PDA

View Full Version : fuell pressure and scan tool?



BUIZILLA
10-19-2004, 08:37 PM
ok, ok..

what is the fuel pressure for a 7.4-310, '98 vintage? at idle with vacuum reference signal on, or off?

how's a bout a used Mercruiser scan tool available to purchase/borrow/barter?

are the cool fuel electric pumps supposed to be noisy? with a steady *buzzing* sound??

JH

Lenny
10-19-2004, 09:43 PM
Jim, you don't mention if you have a "surging" issue with it or not, or if it is just GREAT !!! and you are just curious. I am assuming the former with this reply and hijacked knowledge.


SENSITIVE SENSOR
Q: I can use some help with my 7.4 MPI MerCruiser. The engine started to surge a month ago. I changed the fuel filter, then the fuel pump. I had a MerCruiser dealer check the computer for fault codes. Everything checked out. Then I checked the fuel vent to make sure it wasn't obstructed. It wasn't. Next, I changed the throttle and checked for vacuum leaks. No improvement. Most of the surging occurs from 2800 to 3600 rpm. I also removed the distributor cap and checked the rotor. I'm at a loss.
DAN HARTWELL
(VIA E-MAIL)

A: Since you've tried almost everything else, let me tell you what it sounds like. Either the MAP (manifold absolute pressure) sensor or the throttle position sensor are at fault. The better bet is the MAP sensor. This device measures the vacuum in the intake manifold so the computer knows how much load the engine is under. It does this by converting pressure into a signal that the engine management computer can read.

I think the codes need to be checked again. A MerCruiser dealer with the correct diagnostic equipment can read the fault codes and pinpoint the problem in a flash. By the way, even though a lot of boats have newer engine management systems with computers, the laws of physics haven't been suspended. A vacuum leak is still a vacuum leak.

Engine management systems are sophisticated, but they can't pinpoint things they don't have sensors for. Such a system may not be able to find something as simple as a corroded ground lead or a pinched wire. Only a person can troubleshoot those problems.

The first step in any troubleshooting endeavor, even with a computer-controlled engine, is to check the easy stuff. Corroded or damaged wiring and connections are a good place to start. So even if you're running a newer engine, don't hang up those old-fashioned skills yet. They're just as necessary now as they ever were.

and a better one, :)


That would be typical of a restricted fuel supply or a fuel pump failure.
There are two basic tools used to test the 7.4L
The Digital Diagnostic Tool (DDT) and the Fuel Pressure Gauge.
Fuel pressure is critical. Poor quality fuel or contaminated fuel can
eventually ruin a fuel pump.
So can sitting. Depending on the type of fuel... a fuel pump can go bad
if it's not run in about 8 months.
Your fuel rail has a shrader valve at which you can install the Fuel
Pressure Gauge and test pressure.
If the manual calls for 47 PSI, you MUST have 47 PSI... not 46 or lower.
90% of performance issues are fuel replated.
It might be something as simple as a sticking antisyphon valve in your
pick-up tube fitting on the fuel tank.

roadtrip se
10-20-2004, 10:10 AM
high-pitched, squeal coming from the fuel pump, I have ever heard.

I was told it was "normal", during the only z-care visit I ever made. Fuel pressure checked out fine.

I could hear it even when the boat was on plane and dogs would cover their ears and go into hiding whenever I started her up.

Of course, I tossed that sucker out after 67 hours and the problem went away. The 500 makes it own set of strange sounds, the most disconcerting being the flapper clatter that sounds like something much worse...

:eek: :eek:

Not a solution to your problem, but maybe a reference point.

So when you going to start culling that herd of boats?

Todd

BUIZILLA
10-20-2004, 11:15 AM
Here's the skinny.... boat only has 61 hours now in 6 years since new... ran boat 4 hours on Saturday, down 400 rpm from spec... removed fuel filter (*original*) and moisture was obvious.. changed filter on Saturday night, ran boat again Sunday for 5 hours, still 400 rpm low, pump pretty quiet and smooth all day, engine could have been a tad smoother off idle though... yesterday removed all 8 injectors for ultrasonic clean and flush and flow balance here in my shop, rust stains/particles in all 8 injector inlet filters and even in plastic fuel rail... 4 were terrible but they came around acceptable after cleaning 6 times, 5 within 1%, 2 are 2% low, one is 4% low, may still need one... put everything back together nice and clean, and now it feels like a slight miss at idle, didn't miss before, pump is NOISY as hell all of a sudden.... need to check fuel pressure and scan engine before I rip back into it.... new pump and another Racor element on order... map sensor feels loose in grommet too... oh joy...

JH

Ranman
10-20-2004, 05:32 PM
Jim,
I'm most likely no help here, but my 97 MPI SBC rund ~24psi at idle fuel pressure. This is to spec in the Merc book. My EFI fuel pump, mounted nexto and above the starboard front engine mount is the same as the BBC's makes a very distinct whirring/whining noise. It sounds to me like the alternator bearing (just above the pump) is going bad but it's the fuel pump running..

BUIZILLA
10-20-2004, 09:15 PM
Ran, today I found that the Merc parts book says 43# on the regulator pressure for a 7.4 MPI with vacuum source disconnected or at WOT, 35# connected at idle...
I took it back apart today and #7 injector is static all the time on the flow bench, at 6 ms and 2500 rpm it pumps fuel without the wires attached while under pressure, and I mean pumps 2 1/2 times what the spec calls for with the trigger voltage connected :eek!: 35cc vs.82cc ... I ordered one new injector today, installed new fuel pump already, adjusted TBU base setting correctly, checked MAP seal, changed starter solenoid, cleaned fuel cooler, trimmed restrictive TBU gasket. Gonna re-assemble tomorrow and check fuel pressure again.

JH