Mercruiser Leak Down Test

Thomv

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I am still experiencing hard starting on both of my Mercruiser 7.4 MPI engines after they have been shut off for about three hours on the water. Someone suggested I do a leak down test on the fuel pressure system. Does anyone know, once I attach the gauge to my schrader valve on my fuel rail, what pressure I should be reading when the engines are running? That would be a good starting point to hopefully fine that fuel is slowly leaking into my combustion chambers causing a flooding condition due to some faulty fuel injectors?
 

alldodge

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I am still experiencing hard starting on both of my Mercruiser 7.4 MPI engines after they have been shut off for about three hours on the water. Someone suggested I do a leak down test on the fuel pressure system. Does anyone know, once I attach the gauge to my schrader valve on my fuel rail, what pressure I should be reading when the engines are running? That would be a good starting point to hopefully fine that fuel is slowly leaking into my combustion chambers causing a flooding condition due to some faulty fuel injectors?

It sounds more like a vapor lock issue. The leak down test tells you if your valves, head gasket and combustion chamber is in good shape. If you had weak compression your engine will still run. If you think it is flooding, move your throttle handle forward in throttle until you start to fill the secondary's start to open. This puts your engine in flood mode and the ECM will stop all fuel going to the injectors. Crank it over a bit to expel fuel. Return throttle handle and try to start.

Now if it's vapor lock, find the Schrader valve on the fuel rail and push in the center of the valve with a rag over the end. Turn key to run a couple times to build pressure and repeat.
 

Bondo

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The leak down test tells you if your valves, head gasket and combustion chamber is in good shape. If you had weak compression your engine will still run.

Ayuh,.... That was My 1st thought, but I believe Thomv wants to know what pressure is in the fuel rail, motor Off,....

Not a cylinder leak-down test,....
 

alldodge

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Ayuh,.... That was My 1st thought, but I believe Thomv wants to know what pressure is in the fuel rail, motor Off,....

Not a cylinder leak-down test,....

If that's the case then need to do a fuel injector balance test. Should be 38psi at start and reduce to 19psi (300ms) using scan tool. Above 19 will run rich, below runs lean
 

achris

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So why is the thread title "Mercruiser leakdown test"....??? Fuel rail pressure and leakdown are 2 different fish.
 

alldodge

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Guess it could be from one of thise cartoons I use to watch when I was a kid "search me blab" Badee, bade, bade ... that's all kolks :D
 

Thomv

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Sorry for not getting back sooner. I had a major computer breakdown which has finally been corrected. What I am trying to do is a fuel pressure leak-down test on my injectors. I did purchase a fuel pressure gauge from Advance Auto. Upon hooking it up I was not able to get a reading from the gauge. Looks like the gauge fitting is not compatible with the Mercruiser schrader valve connector. The fitting does screw down on the threaded schrader valve but no reading. I think Mercruiser has their own style of valve that only will work on "their" own fuel pressure gauge. And yes, I did word it wrong on my title for the thread.
 

alldodge

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Sorry for not getting back sooner. I had a major computer breakdown which has finally been corrected. What I am trying to do is a fuel pressure leak-down test on my injectors. I did purchase a fuel pressure gauge from Advance Auto. Upon hooking it up I was not able to get a reading from the gauge. Looks like the gauge fitting is not compatible with the Mercruiser schrader valve connector. The fitting does screw down on the threaded schrader valve but no reading. I think Mercruiser has their own style of valve that only will work on "their" own fuel pressure gauge. And yes, I did word it wrong on my title for the thread.

They sure do, go figure anything to make another buck and cost boaters more. The part you need is 91-806901 and it cost around 27 to 29 bucks
 

Thomv

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My Mercruiser 7.4 MPI already has a connection on the front of each motor to hook up a gauge. When I tried using the Advance Auto gauge it did screw onto the connection but, for some reason, it did not compress the schrader valve pin plunger, very much like what you would find on a car tire. Because of this I could not get a reading. The other part looks like it would only be used if there was no fuel injection schrader valve to tie into.Am i correct or is this part from Mercruiser needed for the hook up of the fuel gauge?
 

alldodge

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Google "mercruiser 91-806901 " and it will come up with a few places that sell it
 

Thomv

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I did the fuel pressure leak down test. When I started the one motor my reading was around 48PSI. When I shut off the motor the reading fell to 35 PSI. After about 1,5 hours the reading fell down to 15 PSI. Does it sound like I have some injectors leaking fuel into the cylinders. If this is the case causing my hard starting how do I isolate which injectors are leaking?
 

alldodge

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I did the fuel pressure leak down test. When I started the one motor my reading was around 48PSI. When I shut off the motor the reading fell to 35 PSI. After about 1,5 hours the reading fell down to 15 PSI. Does it sound like I have some injectors leaking fuel into the cylinders. If this is the case causing my hard starting how do I isolate which injectors are leaking?

The test isn't done with the engine being shut off, but in either case I don't see an issue with your fuel pressure.

To do the balance test you need the scan tool, and is found in manual 23 http://boatinfo.no/lib/mercruiser/manuals/mercruiser23.html#/642 Injector Balance Test
Test Procedure
(Refer to the following figure for test set-up.)
The injector balance tester is a tool used to turn the injector ON for a precise amount of time, thus spraying a measured amount of fuel into the manifold. This causes a drop in fuel rail pressure that we can record and compare between each injector. All injectors should have the same amount of pressure drop. Any injector with a pressure drop that is 1.5 PSI (or more) greater or less than the average drop of the other injectors should be considered faulty and replaced. Injector testers are available for various manufacturers. For 454 and 502 engines: the tester must be capable of selecting an injector pulse width in the range of 200-400 milliseconds (m sec). The recommended starting point for these engines is approximately 300 m sec. In any case a pulse width that drops the fuel rail pressure to half the normal operating pressure, should be used.
STEP 1
Engine cool down period (ten minutes) is necessary to avoid irregular readings due to “hot soak” fuel boiling. Relieve fuel pressure in the fuel rail as outlined in “Fuel Pressure Relief Procedure” in “Repair Procedures.” Remove plenum as outlined in “Repair Procedures.” With ignition OFF, connect fuel pressure gauge to fuel pressure tap. Disconnect harness connectors at all injectors, and connect injector tester to one injector. Use adaptor harness furnished with injector tester to energize injectors. Follow manufacturer’s instructions for use of adaptor harness. Ignition must be OFF at least ten seconds to complete ECM shutdown cycle. Fuel pump should run about two seconds after ignition is turned ON. At this point, insert clear tubing attached to vent valve into a suitable container and bleed air from gauge and hose to ensure accurate gauge operation. Repeat this step until all air is bled from gauge.
STEP 2
Turn ignition OFF for ten seconds and then ON again several times to get fuel pressure to its maximum. Record this initial pressure reading. Energize tester one time and note pressure drop at its lowest point. (Disregard any slight pressure increase after drop hits low point.) By subtracting this second pressure reading from the initial pressure, we have the actual amount of injector pressure drop.
STEP 3
Repeat Step 2 on each injector and compare the amount of drop. Usually, good injectors will have virtually the same drop. Retest any injector that has a pressure difference of 1.5 PSI (10 kPa), either more or less than the average of the other injectors on the engine. Replace any injector that also fails the retest. If the pressure drop of all injectors is within 1.5 PSI (10 kPa) of this average, the injectors appear to be flowing properly. Reconnect them and review “Troubleshooting.”
 

Thomv

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After work today I noticed a 5 PSI drop after ten minutes of shutting off the starboard motor. It was holding at 36 PSI upon shutdown and then dropped down to 31 PSI. Hopefully I will perform the Injector Balance Test this weekend.
 

Thomv

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How do I get access to the fuel injectors on my motor?
 

alldodge

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How do I get access to the fuel injectors on my motor?

I don't think you have an injector problem, I still think you have a vapor lock issue and the balance test won't help. Your pressure is not dropping much so I believe your injectors are fine. Where do you get your fuel? Does this happen more on hot days then cooler ones?
 

Thomv

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I don't think you have an injector problem, I still think you have a vapor lock issue and the balance test won't help. Your pressure is not dropping much so I believe your injectors are fine. Where do you get your fuel? Does this happen more on hot days then cooler ones?

Down here in Tampa Bay there are very few cool days. We have not experienced any cool days while out and about. I believe you are right about the vapor lock issue. The next time we go out maybe it would be a good idea to run both bilge blowers a while to let some of the heat escape in the engine department after shutdown. I still would like to test each injector using my Rinda scanner but again I need to know how to get access to the injectors wire connectors.
 
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