1999 7.4mpi fuel pump issue?

1

1990dtgl98

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Jul 17, 2011
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So im chasing a rough idle and misfire. I bought a boat thst was in decent shape minus it running rough. It had bad gas in it, so I siphoned it out, replaced the feed lines to the filter, filter and anti siphon valve.

Looked over everything and replaced the cap, rotor and coil.

Went to set base timing it it still won't run right. It'll fire off and stumble and die without giving me enough time to set the idle.

Got a cheap fuel pressure gauge and hooked it up. Key on engine off it will shoot to 60psi while the pump primes (you can hear it running). Once the pump primes it settles at 28psi. Bleed off is maybe 1 psi over 5 min.

Sounds like a fuel pump issue? If so it looks like a nightmare to get to. Would it be wise to just do the pump and pressure regulator while I'm down there. Could it be a relay? It primes to 28psi consistently. I'm also not sure if thst initial 60-70psi spike is normal?

Thanks for the help in advance everyone!
 
alldodge

alldodge

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Coming up as an L29
Fuel pressure should be 43 psi running, and 28 would give it a lot of issue to run. Just wouldn't get enough fuel

The 60 to 70 psi spike would give me the thought that the Regulator would be first suspect. Could remove the Reg and test it with air to see when it opens

The pump is hard to get at as you can see. The plastic cover saps off, then there are 2 nuts which fasten the pump assembly to it. Can remove just the pump and cooler without removing the entire assembly
 
1

1990dtgl98

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Jul 17, 2011
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Yes, it's the l29.

Is the regulator mounted on that cool fuel assembly as well? If so, I think I would preemptively do both since the assembly looks sandwiched between the block and stringer, directly under the engine mount. 🙄
 
1

1990dtgl98

Seaman
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Jul 17, 2011
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You said 43psi running. Would that be the same as key on, engine off? Kind of hard to get a reading right now
 
1

1990dtgl98

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I there's 2 relays on the port side of the motor, under the plenum cover. Is one of those the fuel pump? Would it be beneficial to try to replace them before launching into the cool fuel?
 
alldodge

alldodge

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The fuel pump is working so the relays are working. If the pump had trouble turning ON, then first thought would be swap the two. One is Main Power Relay (MPR) the other is FP

Having low pressure would not be a relay problem
 
1

1990dtgl98

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Jul 17, 2011
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Thank you!

While I'm here....electrical question.

Went over everything with a fine tooth comb

I have 2 plugs that terminate in loops (looks factory). Both are at the back of the engine. One is a yellow w/ black stripe and black wire. That terminates with a white loop plug. The other is a white wire and black wire that terminate with a black loop plug. Is that normal?

Finally....

I have 2 broken wires. One is tucked under the ecm on the starboard side under the mercathode system.. It's a grey wire.

The other is on the back on the back of the block. It's a blue (or purple) with a tan stripe.

Do you know what these go to? Now I'm wondering if one of these could be causing an issue.
 
alldodge

alldodge

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If its a Purple it could go to the oil pressure switch
If its Dark Blue this would be the knock sensor

Unable to find anything for the Gray or loops, but the diagrams for the L29 are not great. Being at the rear of the motor, one may be used if Alpha drive was used
 
1

1990dtgl98

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Jul 17, 2011
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Screenshot_20210615-111012_Firefox.jpg

I was able to find a possible image. Looks like the wire might be to the distributor (purple/white) and grey shows possibly the throttle position sensor. Would the boat even run if the tps wire wasn't attached?
 
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