Howdy, Stranger!

It looks like you're new here. If you want to get involved, click one of these buttons!

12" White Stickers
BallOfSpray $5 Donation
BallOfSpray $10 Donation

Fuel Pressure Check

2Valve2Valve Posts: 302 Solid Baller
I'm helping a buddy diagnose a fuel pump issue ('96 ProStar, 5.7L TBI) and we can't seem to locate the Shrader valve to hook up my fuel pressure gauge.

Location info would be appreciated.

Symptoms are:
Boat starts and idles fine
Pull up a skier, runs fine and then suddenly it bogs and runs at idle only
Changed fuel filter, seemed to improve but only for a day or so

Thanks in advance!


  • Orlando76Orlando76 Posts: 1,244 ★★★Triple Panda Award Recipient ★★★
    Is there a shrader on it? I don’t recall my old GM TBI in old truck having one. In my experience your symptoms point to ignition. How do you do on hot starts?
  • 2Valve2Valve Posts: 302 Solid Baller
    We can't find a Schrader valve was wondering if there was even on there.

    I think it's fuel only because it never dies. When pulling a skier (34mph), it takes off great with plenty of normal power. A few minutes into the pull, the RPMs' simply drop to idle.

    If you switch to neutral, it still idles normal.

    I have a MEFI-4 code scanner so I think there's a port for that.

    Will know more this evening.

    Shame since the wake at 32 off is amazing!
  • skihackerskihacker Posts: 358 Solid Baller
    Take the spark arrestor off an take a long look at the feed line, most GM's of that era had one, possible that this particular marine version doesn't. You'll need a bigger fuel pressure kit if not,
  • ZmanZman Posts: 1,592 Mega Baller
    I had a crankcase position sensor go bad. It only acted up after boat warmed up, after about one set.
    Boat would go to safe idle. And, not restart once turned off..
    Fortunately, a very easy fix and sensor available at auto parts stores.
  • BraceMakerBraceMaker Posts: 4,229 Mega Baller
    The typical MC route is to install a T with a test fitting port. As at local auto parts store and they should have a GM throttle body test port kit.
  • ZmanZman Posts: 1,592 Mega Baller
    edited May 2019
    Does the MC have the dual fuel pumps in a cannister inside the fuel tank, like Malibu?
    Maybe not in 1996.
    If it does, check to be sure the two fuses are still good.
    With 2 pumps a ski boat can almost do ok on the main pump, until under a good load.
    If both fuses are good, check your pressure with one fuse at a time pulled to be sure both pumps work.
    Again, IF you have the dual pump system.
  • 2Valve2Valve Posts: 302 Solid Baller
    Good info. I was thinking we could splice something together as well and make our own connection.
  • 2Valve2Valve Posts: 302 Solid Baller
    I don't see a separate pump in the tank. Will do more troubleshooting this evening.
    After skiing of course.....:) (We'll use my boat)
  • BraceMakerBraceMaker Posts: 4,229 Mega Baller
    edited May 2019
    @2Valve Its not a bad idea to make up your own connection, the factory style pump for these boats has become a bit NLA and as such the new Indmar replacements are $$$ - think a 600$ part.

    But it is a very common style fuel pump if you're willing to cap off return line fittings and a few things - in so doing the best way to go is to have a place like NAPA make up fittings and lines for you with a test port - and if I were doing this again tomorrow I'd install a fuel pressure sender and gauge, it is just such a common culprit of issues with these boats with the thread going something like "so I was skiing and I just lost power" and then people have to argue that they can have fuel pressure at idle and still have fuel pump issues - should see if I can find a sender that turns on an LED if the pressure goes below a certain amount and sell those.
  • AndreAndre Posts: 1,364 Mega Baller
    There's a screen in the fuel pump inlet that often get clogged.Symptoms are exactly what you describe and it happens every 3-4 years on my 94.Carpet fibers ,dog hairs and other shit will reduced fuel flow when under load but running fine at idle or revving in neutral.Lots of threads about this on Team Talk.
    Easy to check
  • 2Valve2Valve Posts: 302 Solid Baller
    edited May 2019
    we had the boat out last night.
    Inlet screen in tank is clear. The fuel pump was taken apart and the screen inside was cleaned. Some small debris there but nothing major.
    Decided to ski. Pulled my buddy up and a min later into his run, the motor bogged down.
    I left the throttle un-touched and it doesn't die, just bogs down, with rpm continuing to drop.

    Throttling back to idle and all is normal. We immediately skiied again and this time I noticed it bogged on his 3rd cut, motor rpm dropping about 100rpm. When it happens, the motor smells very, very rich. Seems weird that it only happens pulling a skier.

    There is no connection for a fuel pressure check, so at some point we'll need to install a T.

    We stopped skiing and just went around the lake, 34mph, 3500 rpm and no problems for a 3 mile run. We tried varying rpm and still no problem.

    I then watched the clear fuel filter that's installed inline near the tank, and there was no starvation there, speed at 34mph.

    We also hooked up my CodeMate and it showed no codes, except of course 12, meaning all was good.

    I have to say, we're stumped.

    My mind thinks it might be something related to the throttle position sensor, but again no codes. It got dark so we didn't have time to check the connections, but I recommended he pull and clean every connection on the motor.

    Any recommendations appreciated.
  • skihackerskihacker Posts: 358 Solid Baller
    Is there a MAP sensor on that motor?
  • 2Valve2Valve Posts: 302 Solid Baller
    @skihacker I don't believe so. It just has a spark arrestor with an older Rochester 2 venturi throttle body with 2 injector's at the top, squirting down into the venturi's.

    Very similar to a mid 90's Chevy truck engine. Maybe an LT1?
  • skihackerskihacker Posts: 358 Solid Baller
    There are few sensors on that motor that will drive it extremely rich or lean, the tps obviously the most critical, I would also bet there is a map sensor, possibly mounted remotely with a vacuum line to it, they can be tested with a dvom or a scope or just swapped out, it is pretty much an early 90's chevy truck motor. A scan tool that shows live data can be really useful too if available.
  • 2Valve2Valve Posts: 302 Solid Baller
    edited May 2019
    My mistake. I was thinking MAF (mass airflow sensor), as found in the air stream between the air filter and throttle body on most modern cars.

    A bad MAP sensor might be the culprit. If I recall, it sends info to the ECM so that fuel/air ratio, ignition timing can be set. We'll look into that and thank you!
  • AndreAndre Posts: 1,364 Mega Baller
    edited May 2019
    Without fuel pressure reading,could be a bad pick up coil,ignition module and couples more things.
    Rich smell could indicate fast intermittent spark problems but you would loose more then 100 rpms...i think...?
    You can drive around without engine cover and flame arrestor and check fuel spray when it happens under load...put one of those cheap inline spark tester on each plug to check if you're loosing spark...
    You mentionned clear filter near tank but on my 94 fuel filter is AFTER the pump before tbi...could be different on a 96,don't know.
    Good luck!
    Edit:There is a MAP sensor on that engine .
  • 2Valve2Valve Posts: 302 Solid Baller
    We haven't looked at the distributor or internal component's yet but I'm definitely leaning more towards the electrical side of things.
    On the fuel filter, yes....there's the stock one between the pump and throttle body and this was replaced. The P.O. probably installed the additional clear glass filter on top of the fuel tank.
    Thanks on the confirmation of the MAP sensor. I know where to look now.

    Looks like we're using my boat to ski tonight. :)
  • DragoDrago Posts: 1,588 Mega Baller
    If you smell gas, one would think its an electrical problem.
    Different boat, but in 97 i was part of a co-op boat that ran great for everyone but me. I was the only 36 mph skier and with more load. After a couple of weeks of diagnostics, it was ultimately one spark plug wire.
    Check the simple stuff...
    SR SL Judge & Driver (“a driver who is super late on the wheel and is out of sync”)
  • DanEDanE Posts: 909 Crazy Baller
    @skihacker MAP sensor is more critical than TPS.
  • BraceMakerBraceMaker Posts: 4,229 Mega Baller
    Go to the last paragraph on page 11 and then read page 12

    MEFI will run w/o most of its inputs because it uses those inputs to modify a preset mapping. For instance it knows how lean/rich it should be with no sensor - it uses the sensor to enrichen for cold starts (choke) and to lean out for warm running from the baseline. But a failed sensor can tell it its cold when its not or tell it its hot when its not. For TBI boats that often means hard starts or rich running.
  • 2Valve2Valve Posts: 302 Solid Baller
    Thanks for the info. Then as a possible technique, could we disconnect the MAP sensor and see if the problem occurs? If not, then conclude that the MAP sensor is sending an incorrect signal to the ECM when the fault occurs?
  • BraceMakerBraceMaker Posts: 4,229 Mega Baller
    edited May 2019
    Correct, if you unplug the MAP sensor when running you should then set a DTC 33 and as a result it shouldn't run "better" but it should change the drivability issue - that being it should go to a more basic condition which would let you troubleshoot if the MAP might be the issue - which another aspect to that is if you have a Vacuum leak it will act like a MAP issue.

    MAP varies resistance by manifold vacuum pressure - as such a vacuum leak will give the same faults as a failing MAP - and again if the numbers are in the realm of "normal" but are just wrong that's when you get weird faults with no DTC set.

    So with a MAP you can back probe the harness and read voltage at the MAP sensor which along with a vacuum gauge hooked up you could then directly read the result, or if you pop the sensor up you can hook a mityvac to the back of it, pull vacuum (chart in that link) and read the voltage with a DVM.

    Depends how you like to roll. Being a 100$ part and possibly being more of a vacuum leak (particularly look at the oil fill and the vacuum line to the valve cover hint hint) - it can sometimes be better to actually troubleshoot the part.
  • 2Valve2Valve Posts: 302 Solid Baller
    Got back on this late last night.
    Using my handy dandy Fluke meter, I disconnected the TPS and measured the resistance on the TPS. Varying throttle position revealed values all over the map, from 2kohms up to infinity, back to 5 ohms, back to infinity.

    Also did some voltage checks with the connector plugged in (5vdc reference) and the output voltage was very inconsistent as well. So we'll be replacing the TPS.

    Also found the fuel pump relay housing (which was laying on the intake manifold), was burnt. Even though the fuel pump powers up, we're replacing the relay and will route it away from any heat sources.

    Thanks for your help everyone. I'll report back once we get it installed.
  • 2Valve2Valve Posts: 302 Solid Baller
    Thank You, We're going to swap out the TPS first and see if that does it. I have a MityVac that I use to bleed brakes with so if needed, I can use your suggestion to troubleshoot the MAP.
    Now let's ski!!!
  • 2Valve2Valve Posts: 302 Solid Baller
    The saga continues.

    New TPS didn't solve the problem. Pulled a skier last night and the engine bogged down as before.

    However, THIS time I was able to observe the engine temp gauge do something very weird.
    When the engine started to bog, the needle went all the way to full hot (220F).

    I watched it as I left the throttle in the same position and it stayed there, until I pulled back to idle. As soon as I did that, the needle came back to it;s normal position a bit below 160F.

    We then tried duplicating without pulling a skier and it happened again.
    Same thing. Nice smooth power planing out to 34mph and then a couple min. later, temp gauge goes full. Stays there until I pull back throttle and then the gauge returns to normal.

    My first thought is a bad temp sensor, telling the ECM that the engine is overheating. I'm guessing the algorithm is to reduce power at that point until the engine temp returns to normal.

    Or maybe a thermostat that's somehow opening and closing intermittently?

  • skihackerskihacker Posts: 358 Solid Baller
    Most boats have an audible alarm, oil pressure, temp and sometimes trans temp will set it off, not sure if it will go into some kind of limp mode.
  • FLeboeufFLeboeuf Posts: 70 Baller
    Or maybe the thermostat is not opening or not opening completely meaning the engine is really overheathing which is not good.. I have changed my thermostat once after a little over 1000hours. It was rusted and would not open at all but I can’t remember if it made the engine overheat. I am guessing it could.
  • AndreAndre Posts: 1,364 Mega Baller
    There's a temp sensor for the gauge and a a second one one for the ECM.
    For the needle to go 220 and for the engine to go into limp mode you must have a real overheating problem.But the gauge shouldn't go back to normal instantely when pulling back the throttle...
    Weird...Could the rubber impeller spin on the brass hub under certain conditions...?
  • 2Valve2Valve Posts: 302 Solid Baller
    thanks everyone. The plan is to look at the impeller first, then the thermostat, under the assumption that the boat is really overheating intermittently.

  • BraceMakerBraceMaker Posts: 4,229 Mega Baller
    Mastercraft forum has values for ect resistance. Pull it, fluke to ohms pot of water on stove with a thermometer and check values cold to hot.
Sign In or Register to comment.