Update on FMU problem: Resolved!
 

Sorry if I haven't posted in a while; my father passed away on Feb 27. About 4 weeks prior I had a friend install my Vortech SQ series supercharger in my Mustang Cobra. Since the car was completed I got the reprogrammed computer back but the darn thing detonates at around 5000 rpm. Without a fuel pressure and boost/ vacuum gauge I just decided to drive it easily till I could narrow down what the problem is.
Just installed the gauges last weekend and herein lies the problem. When boost kicks in the FMU doesn't. It has an 8:1 ratio FMU, meaning that for every 1 pound of boost, the FMU should increase fuel pressure 8 psi. Fuel pressure stays at 42 psi no matter what, that's normal WOT pressure. I checked the inlet and return line that connects to the FMU and they are not reversed, checked and made sure that the FMU was not damaged by checking it with a pressure/vacuum hand pump, pressure spikes up when applyng pressure to it while engine is idle, like over 100 psi, so that works. Vortech guy said to check the line that goes to the FMU and make sure that no other line is connected to it, may be messing up the vac signal that way, there was a line running from the diverter valve so I rerouted that and ran a single line from the FMU to the fuel pressure regulator like the manual says, still nothing!
Then I ran the line to another vac source-Nada still!! WTF!! I am running out of vac sources under the hood. I am trying not spend more money by ditching the FMU and getting the 'puter tuned with new injectors and mass air but if I have checked everything properly, this should not be happening. I was looking at my Ford service manual cd, I may try another stronger vac source, even my mechanic friend is scratching his head on this one, he said that he will be coming over Sunday to look at it and see if he can come up with any ideas.

have you looked for any vacuum leaks in the lines?

Is it a scroll type supercharger that sits on the intake manifold or a roots type that bolts to the block and uses an impeller to push the air through to the intake?

If it's a scroll type, then check for vacuum leaks around the base where it bolts to the heads.

If it's a roots type, make sure all the hose routings after the MAF are securely tightened. If not, it's an unmetered air leak (read vacuum leak).

What does a vacuum leak mean to your system? it means that the ECU doesn't see the extra air charge and the FMU can't jack the fuel pressure accordingly.


Sorry to hear about your dad. RIP. :(

im sorry about your loss, my condolences FoxFour.

unfortunately, i dont know much about such things; but i'll ask some guys i know... see if they have any good ideas.

It is a scroll type (centrifugal) According to my vacuum gauge, I show strong vacuum but I will definitely look for such anomolies. Thanks guys for the condolences.

I am completelly lost when it comes to car technology babbl, but I'm so so sorry to hear your father passed away. May he RIP :(

Vacuum should show 18-22 in-hg at the manifold. If it's in that range, and steady, it should be fine. Where are the sources that it tells you to plug into and where else have you tapped?

I'm not all that familiar with FMUs (maybe i am, but just under another name) is it an electronic unit?

foxfour, have you asked this same question @ svtperformance.com or modularpot?

I can't answer your question but I want to say i'm sorry about your loss.My condolences to you and your family.RIP.

Vacuum shows a rock steady 20 inches. The FMU is not electronic, it is a pressure/ vacuum operated device. It has a disc inside that operates on pressure. It basically restricts flow to the return line, thereby increasing fuel pressure to the inlet.
I should. I know though, that a lot of answers that will come back will say 'Ditch the FMU' and I don't want to deal with that right now. I have a lot more pressing matters at hand right now. Oh well, might as well post over there, I'm sure someone may hold the answer. You guys have been great, you really have helped. Time to go outside once again and play with it some more.

i'm wondering if there's not something you need to adjust, like a set screw or something.

I'm really sorry to hear about your dad, losing someone that close is a difficult thing even in the best of times.

Unfortunately I'm not all that well versed in supercharger installs, so I don't know that I'd be much use except to suggest that assuming that the car is running a reasonably modern, "Smart" ECU, then it will "learn" to deal with new A/F ratios after 3 startups. I know it worked that way on my mom's Contour SE which developed the dreaded, "hooting and moosing" problem that eventually got so bad that it was cycling so low at idle that it was dying. The fix for that was to figure out a way to meter the airflow past a specific sensor in the intake plenum such that it didn't have the resonance problem that the car had developed. It still tried to die for 3 starts after I'd made the adjustment, but after that, the car figured out what was up and began running smoothly again. I doubt that it's something this simple for you, as you've probably run the car quite a bit with the unit attached at this point, and the car hasn't made the requisite adjustment apparently. Anyway, hope you figure it out!

Update: Finally have the issue resolved! It seems that I did have a vacuum leak. The line that connects to the top of the fuel pressure regulator was bent in such a way that it was leaking when put under boost. It was repositioned to make room for the line that goes to the FMU, now the pressure rockets up when boost increases. Yesterday was the first time that I was able to really romp on it- what a thrill. Thanks for all the help guys.