I am getting the P0090 fuel pressure regulator control circuit code every 2nd startup after I clear it. Recently a friend accidentally had a fuel injector plug and the plug for the fuel pressure regulator switched, so I thought maybe the regulator had blown or gone bad. I just replaced the regulator and I am getting the same symptoms. I also checked for blown fuses and found none.

I recently had to take my head off and ended up changing out a couple of parts along the way. The major changes were mild cams and full 3" intake and exhaust with O2 housing. Car is mostly stock otherwise and I am still on the stock rom. Since all this was done at the same time, and the first startup was with the cables swapped, is there a chance the code is just from the new parts?

The car seems to run fine all things considered. I am planning on getting a tune but I would like to figure this out first, mainly because I have to pass inspection here in ma. Any suggestions would be appreciated. Thanks.

 

Just found these two threads where the guy made the same exact mistake and is experiencing the same thing.

https://www.evolutionm.net/forums/ev...d-circuit.html
https://www.evolutionm.net/forums/ec...rcuit-ecu.html

Unfortunately the fix is unknown and he just removed it from the periphery bits in the ECU which we can do but I'm not sure the implications this would have on MA inspection.

Given that someone else did the same exact thing and had the same result we can pretty much conclude that its not from swapping cams.

Has anyone else ran into this and determined a way to alleviate the issue?

This is my good buddies car and I wanna help him get his car back in tip top shape so he can come race with me! And no I wasn't the friend that made the mixup of the connectors, thankfully...

 

So my car is past inspection and I need to fix this. If/when I find a solution I will make sure and post it. Any help is appreciated...


Bump.

 

Up, anyone?

Within the fuel pressure regulator circuit what's involved? Im guessing it simply ties into the main engine harness. If its not the actual solenoid and someone else tried swapping ECU's to no avail then what else could possibly be compromised?

Is there something simple like a fuse or relay that could have fried?

I know this isnt a common problem but Im hoping someone here can chime in that is familiar with the vehicle's wiring. Its gotta be something stupid but without knowing all the pieces to the puzzle tracking it down is pretty tricky. Would love to go at it with a multimeter but wouldnt even know where to begin...

 

The solenoid is commanded open when the engine is hot venting the fuel pressure regulator to atmosphere instead of manifold. The result of doing this is increased fuel pressure in the fuel rail preventing fuel vaporization (vapor lock). The solenoid is supplied power by the MFI relay. In other words, it always has power. The other end of the circuit is the ECU which provides a ground. The ECU controls and monitors the solenoid. It monitors the voltage produced when the coil is energized and de energized. The bit of temporary voltage produced by the collapsing magnetic field is supposed to be about 2 volts.

What sets the code is the solenoid surge voltage (2+ volts) not being detected when the solenoid is turned off.

 

Hey, thanks for chiming in! Im not sure he has checked the MFI relay so that sounds like it could be something to look at.

Is there anything else that could possibly cause the issue? Knowing that the connector was plugged into a fuel injector temporarily which has a different resistance from the fuel regulator solenoid that would result in a different voltage which could have "popped" the relay. Does that sounds feasible? I have a very limited knowledge of or wiring.

I find it unlikely that the ecu got fried, the solenoid has been replaced, and the wiring without fully unraveling it from the main harness appears to be intact, it seems there isnt any other pieces that it could be...that is unless Im not fully understanding what you just explained...

 

The fuel pressure solenoid has been around for awhile. There was one on my 1992 DSM. It still started hard on a hot day. But back then no test to check if it was working. So if you wanted to pull it no CEL.

The MFI relay powers not only the fuel pressure solenoid but also most of the important stuff on the engine. So, if the relay were bad you would have more problems than a CEL, the engine would not run.

Anyway, I wish I could help you more but I have never encountered this problem. When you say the code appears every other startup, could it be that it appears not first thing in the morning but after the vehicle has been driven and is hot?

 

Thanks for responding! It may only trip the cel after the engine is warm. It seemed to be every other startup but there was once or twice where it took three startups, so I can't be sure but it may have been after every time the engine was running for more than a few seconds.

I did replace the fuel pressure regulator and it didn't change anything. If I can confirm that it only trips the cel when the engine is warm and not when its cold, what does that tell us? Should I try a 3rd fpr?

Is there any chance the relay could be failing but not dead? P0090 is the only code I'm getting and the car runs fine.

 

What sets the code supposedly is not seeing the 2 volt surge put out by the coil when power is deenergized. This is the same effect that produces spark in the spark coils for ignition however to a much greater extent. So, this is only a check for solenoid operation and nothing else. I don't see the reason why the maker wanted to check if the solenoid is still there and in operation since this is not emission related. So, this pretty much limits what can be causing it - solenoid not getting power or ECU. And since it is ground switched, hard to test.

In the Evo X I have heard they get this code when a wrong map is loaded into the ECM. Anyway, a tuner can turn off the code but I believe it also turns off some other codes.