Engine Backfiring under load/ loss of speed/torgue / top end

[Mateyk]

I have a 04 Outback 320 EFI TBI with 650hrs, owned since new. Put plumbing etc back together and ran out of water with muffs. Engine ran smooth from idle to redline out of gear. At idle chugged or sputtered a bit but figured that it was just the fogging wearing off. Made the 4hr tour to the cottage for our 1st canadian long weekend and wished I had done a water test 1st!

symptoms: I have no midrange power and have lost full rpm and top speed. 3800rpm and 38-40mph. I usually get 4800 and 48mph. Under load the engine backfires. I replaced the fuel filter and checked all of the plugs which were brand new. Plugs were fouled, at least 6 of the 8. Cleaned and put back. Same result. This time starting was tough sometimes unless I took out of gear and floored the throttle.

I'm thinking it's the fuel pump now or there is something blocking the fuel lines? The boat has been bulletproof at over 100hrs each summer. Any ideas, just to be a sanity check for what my dealer is going to tell me the issue is?!

[kaneboats]

I don't see how fuel starvation from a bad fuel pump is going to foul plugs. With fouled plugs usually there is a problem with spark or timing or else the fuel is bad.

[JesseC]

Sounds like there is oil/water in the gas from a bad tank of gas. Change the fuel filter and get as much gas out as possible. I had a car do this (would idle fine but just wouldn't go) and when we bypassed the fuel filter it ran great. We then went home dumped the fuel and changed the filter. You probably have water in the gas from the Ethenol sitting all winter.

[maxpower220]

Fouling plugs is hard to do on a 4 stroke motor. First, check your spark plugs. I have had new plugs that (I assume) I cracked during installation. The spark was going outside of the plug straight to the engine block. It was on 2 plugs, it gave little mid range and no top end power, but idled fine. Also, the plugs obviously weren't firing inside and became fouled. I only noticed it as it was dusk and I happened to see the spark around the plug as the engine was running.

Timing may be another problem, but this should affect idle and all engine rpms. You can get a fuel pressure gauge and install to check your pressure to check the fuel pump.

[AaronWhitt82]

I'm going to say it sounds like some type of fuel issue, the timing is off or possibly you have a bad coil....

[mmandley]

I have a 04 Outback 320 EFI TBI with 650hrs, owned since new. Put plumbing etc back together and ran out of water with muffs. Engine ran smooth from idle to redline out of gear. At idle chugged or sputtered a bit but figured that it was just the fogging wearing off. Made the 4hr tour to the cottage for our 1st canadian long weekend and wished I had done a water test 1st!

symptoms: I have no midrange power and have lost full rpm and top speed. 3800rpm and 38-40mph. I usually get 4800 and 48mph. Under load the engine backfires. I replaced the fuel filter and checked all of the plugs which were brand new. Plugs were fouled, at least 6 of the 8. Cleaned and put back. Same result. This time starting was tough sometimes unless I took out of gear and floored the throttle.

I'm thinking it's the fuel pump now or there is something blocking the fuel lines? The boat has been bulletproof at over 100hrs each summer. Any ideas, just to be a sanity check for what my dealer is going to tell me the issue is?!

First thought as well is Fuel. Even bad fuel the engine can rev up but it wont have any HP to move the boat.

On a side note NEVER NEVER NEVER...did i mention NEVER red line an engine with out a load.

This is a huge stress on an engine and this right here can cause the engine to come apart. Engine are designed to be reved up with no issue but to red line you need to have a load on the engine.

[LSM]

If you get 2 plug wires crossed somewhere it will do the exact same thing.......Ask me how I know. It'll sound great out of gear but when you load then engine it'll run weak and back fire...I had an older MasterCraft before my LSV and I had replaced the plugs and wires and got two crossed up. Same symptoms......

[NCSUmoomba]

Sounds like a fire issue to me. Are any of the plug wires damaged or loose? I know that when a car acts like that it is often plug wire or the coils.

[Mateyk]

First thought as well is Fuel. Even bad fuel the engine can rev up but it wont have any HP to move the boat.

On a side note NEVER NEVER NEVER...did i mention NEVER red line an engine with out a load.

This is a huge stress on an engine and this right here can cause the engine to come apart. Engine are designed to be reved up with no issue but to red line you need to have a load on the engine.

Thanks! I guess I should have clarified that I reved the engine through its range and not to redline. It is possible that I have bad fuel and maybe wrong order with the cap wiring even though we made sure to change one plug at a time.

[Mateyk]

If you get 2 plug wires crossed somewhere it will do the exact same thing.......Ask me how I know. It'll sound great out of gear but when you load then engine it'll run weak and back fire...I had an older MasterCraft before my LSV and I had replaced the plugs and wires and got two crossed up. Same symptoms......

We were very carefull to replace 1 plug at a time, but besides the gas this makes the most sense. The plug wires are not labelled but seem that they can really only go to certain areas on the block with having excess or not enough slack. Where would I find info on which goes where?!