Fuel FMU stuck?

JD1964

there is enough sticking out to grab on to

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#1

• Sep 16, 2025

• #1

The LX had been running great for a couple of months after putting the blower back in service. I was just about to increase the boost a tad with a pulley change but something happened. Shortly after a smidgeon of spirited driving, I noticed a rough and surging idle at the next traffic stop. I look at the wideband and its pegged rich, like 10. I'm guessing the FMU valve got stuck. Is this common? It's a Vortech FMU. I have not yet tried to start it again since limping it home. I wonder if after cooling down it will pop itself back into order.

 

Noobz347

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#2

• Sep 16, 2025

• #2

JD1964 said:

I'm guessing the FMU valve got stuck. Is this common?

Click to expand...

I have never heard of this before but it should not dissuade you from checking it out.

What [is] common is a hole or tear in the diaphragm that gives the opposite issue... Not enough gas.


Thing I might suspect at the start:

Fuel Pressure Regulator - These can and do fail closed
Vacuum lines
FMU
Return lines

Dead cylinder (for whatever reason - spark, fuel, etc.) - ECU senses free air causing ECU to enrichen others in the bank.
Leaky injector - Most dangerous because one cylinder burning [fat] can make the EEC lean out all others in the bank.

 

nicholase

I lubed and pushed, until it was all the way in

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#3

• Sep 16, 2025

• #3

I'd put a fuel pressure guage on it. Then you can see what's up as far as the FMU goes. You can also put a hand vacuum / pressure pump set to pressure and apply it to the FMU to see if the fuel pressure swings as it should.

You can rent a schrader valve FP guage from the local auto parts store if you dont have one.

Also pop the vac line off the FP regulator and see if it's full of fuel.

 

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JD1964

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#4

• Sep 16, 2025

• #4

Thanks for the suggestions. First I’m going to start it back up today and see how it’s behaving.

I have a fuel pressure gauge and a vacuum/ pressure pump if needed for diagnosis.

For the record, it has a Kirban adjustable FP regulator. It’s been on there for several years. I did pull the vacuum line off and it was dry so I doubt the diaphragm is ruptured.

 

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JD1964

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#5

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• #5

I found the problem. Duh! My cheesy bypass valve setup blew the hose off.

 

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gkomo

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#6

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• #6

How many lbs of boost are you running?

 

nicholase

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#7

• Sep 16, 2025

• #7

Well that'll make it run pretty bad! I'm suprised you could even limp it like that.

 

JD1964

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#8

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• #8

gkomo said:

How many lbs of boost are you running?

Click to expand...

Not much. About 6 right now. Maybe 7 when I change to a smaller pulley on the blower. Gonna go from a 3.33 to a 2.62.

 

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JD1964

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#9

• Sep 16, 2025

• #9

At minimum I need to ditch the poly nipple to a brass one. The poly nipple is starting to deform from the heat.

 

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JD1964

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#10

• Sep 19, 2025

• #10

I think I will change out the bypass hoses to a different configuration. My goal will be attempting to get the valve as far away from the hot exhaust as reasonably possible. Currently its pretty close to the header

 

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from6to8

There's suction so I used that end O_O

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#11

• Sep 19, 2025

• #11

Noobz347 said:

I have never heard of this before but it should not dissuade you from checking it out.

What [is] common is a hole or tear in the diaphragm that gives the opposite issue... Not enough gas.


Thing I might suspect at the start:

Fuel Pressure Regulator - These can and do fail closed
Vacuum lines
FMU
Return lines

Dead cylinder (for whatever reason - spark, fuel, etc.) - ECU senses free air causing ECU to enrichen others in the bank.
Leaky injector - Most dangerous because one cylinder burning [fat] can make the EEC lean out all others in the bank.

Click to expand...

I replied to a thread from 2011, yes super old but when you see it and have time check it out plz. I guess I can just post it here but the questions are more relevant to the other OP's " Adding a Supercharger what else will I need" thread....

 

JD1964

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Sep 24, 2025

#12

• Sep 24, 2025

• #12

Ok I have the bypass valve relocated at least 4” farther away from the exhaust header. I have it wrapped with heat shielding fabric. I used 90 degree elbow fittings of stainless steel in both air pipes. It still looks cheesy but I promise you that it’s rock solid and won’t blow the hose off any more.

 

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