Breaks up and misses under load-FPR?

[monkeyspunk79]

Hello folks, once again I have an annoying problem I'm trying to track down with minimal time & tools at my disposal. But here goes. Feel free to chime in if anyone has any advice...

My 98 Cobra has developed a problem where it breaks up, bucks, and pops & misses under load or WOT. It only happens past about 3K rpm, and sometimes will happen higher or lower in the powerband depending on temperature or what gear I'm in. Lower gears are worse. This is only when the car has warmed up as it runs 100% fine when its cold or after the first 5 minutes of driving. I have done the following to try to improve this with no luck:

1. Checked IMRC cables. They both move when engine reaches 3250 rpm as they should. How far they're moving I can't tell, but it feels like they're doing their job

2. Replaced plugs & wires with OEM parts.

3. Swapped aftermarket coil packs back to OEM ones.

4. Changed fuel filter, cleaned MAF.


Another clue that may help...it takes a while to fire first thing in the morning. Like I have to cycle the key to build fuel pressure twice before I get it to fire up, otherwise I have to crank and crank until it finally sputters to life. The rest of the day its fine until it sits overnight.

What's my next step? Should I put a fuel pressure guage on it and see what she does? Could this be the fuel pressure regulator? How would I check this? Also if I do check fuel pressure, how can I do it to see what happens under load. All the kits I've seen have really short hoses that hook to the fuel rail, but I want to drive it and check fuel pressure under load...not just at idle.

Thanks as always, this place is THE site for SVT advice.:beer:

 

[monkeyspunk79]

Bump for folks who check SVTP during the work week.

 

[SNAKEYE]

When was the fuel filter last changed?

 

[vettez062002]

GAP your plugs!!! my brothers SRT4 ran like crap for months when he would go into boost... turned out the idiot didnt gap his plugs.

 

[Wynn]

I think your fuel pump is going bad.

 

[SVT CAMR]

Yeah you need a long hose for the fuel pressure gauge. That would be a good thing to do. Or you could just throw it on a dyno and check the fp under load too!

For giggles change out the filter if you have not in the last 4+ years. Also unplug the crank position sensor and make sure it is clean inside the connection.

It does sound like a bad pump though. don't forget to check the CPS for a clean connection. I would also look at the mass air sensor and make sure it is clean as well.

BTW our cars b-days are 2 days apart! 10/15/97 for me. T.Vagi did my motor. Who did yours?

 

[monkeyspunk79]

Thanks everyone, fuel filter was the first thing I changed two months ago, but it was acting up before I changed it as well. Plugs are gapped to factory specs and the MAF was clean. I think it may be the pump on its way out...which means its time to rent a pressure gauge and confirm the bad news. I also have some other things to look at now, so thank you again.

I appreciate the advice and we'll try to update once i have time to work on her.

SVT: That's really cool, small world, eh? My motor was built by T. Vagi as well and I can't make out the other name. Great work though as I have 100K on her and she still sings. When she's not breaking up under load I used to live in Chicago burbs, BTW.

 

[SVT CAMR]

The other guys name I can not make out either! It's James ????????
The cars birthday is coming up this month. I'm hoping to take mine to the track and let her breath a little some fresh air. BTW if you need a fuel pump... I'm not sure what is done to your car but if it's a stocker like mine you can get a FRPP pump for a supercharged 1998 mustang GT. Plug and play. 170 liter pump and is what I'm using for 10 years now. just a thought. Hope you figure it out.

 

[monkeyspunk79]

*Updated* Breaks Up Under Load: Now with fuel pressure data

UPDATE 10/17/10: I went out and bought a fuel pressure gauge set and now have some data, but could use a hand deciphering it. Anyone with a Ford manual feel free to chime in, or any other knowledge lay it on me!

Fuel pressure at the shraeder valve on the fuel rail at idle is 29 psi. When the throttle is raised it bumps up to 40 psi, then drops slowly back down to 29-31 psi and stays there.

When I pull the vacuum hose off the FPR, the pressure raises to a constant 41 psi and stays there. I didn't do this long or rev the engine without the vacuum hose for fear of flooding it or doing any harm. Hope that is okay.

1. What should the fuel pressure on these cars be? My hunch is higher than 29psi.

2. If 29-31psi isn't within specs, does this mean the FPR is bad since it puts out a higher psi without the hose connected? Or does it mean the fuel pump is going?

3. Any other tests I can do?

Thanks as always for your thoughts SVTP'ers.

 

[Lightning&Mach1]

Seems that you have a fuel pump getting weak. Give that a try.