A recent discussion about the DeLorean frequency valve and the two common methods of replacing the original, 40+ year old, brittle nylon fuel lines prompted us elaborate on our response and explain our rationale for using pressed on nylon fuel line vs a rubber hose and clamp.
On the fuel lines at the fuel pump, the original nylon lines are often brittle. We use fuel injection-rated rubber hose with fuel injection-specific (not worm) clamps as replacements. The environment in this area is not excessively warm or prone to the persistent vibration seen at the engine, and, while they may not last as long as OE nylon lines, they are easily installed and inspected.
While braided stainless lines are often recommended in the engine compartment, the frequency valve has its own set of issues. In 2023, we switched our rebuild process from all stainless lines, to a nylon low pressure line, and a stainless high pressure line. The reason for this change is due to the high number of frequency valve failures we were seeing as a result of the normal crimping action involved in attaching the stainless braided line onto the low pressure side of the frequency valve. When this failure occurs, the frequency valve no longer sprays and cannot be repaired - even if the added stainless line was subsequently removed. Frequency valves are becoming increasingly rare and their value is increasing correspondingly. We did not want to continue to place these precious and vital parts at risk of complete failure by continuing to crimp on a stainless steel braided line. We have arrived at what we feel is the only solution - rebuilding with a combination of new original-specification material for the low pressure side, and continuing to use upgraded braided stainless line for the high pressure side.
When replacing the low pressure line for internal or mail-order use, we use a modern, gasoline-rated, new nylon line pressed onto the frequency valve with a tool specific to this task. The nylon line is also protected by a thick rubber sheathing, similar to the original. As it’s an area with significant heat exposure, and as the car originally came with a sheathed nylon hose - most of which have lasted for 40 years or more, we feel this solution will achieve similar longevity.
Others perform/advocate a method of attaching a short length of rubber hose directly to the frequency valve with a hose clamp. This hose is then connected to a stainless braided line that runs to the fuel distributor. We strongly discourage the use of a combination braided stainless/short rubber hose and hose clamp on the frequency valve.
The barbed end of the frequency valve was never intended for use with a rubber hose and hose clamp. The barb is too short to properly accommodate the width of a typical hose clamp and too aggressive for a soft rubber hose. Both of these factors mean that it is not reliable and therefore not safe. One would never see this in an OE application. On the contrary, nylon fuel lines are used throughout the automotive industry to this day as a proven and reliable solution.
The combination of rubber and stainless braided hose is only as strong as its weakest link. That is problematic, as the rubber hose can come in direct contact with the hot rocker cover and experience abrading due to normal engine vibration. Worsening the issue is the fact that the rubber hose is obscured by the air filter housing when installed, which leads to a false sense of security with only a braided SS hose visible.