Hey guys. Fuel Pressure is dropping all the way out under WOT/boost conditions. It drops all the way down until the car shuts off. A turn of the key to off (pump cycle) gets the car started again. No issues when not boosting. Thoughts? Thinking maybe the inline ATI pump is out/bad ground. Let me know what you think. Already changed pump and filter, no restrictions found in lines. Screwed with FMU a little, could a turn in the wrong direction do this?
-Dave
96', LT4, Procharger

 

drochau,
My apologies for not replying to your direct email. I sent to work so that I could reply, and I have been swamped.

On to the issue. Has the car run under boost prior and "developed" this problem, or is this following a blower install? Are you running the ATI supplied FMU (either standard FMU or ATI "super FMU")? If so, the FMU is a linear response unit, and you should be able to adjust the needle screw in or out and see an affect on fuel psi. I have a graphic detail of the linear adjustment on the super FMU and what impact the position of the screw has on the resultant fuel psi (unfortunately, its at work, but can be had next week).

Are you running a fuel psi guage so you may be able to see fuel psi under WOT? You speak of it dropping out until the car dies. What is the actual fuel psi dropping from/to? The ATI booster pump is merely a high psi unit that allows the psi to spike when the return is restricted vs the factory pump which pushes volume.

Yes the ATI pump would not psi the system under boost if it were bad, but your factory pump should at least keep the factory rating (i.e. 50-55 psi), unless the drain on the system is so far beyond what the factory system was designed to handle. The factory pump should be able to readily handle 460RWHP.

What mods are done on the car? What is a realistic estimate of the RWHP of your car? Did you change the undercar filter, or are you referencing the screen on the fuel pump that you changed?

Let us know the answers to the above, and possibly we can lead you in the right direction.

Off to mow the grass.

Aaron

 

Mods: LT4 stroked to 383 with forged internals. Large injectors -42# I believe, though I cannot find the reciept at the moment. Exhaust, smaller pulley on crank, I think a 2.8? Car ran exceptionally well for the past two months. Had a no spark condition, repaired and then a fuel pressure issue that I am currently dealing with.
This is not a new install. The car was put back on the P1SC (previously a D1) due to head unit problems (sounded really, really messed up and wasn't making very good boost. Sent back to ATI for diagnosis). I changed the in-line fuel pump under the car. With the D1 running at 15 psi we saw a peak rwhp of 530rwhp. I'm not sure what the P1 is making for rwhp, but before the stroker motor I was seeing 453 rwhp at 8-9 psi of boost.
This problem came on about three weeks ago. Some wiring (stupid scotch adapters) came loose and I sent it into the shop to get the entire harness under the hood soldered to avoid future problems. Soon after, I had fuel pressure problems with the pump not turning on while the car was running. I replaced the fuel pump with a stock pump (the one I pulled looked to be stock as well) and the car idles and revs fine in neutral.
I am certainly not seeing 55 psi of fuel pressure even at idle. It seems to be between 42-46 psi at idle with it being slightly higher when the lines are charged at key turn-on.
I haven't noticed an appreciable change at idle from turning the screw on the FMU. Per ATI's installation tuning page, I started the tuning process at two turns from closed. Idle fuel pressure did not change, I don't think the boost fuel pressure went up either, though I will check again.
To reiterate, the car will go to redline fine in the garage, only under WOT conditions, to 6 psi of boost, while driving does the fuel pressure (as indicated by the in dash gauge, verified to be operating correctly by autometer yesterday) drop all the way down until the car shuts off. A turn of the key to off and then on recharges the line to 44 psi and the car starts right up.

Thank you so much for the assistance, I have been supremely screwed over by my speed shop and will not be wasting anymore money on their "good enough" installation and repair jobs.
-Dave

 

Dave,
This sounds like a multitude of problems. From your description, it sounds like the booster pump is not coming on and the stock fuel pump would be an issue as well.

Let's step back for a moment. The D1 combination produced 530RWHP, so I would imagine that the P1SC would be very close to that as well (maybe 490-520RWHP). The stock pump should handle about 440RWHP. The pump you pulled out was probably a Walbro, which looks very similar to stock. You need to put an aftermarket pump back in the tank. I ran twin Walbros on a -8AN line feeding my old combination that was good to 570RWHP.

Secondly, you speak of the schotch adapters causing problems and then having all connections soldered. The feed for the ATI booster should come directly from the (pink, I believe) feed to the intank pump which energizes the relay. The actual pump voltage should either come from the battery junction block or directly from the alternator discharge (i.e. 14+VDC). It sounds like the booster is not coming on. You should be able to probe the wiring to determine if it is on even at an idle (engine must be running to have the pumps running). If the booster is not running, then it acts as a restriction in the line. This restriction, along with the large draw will cause the fuel pump to be outside of its capacity and will overload causing it to "thermal" out (shuts off itermitantly).

The fuel psi should be 39-44 psi at an idle. You should see an additional 3-4 psi going from high vacuum (idle) to atmospheric (WOT, no boost), and an additional 1 psi for every pound of boost that is referenced. Total fuel psi for your combo should be say 42 psi at idle and 55 psi at 10 lbs boost, 60 psi at 15 psi boost. No matter what you do with the FMU, it will do nothing at an idle. The FMU restricts the return line only when positive manifold psi is seen. Set the FMU to ~2.5 turns out when starting the tune (only after installing a decent in tank pump).

Additionally, no more WOT rap to redline in the garage. That is not only hard on the engine (no load), but diagnoses nothing due to loading.

Bottom line is, check the booster wiring and get a decent pump back in the tank. I would hate to see you waste a nice 383 due to a $100 pump.

I hope this helps. If not totally clear, feel free to call me at the number I have provided.

You will get there,
Aaron

 

Checked out my old fuel pump. It doesn't have black tops/bottoms like the Walboro and there is "AC" molded into the bottom peice. It looks like it used to be white on the top and bottom, though now it is a beige color. The sock that was on the unit was black and thick. I ordered the 255 lph Walboro kit from Racetronix. No idea when that will show up. I checked to see if the fuel pressure is going up under boost (no more WOT till racetronix kit arrives). It is steadily rising with the boost levels. I didn't go to full boost for obvious reasons though during vaccum after a boost run the fuel pressure dropped suddenly and I voluntarily shut the car off when it dropped to 20 psi. Is this a symptom of a failing fpr?
Car also died at idle today while I was inspecting the ATI inline pump. I have voltage on the pump and it audibly seems to be running as usual. The a/c had previously been on, but the fuel pressure dropped and the car died with the a/c off (perhaps a minute or two later, I'm assuming unrelated.)
-Dave

 

So would the stock pump be overloaded even at idle? I'm confused because everything seems to be normal and then at times (like yesterday in the driveway) the car lost fuel pressure in the driveway(incline) and died. What would be the symptoms of the fpr going down? I'm a little confused.
-Dave

 

Found something weird. The inline pump had been relocated to beside the spare tire carrier and wrapped in header wrap right next to the exhaust! Now this doesn't explain my boost fp loss issues, but I knew that it seemed like a silly place for an inline pump so I relocated it to behind the license plate.
Update on FPR: I can hear the FPR making "squishy" sounds when I turn the key on before start up. Could this possibly be failing? no one seems to have much experience with a FPR on it's way down the drain, only when there is gasing spewing through the vacuum line.
-Dave

 

This is a great big THANK YOU to everyone who helped me out with my problems. She's running excellent now(though a little rich) and pulls just as fast as before. No more stalls, no more fuel pressure issues. Thank you all! Should any of you make it out to Albuquerque NM anytime look me up and I will buy you a frosty beverage of your choice!
-Dave