12a Blowthrough... Erratic Fuel Pressure
 

Ok guys... need some help/opinions here.

So I've finally got the car up an running and I've been fighting it now for a few weeks. I've had a couple boost/fuel leaks from different areas of the carb/carb hat, but all of those have been resolved. The issue I'm running into now is a little more involved.

Basic run down:
12a Large Street Port
OER 47mm carb (prepped by Rotaryshack)
Mallory 4309 (boost referenced from carb hat)
Walbro 255 lph pump mounted in the tank using a 2nd gen pump assembly
1/2" hardlines from tank to engine bay for feed and return
AN -6 stainless lines for feed and return in engine bay
NGK B9EG plugs gapped at .035

I have been fighting issues of inconsistent fuel delivery, I believe. Previously I had the FPR referenced to the intake manifold, and that didn't work so well. I was having over fueling issues and generally poor driveability.

Last weekend I moved the boost reference to the carb hat and it drives immensely better. BUT I'm still having issues. Basically, fuel pressure is very erratic.

I set the base pressure to 3 PSI, and it rises steadily as the car comes off vacuum and begins to build boost. AFR's under vac are a nice 11.9-12.3. However, as car begins to come off vacuum and sees positive pressure in the manifold (boost gauge is hooked up to manifold still) the car starts to lean out and fuel pressure oscillates back and forth by a swing of about 3 psi, car cuts out, and I can smell raw fuel. If I floor it the car will boost up to 10 lbs, but AFR's start to lean out and I have to get out of it. When it does fully boost and I change gears the car spits and sputters and pegs the wideband lean. I have to let off of the throttle and give it a second, then I can roll back into the throttle and I have throttle response.

Basically it feels as if the float bowl is running dry under boost and takes a second to fill back up. Or that perhaps the float bowl is not being pressurized quickly enough and boost is forcing fuel back through the feed line? Basically I have no boost, or WOT and full boost, no part throttle boosting.

I made a quick video last night driving home. This is 5th gear, doing about 60 and rolling onto the throttle. I went from 5 in/hg vacuum to full boost. You can see how wildly the fuel pressure oscillates. BTW, this is a mechanical gauge with a -4 stainless line feeding it directly off of the FPR.

http://www.youtube.com/watch?v=qfLYw...&feature=feedu

Switch to 480p to rotate 90 degrees for better viewing.

Your setup seems good. Have you checked your fuel pump wiring? Are you powering it straight from the battery with a relay?

Yes... battery is mounted in passenger bin, 10 gauge wire feeding relay, 10 gauge wire to pump. 10 gauge ground directly to battery for both relay and pump.

Here's everything I could think of that could cause those symptoms:

- Fuel pump going out (unlikely if the pump is new, but it could happen)
- Fuel pickup or filter clogged
- Fuel lines blocked or kinked or something
- Needle valve or carb fuel inlet blockage
- Fuel pump wiring (doesn't seem to be the problem)
- Problem with the FPR or reference line
- Something I haven't thought of :)

I especially like the last suggestion... :)

I'm wondering if it's the FPR. I replaced the diaphragm before installing it and it came with some little gasket that said was to be used with "old style regulators". The instructions were a little lack luster to say the least, so I put the gasket in... BUT there wasn't a gasket installed in the regulator when I tore it apart.

This could be your problem

I know Rob is not the most popular guy on this board, but I've never had a problem with him other than being slow to ship parts. I've called him a few times with random issues and he's helped out. So I'm pretty confident the carb is not the issue.

Besides all personal opinions aside, there's no one else that I know of with more blowthrough 12a experience than Rob, and I'm no idiot. I checked the carb and it was properly boost prepped.

Try taking that gasket out. Seems like it could be your problem, maybe.

Going to give it a shot... here's the instructions I was referring to if anybody has any interest.

http://prestoliteperformance.com/Por...m_1082_all.pdf

I say it is:

By the way, boosting in 5th gear (under load) is a GREAT way to blow your un-tuned engine.

You have alot of load in 4th/5th gears, and will destroy your engine real quick if there is
something wrong with your tune.

Not to mention, you're running a carb that will run out of fuel
relatively quickly even in earlier gears. (I know I've said that before)

It's kinda hard to mess up boost-prepping an OER, considering they're boost-prepped from the factory. All you have to do is get a regular rebuild kit for it.

I agree with Directfreak, at this point I think the FPR is your most likely culprit.

defiantly the fpr .

Well aware of the dangers DF, but thanks for the heads up. I actually dyno-tune cars for a living. Unfortunately, right now we're in the middle of installing our new all wheel drive 424 xlc dyno jet and moving our 224 xlc from our old building. Otherwise I would have strapped it down and had a better view of what's going on.

When I roll onto it in 4th and 5th it actually maintains a pretty safe AFR, between 11.5 and 12.3, with an exhaust leak at the manifold flange. So I'm definitely being careful.

Just to update everyone, in case someone else is having the same problem. After doing some homework over on the turbo forums, (mostly a blow through turbo mustang forum), there where thoughts that the pulsing could be compressor surge. So as a test I turned the fuel pump on and disconnected the reference line. Then I blew into the line and watched the fuel pressure gauge. 3-6 psi, rock steady.... 7-13 psi, bouncing like a strippers tits..... 14 psi on, smooth as silk.

So looks like the FPR is having trouble maintaining pressure between 7 and 13 psi. So I just ordered up one of these [IMG] http://www.carshopinc.com/images/AFS/13301.jpg [/IMG]
seems to be the FPR of choice among the turbo mustang guys that are making less than 500-600 whp. So I'm hoping it will solve my issues.

Well aware of the dangers DF, but thanks for the heads up. I actually dyno-tune cars for a living. Unfortunately, right now we're in the middle of installing our new all wheel drive 424 xlc dyno jet and moving our 224 xlc from our old building. Otherwise I would have strapped it down and had a better view of what's going on.

When I roll onto it in 4th and 5th it actually maintains a pretty safe AFR, between 11.5 and 12.3, with an exhaust leak at the manifold flange. So I'm definitely being careful.

Just to update everyone, in case someone else is having the same problem. After doing some homework over on the turbo forums, (mostly a blow through turbo mustang forum), there where thoughts that the pulsing could be compressor surge. So as a test I turned the fuel pump on and disconnected the reference line. Then I blew into the line and watched the fuel pressure gauge. 3-6 psi, rock steady.... 7-13 psi, bouncing like a strippers tits..... 14 psi on, smooth as silk.

So looks like the FPR is having trouble maintaining pressure between 7 and 13 psi. So I just ordered up one of these http://www.dieselstorm.com/images/Aeromotive-13301.jpg
seems to be the FPR of choice among the turbo mustang guys that are making less than 500-600 whp. So I'm hoping it will solve my issues.

What did you have to begin with?

Mallory 4309.... bought it used and replaced the diaphragm. BTW, I pulled the gasket out last night and made no difference in the pulsation. I've read a couple posts on the turbo forum with guys saying the new diaphragm design for the 4309 is crap and is known for causing issues.


That S4 T2 motor I have torn apart in my garage and the megasquirt sitting in my closet are starting to look better and better... :) I'm sure my kid brother won't miss them at all.

nice alex :)

Blow-through setups become kind of hard to justify once you've actually tried it, don't they? :)

blow thru works well for alot of guys
this one lives at 14 psi all day long.................

http://www.youtube.com/watch?v=fPj2-afWOg0

I'm watching this thread like a hawk. I was about to order a 4309 for my own build. I may rethink that if the Aeromotive FPR works out better for you.

I'm unfamiliar with the carb you're using so I can't give you any advice there. Maybe you could try taking your boost reference closer to the turbo. That way any surge that comes back from the carb will get filtered out a bit on their way back through the system, especially in the intercooler.

I wouldn't recommend that because of the pressure drop across the intercooler. The closer to the float bowl you can take your reference, the better.

I don't know how his carb is set up... is there a source on the carb itself? If not, you could probably mod it to add one.

Cool Video...Except the last 3 seconds..

http://myspace.roflposters.com/image...5D.myspace.jpg

There isn't, but I wouldn't bother with that. The carb hat works fine as a reference.