Imo000

Last night I hooked up the FMU I bought from another Rennlister to my SC'd 928. The engine responded very well especially under partial throttle. However as soon as I went WOT the A/F gage went to rich and the motor lacked the power it had before the FMU. Before the FMU, the A/F gage showed the engine really leaning out at WOT.

If I feather the throttle and keep it 90% open the problem is not there. This only happens at WOT. I can hear the tone of the engine change when it starts to run rich. The FMU supposed to have an 8:1 diaphragm in it. I’m suspecting the 8:1 ratio might be a little too much for the 24lb injectors. Is this correct? Is there anything else that can cause this condition?

Now that I think of it, I forgot to re set the PCM after the FMU install. Is it necessary to re set it or will the PCM learn the new setup over time?

Gretch

Did you install a fuel pressure guage? Do you know exactly what your FMU is doing? Do you know what you wanted it to do? By FMU do you mean rising rate fuel pressure regulator? What is the make and model of the unit......These are just a few of the questions you need to answer for yourself.

shaaark89

like gretch says, it's hard to tell what's going on without other data. the fmu will increase your fuel pressure under boost by 8 lb for every lb of boost. you need to at least know what your baseline fuel pressure and boosted fuel pressure is. most likely you will need an adjustable fuel pressure regulator as well so you can change the fuel pressure to fine tune the baseline value. even then, it becomes a bit of give and take because ideally, the increase in pressure shouldn't necessarily be linear to be optimal. unfortunately, the only way to get a more non linear increase is with more complex fuel and/or ignition management systems.

Imo000

Gretch,

Here are some of the facts I know of my setup and the FMU:

Andy sent me a PM and pointed out that an 8:1 FMU is way too high for the ’85-’86 engine. They only require a 2:1 setup.

By an FMU I meant a Fuel Management Unit and not a RRFPR. I had an FMU on my SC’d Mustang years ago and I am familiar with its operation. They gradually pinch the return line to raise the fuel pressure under boost. The ratio determines the rate of restriction for every pound of boost. Is this correct?

I have a fuel pressure gage on the end of the right bank fuel rail. I can’t recall at the moment what the pressure was at idle. I think it was somewhere around 35-40 psi . The FMU is definitely working under boost. Except it's running extremy rich at WOT.

The FMU supposed to be a Vortech unit. Looks like one except it’s not engraved with the Vortech logo. Also, supposed to be calibrated to an 8:1 ratio. The data about the FMU was given to me by the PO. I did take the unti apart and inspected it before installation. All internal parts were in great condition. The FMU was only used for a short period of time my the PO.

Imo000

Tom and Gretch,

Thanks for the advice. I will take some baseline FP readings tonight.
I do have a Carroll Superfueler system left from my Trans Am’s kit. It’s an adjustable stand alone supplementary fuel injection system (3 injectors), that uses a MAP sensor and can be tuned to exactly what a boosted engine needs. I am planning on fitting to this setup during the winter months, but till then I wanted to use a FMU.

Here’s my roungs calculation form the info you guys gave me. Lets say my baseline is 40psi and the FMU at full boost (7lb) will increase the fuel pressure by (8x7) 56 psi. Adding that to the baseline will make the FP 96psi. 24lb injectors at 96 psi of pressure will flow a lot of fuel. Now it’s pretty clear to me why the A/F gage goes to the rich side at WOT.

Tim Murphy

I would be willing to bet you have it hooked up backwards. BTW, I think 8:1 is a good place to start with your car.

Imo000

Hmmm… it’s possible the lines are backward or installed incorrectly. However I did pay attention to make sure they were hooked up correctly. At least to what I thought was correct.

I spliced into the fuel line at the back of the engine, to line that goes into the passenger side of the fuel cooler.

The route of the fuel is as follows: leaves the fuel dampener at the back of the engine then enters to the inlet side of the FMU. After passing through the FMU it leave on the exit side of the FMU and enters the left side (passenger side) of the fuel cooler. Is this the correct way ?

I know it might be difficult to picture. If needed, I can take pics of the set up tonight.

Tim Murphy

Can you open the attachment in my last post? I'm not sure why the picture doesn't show like it usually does.

The way the FMU is hooked up is a little misleading (at least it is for me). The return line from the regulator goes into the side port of the FMU and the bottom port of the FMU goes to the fuel cooler on the firewall.

Hope that helps.

Imo000

Tim,

Thanks for the diagram. Yes, I was able to open the attachment. The file is in adobe format. Maybe that's why it doesn't show.

To be honest, I can't recall how I hooked it up last night, but with the diagtram I'll check it again tonight.

Thanks for the help everyone, I'll let you know the oucome.

SilverSFR

Imo000,
I am using Vortech 10:1 @ .5 bar of boost. with out any problems. Tim's diagram is a great ref. thanks Tim. The only change I made from the diagram was to pull boost preasure from the hole in front of the spider instead of from the fuel regulator. check your install and report back.

SilverSFR

Imo000

Well, I’ve checked a few things last night.

The FMU fuel lines were indeed installed correctly. Just as in the diagram posted by Murf. I was hoping the problems would be mixed up lines, but that wasn’t the case.

I’m pulling the vacuum off the front fuel dampener (the blow off valve is also spliced into that vacuum line).

Silver, My engine is the 32V US type, the intake is different than the older spider design.

Baseline fuel pressure is around 30~32psi at idle. Last night the weather was much better than the night before. It was cooler and a lot less humid. The engine seemed to behave alto better last night. I’ve only did one strong pull during the test drive. I want to test the set up slowly before going to full boost on a regular basis. The boost gage showed 6psi. and the A/F did show a slight rich condition. I know these A/F gages are not that accurate and I’m only using them as an indicator and not a true gage.

I did notice one odd thing. The FMU seemed to be seeping some fuel around the top edges of the upper diaphragm. The fuel seepage was very minor, not even enough to form a drop. I’ve tightened the bolts just a bit to stop it. I’m too afraid to over tightening and damaging the diaphragm.

Should I re set the PCM or will the PCM learn the way the FMU adjusting the fuel pressure?

Gretch

Re your last question, any time I tinker(ed) with my setup, I pulled the ground strap......attempting to limit the number of variables at work......I can't see how that can be a bad idea.