Fuel pump only on when cranking, MS3x issue? - Miata Turbo Forum - Boost cars, acquire cats.

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Old 05-31-2015, 05:37 PM   #1
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Default Fuel pump only on when cranking, MS3x issue?

Hey everyone,

I've been having a lot of trouble getting my car running after my turbo install. Here's a little bit of information about the car.

92 Miata with a 1.6
Most of a BEGI Shanghai turbo system with "2854 chinacharger" and intercooler
AFM delete with a GM IAT
BEGI 550cc injectors
Wells VTPS
5xRacing adjustable FPR
In line fuel pressure gauge
Walbro 190 fuel pump
Brain built MS3x
3" custom exhaust with Magnaflow cat and glasspack
AEM wideband
Trackspeed Toyota COP kit
Injectors and COP wired for full sequential
If I left anything out, just ask me.

So the turbo install went well. We put everything together with the stock computer and AFM still attached and it fired right up with no problems. I installed the VTPS, injectors, and the MS3x, and put the piece if wire in the AFM connector, and now the fuel pump only wants to be on when the key is in the cranking position. At this point I thought maybe the fuel pump was having issues with the bigger injectors, so I put in the Walbro 190, AFPR and fuel pressure gauge. Now I can see the fuel pressure gauge getting pressure when the car is cranking, but the pressure goes to 0 when it starts, then the car sputters and dies. If I turn the key to the crank position but turn it back before the car starts, the gauge will hold a constant pressure, so there's not a fuel leak. That made me believe that since I was only experiencing this situation after the MS3x, injector and VTPS install, it would be one of those things. I disabled the VTPS, same issue. I swapped the stock injectors back in, same issue. Now I'm thinking, through process of elimination, that there's an issue with the MS unit itself. Is there anything else I can check before cracking open the MS and checking everything? I was considering putting a fuel pump toggle switch in the car to see if it will fix the issue with the fuel pump being manually turned on, but I figured I would ask you guys before I did so. I'm out of town right now visiting family so I won't be able to post a copy of the tune or data log until tomorrow.

Any input is appreciated, I'll try anything at this point!

Thanks for the help,
-Brandon L.
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Old 05-31-2015, 06:16 PM   #2
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I'm sure Scott will be along shortly, but the issue has to do with how the fuel pump is triggered in the 90-93 cars. It triggers off the airflow meter, and it only comes on when it sees airflow. To start the car, there's a circuit which powers the fuel pump when the key is in the "start" position, but not in the "on" position. In '94, they moved the fuel pump control onboard the ECU, so the ECU can prime and run the pump regardless of the AFM.

There is a wire that goes from the ECU (1C) to the fuel pump relay, but it's also connected to +12v via the ST SIGN fuse. All the PnP boxes have you remove this fuse, and then they use that pin to power the fuel pump relay with a highside driver. Did your instructions tell you to remove the ST SIGN fuse?
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Old 05-31-2015, 06:36 PM   #3
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Which pins did you jump in the AFM connectors, do you have a picture you followed?
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Old 05-31-2015, 06:38 PM   #4
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Thank you so much for the quick reply!

Before I purchased the MS3x, I had planned on getting the MS2 PnP and read the install instructions over and over. I remember the part where you had to remove that fuse, but I figured since this was a different unit that wasn't a PnP, you wouldn't have to do that. I suppose that's my fault for assuming. I didn't receive any install instructions with the unit, just the unit itself, so I plugged it in, wired in the IAT, used the jumper wire on the AFM connector, put the resistor in the diag box, and went from there. I'll try removing that fuse and I'll let you know if it fixes the problem!

Thank you again for your help,
-Brandon L.
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Old 05-31-2015, 06:40 PM   #5
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I know with my ms3x which was built loosely for brains instructions I didn't have to pull the fuse. Don't just go pulling fuses without solid instructions.
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Old 05-31-2015, 06:42 PM   #6
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Quote:
Originally Posted by aidandj View Post
Which pins did you jump in the AFM connectors, do you have a picture you followed?
I do have a picture on my laptop that I followed, but unfortunately I'm posting from my cell phone in a hospital room. I'll try to find it when I get back home later tonight and I'll post it.

Thank you for the help!
-Brandon L.
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Old 05-31-2015, 07:11 PM   #7
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The pins he has jumped are the same that I have in my car that works.

Here is a pic I literally just took of BrandonL's car:

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Old 05-31-2015, 07:22 PM   #8
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Here is the current tune on his car, although it probably isn't too helpful solving why the fuel pump doesn't seem to stay on.

Since this is after switching back to the stock 240cc injectors just to test if his 550cc injectors were the problem.

However, the datalog is from when it was on the 550cc's and it would barely idle.
Attached Files
File Type: msq CurrentTune.msq (225.0 KB, 51 views)
File Type: msl barelyidles.msl (161.6 KB, 34 views)
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Old 05-31-2015, 07:25 PM   #9
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Thanks for posting that, Carbon! Hopefully we can get a solid answer now, I can't wait to drive the car again.

-Brandon L.
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Old 06-01-2015, 12:47 AM   #10
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Can we just jump the F/P and the GND in the diagnostics box with a switch in between with the Brain MS3x setup without frying anything?

As far as I am aware, it should be fine. Just looking for some confirmation before doing it.
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Old 06-02-2015, 03:23 PM   #11
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The car is up and running, finally! We ended up doing as Carbon suggested and grounded the fuel pump pin in the diag box. Fuel pump fired right up, had perfect fuel pressure and drives like a champ. Now I wonder why we had to do that in the first place, the fuel pump shouldn't be having issues turning on, it's brand new and everything else is operating as it should be. Hmm.
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Old 06-08-2015, 10:52 AM   #12
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I wire all my 90-93 units to activate the fuel pump through the AFM connector at pin 2O.

This sends the FP ground signal through the AFM connector through an unused wire. The user then has to install a jumper wire between the two pins (as seen above) to bring that signal back to the FP relay.

I find this a much better approach than the MSPNP activation. The user does not have to worry about pulling the ST_SIGN fuse or blowing transistors.

I'm unsure why the jumper (as seen above) did not work, and how it was getting fuel during startup at all if it didn't stay activated after that. The first things that come to mind is that I either built the harness incorrectly and put the FP output on the wrong pin or built the mainboard incorrectly somehow. I'd need BrandonL to poke around to confirm this--I'm leaning towards the latter. I might have put a resistor on R39 instead of R40 by mistake (I had this funny feeling that a unit I built I might have done this, this may have been the one). Looking at the schematics, it appears it would run the pump in reverse without that resistor to ground.
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