7.4l mag efi need help
 

Looking for help I have 1994 power quest 290 with twin 454 magnum efi motors. The starboard motor was running great and started having it over heat found old owner had stat in wrong place and a 195 in it. Removed stat and installed 160 in the stat housing along with replace the sea water impeller. Since then the motor has not run the same, when motor is started it runs fine for a min the idle starts to get alittle choppy and drops right under 600rpms, and u give any light throttle to motor it will spit and sputter and surge, but if you free rev motor it will rev right up to 2000rpms fine. I have done a compression test 1,6,7 came back at 140 and 2,3,4,5,8 came back at 150. I have changed spark plugs, and cleaned vst tank and screen. I put a fuel pressure gauge on the rail and when key is turned on psi will go to 10 and once motor is started and running will sit at around 28. Trying to see where to go from here

Which engine are we actually talking about here? Does it say 7.4 or 454 on it? And does it say EFI or MPI on it? It may sound like all the same thing, but it matters.

Does it look like this?


https://cimg2.ibsrv.net/gimg/www.off...2b17440df9.jpg

The one sticker has came off but the other motor says 454 magnun
mercruiser efi
crossram induction system
tried to upload picture but says I haven’t made enough post yet

DieselBoy,

Just spam this thread with "8", "7", etc, until you reach ten posts. this post away with your pics.

Thanks. Brad.

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https://cimg9.ibsrv.net/gimg/www.off...c97855568.jpeg

OK, so you have the first gen of the MPI system that they called EFI (1994-1996ish).

I think the fuel pressure is a clue - seems way too low. I think spec is 37 psi, so your 10psi key-on and 28 psi running sounds way too low.

Weak fuel pump, vs faulty regulator?

So I swappped high pressure pump from the good motor and it’s doing the same numbers
https://cimg8.ibsrv.net/gimg/www.off...00e669663.jpeg
First turn of key

https://cimg1.ibsrv.net/gimg/www.off...2bde33272.jpeg
Sitting at idle

If I take the regulator hose off ibrake it jumps up to 30

What does the pressure read on the good motor?

Same numbers and on the motor acting up if I click the throttle forwards 2 times it will almost die out and comeback then start surging but if I give it for clicks to get to 1000 rpm it will just sit there and run after it sputters for a second

Check for vacuum leaks?

Vacuum leaks would be the yellow hoses from intake manifold to the intake correct iv been trying to find a Manual to look it up but I can’t find a serial number on mine it’s rubbed off the top of the electric box

Vacuum leaks can occur anywhere in the lower intake manifold, the gaskets between the lower and the upper plenum, and between the upper plenum and throttle body. Or any vacuum hose/line.

There are smoke machines that can be used to test for leaks, or, the other option is a propane cylinder with the valve open but the flame not ignited, aimed around all the gaskets and joints of the intake system, looking for any location where the idle speed increases when exposed to propane. You can look up a more detailed procedure.

Your serial number likely falls in this range:


https://cimg2.ibsrv.net/gimg/www.off...398b9471b9.jpg

Ok I replaced the intake manifold gasket the guy I got it from did not do it correctly and it was leaking water but the motor was running fine after I did that and iv fired it up 6 or 7 times it was after I seen it was getting hot cuz of thermostat and I replaced stat and impeller is when it started acting like this and that’s what confused me on where to even start

Could be leaking from intake gaskets even if it's not leaking water... also lower injector o-rings can be a source of vacuum leaks.

Keep in mind, these aluminum intakes will corrode over the years, especially if used in salt, and the sealing surfaces might not be so good anymore.

1 fuel tank or 2? Check your fuel tank vents and anti siphon valves. Im pretty sure fuel psi should be 35 psi or more

Ok I’ll look into that and see what I can find

Has 1 big tank that feeds both motors and that was my next check was anti syphon and from what I read it says fuel pressure suppose to be 35 but was not sure on for my motor I guess if both pump motors are acting that way maybe it is anti syphon or something to affect both

DieselBoy,

I've personally twice witnessed a tank check valve that was corroded/clogged to the point where adequate FP would be observed at idle but would restrict flow to the point the engine would lean out even before WOT. If you haven't changed yours, assume they need changed. They are cheap. The fact that you get increasing FP readings with each subsequent key-on, makes me suspect the check valve is at least part of the issue, or an issue unto its own.

Thanks. Brad.

Ok I will def check that yeah on both motors first turn of key is 10 psi if I keep turning on off it will keep rising to 25-30

sounds like you have a fuel problem.use a gas can and plumb it into the system,eliminating everything before your fuel filters.you have a new fuel filter on these motors ,,right?lets see if the pressure comes up.

Yes I have changed them

Gas fuel injectors are rated at 43psi for flow. The fuel pressure regulator is often controlled by vacuum. The less intake vaccum, the harder the engine is working and using more fuel, so the regulator increases flow to ensure adequate fuel pressure. Fuel pressure is directly related to fuel metering flow.

Low fuel pressure, means less fuel flow. If the EFI doesn't have closed loop feedback, it doesn't know its only putitng in a small amount of fuel. Bit throttle blips need a massive amount of fuel to get the engine mass moving. Just like a carb's accelerator pump shots. If you can't deliver that, the engine will fall on its face as its too lean...with no energy to rev up.

Lots of reasons for low fuel pressure. I wouldn't look at anything else as an issue until you get the pressure up.

Not to mention...the injector atomization at low pressure is poor, amd makes less of the fuel available to burn, so even though you have fuel in the cyl...its seen as even leaner.

Ok thanks for that I’ll definitely look into restrictions and the check valves

Having the wrong stat makes me think your cooling is off. Does that motor have the crappy rubber piece for water intake? You get surging and rough idling if it’s not getting water to cool.

also that fuel pressure is super low for EFI. Something else is probably going on there too.

what do you mean by crappy rubber piece and motor now has 160* stat insatalled

your thermostat and it’s the impeller of your water pump.

Ah I haven’t pulled the water pump off to see

the fuel regulator are 1:1, have a hose to de intake for referense?... At idle or partthrottle there is a lot of vacuum in the manifold and the regulator causes the fuel pressure to drop to maintain a constant flow.