need help
 

i have a 1998-2000 7.4l mpi 454 330hp and it is doing some weard sh?? i have tryed just about very thing had a couple people try to work on it but still running like shhhh its kinda hard to type what it is doing so if there is any way some one could call me ill explaine what it is doing , NO disrespect to anyone but looking for someone like a mercruser tech. i have had a lot of joes say thay know but they don't know any thing so lets see if we have someone really smart on OSO that can fix my prob. 252-646-2751 thanks :drink:

Try typing what its doing. I GUARANTEE someone on here has had/dealt with your problem. These are some smart and experienced guys. :drink:

What did the joes say it was that way we can rule that out

help
 

when you give it gas it has NO POWER and is poping throw the intake kinda like a back fire i guess, and when it is at idle it is sucking air REALLY load throw the intake so load it herts your ears some times, will not idle for more then a min. or two, one guy said it was triping a code for the map and temp sensor and thay run off the same grownd and if that didn't work it was the ecm so i replaced all of them and nothing, still messing up, then he said valve guides were leaking, thought it was a valve guides so pulled the head and had the valves reworked and new valve guides put in that did not work eather. and i don't know anyone around here that has a dynics computer for mercruiser that worth a crap. or thay even really works right. O yeah i had another guy say he thinks its timing but i have never mest with the timing ever, and it ran grate untill all of a sudden one day it started acting up.

Wow, I am no expert but have had alot of time in and around this motor. First the sucking sound through the intake is normal. It may have gotten louder recently if someone took out the small "muffler" covering the IAC intake. As for the backfiring, there are a number of things that can cause that, coil could be going bad, dist cap corroded, timing, you said the heads were rebuilt so valve is out, and a few more items. Second you are pulling codes for temp and MAP. the temp sender uses the block for its ground and the map sensor uses a closed loop and varies a voltage to tell the ecu what the map is. they are not on the same circut. It almost sounds like you have the cam timing off. Did you have the cam out before all this started? You have replaced the MAP sensor, temp sensor, ECM, and had the heads done. Who is working on your boat? That is alot of coin to spend with no results. :eek:

It may have lost an exhaust lobe on the camshaft. Those symptoms lead me there. Hope I'm not right.

Fixx
 

the banging and poping noise is caused from weak fuel pressure..

i have 35+psi at the fuel rail i put a new cool fuel pump ang reg. on last year

i have not have the cam out time thinking its timming im going to look at that today.

Fixx
 

i guess it could be timing but you said that you already had the heads off and that should have been checked when it was put back together..

35 psi is ok but it calls for 37 psi..have you checked the mechanical pump as well? 3-7 psi @ 2000 rpm's..even tho you have 35 psi of fuel pressure you may not be getting enough fuel to the cool fuel pump..did you check pressure idleing or out on the water..

a bad injector will act like a fuel pressure problem also..

nope timming checked out so its not that, there is no mechanical pump on a gen6 just the cool fuel, And when i replaced ther fuel pump last year i put to new injectorin to ...yeah i know alot of money in parts not to fix the prob. thats why im so irritated over the hole thing.

Dont worry, your not right!!!


you said the ignition timing was set correctly, have you checked the cam timing? what do the plugs look like?
Honestly, sounds like a huge vacuum leak to me. need a scanner so you can see what the map is actually saying and see fuel trims as well

yeah i know im looking for someone around here that has a good one. I tried one computer scan tool it said map voltage high and temp volt hight when cold, clear the code and it pops right back. code 22 and 14, secret 412 witch is the grownd for the map and the temp sensors that are on the same grownd secret.

Fixx
 

so your saying theirs no fuel pump on your sea pump???? looking at a 97 formula as we speak and theirs one on each mpi engine's sea pump.. the electric pump in the cool fuel cant suck the fuel from the tank to the resivour..their should either be a mechanical fuel pump or a inline electric pump to feed the cool fuel pump..

nope no pump on the sea pump just the cool fuel ...on 97-00 gen6 there is no pump on the sea pump...are you looking at a 454mpi OR a 454 mag mpi there is a big diff. the mag mite have the fuel pump on the sea pump.

My 98 7.4 MPI (L29) only had cool fuel pump as well. That is normal.

Good to know. Have you ever heard an engine with flat cam lobes? Just curious.

Fixx
 

1997 formula 312 gen VI 330 hp mpi,, and their are fuel pumps on thease engines..

just like it shows in the pic under fuel pump..

http://www.mercruiserparts.com/selec..._nbr=809994965

I have and it does sound like he described. The loudness would be because an exhaust valve isn't opening, then when the intake valve opens the compressed combustion charge gets puked back into the intake plenum. But I believe this vintage merc engine would be roller cam which is far less likely to have wiped a lobe.

Anyway, some basic mechanical diagnostics like comp test, leak test and cut open the oil filter to look for metal would be my next move.

BT

Did you put it in base mode to time it. Could be the time chain jumped a tooth, you can check with a valve cover off. It could be ignition module, coil, vacuum leak, FUEL PRESSURE LOW/ lean condition, bad ground. I'm with Blue Thunder and some basic diagnostics are needed.

I thought they did all that basic testing. The ECM harness has two small ground wires that terminate on the rear bellhousing mount studs, they are twisted together into a common 3/8 ring terminal. The studs are on each side of the bellhousing where the curve transitions to straight down. They are fairly small guage wires and the ring terminal is steel and could corrode. Check these to see if they are loose on the stud, there may be a negative battery cable over them on the stud. If there is signs of corrosion inspect them carefully for continuity. The ECM circuit grounds through these small wires. Good luck.

Pull your timing chain cover off and check for wear, like was said it may have jumped. How many hours are on the engine?

ill have to check this tomorrow, thank you so much every one for your help, ill for sure let everyone know when i fix it and what it was.