Members | Sign In
ScannerDanner Forum > Post your repair questions here
avatar

Mazda 3 Returnless Fuel Pump Prime

posted Apr 18, 2016 01:07:46 by JohnC
2005 Mazda 3 with the 2.3L & MTX. 150kmi. Return less fuel system.

Checking fuel pressure today in order to investigate a long crank issue. I was thinking rest pressure problem. Hooked up fuel rail adapter and pressure gauge and went KOEO. Fuel pump kicked on and went off. Pressure gauge showed zero psi. I tried it several times, key on, pump runs but no subsequent pressure increase. Gauge just shows zero till you start the engine.

KOER and pressure went to 54psi, in spec. Bleed off was rapid but I'm checking to see what the spec is. After 30 minutes it was around 23psi and still dropping.

Anyway, my question is, With a return less system will there be a pump prime every single time you turn the key on and the pump runs?

I have Paul's book and on page 16-2 he says that not all return less systems provide a prime. However, I hear the pump running after every key on, so, I'm a bit confused.

Thanks for the help!

John
page   1
7 replies
avatar
Tyler said Apr 19, 2016 02:24:41
Hooked up fuel rail adapter and pressure gauge and went KOEO. Fuel pump kicked on and went off. Pressure gauge showed zero psi. I tried it several times, key on, pump runs but no subsequent pressure increase. Gauge just shows zero till you start the engine.


This behavior is somewhat normal after hooking up a pressure gauge, especially one where you have to tee in to the line. All the air that gets introduced by the gauge can make it difficult for the fuel system to build pressure correctly, until the engine is run and the air is bled out.

After you installed the gauge and ran the engine, did you have a chance to recheck for any kind of key-on prime? Having the gauge set up will give you a much better idea of what's going on.

KOER and pressure went to 54psi, in spec. Bleed off was rapid but I'm checking to see what the spec is. After 30 minutes it was around 23psi and still dropping.

Anyway, my question is, With a return less system will there be a pump prime every single time you turn the key on and the pump runs?


Kinda depends on the returnless system at work, honestly. Sorry, I know that's vauge, but it's true! Lots of systems will have a 'timer' at work, which won't prime the fuel system again if the key was cycled within a few seconds. Others will prime every time. My car uses a returnless system, and never primes anything...

It sounds like your Mazda produces a prime after every key cycle, though it may be that the fuel pressure bleeding down is making it tough for the system to prime correctly. The bleed off sounds too fast to me, but there's many different rules-of-thumb for that, too.

You can use a shut-off valve to isolate the fuel rail from the pump, and determine if you have a leaking injector, or a leaking fuel pump check ball.
avatar
JohnC said Apr 19, 2016 03:21:15
"This behavior is somewhat normal after hooking up a pressure gauge, especially one where you have to tee in to the line. All the air that gets introduced by the gauge can make it difficult for the fuel system to build pressure correctly, until the engine is run and the air is bled out."


Good point, I should have thought of that. My first time checking fuel pressure though.

"After you installed the gauge and ran the engine, did you have a chance to recheck for any kind of key-on prime? Having the gauge set up will give you a much better idea of what's going on."


Yes, I checked multiple times which, I guess, eliminates the problem of air entrapment mentioned earlier.

"Kinda depends on the returnless system at work, honestly. Sorry, I know that's vauge, but it's true! Lots of systems will have a 'timer' at work, which won't prime the fuel system again if the key was cycled within a few seconds. Others will prime every time. My car uses a returnless system, and never primes anything...

It sounds like your Mazda produces a prime after every key cycle, though it may be that the fuel pressure bleeding down is making it tough for the system to prime correctly. The bleed off sounds too fast to me, but there's many different rules-of-thumb for that, too."


Yes, I'll have to consider some of what you've pointed out about timing and key cycles. Knowing that you have a no prime returnless gives me some peace of mind.

This rapid bleed down has me concerned. However, Mazda FSM states that after 5 minutes the pressure should be 36psi or higher. That's almost 20psi in 5 minutes which is pretty quick! lol

"You can use a shut-off valve to isolate the fuel rail from the pump, and determine if you have a leaking injector, or a leaking fuel pump check ball."


Since I don't have a shut-off valve in-line I intend to pinch the hose of my pressure gauge off on injector side and then pump side of the gauge to see where the bleed off is happening.

Thanks very, very much for taking the time to respond. I greatly appreciate your feedback!
[Last edited Apr 19, 2016 03:29:27]
avatar
Tyler said Apr 20, 2016 00:18:17
Yes, I'll have to consider some of what you've pointed out about timing and key cycles. Knowing that you have a no prime returnless gives me some peace of mind.


Aye, there's so many variations of pump control out there. FYI, Toyota products are usually prime-less. For whatever reason, lol.

This rapid bleed down has me concerned. However, Mazda FSM states that after 5 minutes the pressure should be 36psi or higher. That's almost 20psi in 5 minutes which is pretty quick! lol


I'm with you, sir, that spec seems really generous! I'd be happy with 20 PSI in half an hour.

Since I don't have a shut-off valve in-line I intend to pinch the hose of my pressure gauge off on injector side and then pump side of the gauge to see where the bleed off is happening.


That'll work! Let us know what you find.
avatar
Noah said Apr 20, 2016 11:54:29
Just be careful pinching lines, you can easily compromise a hard plastic line. If they're just rubber flexible lines, pinch away.
Massachusetts, USA
avatar
JohnC said Apr 25, 2016 01:32:01
Just be careful pinching lines, you can easily compromise a hard plastic line. If they're just rubber flexible lines, pinch away.
Thanks Noah, I would not pinch one of the plastic lines unless I had a replacement standing by!

That'll work! Let us know what you find.
So, I was wrong, there is a prime. I was more careful to bleed the air out of my gauge this time and with my new OTC gauge I saw that there was indeed a prime with the key turned to the On position. Every time regardless of how often I turned the key to the On position there is a prime to 56psi which then drops to 53psi. That eliminated any questions about time outs or key cycle time. I feel better knowing that the pump is running for a reason each time the key goes to On! lol

Running pressure was as before, ~53psi and steady with throttle blips.

On to checking rest pressure. Per the FSM, I ran the pump for ten seconds (via the FP relay) and then monitored rest pressure for 5 minutes. FSM spec is >36psi after 5 minutes. Sadly, the best I had was 28psi. Now to see if it was the FP check valve or an injector(s). I ran the pump again for 10 seconds and then quickly clamped off the fuel rail side of my setup. After three tests the pressure dropped on the FP side to an average of 28psi in five minutes. Since it was plastic line on the FP side I was not able to isolate the injectors. I'll have to upgrade my adapter.

Despite the fact that I was only able to test the pump side, I think the results are pretty conclusive, the FP check-valve is not holding.

I hope this helps others but particularly Mazda owners since for some reason info specific to Mazda's seems unusually hard to find. Thanks for the feedback and advice guys!
[Last edited Apr 25, 2016 01:39:26]
avatar
Tyler said Apr 26, 2016 02:27:04
So, I was wrong, there is a prime. I was more careful to bleed the air out of my gauge this time and with my new OTC gauge I saw that there was indeed a prime with the key turned to the On position. Every time regardless of how often I turned the key to the On position there is a prime to 56psi which then drops to 53psi. That eliminated any questions about time outs or key cycle time. I feel better knowing that the pump is running for a reason each time the key goes to On! lol


Congrats on the new fuel gauge! It's a huge pain when OEM's don't make info like that easily available, when they should.

On to checking rest pressure. Per the FSM, I ran the pump for ten seconds (via the FP relay) and then monitored rest pressure for 5 minutes. FSM spec is >36psi after 5 minutes. Sadly, the best I had was 28psi. Now to see if it was the FP check valve or an injector(s). I ran the pump again for 10 seconds and then quickly clamped off the fuel rail side of my setup. After three tests the pressure dropped on the FP side to an average of 28psi in five minutes. Since it was plastic line on the FP side I was not able to isolate the injectors. I'll have to upgrade my adapter.

Despite the fact that I was only able to test the pump side, I think the results are pretty conclusive, the FP check-valve is not holding.


Conclusive enough for me! That's a really fast bleed-down, and you'd have a horrible single cylinder misfire if there were an injector leaking that badly. Unfortunately, now it's fuel pump time :-(

avatar
JohnC said Apr 26, 2016 03:08:20
It's a huge pain when OEM's don't make info like that easily available, when they should.
Not a single sentence about a FP prime or KO behavior/pressure at all in the FSM. Jeez!

you'd have a horrible single cylinder misfire if there were an injector leaking that badly
Agree about the misfire. My fuel trims are excellent and it runs fine so I would have been quite surprised to find an injector issue.

Unfortunately, now it's fuel pump time :-(
Yeah, it's pump time. This is a drop the tank job so should be fun! Thanks again Tyler.
Login below to reply: