In this photo you can see the fuel pressure regulator. When installing the '99 Miata head on the '94 block, it's necessary to use the '99 fuel rail with the '94 fuel pressure regulator (FPR) because the '99 is a returnless system, while the '94 uses a return to the fuel tank from the FPR. The '94 FPR can only be oriented one way, due to a vacuum port that has to face out. That's no problem, but it dictates the routing of the fuel hoses. You can see this one sticking straight up, and the other one sticks straight down. The '94 fuel rail has an input that has a flared end on it so the fuel hose with a clamp on it will be nice and tight. The '99 uses the newer style fittings that click into place and require a special tool to remove, so the input to the rail has NO FLARE. The '94 FPR has an output on it that is flared, and so must be a clamped connection. Being ignorant when I embarked on this project, I blithely bought the first spool of high pressure fuel hose I found online and simply clamped the hoses to the fuel rail connections and on the other end to the fuel hardlines from the tank. It worked for 2000 miles.SAE 30R9 fuel hose (Gates P/N 27085), so the clamped connections will work. For the '99 fuel rail input, the 30R9 hose is fine, but you need to make the connection with one of these. It's DORMAN part number 800-081.5. It's a nylon 5/16 inch 90 degree barbed fitting. It needs to be 90 degrees because the space under the fuel rail is very tight and a straight fitting doesn't fit. I got 5 of them from Autozone online for about $12. It was a special order item so I'm not sure if you'll find them at your local store.
Saturday, March 24, 2012
Update to '99 Miata Head on '94 Engine - Fuel Rail Issues
I've been waiting to post this until I had a definitive solution, but if you look a couple blog posts back you'll see my car on the back of a flat bed. The issue that cropped up was the fuel lines simply let go from the fittings on the fuel rail and I couldn't get them to hold on no matter what I tried. This led me down a path of learning more than I ever wanted to know about high pressure fuel injection hose and what type of hose is proper for this installation. Maybe this was obvious to everyone else who has done the '99 head swap but in all my reading about the subject I never saw it mentioned and I did find a few posts from people who were having trouble with leaking fuel hoses.