How do you pressure / vacuum test a 5.7L Mercruiser ?

FixIt4Life

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I have a quick question . I have a 1991 250HP 5.7L Mercruiser and i want to pressure thest the fuel pump but dont know how or were to even look . Can some one please send attach a picture here so i can see what i should look for to connect the fuel pressure gauge and also somehow explain to me how it works ? Thanks :)
 

alldodge

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Re: How do you pressure / vacuum test a 5.7L Mercruiser ?

You need a vacuum gauge and fuel pump tester like this:
Performance Tool Vacuum and Fuel Pump Tester, Model# W80594 | Automotive Diagnostics| Northern Tool + Equipment

Need to insert a tee fitting in line between the fuel line and the carb. Disconnect the fuel line going to the carb and connect a piece of 3/8 fuel line over the end, insert other end to tee fitting. Other side of tee fitting, insert into another piece of 3/8 gas hose. Use a fitting something like this to reconnect to carb:
http://www.summitracing.com/parts/mor-65140/overview/

Connect Vacuum/pump tester gauge abd use hose clamps on all hose fitting ends. Sorry don't have a pic
 

FixIt4Life

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Re: How do you pressure / vacuum test a 5.7L Mercruiser ?

Thank you . I think I understand now . So I should disconnect the fuel line that goes from the carb to the fuel pump and connect one of the ends of the t fitting onto the hose and the other to the carb and thats it ? also all i have to do is just start the motor up and see what the pressure is as said in the manual and compare it that way ? And if the pressure is lower then it should be that means i have to replace the fuel pump ??
 

achris

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alldodge

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Re: How do you pressure / vacuum test a 5.7L Mercruiser ?

Thank you . I think I understand now . So I should disconnect the fuel line that goes from the carb to the fuel pump and connect one of the ends of the t fitting onto the hose and the other to the carb and thats it ? also all i have to do is just start the motor up and see what the pressure is as said in the manual and compare it that way ? And if the pressure is lower then it should be that means i have to replace the fuel pump ??

Yes you have it right. The pressure should be netween 4 to 7 PSI. You need the Vacuum.Fuel gauge not the higher price and higher pressure gauge used by fuel injected engines.

Fun Times found the post I forgot to look for.
Thanks Fun Times
 

FixIt4Life

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Re: How do you pressure / vacuum test a 5.7L Mercruiser ?

do you know if i can make one out of a cylinder test compression gauge ?
 

alldodge

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Re: How do you pressure / vacuum test a 5.7L Mercruiser ?

do you know if i can make one out of a cylinder test compression gauge ?

I'm sorry but no, You need to be able to read around 10PSI and a compression tester starts somewhere around 50PSI

Edit: changed no more to around
 
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thumpar

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Re: How do you pressure / vacuum test a 5.7L Mercruiser ?

Why are you testing it? Do you have an issue or just checking?
 

achris

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Re: How do you pressure / vacuum test a 5.7L Mercruiser ?

do you know if i can make one out of a cylinder test compression gauge ?

No, the pressure reading for a compression gauge is way too high, and you also don't get the vacuum part of the gauge. Vacuum has a few uses. 1. Checking engine intake vacuum while running, 2. checking for fuel delivery restrictions to the pump., 3. as a vacuum test for drives..

And besides, that pressure/vac gauge is only $20.... You'd spend more than that on beers for a weekend....

Chris.....
 

FixIt4Life

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Re: How do you pressure / vacuum test a 5.7L Mercruiser ?

You have a point HAHAHA ! :laugh:
 

FixIt4Life

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Re: How do you pressure / vacuum test a 5.7L Mercruiser ?

I have an issue . My boat does not go full speed as before at full throttle . And sometimes at full throttle i hear the engine rev up and down . Sometimes the rpm would jump from 3800rpm - 4100rpm and then it fells like the engine is about to slip out of gear so i back down on the throttle. But the boat preforms bad it used to go anymore from 4000-4500 RPM and now it barely goes 3500rpm and sometimes it comes up to 3800rpk if im lucky . As far as i know it could be a bad fuel vacuum so i am checking for that . And also as far as i know if the vacuum psi is low that means i have a bad fuel pump and as far as i know that means thats a 100 bucks out of my pocket for a new one !!! :mad:
 

alldodge

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Re: How do you pressure / vacuum test a 5.7L Mercruiser ?

I have an issue . My boat does not go full speed as before at full throttle . And sometimes at full throttle i hear the engine rev up and down . Sometimes the rpm would jump from 3800rpm - 4100rpm and then it fells like the engine is about to slip out of gear so i back down on the throttle. But the boat preforms bad it used to go anymore from 4000-4500 RPM and now it barely goes 3500rpm and sometimes it comes up to 3800rpk if im lucky . As far as i know it could be a bad fuel vacuum so i am checking for that . And also as far as i know if the vacuum psi is low that means i have a bad fuel pump and as far as i know that means thats a 100 bucks out of my pocket for a new one !!! :mad:

Looks like you got several problems goming on here. But a BOAT is "Bring On Another Thousand", so if your willing we can figure them out
 

FixIt4Life

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Re: How do you pressure / vacuum test a 5.7L Mercruiser ?

Yea unfortunately thats what it stand for so only 1 thing left to do is bring out that thousand !! What do you thinl the problems can besides bad fuel pump ? Than You for your tips . Very Helpful !!!
 

achris

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Re: How do you pressure / vacuum test a 5.7L Mercruiser ?

Do you have a auto parts store near you that will lend out/hire out the tools? If so, that might be the easiest way to start reducing the possibilities...

Chris.....
 
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alldodge

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Re: How do you pressure / vacuum test a 5.7L Mercruiser ?

With some umknows about the state of your engine I'll suggest doing a compression test. Then check out your fuel system, pump presser, flow rate, filters and carb. Check plugs cap and rotor while your at it. Let us know what you find.
 

FixIt4Life

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Re: How do you pressure / vacuum test a 5.7L Mercruiser ?

I have replaced the spark plugs , Cap and Rotter and rebuild my carburetor . Logically speaking , can it be because of my batteries being so low on volt because my alternator is broken ? I mean if you think about it . The fuel pump needs 12V at all time to pump fuel in the engine at the same rate velocity but in my case my fuel pump is not getting 12V because my alternator is broken . I mean the volt gauge only shows that its getting 8V which is bad . What do you think ?
 

FixIt4Life

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Re: How do you pressure / vacuum test a 5.7L Mercruiser ?

Maybe that's why my boat is going so slow because of the lack of power coming into the fuel pump and the carburetor not getting enough fuel to go fast ?
 

achris

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Re: How do you pressure / vacuum test a 5.7L Mercruiser ?

If you have a 1991 5.7L engine, you have a mechanical fuel pump, not an electric. But if you battery voltage is that low, your ignition system will be cutting out...

It might be a good time to do an audit on your boat and engine, and list all the things you know are not working/faulty, and a list of things of symptoms. Either way, it would be a good idea to stop using the boat and bring it back up to an acceptable level of maintenance and repair so it doesn't leave you stranded and endanger you/your family and friends, and the people who have to try and get out to try and save you...

I'm sorry if I sound harsh, but I have attended too many coroner's inquiries to make light of boats and engines that are not seaworthy... The simple truth is 'people die at sea', and it is incumbent on all of us who go to sea to take every possible measure to ensure not only our own safety, but that of our passengers. That not only includes where we go and when, but what condition our craft is in.... And just saying 'it floats' is not good enough....

Chris.....
 
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