7.4 L will not throttle past 3100 RPM

Capt.Will

Capt.Will

Active Member
Aug 15, 2016
148
Lake St. Clair
Boat Info
1997 Sea Ray 330 Sundancer
Engines
7.4 MPI Mercruiser Inboards
Vessel: 1997 330 Sundancer

Engines: 7.4 L Mercruiser MPI inboards (V drives)

Issue: STD engine will not increase RPM past 3100 RPM. Throttle will move forward but RPM's will not pass 3100.

Recent preventative maintenance: New spark plugs/cap/rotor. Refinished props. New fuel filter/water sep. Just hooked up scan tool and no codes show up. Timing, temperature and everything else checks out A-OK.

Any help here would be great.
 
Have you checked the fuel pressure at the fuel rail? Fuel problems don't normally trip codes.
 
Check the knock sensors. 3100 is about the RPM you'll get with fully retarded timing, which the computers revert tó when they dó not get an inut from the sensors. . Not sure if they throw codes when they go bad or become disconnected. With all the work that was just done the wires could have easily been pulled off the sensors.
 
JohnGordon said:
Check the knock sensors. 3100 is about the RPM you'll get with fully retarded timing, which the computers revert tó when they dó not get an inut from the sensors. . Not sure if they throw codes when they go bad or become disconnected. With all the work that was just done the wires could have easily been pulled off the sensors.
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Where exactly are the knock sensors located?
 
Fuel pressure as mentioned above. Also check to see that your new SP wires are seated tightly. Had same issue and it turned out to be a bad wire, took a while to figure that one out as wires usually are not the culprit. Also - my SC never threw a code.
 
Knock sensor location:
kn.jpg
 
I had the same issue on my last boat. I did a full tune up and then one of the engines wouldn't run up like it should have. Turned out to be a bad batch of distributor caps from Merc.

I would start by putting the old cap back on to see if the issue goes away. If that doesn't work I would start looking at the plug wires.

Are you sure you have the correct spark plugs w propper gap?
 
One other thought......how was it running before the "preventive maintenance" was performed?
 
JohnGordon said:
Check the knock sensors. 3100 is about the RPM you'll get with fully retarded timing, which the computers revert tó when they dó not get an inut from the sensors. . Not sure if they throw codes when they go bad or become disconnected. With all the work that was just done the wires could have easily been pulled off the sensors.
Click to expand...
Or the wires were possibly put back on wrong. That happened to me by a "professional"yard mechanic who swapped out a dist cap while I was out of town. He (or more likely his helper) got 2 of them swapped. Engine ran, but not well. I wound up going through 50 more things before I took a hard look and found the mistake.
 
Tested the boat yesterday. I changed the spark plug wires and swapped the high pressure fuel pump between the two engines. It seemed slightly better but still the same. I. believe the issue to be a fuel issue. Just not sure why at this point. I will burn some fuel this weekend. Maybe it will get better. We shall see.
 
I had same exact issue, turned out to be a couple clogged fuel injectors, sent out to be cleaned and flow tested and ran perfect when back.
 
sccchiii said:
I had same exact issue, turned out to be a couple clogged fuel injectors, sent out to be cleaned and flow tested and ran perfect when back.
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Can you let me know how you troubleshot that and found out that was the cause? Did an more sophisticated scan tool detect that?
 
22 year old injectors are probably your issue. They don't last forever. You have'nt said anything about your fuel pressure when it hits the wall.
 
Capt.Will said:
Can you let me know how you troubleshot that and found out that was the cause? Did an more sophisticated scan tool detect that?
Click to expand...


No other than getting fuel pressure reading......in my case i checked all the plugs and compression first, then moved straight into fuel pressure, once i verified fuel pressure was good I realized that injectors had to be culprit. They had been on engines for quite some time so i figured they really needed to be cleaned anyways but once they came back boat ran the best its ever run so my guess is they had not been very efficient even before the misfire/stuttering at 3k rpm and up issue presented itself.
 
Couple things. First, take off the flame arrestor and run the boat. I had the identical issue with a 7.4 MPI. We changed props, fuel injectors, plugs, wires, swapped ECMs, etc. etc. etc. etc. etc. I finally scanned the motor while running it and sent the data to this guy (I used his software to pull the data): http://mefi3.com/ He is a veritable expert on the mefi3 computer on these engines and the data they produced. Based on the data, he concluded that the engine wasn't getting enough air or at least thought it wasn't getting enough air. We checked the manifold pressure sensor, but it was reading barometric pressure at ignition on, which it should have been. I inspected the flame arrestor and throttle body and noted no obstructions. One night, it occurred to me to just take the damn flame arrestor off and run it. The goddamn thing ran right up to 4800 RPM. The arrestor didn't look corroded at all, but must have been internally. May well not be your issue, but I would try it.

Indeed, before you buy any more parts, I would scan the engine while running at idle all the way to full throttle, record all the data and send it to the guy above. Had I done that at the outset, it would have saved me thousands of dollars and 2 frustrating months of monkeying with this issue.
 
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Capt.will any updates? Did you find problem?
 
Ok. After many sleepless nights over this issue, I finally repaired it. I was thinking it was a fuel issue at first so I did remove the injectors and had them cleaned ($255) and no difference. I performed a fuel pressure check and found that the low pressure fuel pump was not the greatest so I changed that ($210) and no difference. Once again, had no codes showing on the Rinda scan tool, but after a suggestion from a mechanic friend of mine who claimed that the distributer module/pick up coil is guaranteed to be the issue I want ahead and changed that ($128). PROBLEM SOLVED!

IMG_5361.jpg
 
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