1995 454 Magnum EFI Stumbling and rough cold start
03-20-2025 | 09:50 AM
#11
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Joined: Apr 2013
Posts: 124
Likes: 9
I’ve got the same exact engine.
bought this boat this winter because the guy I bought it from couldn’t get it running.
After all this cold/bad weather i finally got around to playing with it.
I checked compression.
mines a bit different than the above. Cylinder 1 190
Cylinder 3 170
Cylinder 5 175
Cylinder 7 165
Cylinder 2 175
Cylinder 4 185
Cylinder 6 180
Cylinder 8 190
next checked spark…got spark
all plugs looked decent and wires are connected to the correct holes.
next tried to fire her and she will fire on starting fluid but falls off
checked fuel pressure at the shrader valve…zero
next pulled fuel pump
it’s blown the hammer/foot that rides on the cam is worn down to a flat nothingness and the little bearing piece that rides on the cam is split in 2 down the middle..the pump wasn’t pumping at all
got a rebuild kit for the seawater pump and brand new fuel pump coming my way and hopefully I’ll get this big engine running the next few weeks
im betting yours is fuel pump related or check that little bearing on that cam
I would check fuel pressure at the shrader valve too
bought this boat this winter because the guy I bought it from couldn’t get it running.
After all this cold/bad weather i finally got around to playing with it.
I checked compression.
mines a bit different than the above. Cylinder 1 190
Cylinder 3 170
Cylinder 5 175
Cylinder 7 165
Cylinder 2 175
Cylinder 4 185
Cylinder 6 180
Cylinder 8 190
next checked spark…got spark
all plugs looked decent and wires are connected to the correct holes.
next tried to fire her and she will fire on starting fluid but falls off
checked fuel pressure at the shrader valve…zero
next pulled fuel pump
it’s blown the hammer/foot that rides on the cam is worn down to a flat nothingness and the little bearing piece that rides on the cam is split in 2 down the middle..the pump wasn’t pumping at all
got a rebuild kit for the seawater pump and brand new fuel pump coming my way and hopefully I’ll get this big engine running the next few weeks
im betting yours is fuel pump related or check that little bearing on that cam
I would check fuel pressure at the shrader valve too
03-20-2025 | 12:51 PM
#12
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Joined: Aug 2019
Posts: 1,249
Likes: 420
From: BC
The same symptoms actually running? Or is this just on the trailer idling amd reving with no load?
Stumbles from throttle blips are often caused by ultra lean A/F charge. Where the accelerator pump or EFI injector charge of fuel is not sufficient.
If your EFI fuel pressure is low...the injectors will flow enough for a progressive thrittle increase, but unable to discharge on a throttle blip.
On that note, it's way harder to ignite a lean mixture...so weak spark is also a factor.
Lean mixtures have a longer burn time, so you get less cylinder pressure and less power.
Check your ignition timing to ensure it's within spec at idle and rev'd rpm. If the timing doesn't advance, a lot of the cyl pressure just goes out the exh valve.
Stumbles from throttle blips are often caused by ultra lean A/F charge. Where the accelerator pump or EFI injector charge of fuel is not sufficient.
If your EFI fuel pressure is low...the injectors will flow enough for a progressive thrittle increase, but unable to discharge on a throttle blip.
On that note, it's way harder to ignite a lean mixture...so weak spark is also a factor.
Lean mixtures have a longer burn time, so you get less cylinder pressure and less power.
Check your ignition timing to ensure it's within spec at idle and rev'd rpm. If the timing doesn't advance, a lot of the cyl pressure just goes out the exh valve.
03-20-2025 | 01:01 PM
#13
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Joined: Jan 2024
Posts: 45
Likes: 17
From: So Cal
The same symptoms actually running? Or is this just on the trailer idling amd reving with no load?
Stumbles from throttle blips are often caused by ultra lean A/F charge. Where the accelerator pump or EFI injector charge of fuel is not sufficient.
If your EFI fuel pressure is low...the injectors will flow enough for a progressive thrittle increase, but unable to discharge on a throttle blip.
On that note, it's way harder to ignite a lean mixture...so weak spark is also a factor.
Lean mixtures have a longer burn time, so you get less cylinder pressure and less power.
Check your ignition timing to ensure it's within spec at idle and rev'd rpm. If the timing doesn't advance, a lot of the cyl pressure just goes out the exh valve.
Stumbles from throttle blips are often caused by ultra lean A/F charge. Where the accelerator pump or EFI injector charge of fuel is not sufficient.
If your EFI fuel pressure is low...the injectors will flow enough for a progressive thrittle increase, but unable to discharge on a throttle blip.
On that note, it's way harder to ignite a lean mixture...so weak spark is also a factor.
Lean mixtures have a longer burn time, so you get less cylinder pressure and less power.
Check your ignition timing to ensure it's within spec at idle and rev'd rpm. If the timing doesn't advance, a lot of the cyl pressure just goes out the exh valve.
I took the boat on a very brief sea trial a month ago (sun was setting and I didn’t want to be the last one at the ramp) and in gear it seemed to accelerate well and plane out although I may have noticed that the engine in question required more throttle to match the Rpm’s of the other engine.
I’ve got the cap,rotor and new plug wires on the way. I’m going to swap them soon.
how can I verify the efi fuel pressure? I’m not near the boat currently but I’ll go look under the hatch for an access point later.
03-20-2025 | 07:12 PM
#14
Registered
Joined: Aug 2019
Posts: 1,249
Likes: 420
From: BC
Most EFI fuel supply lines on engine will have a schreader valve test point. Find the fuel line routing and the stainless fuel tube. It will be on that. You need to have a fuel pressure tester gauge, that screws on and depresses the schreader valve.
The fuel pressure regulator may also be an issue.
Low fuel pressure issues can be from clogged fuel filters or water in the filter etc.
The fuel pressure regulator may also be an issue.
Low fuel pressure issues can be from clogged fuel filters or water in the filter etc.
