mmdehaven

Has any body seen this before?
I currently run a ’94 Trans Am with the following setup. Short block pulled from ’94 Corvette all stock, not an LT4 either. 1.6:1 roller rockers, 42lb. Bosch injectors, Walbro 255LPH in-tank pump, Aeromotive Adjustable fuel pressure regulator, Hooker shorty headers with Edelbrock Cat-back less the cat, Jet MAF sensor, MSD 6BTM ignition, Pro Charger P1SC 7lb. boost with single cooler, Snow Stage 2 Methanol injection system, Moser 12 bolt with 3.73:1 rear end and a T56 6Spd transmission.
I have reprogrammed the ECU with various different tunes including one custom built for the above description by a reputable source. The issue I am having that I cannot seem to resolve is this. At full throttle or at least more than 85% throttle, the car will just fall on its face a 4500 RPM. There is no missing, no sputtering, no weird noises at all. It just goes from 4500 RPM down to 2000+ RPM and loses power. Both the boost gauge and fuel pressure gauge drop from about 7 PSI boost back to vacuum and fuel goes from about 50 PSI to below 30 PSI. I have replaced all of the following parts as a test and varied the fuel pressure from 43 PSI to 50 PSI to try and compensate. MAF sensor, MAP sensor and the ignition module.
I can run the RPM up past the 4500 RPM mark into the 5000+ area if I do not exceed the 85% throttle mark. If I put a 5000 RPM pill in the BTM it will bounce the limiter and pull the ignition as expected giving me the indication that the coil and ignition components are working properly.
If I prime the pump with the ignition in the ON position, I have the regulator set to 48 PSI the pressure will drop almost immediately. I read that is because the Walbro pump does not have a check valve in it. Is there a way I can test the fuel system to see if it is unable to feed the motor at full throttle conditions, or is there something else I am missing?
Any help would be greatly appreciated.

hellbents10

iTrader: (10)

If the fuel is dropping to 30psi, then you have found your problem. It is a bad pump, or voltage problem to the pump.

mmdehaven

I know that sounds like the obvious answer, but the problem I have in identifying the actual cause is I am not sure if the pressure drops because of the immediate drop in RPM going back to a vacuum from pressure etc...
Do you know of a good way to verify the fuel pump? Is there a good way to test it under a high load? Also, do you think the Walbro 255LPH pump is sufficient for my setup?

Thank you very much for the input!

Striker289

iTrader: (1)

My Buddy had a problem with leaning out bad with his 255 on his LT1. Found out it was the rubber hose supplied with the pump expanding. Does your pump have a rubber hose, if so is it rated for being submerged in fuel?

mmdehaven

I did find a few articles while searching the web for a little help. I guess that looks like a good culprit at this point. I will look at the pump and see what I find. I will follow up on this post afterward.

Again, thank you for the advice.

Twin turbo c5

I've had the same problem before. And yes your fuel setup is fine for your setup. Are you sure its leaning out i always had trouble on my turbo lt1 with the opti spark. My car wouldn't get on the boost at all with the stock ignition it would blow the fire out and fall on its face around 4500.

mmdehaven

What did you do for the ignition? It was the same issue at 4500 RPM? I do hit 5+ PSI of boost, then 4500RPM flat on its face. Like I said in the initial post, no misses to backfires nothing. Just sound vicious one second, then falls on its face. I get out of the throttle and normal operations like nothing happened. That is why I am so confused.

mmdehaven

I replaced the in-tank fuel pump with a Walbro HP 255LPH and I still see the same issue. I do think the RPM that it falls on it's face may be a few hundred RPM higher now. Maybe around 4800-4900, but the same issue happens. The boost drops to a vacuum and the fuel pressure and RPM drop. I get out of the throttle and its business as usual. It does no coughing, sputtering or missing, RPM just drops till I get out of the throttle. Does anybody else have any ideas? I may look at the Aeromotive adjustable pressure regulator now, but replacing all these parts with no success is getting a little expensive and troubling.

05_quick4door

iTrader: (1)

fuel filter?

mightyquickz28

iTrader: (8)

Are you logging this at all to see what is transpiring? I'd like to see that your O2 readings are at the point it falls flat. Also, pulse widths, KPA and MAF readings upto that point and beyond.

Are you running a 1 bar MAP sensor or 2 bar?

I was experimenting with a SD tune and going back and forth with the 1bar sensor MAF tune and the 2 bar SD tune and I encountered what you have seen. Not saying it's the same thing but just found it interesting that your engine is behaving that way. Have you tried changing anything about the tune?

mmdehaven

Yes, I did also replace the fuel filter.
I am now running with a one BAR MAP. The gentleman that I had build my tuning file recommended I drop my two BAR MAP as he stated it is easier to tune. I am very confident it cannot be anything with the tune, as I have had at least 6 different files loaded on my ECM from various sources including my stock file. I have had three different MAP sensors on the car with the exact same symptom.

I do have logging data, but I will need to make another short run and I will post the results no later than tomorrow to this blog. Hopefully somebody can help me figure it out.

I am going to look at my Aeromotive fuel pressure regulator again tonight. I was told be a local source there have been quite a few issues with them in the past. He believes the boost may not be being compensated by the regulator as it should. I just think it has something to do with electrical as opposed to mechanical due to the lack of any back fires or misses etc... seems like it just shuts down. I plan on trying to put a mighty-vac pump on the regulator to simulate boost and see if the pressure will continue to rise or drop at or around 7 to 9 pounds.

S10LSa

Whats the wideband read? What do the narrowbands read as well while you are driving around?

mightyquickz28

iTrader: (8)

I would definitely like to see a datamaster log of the seconds before, upto and after the point where it falls over. I think that will tell us a lot.

I've got over 80 log runs with mine and in some iterations of tunes, as soon as the MAF maxed out (471) the car would nose over, the duty cycle/pulsewidths would be something ridiculous(impossible number) and the engine got no fuel till I let off the gas. No popping or sputtering. It was like a switch was thrown.

Looking forward to the log so we can view it.

Right now, my tune is pretty much perfect. It's a MAF tune, 2 bar MAP sensor, my prior mail order tunes VE tables, my mail order tunes spark tables but compressed 20 - 100 KPA into to the 20 - 50 KPA and made 55 - 100 KPA all the same as the 100KPA was. I also left the Injector constant as the poundage they really are, not halved like the SD tune.

Result: a tad lean at idle but under acceleration, cruise, its right at 14.5 on my wide band and 10.5 under boost at 12 psi. A rocket to ride

mmdehaven

I currently do not have a wideband on the car, that should show up today or tomorrow. As far as logging I will get the those posted later today. I appreciate everybody's help and input on this too!

Old Geezer

iTrader: (3)

Ck what TJ says...
If you have the fp set at 58, and it drops to <30. That will, no doubt be a good chance at being the problem..
How is the system plumbed?

Bob@BruteSpeed

iTrader: (16)

I wouldn't mess with it until you get the wideband installed, as you want to make sure you're not going lean under boost. Bob

helga203

Any progress??? Mine is doing the same thing but at 6100rpm just shuts off. I have 1999 c5 corvette. Any ideals? I was logging atg the time also but didn't show any info that helped. Just shuts off then comes back on like nothing happen. No codes nothing. Wierd.

mmdehaven

I just got back from an out of town emergency trip, so I am hoping I did not lose any support on this post. I still do not have the wideband installed, but hope to get it done today.

I've included a datamaster .uni file to check values if anybody can help. It's in a zip file format.

I really appreciate any help anyone can offer.

conor.davis

iTrader: (20)

Just gotta ask ...where's your FPR plumbed

mmdehaven

I am using an Aeromotive adjustable unit, and it is located in the same location as the original at the back of the fuel rail. Does this matter with what I have going on or are you just looking for ideas?