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How to troubleshoot your fuel system

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Old 10-30-09, 06:59 PM
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How to troubleshoot your fuel system

This thread is meant to help rx-7 owners troubleshoot and fix their fuel problems. Always be extra safe and wear protective glasses and clothing. A face full of gasoline is never good. Feel free to add to this thread if you have more technical advice.

To start out, make sure your fuel lines are routed properly. There are 3 main lines, the feed, ventilation, and return line. Here is the fuel system diagram.
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Once you have verified you that your fuel lines are correctly routed. You want to listen for the fuel pump to come on. Sit in your car and turn the key to the on position, you should hear the fuel pump turn on for 2 seconds. You can further test this by taking a piece of wire and with the key still in the on position, touching one end of the wire to the pin labeled f/p and the other end to the ground.
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The diagnostic box is located on the drivers side of the engine bay close to the battery. It is a small plastic black box that should say “diagnostic” on it.
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If after 5 seconds or so, you can’t hear the fuel pump, chances are it seized up and will need to be replaced.

Once you have verified that the lines are routed correctly and the fuel pump is pumping you need to verify that fuel is making it’s way to the rail. If you have an aftermarket fuel set up, it should of included a fuel pressure regulator with a fuel pressure gauge. Once you have ran the fuel pump, the gauge should read somewhere inbetween 36-38psi. Some say 40psi and above. Make your own decisions based on your setup. Here is a video of the fuel pump being turned on and pressure coming up:

http://www.youtube.com/watch?v=Tg6QH...layer_embedded

*Warning* before you fix any leaks, you need to relieve the fuel pressure in the system, turn the key off, and disconnect the battery as a safety precaution.

Once fuel pressure comes up, you need to verify that it stays up. If the gauge shows pressure is dropping, you have a leak. At this point, you want to look at the primary and secondary rail and look for leaks. Feel around gently with your hands around all fittings, rails, and injectors. A very common point of failure on a stock fuel system is the fuel pulsation dampener. If it is leaking, replace it. Common causes for leaks on injectors is O-Rings. If you notice any type of tear in the o ring, it needs to be replaced. You can either order one from someone like KG Parts(aftermarket) or if you are still using the stock system, Ray at Malloy Mazda. I usually just take the old o-ring to the parts store and have them match it with a fuel rated o-ring. Once you have the new o-ring, put a light coat of oil or grease on it(once again, all kinds of preferences on what to use), and slide it back into the rail. I normally wiggle mine back and forth until they slide in and properly seat.
A common cause of the primary injectors leaking is leaving the small spacer off from underneath the primary injector rail causing the rail to cant and leak slightly. If you do not have one, you can either order one from Mazda, the part number is N326-13-158 and should be less than $10. You can also make one yourself, the spacer is one hundred and sixteen thousands thick.
This is an easy fix:
1. unbolt the two 12mm bolts
2. pull the rail out
3. Take the fuel injector insulators that sit in the block and make sure they are properly in place and are not damaged.
4. Replace fuel rail with spacer
5. Replace 2 12mm bolts

Your lines may also be leaking. The common causes for this is either a cut/hole in the line, bad clamp where it connects to the hardline, or a warn out fuel line. Simple fix here, remove and replace weak link, whether it be a clamp or a hose.

Once you have sorted all of your fuel leak issues, reconnect the battery and turn the key to the on position and let the fuel pump pressurize the system. Verify the fuel system is holding constant fuel pressure of 36-38psi. Mine is a little low in this picture, but you get the idea.
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At this point, you have fuel getting to the injectors. Now you need to verify that the injectors are opening and closing. The rx7 injectors work off of two wires. If I remember correctly, the top wire has constant power on it. The bottom wire is the ground. When the ecu sends out the signal and grounds circuit, the injector opens and closes. I would recommend having your injectors cleaned if you are having issues. I had one instance where I installed a new set of primary injectors that I verified were working, only to find out the next week that one was stuck.
To verify your injectors are opening and closing:
1. Get a small set of jumper clips.
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2. Place the one clip on each injector prong(order does not matter) Place the other end of the clips, one on a ground in the engine bay and touch the other clip to a 12v source(positive side of the battery for example) and then remove it quickly.
3. You should hear the injector click. If you do not hear a click, you have a stuck injector. It needs to be cleaned or replaced.
4. Repeat the process for the other 3 injectors.
Here is a video of what the injector clicking sounds like:

http://www.youtube.com/watch?v=BvTab...layer_embedded

If you still are not having any luck, it is now time to check to make sure the injector connectors on the wiring harness are working properly. As described above, there are two wires on the connector. One wire is a constant power source. The ecu sends a signal and grounds the other wire causing the injector to open and close. The easiest way to verify this is to go to advance auto and buy a node light set(big up’s to David Jerome for telling me about them). Here is what they look like:
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All you have to do is:
1. Take the node light labeled “bosch“ out of the set. This works on aftermarket and stock injector connector clips.
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2. Remove the injector connectors from the injectors.
3. Plug the node light into the injector connector.
4. Have someone turn the car over as you watch the light.
5. If the light flashes on and off, you connectors are working properly. If not, you have a problem. If everything else if functioning properly, you could have a bad ecu. Also you could have a loose wire.
Here is a video of what it should look like:

http://www.youtube.com/watch?v=PPCEo...layer_embedded

If you are still not sure you are getting fuel into the engine, you can check the spark plugs to see if they have fuel on them or disconnect the primary fuel rail(after relieving fuel presure) and lift it out. Once you lift it out, physically look at the bottom of the injectors to see if they are wet.

That’s it, soup to nuts. Start the car up and enjoy! I’m sure I have left some things out. This is just meant to be a good starting point.

Edit: I originally did this write up a couple of years ago. One other issue I have encountered is a bad ECU/burned out injector driver. You can test this by changing out ECU's.
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Old 10-30-09, 09:26 PM
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Good info, thanks for this.
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Old 10-30-09, 11:09 PM
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So I removed my catch tank and purge control during my swap. When I change the fuel filter I am also going to remove the carbon canister. What should I do with the line going TO the carbon canister from the gas tank? Just leave it venting there? Should I lave the fuel vapor valves, separator and 2 way check valve on the tank?
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Old 10-31-09, 04:25 AM
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Im sure this will help a lot of people, nice job
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Old 11-01-09, 06:47 PM
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I'm having a problem with my fuel pump. I have an s5 fc. It seemed to be working but there wasn't enough fuel being delivered, but if i held in the airflow meter just a bit, the car would idle until i let to, or pushed it in too far. So i relieved all fuel pressure and disconnected the fuel pump connector to replace the fuel filter, and after reconnecting the fuel pump connector, the fuel pump doesn't work.
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Old 11-02-09, 10:55 AM
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I would post your question in the 2nd generation forum for better results. I'm not familiar with the FC's fuel system, but I would start by checking to see if you are getting power to the pump. If you are, then it sounds like your pump is seized.
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Old 11-02-09, 11:52 AM
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One thing I want to point out is that the fuel pump diagnostic connector can be effectively used to diagnose fuel leaks with the UIM removed. You can feel around for fuel dripping or spraying as the fuel system is pressurized.


Originally Posted by mothdawg View Post
I'm having a problem with my fuel pump. I have an s5 fc. It seemed to be working but there wasn't enough fuel being delivered, but if i held in the airflow meter just a bit, the car would idle until i let to, or pushed it in too far. So i relieved all fuel pressure and disconnected the fuel pump connector to replace the fuel filter, and after reconnecting the fuel pump connector, the fuel pump doesn't work.
On the second gen, the fuel pump diagnostic connector is a yellow (sometimes with insulating foam around it) two-prong connector on the passenger strut tower. Jumper that with the key in the ON position and engine off. In the fuel pump circuit, this diagnostic connector supplies the same ground that the airflow meter would normally supply with the engine running.
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Old 11-04-09, 09:32 PM
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Originally Posted by arghx View Post
One thing I want to point out is that the fuel pump diagnostic connector can be effectively used to diagnose fuel leaks with the UIM removed. You can feel around for fuel dripping or spraying as the fuel system is pressurized.




On the second gen, the fuel pump diagnostic connector is a yellow (sometimes with insulating foam around it) two-prong connector on the passenger strut tower. Jumper that with the key in the ON position and engine off. In the fuel pump circuit, this diagnostic connector supplies the same ground that the airflow meter would normally supply with the engine running.
i did that. but i just realized this may be causing the problem. i've been using a paperclip as a jumper wire. will that work?

lol
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Old 11-09-09, 01:28 PM
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i have something to add and a question,

add: i found out when i installed my engine that only connecting the F/P diagnostics connector doesnt bring the pressure in the lines up to pressures seen when the engine is running, to get the best leak test, use some plyers and sqeeze the return line, that'll put full pressure on the lines.

question: i just got back from deployment and my fuel lines are leaking, i used worm clamps on the lines because the stock spring clamps werent clamping tight enough and they were leaking too, the worm clamps worked ok, but when i took them off you can see a spot where they dont put any pressure (thats where the fuel was leaking out). do you guys have any better suggestions for a better hose clamp that will clamp evenly and tightly?

thanks, kyle.
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Old 11-09-09, 05:36 PM
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just found these, they look like they would work pretty well.
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Old 11-09-09, 07:18 PM
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Doh, I'm getting 41 psi in my fuel. Maybe that's why I failed emissions for the first time ever. I'm running too rich.
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Old 11-09-09, 08:46 PM
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Originally Posted by kyleaudio View Post
i have something to add and a question,

add: i found out when i installed my engine that only connecting the F/P diagnostics connector doesnt bring the pressure in the lines up to pressures seen when the engine is running, to get the best leak test, use some plyers and sqeeze the return line, that'll put full pressure on the lines.

question: i just got back from deployment and my fuel lines are leaking, i used worm clamps on the lines because the stock spring clamps werent clamping tight enough and they were leaking too, the worm clamps worked ok, but when i took them off you can see a spot where they dont put any pressure (thats where the fuel was leaking out). do you guys have any better suggestions for a better hose clamp that will clamp evenly and tightly?

thanks, kyle.
Much of the retention in the stock fuel lines comes from the lines themselves. They have a tenacious grip on the tubing ends. The clamps are backup. It sounds like you need to replace the lines.

David
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Old 11-09-09, 09:10 PM
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i replaced all of the fuel line when i put my new engine in, the stuff i used says 7.9mm, seemed to fit on there pretty good.
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Old 11-09-09, 09:32 PM
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Awesome tech thread
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Old 12-23-09, 05:37 PM
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My fuel pressure is reading 25psi when primed...I know this is too low, but when I try to adjust it its not moving up or down. Does this mean my fuel pump is going out?
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Old 12-24-09, 08:28 AM
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Before replacing the FP, I'd verify that it's getting full voltage. If so, then replace the pump.
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Old 12-24-09, 09:34 PM
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Like David said, check your voltage and that'd be the first place I'd check. A couple other suggestions would be fuel pump like you said... or possibly a bad fpr. I'm not sure what yours is able to adjust down to, but if it is able to adjust below 25psi, see if you can. If the specs say it is able to adjust below 25psi, and it can't... that should indicate it's not regulating. As I'm sure you know, the FPR is very simple piece with a diaphragm on the inside.
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