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'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...

​​​​​​​Is the above on the trailer or while on the water under load ?

​​​​​​​This is mostly while on the trailer. I have tested it in the water with the gauge on and it reads a few lbs more when under way (around 30-32)

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.

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.

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.

Doesn't surprise me on the filter. Bought the boat with the filter already on.

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.

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.

Installing a new IAC early tomorrow morning and water testing with gauge on. Thanks for the suggestions... keep em coming! Got next week off and I want this thing working!

I think I may have stumbled on something...

While replacing the IAC, I found the hose that exits the bottom of the throttle body where the IAC sits is T'd off into the mechanical pump vent line and the VST vent line. This doesn't make sense to me. When I rebuilt the motor, I replaced all of the old cracked vent lines with new lines. Those old lines were completely blocked with sludge. I routed all of the new hoses in the same manner that I had found them and really didn't think twice about where things were routed as I assumed it was correct. I have been searching trying to find a diagram of the vacuum and vent hose lines, but I can't seem to find one. So...

Can anyone tell me the correct routing of both the vacuum and vent hose lines??? Is there anything that is supposed to be T'd into the line running from the fuel pressure regulator to the intake plenum? Right now it's just one short line running from regulator to intake plenum.

Specifically, the hose barb directly below the IAC is what is currently T'd off to the mechanical vent (overflow) and VST vent lines... Is that correct?
http://www.mercruiserparts.com/conte...43A12_550.jpeg

Yes that is the correct routing. There is a yellow fuel vent line from the mechanical pump diaphragm dry side that goes to the T-fitting. There is also a yellow vent line from the top of the VST diaphragm dry side that goes to that T fitting. From there it plugs into the throttle body. Under normal conditions those lines should be dry and free of liquid (fuel).

The line from the mechanical pump is to allow fuel from a ruptured mechanical pump diaphragm to flow into the intake manifold rather than your bilge.

The line from the VST is basically the same safety net as the one from the mechanical pump. Allows for an over pressure in the VST to bleed into the intake manifold (or as in the case I mentioned previously when that inner VST diaphragm degrades).

Check to see that your vacuum line from the fuel pressure regulator on the fuel rail is in good shape and connected to the intake manifold. Without that working properly your fuel pressure could be lacking. There will be nothing T'd into that line. Its simply referencing manifold pressure.

Thanks! Will do...

Saga continues!

Got everything back together with the new IAC installed and it idles beautifully. MUCH better than before. I can also chop the throttle back and it settles right in with zero surge or stutter, so thanks for getting that figured out guys!

On to the the new issue... If I bring the throttle up past idle, the fuel pressure starts to come up and down and then plummets to zero and the engine dies. Re-prime the fuel pump with the key, pressure back up to about 38 (and holds). Idles great, but plummets again throttling up past idle.

Could air in the fuel system cause this? I ran a lot of air out via the fuel pressure gauge relief button while continuing to prime the pump. It's solid fuel running through the relief valve with no bubbles...wondering if air trapped somewhere could cause this?

Thoughts?

Went over all fuel line connections and everything was tight. I measured the volatge at the high pressure pump when the pressure drops to zero and the engine dies and it still maintains a solid 12 volts. So the pump is not stopping.

Another interesting detail...when the pressure drops to zero and I throttle up even higher, the fuel pressure will recover, come back up to 35, hold for a second and then dive to zero again. Its like the pump catches up and then it drops again.

Pulling my hair out over this as I was planning to use the boat all week!

Anyone? Ideas?

pump is not losing voltage. doesn't mean it is not stopping. I don't think a regulator will pop open and show 0 psi. but might be a good time to put clear howe on return and watch return flow. if high flow and no pressure, return-regulator issue. if no psi and no return, no fuel.

In case someone stumbles across this thread in the future, the problem was the high pressure pump in the VST tank. I swapped the old pump back in and pressure was back up and holding. Not sure if the new pump was faulty or incompatible. Had the boat out the past two days and it ran excellent. Best it ever has with the new IAC sensor.

Thanks for the help!

Great feedback. Thanks!

Great to see results posted and glad you git it figured out.