502 Low Fuel Pressure. HELP!
06-29-2017, 09:03 PM
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502 Low Fuel Pressure. HELP!
Hey guys,
I'm chasing low fuel pressure on a stock gen 6 VST style 502. I have read and read and searched and tried everything I can think of or read about with no avail. Hoping someone can shed some light on this! Specs: 1996 502 mag with VST fuel system. Engine rebuilt about 100 hours ago down to the crank bearings.
I was experiencing what I thought was the classic vapor lock symptoms lately. Hard (or no) starting after sitting a couple hours at the sandbar etc. Fires up perfect once cooled. I was also getting the occasional stall when throttling down quickly coming off plane.
Put a fuel pressure gauge on because of the stalling and I'm seeing about 34 lbs at idle. When I throttle up a quarter and continue increasing RPM, the pressure drops to around 28 lbs and holds there while on the throttle. Pull back to idle and I'm at 34 again. I know its supposed to be around 38 or so at idle and up around 41ish at RPM. Here's what I've done so far...
New fuel separator in the fall (maybe have 5 hours on so far this summer)
Replaced mechanical fuel pump
The engine already has an inline electric booster in front of the mechanical fuel pump
Replaced raw water sea pump with a new merc pump (had gas in the raw water pump) - no change
Replaced VST fuel pump and filters with new - no change
Replaced fuel pressure regulator - same symptoms with the original - no change
When I pull off the fuel pressure reg hose and put a thumb over it, the pressure instantly pops up about 5 or 6 lbs and holds there. Pressure jumps to about 39ish with the hose covered at about 1/4 RPM. The engine does have a K & N aftermarket flame arrestor. Not sure if that could play a role in manifold pressure to that degree...
I'm at a loss here. I've replaced everything that I can think of. Anyone have any other suggestions?? Adjustable fuel pressure regulator??
Thanks!
I'm chasing low fuel pressure on a stock gen 6 VST style 502. I have read and read and searched and tried everything I can think of or read about with no avail. Hoping someone can shed some light on this! Specs: 1996 502 mag with VST fuel system. Engine rebuilt about 100 hours ago down to the crank bearings.
I was experiencing what I thought was the classic vapor lock symptoms lately. Hard (or no) starting after sitting a couple hours at the sandbar etc. Fires up perfect once cooled. I was also getting the occasional stall when throttling down quickly coming off plane.
Put a fuel pressure gauge on because of the stalling and I'm seeing about 34 lbs at idle. When I throttle up a quarter and continue increasing RPM, the pressure drops to around 28 lbs and holds there while on the throttle. Pull back to idle and I'm at 34 again. I know its supposed to be around 38 or so at idle and up around 41ish at RPM. Here's what I've done so far...
New fuel separator in the fall (maybe have 5 hours on so far this summer)
Replaced mechanical fuel pump
The engine already has an inline electric booster in front of the mechanical fuel pump
Replaced raw water sea pump with a new merc pump (had gas in the raw water pump) - no change
Replaced VST fuel pump and filters with new - no change
Replaced fuel pressure regulator - same symptoms with the original - no change
When I pull off the fuel pressure reg hose and put a thumb over it, the pressure instantly pops up about 5 or 6 lbs and holds there. Pressure jumps to about 39ish with the hose covered at about 1/4 RPM. The engine does have a K & N aftermarket flame arrestor. Not sure if that could play a role in manifold pressure to that degree...
I'm at a loss here. I've replaced everything that I can think of. Anyone have any other suggestions?? Adjustable fuel pressure regulator??
Thanks!
06-29-2017, 10:23 PM
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I'll also add that I scanned the motor and have no codes.
Additionally, when I first kick the key and the pumps prime, I hit about 40lbs. As soon as she fires over, pressure drops to the low 30's...
Additionally, when I first kick the key and the pumps prime, I hit about 40lbs. As soon as she fires over, pressure drops to the low 30's...
06-29-2017, 10:31 PM
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Put a fuel pressure gauge on because of the stalling and I'm seeing about 34 lbs at idle. When I throttle up a quarter and continue increasing RPM, the pressure drops to around 28 lbs and holds there while on the throttle. Pull back to idle and I'm at 34 again. I know its supposed to be around 38 or so at idle and up around 41ish at RPM.
Thanks!
06-29-2017, 11:08 PM
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06-29-2017, 11:10 PM
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The stalling does still occasionally occur if its running with about a 1/4 throttle and I chop it to idle on the trailer...
Occasionally, I'll also get a slight lope at idle after backing it down until it settles out or I bring the throttle back up and ease it down.
Occasionally, I'll also get a slight lope at idle after backing it down until it settles out or I bring the throttle back up and ease it down.
06-30-2017, 12:48 AM
#6
Let me address several items:
1) This has nothing to do with your fuel pressure but the K&N filter will rob you of power. Arcticfriends (another OSO member here) did extensive testing the the stock flame arresting beat nearly everything out there.
2) The stalling you are referring too could likely be the IAC not 'catching' the idle transition. This may be the root of some of your idle problems. Having said that a super lean idle will also create a similar condition. Have your injectors been cleaned and flow tested? See if you can check the IAC operation.
3) I know the VST system has a nefarious history among many big block owners, specifically with regards to vapor lock. Having said that there is another issue that can arise in the VST that will nearly and perfectly mimic vapor lock but it is not vapor lock. Underneath the cover of the VST is another cap attached to the bottom of the VST cover with three stainless screws. This contains a diaphragm that is likely aged out and cracked. When this happens it will bleed fuel/fuel pressure through those little yellow hoses and into the intake manifold. For long shutdown periods after running fuel will percolate out and into the manifold which floods the engine. On restart it will appear to be vapor lock when its actually simply flooded. SO......it may be worth investigating to see if that diaphragm has aged out or cracked.
4) Testing fuel pressure on the trailer may be problematic because you actually never load the engine up to increase the MAP.
1) This has nothing to do with your fuel pressure but the K&N filter will rob you of power. Arcticfriends (another OSO member here) did extensive testing the the stock flame arresting beat nearly everything out there.
2) The stalling you are referring too could likely be the IAC not 'catching' the idle transition. This may be the root of some of your idle problems. Having said that a super lean idle will also create a similar condition. Have your injectors been cleaned and flow tested? See if you can check the IAC operation.
3) I know the VST system has a nefarious history among many big block owners, specifically with regards to vapor lock. Having said that there is another issue that can arise in the VST that will nearly and perfectly mimic vapor lock but it is not vapor lock. Underneath the cover of the VST is another cap attached to the bottom of the VST cover with three stainless screws. This contains a diaphragm that is likely aged out and cracked. When this happens it will bleed fuel/fuel pressure through those little yellow hoses and into the intake manifold. For long shutdown periods after running fuel will percolate out and into the manifold which floods the engine. On restart it will appear to be vapor lock when its actually simply flooded. SO......it may be worth investigating to see if that diaphragm has aged out or cracked.
4) Testing fuel pressure on the trailer may be problematic because you actually never load the engine up to increase the MAP.
Last edited by Trash; 06-30-2017 at 11:04 PM.
06-30-2017, 06:11 AM
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All you can check for fuel psi on the trailer is at idle.
Why ? Because when you rev up the engine with no load you will increase intake vacuum.
Since the psi regulator is attached to vacuum, more vacuum reduces fuel psi.
However, when in the water and driving, the more throttle the less intake vacuum. This causes the fuel psi to go up. This is why you want to check fuel psi while under way and all the way to full throttle. Fuel psi should keep rising the more throttle you give. More throttle is more load is less and less intake vacuum. If fuel psi actually goes down during this, then you have a fuel volume issue.
Why ? Because when you rev up the engine with no load you will increase intake vacuum.
Since the psi regulator is attached to vacuum, more vacuum reduces fuel psi.
However, when in the water and driving, the more throttle the less intake vacuum. This causes the fuel psi to go up. This is why you want to check fuel psi while under way and all the way to full throttle. Fuel psi should keep rising the more throttle you give. More throttle is more load is less and less intake vacuum. If fuel psi actually goes down during this, then you have a fuel volume issue.
Last edited by SB; 06-30-2017 at 08:15 AM.
06-30-2017, 07:44 AM
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Let me address several items:
1) This has nothing to do with your fuel pressure but the K&N filter will rob you of power. Arcticfriends (another OSO member here) did extensive testing the the stock flame arresting beat nearly everything out there.
2) The stalling you are referring too could likely be the IAC not 'catching' the idle transition. This me be the root of some of your idle problems. Having said that a super lean idle will also create a similar condition. Have your injectors been cleaned and flow tested? See if you can check the IAC operation.
3) I know the VST system has a nefarious history among many big block owners, specifically with regards to vapor lock. Having said that there is another issue that can arise in the VST that will nearly and perfectly mimic vapor lock but it is not vapor lock. Underneath the cover of the VST is another cap attached to the bottom of the VST cover with three stainless screws. This contains a diaphragm that is likely aged out and cracked. When this happens it will bleed fuel/fuel pressure through those little yellow hoses and into the intake manifold. For long shutdown periods after running fuel will percolate out and into the manifold which floods the engine. On restart it will appear to be vapor lock when its actually simply flooded. SO......it may be worth investigating to see if that diaphragm has aged out or cracked.
4) Testing fuel pressure on the trailer may be problematic because you actually never load the engine up to increase the MAP.
1) This has nothing to do with your fuel pressure but the K&N filter will rob you of power. Arcticfriends (another OSO member here) did extensive testing the the stock flame arresting beat nearly everything out there.
2) The stalling you are referring too could likely be the IAC not 'catching' the idle transition. This me be the root of some of your idle problems. Having said that a super lean idle will also create a similar condition. Have your injectors been cleaned and flow tested? See if you can check the IAC operation.
3) I know the VST system has a nefarious history among many big block owners, specifically with regards to vapor lock. Having said that there is another issue that can arise in the VST that will nearly and perfectly mimic vapor lock but it is not vapor lock. Underneath the cover of the VST is another cap attached to the bottom of the VST cover with three stainless screws. This contains a diaphragm that is likely aged out and cracked. When this happens it will bleed fuel/fuel pressure through those little yellow hoses and into the intake manifold. For long shutdown periods after running fuel will percolate out and into the manifold which floods the engine. On restart it will appear to be vapor lock when its actually simply flooded. SO......it may be worth investigating to see if that diaphragm has aged out or cracked.
4) Testing fuel pressure on the trailer may be problematic because you actually never load the engine up to increase the MAP.
IAC makes sense. I was leaning towards the low pressure lean idle being the cause. I'll call around today and see if anyone stocks them. I'll get the scanner back on and try to test it. Injectors have not been cleaned or flow tested.
I'll open up the VST tank again and take a look.
Going to try to get it on the water tonight or tomorrow and test again.
Last edited by RockandRoll24; 06-30-2017 at 07:47 AM.
06-30-2017, 11:32 AM
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You can buy the IAC at any autoparts store. You just need the cross-reference number. The mercury part is 3 times the cost. I have the napa number at home but you can probably search around on here and find the number.
