Fuel Pressure Drops Like a Hurricane!
Thread Starter
Racer
Joined: Oct 1999
Posts: 432
Likes: 19
Fuel Pressure Drops Like a Hurricane!
I just started playing around with my LT1, getting it into boost. I thought I had a fuel filter issue so I changed it last night. I've got a Vortech kit on my LT1 that is basically stock with 30lb injectors and an FMU. I've got a 255lph Walbro fuel pump in the tank. When I go into boost, the fuel pressure drops off significantly. I don't see anything over 55-59psi and if I stay in it, I can watch it drop off. I took the fmu apart to be sure that it wasn't plugged anywhere and when I put a pump on it, it does increase the fuel pressure. I didn't have the engine over more than an idle when I hooked the fmu up to the pump so that obviously would use more fuel. I remember seeing more fuel pressure (at least into the high-60s) at one time with another supercharger so I'm wondering what could account for the difference. Any advice?
I also pulled the fuel pump and sending unit and everything looks good there.
I also pulled the fuel pump and sending unit and everything looks good there.
Thread Starter
Racer
Joined: Oct 1999
Posts: 432
Likes: 19
Highest boost pressure so far is about 6lbs. I've not wrung the car out yet, just getting started tuning the upper end. As for the FMU ratio, how do you go about figuring that out? Is there a way by looking at the inside of the FMU? It is the original FMU from the Vortech kit.
Sorry I don't have more concrete info yet, still working this all out.
Sorry I don't have more concrete info yet, still working this all out.
Pro
Joined: Oct 2004
Posts: 628
Likes: 6
Interesting. I have no experience with FMUs - my 12psi setup uses a FPR on a 1:1 basis so the maximum FP on mine (using 60# injectors) is about 55psi (based on 36psi at idle) - which is supposed to be ok.
I'm going to go out on a limb and suggest that 55psi is more than enough for your setup, and therefore "safe". However, it doesnt explain why your pressure drops off, and I dont have any ideas (outside of the usual things) why that is happening.
(edit) I have heard of people who have problems with belt slip at high RPMs, and therefore the boost drops off, and therefore the Fuel pressure drops off - is it possible that this is happening?
I'm going to go out on a limb and suggest that 55psi is more than enough for your setup, and therefore "safe". However, it doesnt explain why your pressure drops off, and I dont have any ideas (outside of the usual things) why that is happening.
(edit) I have heard of people who have problems with belt slip at high RPMs, and therefore the boost drops off, and therefore the Fuel pressure drops off - is it possible that this is happening?
Supporting Vendor
Joined: Oct 2004
Posts: 761
Likes: 68
If you want to use the FMU you MUST have the in-line booster pump that came with the original kit. If no have then get GSL-392 in-line pump to mount behind license plate. I have made many posts as to why.
If you want to run the in-tank pump only then put a set of 60's in and a new chip.
Remember the GSS-340M ( and most other in-tank pumps like Delco) have a 60 psig relief valve built into them which negates FMU usage w/o the in-line booster pump.
If you want to run the in-tank pump only then put a set of 60's in and a new chip.
Remember the GSS-340M ( and most other in-tank pumps like Delco) have a 60 psig relief valve built into them which negates FMU usage w/o the in-line booster pump.
Drifting
Joined: Dec 2001
Posts: 1,313
Likes: 1
From: providence ut
I'm running a walbro 255 and an fmu ............100 psi is no problem.........I bleed off boost pressure to the fmu to keep it lower.............also no in line boost pump..........but great information thanks.
Safety Car
Joined: Apr 2001
Posts: 3,525
Likes: 2
From: Houston TX
I have run several combos with the 255Walbro (low boost combos), and have had very different results from what leadfoot 85 is describing. I agree with Greg, you need the booster pump (T-Rex I believe for the Vortech) if you want to retain the FMU.
Aaron
Aaron
Drifting
Joined: Dec 2001
Posts: 1,313
Likes: 1
From: providence ut
I respect both your opinions and knowledge..............but I am running a fmu and ONLY an in tank walbro 255 up too 100 psi...............for 6 years now........usually running at 70 psi and when tuning, which I just did @ 100 psi............I guess something is weird with my fuel pump, maybe the older ones didn't have the relief valve.............I am on my secound fmu...........same pump.
Regards Bert
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Melting Slicks
Joined: Jan 2002
Posts: 3,293
Likes: 79
From: Jackson MI
If he's loosing fuel pressure then there are only a few options as to what could be wrong:
1.) Pump is not keeping up with fuel/pressure requirements
-Could be the problem
2.) Clogged fuel filter
-Replaced already
3.) Regulator/FMU is faulty at high boost
-tested, so it should be ok
4.) Injectors are opening full up and dumping all the fuel they can
-even if they opened up all the way the pump should keep up
5.) leak in the system
-This would be very bad and easily noticeable with the car running in the garage
Fluid in has to equal fluid out, so either there isn't enough fuel getting in or the fuel is getting dumped out somewhere. Since the fuel pressure is dropping at high rpm and not staying constant like with an FMU failure I would guess that there is some issue with the pump. Maybe a clogged pump filter, the non-high pressure walbro or something of that matter. Adding the extra pump couldn't hurt and is a good piece of mind on your fuel flow.
1.) Pump is not keeping up with fuel/pressure requirements
-Could be the problem
2.) Clogged fuel filter
-Replaced already
3.) Regulator/FMU is faulty at high boost
-tested, so it should be ok
4.) Injectors are opening full up and dumping all the fuel they can
-even if they opened up all the way the pump should keep up
5.) leak in the system
-This would be very bad and easily noticeable with the car running in the garage
Fluid in has to equal fluid out, so either there isn't enough fuel getting in or the fuel is getting dumped out somewhere. Since the fuel pressure is dropping at high rpm and not staying constant like with an FMU failure I would guess that there is some issue with the pump. Maybe a clogged pump filter, the non-high pressure walbro or something of that matter. Adding the extra pump couldn't hurt and is a good piece of mind on your fuel flow.
Supporting Vendor
Joined: Oct 2004
Posts: 761
Likes: 68
The GSS340 will flow around 30 gph @ 100 psig, that's enough to make around 360 HP. The GSS340 flow chart is on my website under the downloads section. You can epoxy close the relief valve but if you must work at the higher pressures dictated by a FMU (60-100 psig) 2 pumps in series will last much longer than a single pump because they share the load. 2 pumps in series each pump pulls 7 to 8 amps (basically coasting) compared to the 15 amps the GSS340M pulls at 100 psig by itself.
Thread Starter
Racer
Joined: Oct 1999
Posts: 432
Likes: 19
All good information! Thank you to all who posted. I bought a mechanical fuel pressure gauge and noticed that what my electronic (in cabin) gauge was saying was wrong! Way wrong by about 10 psi at idle. I didn't purchase a gauge with a long enough line to be able to close the hood and still see the gauge so I'm going to borrow a friend's gauge so I can check it under load.
I have removed the sending unit/fuel pump and checked for leaks but could not find any. I've tore the FMU apart and couldn't find anything there so, at this point, I'm thinking that the guage may be at fault. Until I'm sure, it's a nice, easy ride around with little or no boost until I can figure out exactly where the problem lies. I'll let you all know.
I have removed the sending unit/fuel pump and checked for leaks but could not find any. I've tore the FMU apart and couldn't find anything there so, at this point, I'm thinking that the guage may be at fault. Until I'm sure, it's a nice, easy ride around with little or no boost until I can figure out exactly where the problem lies. I'll let you all know.
Thread Starter
Racer
Joined: Oct 1999
Posts: 432
Likes: 19
Just asking, but wouldn't that give me MORE fuel, instead of not enough? Would it affect fuel pressure that drastically? I realize that the "larger opening" would be less restrictive but, isn't the pressure really dictated by the FMU.
I have the pump that originally came with the kit and I think I might install it just to check and see if it helps.
A friend mentioned that Rippie sells some sort of monster Bosch pump that fits in the sending unit, has a bottom-entry and is supposed to keep up with the engine's needs. Anyone used this one? Is this the GSS340M that you are referring to Greg?
I have the pump that originally came with the kit and I think I might install it just to check and see if it helps.
A friend mentioned that Rippie sells some sort of monster Bosch pump that fits in the sending unit, has a bottom-entry and is supposed to keep up with the engine's needs. Anyone used this one? Is this the GSS340M that you are referring to Greg?
Thread Starter
Racer
Joined: Oct 1999
Posts: 432
Likes: 19
Update,
I installed the T-Rex pump from Vortech and now I'm seeing a nice increase in fuel pressure. No more drop as the RPM and boost increases. I haven't wrung it out yet as I want to get on a friend's dyno and use his wide-band O2 sensor. But, as most have said on this subject, the in-line pump seems to have solved the problem. Thanks for the information!
I installed the T-Rex pump from Vortech and now I'm seeing a nice increase in fuel pressure. No more drop as the RPM and boost increases. I haven't wrung it out yet as I want to get on a friend's dyno and use his wide-band O2 sensor. But, as most have said on this subject, the in-line pump seems to have solved the problem. Thanks for the information!
Pro
Joined: Dec 2008
Posts: 658
Likes: 0
From: Lubbock for now, headed for Oklahoma
I just started playing around with my LT1, getting it into boost. I thought I had a fuel filter issue so I changed it last night. I've got a Vortech kit on my LT1 that is basically stock with 30lb injectors and an FMU. I've got a 255lph Walbro fuel pump in the tank. When I go into boost, the fuel pressure drops off significantly. I don't see anything over 55-59psi and if I stay in it, I can watch it drop off. I took the fmu apart to be sure that it wasn't plugged anywhere and when I put a pump on it, it does increase the fuel pressure. I didn't have the engine over more than an idle when I hooked the fmu up to the pump so that obviously would use more fuel. I remember seeing more fuel pressure (at least into the high-60s) at one time with another supercharger so I'm wondering what could account for the difference. Any advice?
I also pulled the fuel pump and sending unit and everything looks good there.
I also pulled the fuel pump and sending unit and everything looks good there.
Pro
Joined: Dec 2008
Posts: 658
Likes: 0
From: Lubbock for now, headed for Oklahoma
Supply and demand. You may be using more fuel at the rail than is being delivered through the stock diameter fuel lines. Also you may check for fuel filter crush inside. Even with a new one you may be collapsing it. Lastly either your FMU or FPR could be at fault.
Le Mans Master
Joined: Aug 2005
Posts: 5,375
Likes: 328
From: CA.
St. Jude Donor '13-'14-'15
One question Glad you sorted it out But why the 42 Lb injectors for an un boosted engine ?
Seems very large to me and very difficult to slope the injector curve properly Curious !!
Seems very large to me and very difficult to slope the injector curve properly Curious !!
