Collapsing Fuel Pressure

Topic: Collapsing Fuel Pressure (Read 1088 times) previous topic - next topic

Collapsing Fuel Pressure

April 16, 2008, 10:31:36 PM

I'm relatively certain it's the pump, but thought I'd check for ideas first.

My son has a 1990 MN12 'Bird, and I know this is all about Fox Birds/Cats, but they're very similar except for shape. We've replaced the fuel filter (twice now). He said there was black gritty stuff coming outta the filter when he changed it this time. I put my fuel pressure gauge on the rail, and after revving the engine a few times, I can open it to WOT, and fuel pressure drops below 10 PSI, and the six lunger starts to stumble. When driving it, it will barely pull itself up a hill. Level road and constant throttle, runs like a dream. He just had HG's put on it (after I did a front end gasket set on the block for leaking TC/WP gaskets). I'm busier than a three legged cat coverin up at work right now, and was wondering how much time I need to allocate for getting the tank out to put a pump in it.

I've done a fuel pump on my F-150, and that wasn't any fun even with the tank behind the rear axle. Any tips/tricks for dropping the tank out from under the MN12 which is basically under the back seat? If I remember correctly, the driveshaft goes over the tank, so I don't think I'll have to drop the driveshaft.

Any hints or suggestions are appreciated.

Collapsing Fuel Pressure

Reply #1 – April 16, 2008, 10:45:26 PM

You've diagnosed a bad pump... Dunno anything about replacing one on a MN12, but I've heard the DS has to come out as well...

Collapsing Fuel Pressure

Reply #2 – April 20, 2008, 11:42:10 AM

Yeah, that's what I was afraid of. Not quite as easy as dropping the rear-mounted tank outta the Fox or F-150. Looks like a weekender job.

Collapsing Fuel Pressure

Reply #3 – April 20, 2008, 01:18:58 PM

I changed a fuel pump in a '89-'91 V6 Thunderbird several years ago.
I don't remember removing the driveshaft, but I do remember removing the exhaust.

The labor estimating software I have access to says it is a 1.8hr job.

Collapsing Fuel Pressure

Reply #4 – April 20, 2008, 03:49:15 PM

I used to have an MN12, you do have to drop the back half of the exhaust, but not the d/s, it goes through the tank, like an upsidedown saddle tank.

Collapsing Fuel Pressure

Reply #5 – April 20, 2008, 07:01:14 PM

Ive done it. I did mine with the exhaust and drive shaft in the car.

I siphoned the tank almost dry.

Then, after disconnecting the filler neck, I lowered it onto the exhaust and tipped it towards the passenger side. I used a jack stand to support the exhaust and a jack to lower the passenger side of the tank. There is also a tank equalizing vent that loops over the rear of the tank from left to right.

I cant remember if i disconnected the two rear hangers on the car or not. I know for certain the exhaust was not removed (mines welded into one big hunk).

The lowered/tilted tank gave me just enough clearance to pull the pump/strainer assembly out.

In all, it took me one evening. And about 2 days for the mechanics cologne (gas smell) to wear off.

Note: My 92 has the revised exhaust routing (the one that goes straight under the tank as opposed to around (91 and earlier I believe). I would expect that on the old routing you could perform a fuel pump swap even easier.