7.4L MPI Sputtering at Higher RPM's

C

Capt Brodie

New Member
May 25, 2020
21
East Falmouth, MA
Boat Info
33 Bertram
Engines
454 Merc MPI
Engine starts and idles fine but once it warms up and I bring it up to cruise speed...around 3,000 rpm - it starts to sputter. RPM's drop by 300 - 500 and it bogs down, even if I open it up, she stays at the sputtering rpm's. Then after about 20 seconds of sputtering, it returns to normal operation and I have all the power I want. The returns to sputter mode, then back normal again and so on. I do not think it happens below 2500 RPM.

All ignition (SP wires, SP's, rotor & cap) is new
I have solid 40psi of fuel pressure
All Fuel Injectors have been cleaned and re-installed
I just changed out the Fuel Pressure Regulator
I have good compression in all cylinders
Pick-up coil is new and I have even changed the entire distributor - get the same thing

Going out of my mind - anyone have any idea what's happening ?

Also, I read a few discussions where you guys are using a diagnostic computer to read the codes, which one and is it much $$$ ?

Thanks in advance !
 
I had a miss in one of my 1997 7.4 MPI, I have done several of the same things. The mechanic at our marina told me that the computer controls the timing and when you do a full tune-up, the computer timing should be re-set. May be an option. I have not been back since he set the timing to test.
 
I have the timing tool and it's to 8 degree's BTDC

Fuel pressure is 40 PSI while sputtering and 40 PSI when it returns to normal operation. But when I shut down the engine, it drops to about 35 PSI and stays there. I'm going to compare that to my other engine today. I had all the Fuel Injectors cleaned and serviced this past Spring and they told me that one of them was leaking so they fixed it. Can't help top think that another one of them developed a leak over the Summer but I think the engine would never run normally if that was the case.
 
If I don't find anything else I guess I'll have to pull them again and have them checked. I don't know of anyway to isolate an injector that's intermittent other than pulling all of them and sending them out. They're not cheap to replace either.

But I did have them completely done over five months ago and I can't help to think something else is going on.
 
Hook up the Rinda and check your values while sputtering. If they look normal it's got to be fuel.

What do the plugs look like? How about the primary coil ?
 
No rust on the plugs and they look typically burned. I don't have a Rinda but will definitely look into one. Do they work well with these engines ?

You know, I have a spare coil and thought about changing it. It's an easy change, so that will be next.

Thanks !
 
They work, my engine is a 99. Apparently the l29 has 2 knock sensors. Have you checked your total timing? If not you should. Still sounds like sketchy injectors. I know where you can get a quality set for under $800.
 
What do you mean by "Total Timing" ? I have the timing tool and set that to 8 degree's BTDC. But this sounds like something different...

Honestly I doubt the injectors too. I have one spare injector but with my luck, it would be eight try's ! Where are you talking about fo the set ? Nuwave ?

I got side tracked yesterday and am going to replace the coil now. We've got weather today, so will test drive tomorrow. Won't be much left to replace !
 
8* is initial timing, total timing is timing after about 3000rpm's. Should be up around 29
 
K, I could try that easy enough and BTW, thanks for working with me Scoflaw. I see you're from Cape Cod...I'm in Falmouth.

So I just changed the coil - will see how it goes with that tomorrow. But I also observed the fuel pressure on my other engine (Port). When I engage the fuel pump, it shoots right up to 40 pounds and stays there when I shut off the engine.

The engine I'm have trouble with (STBD), will shoot right up to 40 pounds, but drop down to about 35 within a few minutes of being shutoff. So does that say I have a leaky fuel injector ? Or maybe someplace in the rail ? Either one, I think I'm closer to my problem....
 
Bought me a set of new injectors a couple of years ago, solved several problems. Occasional hard hot starts, rpm's going up and down from 900 to 1200 rpm's and failure to hit max rpm's., but not all the time. What about all those ethanol worn fuel lines? They rot from the inside out.
 
Thanks! Has anyone tried the sets they sell on eBay ? They have refurbished sets for $350 or so. That’s probably equivalent to sending your own set out...
 
Like John says, the Chinese knockoffs are so real looking the only way to tell the difference is to flow test them. The OEM's are made in Italy. If you want to be educated on injectors talk to this guy.

I paid 1k for a set from Southbay who said they were OEM. Lost 6mph. He was a stand-up guy and refunded me, then bought that set from John and got my speed back.
 
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