7.4L MPI Intermittant Loud suck noise then stalls Hot or cold

[rothfm]

Attempting some troubleshooting, and swaps (twin engines) for this issue. So far no avail.

Hot or cold starts, and within 1 min of statring, the Flame arrestor makes a much louder then normal "sucking air" noise, Idle speed goes up a bit, then comes down and motor stalls. This Bravo 3 MPI motor. Its EFI, using standard air intake (not a TBI or Carb)

Can immediately restart, and all is fine! Shut it down- Restart and the problem can happen again or not. Doesnt matter if motor is cold or been running. Can fail either way.

No issue running or at speed at all. Just at start and idling.

Looked at the normal stuff, Compression, Mani/Risers/Temps/Hoses/wiring/Fuel pressure.

Popped in a new AIC-same issue. Going to either swap or replace TPS.

Any other thoughts on this. We wanted a scan from the Marina Tech, but they are bogged down with a lack of techs.

 

[tpenfield]

You can chase the problem with parts replacement, but you really need the computer diagnostics since the issue is intermittent.

It could be the TPS . . . or not. You won't know until you can see the data from the engine while the problem is occurring.

 

[dubs283]

Going to either swap or replace TPS.

definitely swap, if the other engine is sound then you have a fully stocked parts room right in your bilge. no need to waste money guessing the problem

as enfield noted you should use a diagnostic tool to verify engine data/faults, otherwise is mostly guesswork

when was the last full tune up?

 

[2550SX]

IAC?

 

[rothfm]

Thanks All,

Tpenfield, you have assisted me in the past and I know at some point you had a Formula 33. This is the same 33 SS with twin 454's.

Its my brothers boat. I help out. Very much maintained, sched main, and very clean motors and bilge. The Marina guy is backlogged a month due to lack of help. So looking at bringing someone else in with a scan tool.

For the moment thats out. So we are doing some semi-educated swaps to see of we can affect the symptom as its Prime-Time summer here in RI. Yes, there are better methods for sure. But this is what it is, for now.

What I'm asking is (among other causes) could a bad TPS exhibit this type of Very "loud" air suction at the intake, Yes the IAC was swapped to the other motor-same result. The throat was cleaned by me. Im hearing of a mesh type filter media somewhere in the path for the AIC or inside the throttle body. Not sure if all these motors have them. But apparantly this "Muffler" media can be clogged. Didnt see it when I did the AIC but wasnt looking for it either.

Thanks for any input

What "Reasonable" items would cause the above symptoms? We do our own work, so no mechanic charges. Until we can scan or get additional help...We have no issue swapping or replacing the lesser pricey parts. Obviously will not easter egg it with anything complicated or too expensive.

 

[tpenfield]

@rothfm . . . yes I still have the Formula 330 w/ twin Merc 7.4 MPI engines.

When you say 'AIC' . . . I think you mean 'IAC' (Idle Air Control).

I also assume you have the 7.4 MPI (multiport injection) engines (???). Since the problem is intermittent, I would check all of the connections at the sensors.

TPS
MAP
IAT
IAC

It seems that what may be happening is that, based on the sensor's input, the computer is seeing the need to throttle up to maintain desired idle speed. Then it over compensates and leans out the engine causing it to stall.

A scan tool may not show enough information to see the cause of the problem. I always recommend that folks with MPI engines get the full computer program, because you can record the data and go through it afterward. What you are looking for is a sensor reading that goes out of its proper range for the engine's operating condition (i.e. idling w/ no load). For example . . . I had a TPS that would go from a 1% reading to a 5% reading all on its own, causing the engine to rev up . . . even though the throttle never actually moved.

None of these sensors are all that expensive (get the GM versions, not the Mercruiser/V-P packaged versions) So, you could buy them all and save some time swapping. But first . . . Do check to see if the connectors are secure for each sensor, as well as the connectors for the distributor (depending on which version you have)

Got a serial number for the engines?