Lately, I have been seeing more and more people talk about rail caps. What exactly is a rail cap? What does it do and when do you need one? Sorry for my ignorance!

 

The common rail systems have a rail pressure valve installed to avoid spikes in pressure that might damage the injectors. Problem is they are pretty much one time use, so even on a stock truck they have been known to go bad. They were also expensive (haven't priced them lately), but they used to be over $300 each. Thus some people prefer to install a cap instead of replacing the valve; the problem with this is that you risk damage to your fuel injection system from spikes in fuel pressure...

 

How do you know when your valve goes bad?

 

If you have a rail pressure gauge, you will notice a significant drop in rail pressure. You don't need the gauge to realize the problem however. Truck will be sluggish and smoke like crazy, not to mention that the valve usually leaks externally and you can see fuel leaking at the return line from the valve, or at least that was the case every time I had the issue.

 

Also keep in mind that rail caps are to be used for diagnostic purposes only. I would get a rail pressure gauge to be safe.