raenner

I got my fuel pressure gauge hooked up last night and had a question on the readings I am seeing. It's a mechanical gauge hooked up after the fuel filter.

1. The FSM says that if you jump the fuel pump (using the yellow connector) you should see 64-80psi. I am getting no more than 40psi when I jump it.
2. I see about 39psi at idle
3. When I give it some throttle, the pressure drops to 30psi. I would expect it to stay constant (or rise?).

My fuel filter is only a year and a half old. Does this sound like low voltage to the fuel pump?

Thanks.
-Jonathan

psycheus

Hey Jonn,

I don't know a heck of a lot about fuel pressure, but when you step on the gas it should be on a rise. If it's going the opposite direction I'd assume that some wires possible got crossed somewhere along the installation process. What type of fuel gauge did you use?

Sorry I can't help more.

raenner

Well, it's a mechanical fuel pressure gauge, so I don't think any wires got crossed up

My friend has a gauge hooked up in the same spot as I do, and he sees 40-45psi when he gets on it. However, mine falls to 30psi.

Seems like the pump can't keep up w/ the load (even tho it's not really that huge).

Ideas? Thanks for the reply.
-Jonathan

Henrik

The pump will run on low volts (about 10 or so) until the AFM reading gets above a certain value then it should go to 12-13V. Youre reading at idle sounds about right but it should not drop below 36-37psi when running and it should rise with boost. 1.5yrs on the fuel filter means its due for a change, unless you hardly drive the car. Check the voltage to the fuel pump during running, also check the fuel pressure regulator and the vacuum signal to it.

How many miles on the car? Maybe the pump is marginal.

Henrik
87TII

HAILERS

That part that HENRIK said about the vaccum signal to the fuel pressure regulator is not something to overlook. There are two schedules for the regulator and they are controled by the ornange solenoid valve. The presence or absence of the vaccum to the regulator controls the schedule. I'm at work right now and do not have the different pressures in front of me now. Its in the fac manual. Just a thought. Also from reading your post I think I see a flaw. The high pressure reading should be with the gauge hooked directly to the hose coming out of the filter. NOT teed into it. If you are teed into it you cannot read the pressure the pump is putting out at its maximum because the fuel returns to the tank after going thru the fuel rails, pressure regulators etc. Make sense? If not I'll try again when I have the book in front of me.

raenner

Thanks for all the replies. Let me follow up...

The orange solenoid is used only for hot-start conditions. It is only activated during hot-starts. I removed mine and it didn't make any difference at all. See http://www.k2rd.com/FC3S/PROBLEMS/EM/solenoids.htm

Also, I think the pressure reading is supposed to be taken with the gauge tee'd in. This FSM's illustrations show the gauge being tee'd in. Again, I'm not sure on this...I'll double check.

I'm starting to hope that it's just a dirty fuel filter. As soon as I get off work I'm gonna change the filter and if that doesn't help things, I'll check the voltage to the pump.

Thanks again for all the replies. If you have any more ideas, let me know.
-Jonathan

HAILERS

Fuel System section 4B page 4B-69 shows the gauge connected directly to the line coming from the filter.Pressure reading of 71.1 to 92.4. Page 4B-71 the gauge is connected directly into the banjo bolt on the fuel rail opposite the fuel pressure regulator. The pressure is 35.6-37.0 with the orange connector jumpered. Without the orange connector jumpered it is approx 28.4. The book is a 1987 Mazda RX-7 Workshop Manual. Cost approx 75 bucks from Mazdatrix or a free one for a 88RX-7 is on line. The above is for a Turbo. Section 4A deals with n/a.

HAILERS

Section 4A, page 4A-66 shows the gauge connected directly to the line coming from the fuel filter. The pressure is 64-85.3 psi. Page 4A-67 has the gauge connected to the banjo bolt directly opposite the pressure regulator( the regulator is on the opposite end on a N/A vs Turbo. The pressure with the vac hose disconnected is 35.6 to 37.0 psi. With the hose connected it is 28.4. The vaccum is controled by the orange solenoid . The engine is running in the second test. 87 Factory Manual Section 4A.

raenner

I went back and checked my FSM (88) and you're absolutely right. Thanks. As far as checking the voltage at the fuel pump, I was thinking I'd just unplug the connector back by the fuel pump, then hook up a voltmeter to the correct terminals (black and black w/ white strip), jump the yellow check-connector and see what I get. Maybe I should move the AFM door by hand to see if it changes.

Is this the right way to go?
-Jonathan

HAILERS

I just looked at the 88 manual on line and have to admit they go about it a little different than the 87 manual. The 39psi you get at idle seems just right. As for reving the engine and then the pressure drop I do not know. I'm at work right now but in the morning I think I'll put a gauge on mine and see what happens. By the way, do you have a turbo or a n/a. I think the N/A voltage is more or less a constant but the turbo does increase when you get on it with the throttle. I'll look tomorrw if it does not rain. You should not have to move the door on the mass air flow meter if you have the other plug jumpered. The one on the r/h strut tower that I forget the name of right now.

Keith

Hiya, I just wanted to throw my 2 cents in. OK, first thing. If you want to check voltage to the fuel pump, do not disconnect it. You are checking the correct wires, but the only way to know if the fuel pump is getting enough juice is to check with it hooked up. This is because the pump draws about 10 amps, your meter only draws about .0000001 amp. Your meter could be supplied with 12 Volts with a single strand of wire the size of a human hair - but not your fuel pump! See what I mean?

Second, If you have an NA, you have a problem. The system works by supplying fuel at up to 80psi of pressure. The fuel pressure regulator than bleeds back sufficient fuel to the tank to maintain approx. 40psi pressure (unless the orange solenoid applies vacuum, but this should only happen about 50 seconds after starting a warm engine). Now I know where the manual tells you to read these pressures, but I just teed into the supply line, and it worked just as well. Heck of a lot easier to hook in too. Notice, this is voltage related only, engine rpm should not cause more then +- 2psi difference, approximately. Since your system holds correct pressure at idle, it would be reasonable to assume your regulator is ok (least amount of fuel used, greatest amount to be bleed back to the tank). Therefore, you would have 1 of 3 problems: Weak fuel pump, insufficient voltage to the pump, or a restriction in the supply line (tank to fuel rail). Note, a dirty filter can be a restriction.

If we are talking about a Turbo here, all the above applies, with the following exception: There is an extra relay in the fuel pump electrical supply line called a fuel pump resistance relay. What this does is drop the fuel pump voltage about 3.5V - that is, if the pump runs at 12V full voltage, it will run about 8.5V if the ECU picks this relay. So if your fuel pump voltage measures that low all the time, either the relay is defective or the ECU has got it in its head to pick this relay all the time (happened to me!). This should only happen at idle on a hot engine, once you give the engine throttle it should jump back to 12V. I did a fuel pump rewire direct to the battery (my pump is always getting >13V), and I have not noticed a single ill effect. You might want to consider this if your ECU is causing low voltage to your fuel pump all the time. Note: this discussion is meant to identify other causes of insufficient voltage to the fuel pump in TII cars.

Well, enough blabbing for one post! Have a fun day!

Irv, Keith's dad

raenner

Thanks for the comments, Hailers and "Keith's Dad"

My car is non-turbo, so I don't have the voltage resistor -- which should make rewiring the fuel pump much easier if it comes to that.

Thanks for the tip on checking the voltage...I need to do that today. Although it still would be easier if it was just a dirty fuel filter

As far as a weak fuel pump...I don't think I've ever heard of an N/A fuel pump going bad, so hopefully that's not the problem here.

Thanks again for all the replies -- I've learned a lot.
-Jonathan

HAILERS

I just did the N/A. I hooked in like the manual in section 4A( I agree with Irv, there is no real need to do it this hard way except I built this adapter an month ago and by God I'm gonna get some use outa it). Disconnecting the vaccum I get 42psi at idle. I reved the engine and it barely budged maybe a psi max. Then I connected the vac line at the solenoid. The pressure at idle was about 33psi. Reved the engine and it went up to 35psi. Not significant. Now I have a Question for Irv. I know about the 50 second rule for the turbo because I've been down that road with the turbo. But on this N/A the 12v is there all the time. The section 4A under contol unit says this is the way it should be. I did notice the voltage at the organge connector was .76(insignificant) at startup, but just barely nudging the tps gives the 12v reading. It seems they want the fuel pressure to be 28psi for the reading while driving. Sooo if this is true and I have deluded myself into thinking my secondaries(680's) need the extra pressure, do you think taking the vac hose off, now giving me 40 psi, is the thing to do. Half serious.

HAILERS

RAENNER AND IRV.........Forget my last post. I just realized that the pressure solenoid on my car is the wrong one or is working backwards. When it is energized it is supplying a vaccum to the pressure regulator and de-energized it is cutting the vaccum. I've been running for the last two years plus with 28psi instead of the 40 or so psi. Have no idea why the wrong type solenoid is there. Investigating. Duh! If you had not written the post ...........

raenner

Whoa. Good thing you found that Let us know what happens when you get it hooked up properly.

-J

Keith

HAILERS and raenner - have we uncovered some kind of manufacturing error here??? I was just looking at the start of this thread, and it seems raenner's car is doing exactly the same thing (30 is pretty close to the 28.4 spec, don't you think?)! According to the shop manual, the solenoid to be used here is the one where the vacuum fitting pointing up is on the same side of the valve as the cute little air filter (on the other type, this fitting is on the opposite side of the valve with the cute little air filter). Also, the ECU signal (at the blue/orange stripe wire) should be 12V at idle and less than 1V above idle. HAILERS, it sounds like you have the correct valve there, but the voltages are backwards. You might want to swap valves with one of the other types to get it to work right, but why is the ECU wrong here? As soon as Keith gets back from crusin', I'm checkin' this out!!! I would hate for my TII to be running with only 28psi under boost!!! Keep me posted on what you find out, guys!

Irv, Keith's dad

HAILERS

First I'd like to say that I have moments where I am a confirmed functional illiterate. But, I'am at work looking at the 88 manual on line, page 4a-67. It checks the fuel line pressure which should be 28.4psi.Car running. The next step tells you to disconnect the vaccum line on the pressure regulator and the pressure should be 34.1-39.8 . Turn the page to 4a-68 and there is a test for HESITATION AND POOR ACCELERATION which tells us to run the engine and the pressure should be 34.1-39.8. It is not clear in this step if the vac hose is or is not connected. I assume connected. That is the part of the testing that caught my eye in the 87 manual and made me realize something is askew.I plan to look at the wifes 86 tomorrow morning. That car always seemed to run a touch better than the 87n/a and I might have found a reason. Keith, my first thought was a error in the manual, but changed my mind because I know some of these solenoid valves work just the opposite than others. But you know that.I checked the voltage and it was 12v running. Maybe I did not let the car warm up enough? Nah. Mr RANNER, can you check your voltage at the orange solenoid for me please. Car idling and at rpm please. Signed, Confused in Texas.

HAILERS

Another question. Both the turbo and the n/a are supposed to have 12v at idle at the pressure regulator solenoid signal at pin 2M. But in section 4B, Turbo Section, they check with the engine running for 28.4 psi in the fuel line pressure. The next step they jumper the pressure regulator solenoid valve and check for 34.1-39.8psi. Why would you have to jumper if you already have 12v there? See CONTROL UNIT, SECTION 4B, PAGE 4B-32, under 2M. Confused. Must be a reading comprehension thing. Nah. Factory manual is wrong. Any body know. Six Ports?

HAILERS

RAENNER, what do you think about just pluging the vac line and forgetting about it? I'm giving it some thought. Then again how could this be so messed up? Keith, I will take a look at the solenoid valve and the relation of the filter to the hose.

Keith

HAILERS, the manual is correct in both instances. In the non-turbo version of the adjustment, it tells you to remove the vacuum hose. When the regulator sees vacuum, it reduces fuel pressure. On the turbo, however, the darn pea-picken vacuum hose is hard to get at. So, the manual tells you how to do it electrically. The circuit is set up like this: 12V power is supplied to the valve via the black/white stripe wire. The ECU supplies a ground to turn the valve on (blue/orange stripe wire). What's that mean? If you see 12V on the blue/orange wire, the valve is off (vacuum is supplied to the regulator, bacause the ECU is NOT grounding the circuit). If you see <1V on the wire, the valve is on, hence no vacuum is being supplied. So, when you ground the wire in the manual, it turns the valve on, which is exactly the same as pulling the vacuum hose on the regulator! See what I'm sayin', HAILERS?

Now the fun part. I checked my TII 10 minutes ago. Guess what? It was grounded at idle (no vacuum????). Keith gave it throttle, it jumped to 12V, and stayed there, even after returning to idle!!! Valve off, 28psi pressure! This isn't over yet! I'm gonna get to the bottom of this! Will keep you guys posted.

Irv, Keith's dad

HAILERS

Yes, I understand. I just got caught up in a cycle of thought I could not break out of. The lack of the 12v is due to the voltage drop due to the ground at the ECU. The 12v represents a lack of a circuit by no ground at the ECU. Please find the solution. In the mean time I'll plug the vac hose. I'm going to see if I can sense a difference in hp, then challange the wife to a race. And yes, I noticed that the turbo has a 50sec delay on start up and the N/A has a 90 second delay on start up. I seem to be missing this also on the N/A but it could be the lack of engine temp. I my check out with the engine cool so I would not set the car on fire or burn my fingers adjusting things. You scared me. I'm going to put a pressure gauge on the turbo tomorrow and check the pressures.

Keith

raenner and HAILERS - I was looking at the shop manual for my Maxima this morning (Whoops! I mentioned a boinger! Well, it's almost kinda a respectable one????). Well, it has vacuum going to the pressure regulator directly - no valves anywhere. So does my Oldsmobile (Oh no! Did it again! Man, I'm gonna lose all credibility here! Seems most modern port injection systems are set up this way - the system emulates a power valve in a carb by increasing pressure when engine vacuum goes away. It seems Mazda, however, has determined that there are situations (unique to a Rotary, of course) when full pressure is required even though manifold vacuum is high. Hence, we have this confusing pressure regulator control solenoid in the vacuum line to the regulator. Well, not to worry, whenever we slam down the gas, we are going to get full pressure anyway, whether the darn solenoid is on or off (no vacuum, right?). Anyway, I'm going to leave mine alone, that is, I am NOT going to plug the vacuum hose - gas mileadge is bad enough as it is! Besides, every time a put the petal to the metal, the surge of power reminds me that it's all good!

Irv, Keith's dad

HAILERS

Reference IRV, KEITH'S FATHER. When you say *put your foot to the pedal and no vaccum, right* that might be true on a turbo, but what about us poor souls who have n/a's? We still have a vaccum when we put out foot to floor. Also BOBR on the 2nd gen page says his 88 has no vac when at idle. Then again both of my n/a's have a vaccum at idle, at full rpm etc. I'm hoping for more input on the 2nd gen site where I have asked for a response as to whether their n/a's have or do not have a vaccum with a fully warmed up engine. It seems Mr. Raenner who started this post has a car that there is nothing wrong with. Maybe. Maybe not. Still wondering. No since looking at the turbo. Probably just like the n/a. Might run a vac hose to the cockpit and drive on the road and observe the vaccum at all throttles. This car is a hobby in itself. Just thought, RAENNERS friend has a car that acts just the oppostite of ours.

HAILERS

Irv, you are right. I ran a vac hose from the solenoid to the interior of the car on both the turbo and N/A. Went for a ride. Under full throttle both cars virtualy lose the vac which puts the regulator on the high schedule which is 40psi. Let off the throttle and it goes to the low schedule, 28psi. Solenoid is always engaged except during hot start or initial start. Vaccum is controlled by the right foot entirely.