MPI loading up, runs like crap...HELP!
 

Since the first outing this spring, my `98 7.4 MPI has been running like crap. It runs EXTREMELY rich at idle and kinda clears up once past 2500 rpm. I have run about 100 gallons of fresh fuel mixed with injector cleaner through it with no improvement. I feel like I have a ECM problem. 3 plugs were very wet, the others were black but dry. It runs OK once on plane, but sounds like it`s hitting on only 5 cylinders at idle. The problem isn`t as bad when the engine is cold. Fuel pressure reads 30 at idle, 38 WOT. I have a K&N and aftermarket exh manifolds. otherwise stock with 225 hrs.
I have new plugs, cap & rotor, new fuel filter...nothing has helped. I need it going for Memorial weekend. Please offer any suggestions. My Mercruiser mechanic is 2 weeks behind. I need my own scanner......
THANKS!

Sorry I wish I could help. We had the same problem, and took that crap off and put a nice carb on.

Sounds like ignition problem, 3 wet plugs. Check the plug wires and cap on those plugs. New wires can be bad, get multi meter and check each one.
Good Luck.

You really need to scan for codes to get more information. You already have done the basics. Make sure that the vacuum line is hooked up to the regulator. I have seen them come off before. If you have the ability to swap Map sensors with a known good one, that may help.

ok here is the poor mans scan tool, first find the timing/diagnostic lead that breaks out of the harness, pull the protective cap off you will be looking at the female pins inside the connector now, take a paper clip and bend into a U install the paper clip into the end two wires that are black/with white stripe and the white/ with black stripe, this will put the ecm into field service mode, then take a 12volt test light connect the alligator clip of the test light to the postive terminal of the battery then take the pointed end and push into the diagnostic connectors brown wire with a white stripe turn the key on and it will flash out codes through your test light, it will start off flashing a code 12, that will be one long flash then two short flashes, this is telling you diagnostics are working, if no faults are stored it will flash code 12 three times, any codes stored will come after code 12, such as 4 long flashes then 3 short flashes will be a code 43, whick is a knock sensor code, codes go from 12 to 51, good luck and let us know what codes you have

That is some valuable information. I will check for trouble codes tomorrow. Is there someplace on the web or elsewhere that has all the codes listed? I will let you know what I get.
THANKS!

I agree with Mate 229, it sounds like ignition. If you have a timing light, connect it to each plugwire and check for proper spark on all the plugs. If there are misses, then you know where to dig further.

im thinking of doing the samething what could you keep ? do you have a parts list? and what gains if any

I was going to try and read the trouble codes by using the method described above, but I did not see a white wire w/black stripe. I have a 10 pin connector with 2 rows of 5. Here is how mine is laid out:
TOP ROW, left to right-----
BROWN/WHITE , BLANK , BLANK , BLACK/WHITE , BLACK

BOTTOM ROW------
ORANGE , ORANGE , YELLOW , BLACK , BLANK

So, do I connect the black/white wire to the black wire on the top row and probe the brown/white wire?

I am running out of time here....Wish me luck.

Do you have a VST tank on the motor or a cool fuel system, if its a VST tank that is probably what is wrong with the boat and needs to be rebuilt.

was your water temp correct i had a thermostat stuck open and engine would load up @ idle computer stays in cold start mode.

DonMan, I would connect the black/white with the black just beside in the top row. That is where the white black is located on my engine. You may get lucky and find a trouble code that can give you a clue. Still, my experiance is that 95% of all trouble on a marine engine is related to things that do not trigger a troulble code.Then it is back to basics. Would still suggest to check the spark on each plug with a timing light.

I have the Cool Fuel system on my boat.
My water temps seem to be right, running about 175*.
I am going to check the spark to each cylinder. If that is good, I`ll check to see if each injector is hitting. I have a feeling that maybe I have a couple injectors stuck wide open.....?
I think I am going to try and retrieve a trouble code with my test light.
Will let yall know what I find.
THANKS!

Turn the key on and check the fuel pressure if it holds then OK if it bleeds down or falls then an injector is stuck.

on your cool fuel setup it will be the two wires on the end you jump[bridge] black- black/white , [ if you ever need to set your timing you do this step first then hit it with the timing light] after jumping the two wires, then do the test light alligator clip to postive terminal then the pointed tip of light to the brown/white wire with key on, the codes are the same as on an old General motors product from the late 1980's

I checked my fuel pressure a couple days ago, it holds rock steady.
I did the test-light trick on the diagnostic connector and it flashes code 12 over and over (no trouble codes stored).
I pulled the map sensor, just to check it out. It was clean did not have any residue on it.
I put about 2 hrs run time on this new set of MR43LTS plugs and pulled them out for inspection just now. All 8 look very good, no wetness and a slight tan color.
I ran the engine again paying close attention to coolant temp, etc...
It runs just fine while cold, it starts to run rough as the temp approaches 150* and fully gets into it`s rough idle condition after the T-stat opens (160*). I am suspecting a faulty coolant temp sensor. I unplugged it while the engine was running (while cold) and it really made the engine run bad. After the engine was warmed, I unplugged the coolant sensor and it made no difference. Is this my problem?
I am mad at myself for my lack of knowledge of fuel-injection and computer-controlled engines. I am ready to swap to a carb just for the simplicity.
I need a good running engine for at least this holiday weekend. Am I looking in the right direction with the coolant temp sensor?
I went ahead and pulled it, hoping to match it up at the parts store tomorrow.
Any other suggestions????
THANKS!!!!

I would say the coolant temp sender is a good bet, if its faulty it will cause the motor to run rich, but also follow the wires to make sure you don't have a bad connection.

DonMan, It sounds as if the ecu is commanding to injector
pulse width high. The 2 most likely causes would be the
coolant temp sensor curcuit or the map curcuit. If you have a
volt meter you can easily test these. Both run off a 5 volt
reference signal. Unplug the coolant sensor & turn the key on
test the wires,you should have aprox. 5 volts on 1 of them
Then plug it back in. Back probe the other wire (signal return)
as engine temp rises the voltage will go lower, open curcuit
will be 5 volts. The sensor acts as a variable ground. More temp
lower voltage. For the map,start engine on the signal return
you should be at about 1.2 - 1.6 volts depending on rpm. A open
curcuit would be 5 volts. If either curcuit were open the ecu
would command the injectors open thinking that the engine
was cold or at wide open throttle. Also with the map make
sure you have 18in.of vacuum,less vacuum,high voltage. Again
ecu thinks your wide open. The voltage on the map should rise
as the throttle increases. By the way,stick with fuel injection
its a great setup. Good luck, Rick

FFT,do you have a schematic for this system, If you do,look and
see if the black goes to ground or back to the ecu.
If it goes back to the ecu you would probe the black. If it goes to
ground you would probe the yellow. You may be right, I"m an
auto tech by trade & am baseing my info on commom gm obd 2
systems. I've never seen a wiring diagram for the merc setup.

I swapped out the coolant sensor with a new one ($12, auto parts store). ...No change.
I have a new map sensor, but haven`t tried it. I will check things out with the volt meter.
THANKS!

I think you just measure it between the yellow and the black wire. they both go to the ECU to deliver the voltage reference.

FFT, if that black wire is spliced & then goes to map its
a sensor ground,you would test it on the yellow wire.
As the sensor starts to complete the ground the ref
wire voltage will drop. The ecu sees the difference
and uses this to sense engine temp. DonMan,you
said the engine runs extremly rich at idle but gets better
when rpm up. This could be the map, if there is no vacuum at
map or the return ground is open to the ecu,the ecu will
see a full 5 volts on the map signal & the temp signal
this will cause it to run extremly rich. With fuel pressure
o/k something is commanding injectors pulse width higher
to add more fuel, tps could do it, map or coolant but not Iac.
I think you have to test map & coolant sensor first