The Top DSM Community on the Web

For 1990-1999 Mitsubishi Eclipse, Eagle Talon, Plymouth Laser, and Galant VR-4 Owners. Log in to remove most ads.

Please Support ExtremePSI
Please Support ExtremePSI

420A Fuel Pressure not rising even with FMU...

This site may earn a commission from merchant
affiliate links, including eBay, Amazon, and others.

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

cowmilker69

Probationary Member
17
2
Mar 13, 2016
Mississauga, ON_Canada
Hey yall, I got my boosted 420a here and I have a few questions based on the shitty situation I'm in. I'm running a basic, aged 5 psi setup:
T3/T4 turbo
FMIC
12:1 FMU
255lph walbro
DSP Electronics FCD

As of right now the car is running shitty. I drove the car with a fuel pressure gauge hooked up and it read a consistent 60 psi. Even under WOT and boost being made, it stayed the same despite that I do indeed have a 12:1 FMU installed. I'm guessing that this FMU I have was faulty from the getgo? What are some common faults with FMUs(i know there are many)? What do I look for to determine what exactly is the problem?

P.s At first I thought the lean condition was from not having an FCD installed. After installing one the car only got worse. It's normal driving became jerky and now my car is limited to 4000 RPMS. The adjustment screw on the FCD does not seem to do anything. Should I just remove this FCD all togeather?
 
Start with the basics and work your way up. If the car is running shitty even when you drive easy, don't try to boost it just to see if it gets better. That's how you blow shit up. Get the car running well first and ease into boost.

Go over the FMU plumbing and make sure you installed it correctly. You should be getting base fuel pressure at idle and cruising speeds. If your getting 60psi something is wrong and you need to correct that first. Remove the fmu if you have to.
 
The thing is the car was running fairly decent before the FCD was installed. It would occasionally stutter and jerk at cruising speeds and thats why I got the FCD in the first place, but since the day I turboed it the car runs lean under WOT which i'm assuming is because of a malfunction of the FMU somewhere down the line. Btw my car is a 99 and I've seen 99's running perfectly fine with just an FMU, theres video proof on youtube that i'm sure you've already seen.

All I want is for my car to be able to hit boost properly. As of right now it will build boost and start goin faster but once its WOT and full boost the Air/Fuel ratio dives into lean and the car feels like its slowing down. :(
 
first check the vacuum line from the FMU to the engine. if the vac. line is okay, take the 6 allen head screws out of the top of it and check the diaphram for damage.

You must be logged in to view this image or video.
You must be logged in to view this image or video.


if its fuct up you can get a new diaphram HERE
 
Thank you dustyboner. The link you provided for new diaphragms is from vortech. If I needed a replacement, would those replacement diaphragms still work on my FMU despite it not being a Vortech FMU?
 
If its a knockoff that looks like the vortech it will work. If its a cartech or synapse fmu it won't.

Stock fuel pressure is 47-50 psi @ idle. Is your fmu adjustable like the vortech SFMU which allows you to adjust the base fuel pressure?
 
@dustyboner I believe i have the OBX FMU, in which case i dont think it allows you to adjust base fuel pressure. given that, is there any reason why my fuel pressure is always 60 psi? is it because of the FMU? Unfortunately I do not know what the fuel pressure was like before all this turbo stuff...
 
Unfortunately obx isnt known for quality. The ones i have seen are cheap copys of the vortech FMU.

I would check that its installed correctly, check the lines, check the internals of the FMU for damage.

You must be logged in to view this image or video.
 
is your fuel pump the high pressure version?

since you have an adjustable FCD, you need to adjust it so it clamps the MAP signal to 4.7v (upper limit).
 
Yes my fuel pump is the high pressure version. I am hoping that is why my idle and cruise fuel pressure is 60 psi.

As for the FCD, the adjusting pin doesn't seem to actually change anything. Since installation it has limited the car to 4000 RPMs and no matter how much I adjust the pin it does not seem to actually affect anything. I think it's another junk part smh. I have not had the greatest luck with this build but of course it could be worse haha
 
UPDATE: The problem has been solved. I really appreciate all you guys replying. And yes, I do know megasquirt and larger injectors would be ideal, I'm just not that financially stable if you know what I mean hahaha

If anyone else has a similar problem with their boosted 420a, my solution lied directly within the FMU. I would watch this video:
You must be logged in to view this image or video.

I did exactly the things he did in the video and that solved my problem. That video is like a bible for budget 420a builds. I really think it should be on the home page of the 420a Turbo forum or something.
 
Support Vendors who Support the DSM Community
Boosted Fabrication ECM Tuning ExtremePSI Fuel Injector Clinic Innovation Products Jacks Transmissions JNZ Tuning Kiggly Racing Morrison Fabrications MyMitsubishiStore.com RixRacing RockAuto RTM Racing STM Tuned

Latest posts

Build Thread Updates

Vendor Updates

Latest Classifieds

Back
Top