7.4L MPI Engine turns over but immediately stalls

[zavalanche71]

Hello,<br />I am new to this forum. I have a 1999 2330 Chapparal with a 7.4L MPI with only 82 hours. The engine has been running excellent until just the other day, when it stalled and would not restart. Everytime I turn it over it will start up and then stall within 10-15 seconds. I changed out the Water separator/fuel filter but this did not help. I did notice a bit more of a hissing sound coming from the engine the last few times we had the boat out but I am not sure if this has anything to do with this problem. Are there any suggestions? I hate to take this boat in to find out it is something very simple. I do not see any loose hoses but again this engine compartment is pretty cramped.

 

[Don S]

Re: 7.4L MPI Engine turns over but immediately stalls

Volvo or Mercruiser?

 

[zavalanche71]

Re: 7.4L MPI Engine turns over but immediately stalls

Hello Don S, It is a Mercruiser.

 

[newport dave]

Re: 7.4L MPI Engine turns over but immediately stalls

The first thing you should do is check the fuel pressure. Do you have a fuel pressure gauge?<br /><br />The hissing sound you have been hearing is due to a Idle Air Control (IAC) valve that is open very wide. <br /><br />When the engine develops a problem that slows the idle speed, the engines computer (ECM) will open the IAC Valve to try to maintain the idle speed that it "desires". There could be any number of causes for this, but a good place to start is the fuel pressure.<br /><br />Dave

 

[zavalanche71]

Re: 7.4L MPI Engine turns over but immediately stalls

Newport Dave, I do not have a fuel pressure gauge. How much should I expect to pay for one? Where should I check the fuel pressure? There is a tire gauge type of connection in front of the engine on the fuel rail but I believe this is the lower pressure connection since I can push in and get some gas to leak out.<br /><br />Would any of the possible causes immediately pop up? I was able to cruise the lake at 55 mph with no performance degradation before this happened. We parked at a sand bar and later in the day when we went to leave the boat started but died after about 5 minutes. After that, it would not stay started.

 

[snapperbait]

Re: 7.4L MPI Engine turns over but immediately stalls

Faulty fuel pump relay, or a bad connection at that relay would be my first guess...

 

[zavalanche71]

Re: 7.4L MPI Engine turns over but immediately stalls

Snapperbait, thanks for your input. Are you saying that if I have low fuel pressure it may be the relay? If so, where would I find this relay? Is there an easy check to determine if it is this relay?

 

[Don S]

Re: 7.4L MPI Engine turns over but immediately stalls

Before you do anything, you need a service manual. You should get manual 23 from Mercruiser PN. 90-861326--1.<br />There are lots of things that can cause your problem on an MPI engine, and not just fuel delivery problems. Going at this by hit and miss and not even knowing where things are or what they are is going to make it virtually impossible for you to ever fix the engine.<br />If the relay was bad the pump wouldn't run at all, a faulty pump could cause it, but they cost a lot of money to replace. The fuel regulator could be at fault, or even a hose off. Have you checked for any fault codes? There may information there to pinpoint the problem. But without the book, you couldn't even determine what the fault was or how to test the item.

 

[newport dave]

Re: 7.4L MPI Engine turns over but immediately stalls

A fuel pressure gauge is about $40 and can be purchased at just about any auto parts store. It will connect at the shreader valve (the tire gauge type of connection you mentioned) at the front of the fuel rail. Presssure should be 34-38PSI for MEFI 1 or 2 systems and 41-45PSI for MEFI 3 systems(I am guessing that your '99 has MEFI 1 or 2).<br /><br />When you turn the key to the "on" position (don't start) do you hear the fuel pump come on?<br />If so, the fuel pump relay is working. When you crank the engine, power is supplied to the fuel pump from one circuit. Once the engine has started, power is supplied to the fuel pump by a different circuit that goes through a switch that will shut off if there is not enough oil presssure. This oil pressure switch could be the problem. This, however would still not explain the "hissing".<br /><br />Dave

 

[zavalanche71]

Re: 7.4L MPI Engine turns over but immediately stalls

Don S. and Newport Dave, Thanks for your input. I have a Seloc Service manual but if this is not sufficient I will purchase a Mercruiser one as Don mentioned. I did purchase a fuel pressure gauge and will be trying this out shortly. I will furnish updates shortly.

 

[Don S]

Re: 7.4L MPI Engine turns over but immediately stalls

Don't go hooking up a standard old fuel pressure gauge until you know where and how to hook it up or you could cause serious problems. (FIRE)

 

[zavalanche71]

Re: 7.4L MPI Engine turns over but immediately stalls

All, Thanks for your support. I hooked up the fuel pressure gauge. During the first few cycles, I was only getting 10psi. I relieved the pressure after each time. On the Fourth try, the engine started and the pressure jumped to 35-40 psi. I assume that my problem was the fuel Filter/ water separator and that since I did not know any better, I did not think to prime the filter with gas before installing the new filter. There was probably air in the lines and after relieving the pressure from the fuel rail a number of times my problem resolved. I feel silly but this is a good lesson for all you do it yourselfers, even though the there are not explicit directions use some common sense.<br />Thanks alot for all your support. PS: I would have hated to need to change out the fuel pump. It is unbelievably difficult to reach.

 

[rodbolt]

Re: 7.4L MPI Engine turns over but immediately stalls

normally that screaming IAC indicates a lack of fuel. as its running out of fuel the ECM will attempt to increase RPM by increasing the injector pulse width and adding air via the IAC.<br /> most fuel system failures wont show up on the diagnostics. other than an open circuit on an injector on some ECM,s. the rest is done by experience. which is why so many techs get all confuzzled when the engine wont run yet the puter syas its all ok. quite a few techs forget the basics cause the puter says its all good. the filter on the motor is also the vapor seperator and can become airlocked. in my opinion it was and is a poor design. some engine boxes have required me to move the fuel return from the filter to the tank fill line in order to eliminate the "vapor" lock problem.

 

[zavalanche71]

Re: 7.4L MPI Engine turns over but immediately stalls

Hello all,<br /><br />I am back again. What I thought was the problem is probably not the entire problem. The boat ran well for another 1-2 hrs. but after pulling my kids on the tube, the system repeated itself. The engine would act as if it wasn't getting enough gas and konk out. I finally got it running and we went to a sand bar to swim for a few hours. When we were ready to leave, the boat would start and immediately die. We had to be towed back. When I was back at the dock, I played again with the fuel pressure gauge. For the first 3 or 4 cycles the rail showed about 10 psi and after bleeding the system every time, it went back up to 35-40 psi and started up.<br /><br />I am confused. Is it possible that vapor lock is being created in this closed loop system? Should I be checking the impeller to see if it is going causing a slightly overheat condition and causing vapor in my lines? Is it possible the IAC is bad? Or is the pressure regulator bad? What type of further tests should I run.<br /><br />Thanks,<br /><br />Zavalanche71

 

[zavalanche71]

Re: 7.4L MPI Engine turns over but immediately stalls

Any Help?

 

[tommays]

Re: 7.4L MPI Engine turns over but immediately stalls

the filter on the motor is also the vapor seperator and can become airlocked. in my opinion it was and is a poor design. some engine boxes have required me to move the fuel return from the filter to the tank fill line in order to eliminate the "vapor" lock problem.

i no very littel but no that rodbolt is pointing you in the right dirrection<br /><br />tommays

 

[newport dave]

Re: 7.4L MPI Engine turns over but immediately stalls

Originally posted by rodbolt:<br />some engine boxes have required me to move the fuel return from the filter to the tank fill line in order to eliminate the "vapor" lock problem.

Rodbolt,<br />Are you doing this on "Cool-Fuel" systems? I can't see how this would do anything but make the problem worse. <br /><br />The primary function of the Cool-Fuel system is to cool the fuel being returned to the f/w seperator, and keep the fuel in the non-pressurized parts of the system cool to prevent vapor lock. Routing the cooled fuel back to the tank is only going to cause higher temps and more vapor formation in the fuel system.<br /><br />As far as my guess as to the cause of z71's problem - bad fuel pump.<br /><br />Dave

 

[zavalanche71]

Re: 7.4L MPI Engine turns over but immediately stalls

Newport Dave, The system runs fine until after about a few hours of driving and tubing. It sounds like but gas but when I test the fuel rail there seems to be air in the rail. Once I bleed the rail a few times, the system starts up fine. I am not sure why you still think it is the fuel pump. Could you please explain.

 

[newport dave]

Re: 7.4L MPI Engine turns over but immediately stalls

z71,<br />Fuel pump is just a guess based on experience. I definatly could be wrong. <br /><br />Consider that there may be air in the fuel rail due to lack of fuel pressure, not the other way around. <br /><br />Where is the air coming from? It either is due to low fuel rail pressure allowing air to enter the system when injectors are pulsed, or it is fuel vaporizing in the system due to high heat (vapor lock). <br /><br />I don't know how hot it is where you are, maybe it is vapor lock. Are the heatshields still in place (one on the port side exhaust manifold and one covering the f/w seperator)?<br /><br />Dave

 

[waterone1@aol.com]

Re: 7.4L MPI Engine turns over but immediately stalls

Lets start with the fact that Merc. had a series of bad fuel pumps on 99 7.4 MPI's. There is a service bulletin about this problem (99-8). I'm not saying for sure that you have a bad fuel pump, just a high likelyhood of it. Do you know if this pump has ever been replaced ? Have you checked your fuel lines from the tank to the fuel pump to make sure that you are not sucking air before the pump ? Do you have access to a Merc service manual, or have you read the service bulletin about the fuel pump problems with this engine ?