300xs Fuel/Air Pressure Questions

Hello all. Serial #1E051996. Shows 2013 on ID tag.
Anyhow, I wanted to check air and fuel pressures at rails. Made a gauge. My air pressure is 95 psi. The service manual states that 95 is in spec. The fuel pressure is around 111. Again, service manual states that 15psi difference is required. My Smartcraft gauge shows 10.1 psi fuel. That number has never changed. Finally, fuel pressure leaked down about 5 psi after 30 seconds. So, why are my numbers higher than the common 80/90 psi that I see on this site? Why does my Smartcraft gauge show 10.1 psi when the difference is over 15? And, is that leak down pressure within spec?

Thanks in advance. This all started with a rough idle/cut out occasionally.

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Originally Posted by TR1

Hello all. Serial #1E051996. Shows 2013 on ID tag.
Anyhow, I wanted to check air and fuel pressures at rails. Made a gauge. My air pressure is 95 psi. The service manual states that 95 is in spec. The fuel pressure is around 111. Again, service manual states that 15psi difference is required. My Smartcraft gauge shows 10.1 psi fuel. That number has never changed. Finally, fuel pressure leaked down about 5 psi after 30 seconds. So, why are my numbers higher than the common 80/90 psi that I see on this site? Why does my Smartcraft gauge show 10.1 psi when the difference is over 15? And, is that leak down pressure within spec?

Thanks in advance. This all started with a rough idle/cut out occasionally.

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Both pressure must be taken at the exact same time, requires dual glycerin filled gauges 0 to 150 psi, 1% or better gauge accuracy.
Smartcraft cannot report fuel pressures as there is no fuel pressure sensor on the motor, not sure what you were seeing but it is not fuel pressure. Even our scan tools cannot read and report fuel/air pressures.
95 air and 109 fuel + or - 1 psi. 14.2 psi spread. Leak down is fine. I would not be overly concerned at up to 15.5 spread.
I wont except lower than 14.0 spread on the low side as that will make the motor slightly lean. a little rich I am ok with.
Don or Joe may disagree with this, if so, Follow there advice as I consider them as the final word on this.

80-90 is for Optimax motors, not Pro XS or XS motors.
why do you need the pressures, are you having a specific problem?

Leon - Thank you for the response. I am having some rough idling in gear, with occasional engine cut-off. I have read that it could be air/fuel related. Hence the desire to check pressures. I wanted to rule out tracker valve and diaphragms before moving to reeds. I will pay the money for top tier fuel, Quick Kleen/Quick Care at 1 ounce each per 5 gallons fuel, Mercury Racing 2 stroke engine oil, and Premium Plus at about 200/1 with the fuel (lubricate injectors). But, I didn’t want to spend about $600 on a set of gauges. I figured I could get enough troubleshooting information from the cheap gauge I built. It is only a single, no drain valve; but it is glycerin filled, with a screw on fitting.
As for the Smartcraft, I am thinking now that I misread the manual. I think what I saw is actually oil pressure. I will double check tomorrow.

Thanks again!

First things to do:
Compression test
New OE spark plugs, critical that gaps be set exactly to specs, if approaching 1 year oil or 100 hours
Perform all maintenance that is due
Check availble voltage from all plug wire, must bridge at least .500 air gap
wet test wire and coils by spraying water mist while running, look for arching spark and listen for changes in how the motor runs
Do a wiggle test on all injector connectors while running to see if it has any effect on how runs
If 500 hours or more on motor have both direct and fuel injectors flow tested and cleaned (integrity injection, European marine)
Do not take them to just any shop that says they clean them, most cannot service these injectors properly, not at all the same as automotive.
This does not sound like a reed problem to me and the reeds in that motor are very durable and reliable
It would be useful to have the computer history scanned with a printout to read.
Really need the air/fuel pressures as I described above
PM me with a ph number if you would like to discuss this further on the phone

Joe is in Longview Tx, maybe close enough for you!

Thanks again Leon. I checked compression this morning. All between 126 and 131. Motor has 560 hours. I can’t send my fuel system in because it has corrosion on the bottom of the port fuel rail. Boat is used 98% of the time in salt water. Integrity Injection sticky shows a picture of corrosion which would cause the rail to be unserviceable.
Spark plugs are new. Correctly gapped. Spark plug wires are new.
Longview is about 7 hours from me.
I will do some wire checks. According to RPM gauge, it idles between 540 and 610.
Additionally, I looked at the Smartcraft ‘screen’. It shows ‘Fuel PSI’ with a picture of a gas pump. Always reads 10.1. Version 6.01.

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Originally Posted by TR1

Thanks again Leon. I checked compression this morning. All between 126 and 131. Motor has 560 hours. I can’t send my fuel system in because it has corrosion on the bottom of the port fuel rail. Boat is used 98% of the time in salt water. Integrity Injection sticky shows a picture of corrosion which would cause the rail to be unserviceable.
Spark plugs are new. Correctly gapped. Spark plug wires are new.
Longview is about 7 hours from me.
I will do some wire checks. According to RPM gauge, it idles between 540 and 610.
Additionally, I looked at the Smartcraft ‘screen’. It shows ‘Fuel PSI’ with a picture of a gas pump. Always reads 10.1. Version 6.01.

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This would make me concerned to the condition of the direct injectors. If corrosion is present in the air passages Direct injectors will be damaged and could lead to catastrophic motor failure!
Also quite possible of a thermostat stuck open on this motor, pull them to test or replace!

You cannot read FUEL psi or OIL psi on Optimax product, as there are no sensors whatsoever.

If you're seeing them, someone went in and turned on items that weren't automatically active when the Instrument auto-configured to the engine. Of course- if the engine was changed, it's always best to do a Master Reset.

Your pressures are not likely to be playing a role in your complaints and I would NOT recommend tearing anything on those rails apart unless you're prepared to buy a new set.

Let's check something totally different: Using a wire gauge OR a drill bit (locked in a pair of vise grips): Measure the throttle plate airgap at the highest point of the blade (closest to the flywheel). Need to know what size can be withdrawn without moving or "snapping" the throttle plate... and it's important you be as precise as physically possible. .005" can make a difference here.

First of all, thank you both. Second, several years ago I noticed the corrosion on the fuel rail. I cleaned it extensively and regularly inspect, re-clean, and coat in 2-4 grease.
Ok, I measured best I could. 5/64 bit was just a little too big. A wire I had that did fit with ease was about 0.064”. So, the opening is between 0.064 and 0.078. I know that’s a lot more than 0.005” precision.

Use a number drill bit set as they step up in very small increments, in the smaller sizes about .002

Ok. I’ve learned a few new things today. Didn’t realize they made numbered drill bit sets. I’ll have to buy that. And, didn’t know the Opti didn’t have fuel or oil pressure sensors for Smartcraft. Now I know. I messed that up when I reconfigured it.
Thanks!

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Originally Posted by TR1

First of all, thank you both. Second, several years ago I noticed the corrosion on the fuel rail. I cleaned it extensively and regularly inspect, re-clean, and coat in 2-4 grease.
Ok, I measured best I could. 5/64 bit was just a little too big. A wire I had that did fit with ease was about 0.064”. So, the opening is between 0.064 and 0.078. I know that’s a lot more than 0.005” precision.

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If you can verify this, I suspect there may be a problem here.

FIRST: Take a spray bottle of very soapy water, and with the engine idling, spray between the fuel rail and cylinder head, while watching for bubbles. Most likely is underneath Direct Injectors (smaller o-ring), which doesn't affect AIR pressure, but does create havoc with having proper air/fuel ratio, most noticeable at idle.

Problems noted: Correct them (new o-rings on Direct Injectors).

NO problems noted: Remove throttle plate, heat the stop screw to break the epoxy, and reset the plate to attain PRECISELY the correct air gap (.092-.093").

May not seem like much BUT... the difference between .078" and .093" is greater than 15% :eek:

Thank you sir. I’ll check that and repost.

Some updates.
1. I sprayed very soapy water around direct injectors. Did not see any bubbles or hear any change in motor.
2. Took epoxy off jam nut. Set screw did not move easily. Decided to try another route. Set a temporary shim (feeler gauge) under set screw). Ran motor. No change noted.
3. Took coils off. Tested resistance. No significant difference noted between the 6. And I had a new spare to test. I found a 6x6 table with specifications to test a coil. It was in another Mercury Post. I could not find the specifications in my SM. No test instructions either. Based on what I could gather from the other post, my results for all 7 coils were way out of spec. Not sure what is going on. Anybody have the method and specifications?
4. Bought ignition tester. Spark seemed like it might have some inconsistencies. Tested 3 coils. All seemed about the same. Decided to change focus again while hoping to get an answer on coil specifics.
5. Bought a quality gauge. +/- 1%. Retested fuel rails. Pretty steady at 96. Bounced a bit between 95-97. Fuel was 109-110. If we average that split, gives us 13.5. Not good.
6. I still have great compression; not going to take a chance. I pulled the fuel rails. #6 direct injector had discoloration around small o-ring going into cylinder. Looked clean on head though.

I will need a new port rail due to corrosion on the bottom. And I want to get the starboard side cleaned and inspected. And I want to get the direct injectors cleaned and inspected. So, do I just buy a new complete fuel rail (both sides) setup and just send the DIs in to get serviced? Or do I send in my old fuel rails also and see what can be salvaged? New complete rails are about $2100. Port rail is around $1200. By the starboard side is cleaned and tested, might be cheaper to just get the whole new complete set.
I’m leaning toward a new complete set and just sending DIs to Don.

What’s best in the long run?

Thanks in advance.

T

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Originally Posted by TR1

Or do I send in my old fuel rails also and see what can be salvaged?

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Sending the rails and injectors to Don is, in my opinion, is the best course of action. Let him evaluate the entire system before buying any parts.

Thanks Silverfox. When Mr. Don replies, I’ll take his advice.

Send them in - depending on the severity I may have some options to repair. And worst case, I can sell you a new set with a BBC discount (rails).

However: On the throttle plate, I can tell you from experience: You must apply HEAT to the stop screw after breaking the epoxy away. OFF the engine.

Make that adjustment dead-nuts precise, too. JUST making the adjustment may not resolve the problem: If the engine has been idling with 15% less air than it should have, the plugs have been fouled. Most I've corrected ended up with a new set of plugs.

Yes sir. I’ll have them heading your way today.

Ok, we will make it hot…off the engine.

‘Dead-nuts’, haven’t heard that in a coon’s age. Old school. I like it. Dead-nut precise will be done.

Thanks Don. Looking forward to doing business for you. By the way, there was indications of a possible leak around the #6 DI. The o-ring was twisted and pinched.

T