the air. When an engine is forced to sit at idle for extended periods, very little air flows through the radiator and an inefficient system can quickly become overburdened. Another situation involves high performance or racing vehicles that are run really hard and then the engine is turned off. You might think it would cool down because it was shut off, but in fact, the heat continues to build because there is no internal cooling going on. This produces another kind of feedback loop where heat energy stored in the engine compartment has nowhere to go.

Idling for too long is a problem in heavy traffic as well as for parade vehicles traveling at very low speeds. Racing for quarter mile times combines both of these problems, sitting for 15 minutes or more at idle waiting in a lineup, and then blasting full bore down the quarter mile.

Fuel injectors are normally cleaned by the passage of fuel through them. Fuel detergent components clean them and the temperature of the fuel, itself, tends to cool the injectors. This works fine until the engine is shut off. Then, the cooling stops from all sources, and the injector starts to evaporate the fuel. Except that not all of the fuel’s components evaporate. Waxy olefins remain behind and build up over time. Continued periods of heat soak tend to bake these deposits into a hard varnish, which can then block injector nozzles.

What can you do about it?

You can’t stop all engine heat or heat problems, but there are a few things you might try. And again, some models are more vulnerable than others. The last Shelby GT 500 was one, as are some Jeep models and the Mazda Miata, for example. Some injectors appear to be less vulnerable to heat soak than others. In this regard, the Bosch EV14 style injectors have gained a following. And some drivers, particularly those in competition, have benefitted from accessories like the ScanGuage II, which helps you monitor vehicle coolant and system temperatures.

Drivers have also installed louvered hood vents to help dissipate engine heat more easily. But be careful not to get too carried away with the size of these vents, as large ones may also admit a lot of rain water during inclement weather.

If your car can burn E85 ethanol, you might try it if you are experiencing heat soak. Some drivers swear it cured their heat soak problems. But one of the best things you can do if you drive hard and fast, is to let the car cool down at lower speeds for a bit before turning the engine off. And try to avoid those traffic jams where you idle for extended periods of time.

But remember that over time, no matter what you do, inevitably, your injectors will have been subjected to at least some heat soak. That, as well as normal wear and tear will eventually dictate injector replacement.

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