1994 Cobra - Fuel Pressure Drops Revving at Idle

[IconicSilverGT500]

My daughter just purchased a 1994 Cobra convertible and it came with known issues we are trying to diagnose. The car starts great and overall runs great, if you barely give it any throttle, say less than 10% it revs perfectly fine. If you try to give it any more than 10% throttle fuel pressure drops from 38psi all the way down to 5psi and it stumbles hard.

Same thing when driving the car, if you really baby it, it drives fine. If you try to give it any more than say 10-15% throttle it chokes out.

The previous owner tried to different fuel pumps, tried a new fuel pressure regulator (has a SVE aftermarket on it now), new fuel filter, new lines from the tank to the fuel filter and then finally a CCRM. None of this made any difference. I am currently checking wiring to the pump as this seems to be the most likely scenario. But is there anything anyone else can think of?

 

[AeroCoupe]

Did they replace the rubber hose between the fuel pump hanger and the pump?

With them putting a new pump in, new filter, all new lines (hopefully none are linked or pinched), and new regulator that rules out a lot.

Did they confirm the injector rails are clear?

Other than that the only thing I can think of is the pump is not getting enough voltage. Check and see if you are getting 12V or more on the pump side of the inertia switch. If so you can use that to trigger a Bosch 30A relay mounted in the trunk. The relay can then be used to feed the pump via a dedicated 10 ga wire from the battery to the relay. From there use the stock wiring to the pump. This is by no means a complete way to wire it up I am paraphrasing as I’m a few beers deep. If you want a detailed response let us know and if no one gets to it by the morning I will lay it out for you.

 

[IconicSilverGT500]

He did have the injectors off and had those cleaned and tested with no issues found.

He also did braided line from the pump to the fuel filter with an adapter to the stock fuel ine connections. I am going to check from the wiring at the fuel pump in the morning using the min/max feature on the multimeter with it backprobed. My thought was if I see a voltage drop that corresponds to the loss in fuel pressure when revving at least I will know what I am chasing.

 

[IconicSilverGT500]

Ok, i've checked voltage at the pump and intertia switch, with the car running I am seeing over 13 volts and it never drops so that isn't the issue. I then hit the intertia switch to run the lines out of fuel, started the car and it died within 1 second and the fuel pressure gauge was at zero. I definitely have a volume issue.

I then disconnected the feed line at the fuel rail and had someone turn the key on multiple times to prime, hardly got anything out, but the pump is working. I just decided to disconnect the feed at the fuel filter and try to blow air through it (40-50psi), same thing, almost nothing comes through at all. Certainly seems like some sort of clog, but I am not sure if there is some sort of back pressure valve contained within that fuel line spring connection maybe?

I followed the metal lines all the way back, I don't see any kinks or areas where it's gotten pinched. it could be the rubber lines in the engine bay or I suppose even the short jumper of rubber line between the metal lines and the filter.

 

[Noobz347]

I then hit the intertia switch to run the lines out of fuel,

What? I don't understand this piece. All the inertia switch does is cut power to the pump.


Things that I would do if this were my car:

Purchase and install an OEM Fuel Pressure Regulator
Drop the tank and replace the fuel pump and sock and inspect the previous install along with the condition of the inside of the tank.
Bypass that dog, good for nothing, made my amateur hour buttholes, Ford CCRM with a fuel pump wiring upgrade. (said with fondness, of course )

 

[Mustang5L5]

I then disconnected the feed line at the fuel rail and had someone turn the key on multiple times to prime, hardly got anything out, but the pump is working. I just decided to disconnect the feed at the fuel filter and try to blow air through it (40-50psi), same thing, almost nothing comes through at all. Certainly seems like some sort of clog, but I am not sure if there is some sort of back pressure valve contained within that fuel line spring connection maybe?

I

There is no check valve. If you disconnect the hard line at the fuel rain and cycle the key, it should flow a good amount there.

Sounds like you’ve narrowed down the potential source. Possible the fuel hard lines got crimped or pinched somewhere?

 

[IconicSilverGT500]

I checked all down the frame rails and don't see anything. I didn't check behind the wheel well liner, but it seems ok. It's odd, I've never heard of one of these lines getting clogged, but they are 32 years old so anything is possible!

 

[AeroCoupe]

You need to visually check every inch of those lines. As Mustang5L5 stated, if you disconnect the feed fuel line from the injector rail and turn the key on you should get 110 or 155 or 190 or 255 lph (I have no idea what pump you have in the car but if it is a stock replacement then its 110 lph). For comparisons sake 110 lph is roughly 0.5 gpm so this should be a significant amount. If you are not getting any fuel then the first thing I would do is pull the line off the inlet of the fuel filter and check there. If you get nothing there or very little then its either your pump or the hose between the pump and the pump hanger is suspect.

 

[Mustang5L5]

Napkin math. About 6-7 seconds to fill 1 cup of volume with a 110lph pump. Quicker if a higher volume pump.

 

[IconicSilverGT500]

Well, as I suspected, the main line was clogged with something. I disconnected the rubber line from the hardline at the firewall and through some PB plaster down there for a few hours. Then ran some wire from both sides as far as I could. Then blasted it with about 80psi of air pressure, took a few bursts then it blew right now. Now fuel pressure increases when revving. It's funny, i've never really heard of these lines clogging on these cars, but I guess age gets everything.

 

[gkomo]

What ended up blowing out? A chunk of something?

 

[IconicSilverGT500]

What ended up blowing out? A chunk of something?

A lot of little pieces really, I think whatever it was got broken down by the PB blaster mostly.

 

[AeroCoupe]

Next time take pictures...we like pictures so when I say we like pictures...what do we like? That's right booboo pictures. All kidding aside we do like pictures.

 

[Mcmahst]

Boob pictures, and of course, pics of the problem, because this is a serious diagnosis forum.

 

[Noobz347]

There will be no "Boob Pictures" on Stangnet.


In any public facing forum.

 

[IconicSilverGT500]

Hahahaha, fair enough!

 

[Mcmahst]

Next thing you’re gonna read is down with mini skirts . . .