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Dead spot in rpms


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I just bought a 98 response LX with 310 fuel injected motor. I have a dead spot between 2600 and 3000 rpm. Runs great otherwise. It will accelerate smoothly up to 2600 rpm then hesitate, with more acceleration it will suddenly jump up to 3000 rpms. Same thing when decelerating . So far I changed throttle cable,plugs,wires,distributer cap and rotor. Anybody else having issues with this ? Any ideas ?

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of course, it can be alot of things...

but, search for fuel hammer.

i just went through it. best bet, get a fuel pressure gauge and see if it goes away when connected to the schraeder valve... if it does, let me know, i can tell you the next step that worked for me.

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of course, it can be alot of things...

but, search for fuel hammer.

i just went through it. best bet, get a fuel pressure gauge and see if it goes away when connected to the schraeder valve... if it does, let me know, i can tell you the next step that worked for me.

+1

There are several threads on the fuel hammer issue just like what your descibing. It is a simple fix if that is the issue, but definitely get a fuel pressure guage hooked up soon and note what it is a key up, idle, and wot.

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Thanks for the quick answer guys !!! I took boat for a spin this morn with fuel gauge on shrader valve. Flat spot is GONE !!!! What do u recommend now ??? Fuel pressure was 35 with ignition on then held right on 40 with Motor running, held right at 40 up to wot. Thanks again for the help.

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Thanks for the quick answer guys !!! I took boat for a spin this morn with fuel gauge on shrader valve. Flat spot is GONE !!!! What do u recommend now ??? Fuel pressure was 35 with ignition on then held right on 40 with Motor running, held right at 40 up to wot. Thanks again for the help.

Here is what it is supposed to be running: http://bakesonline.com/images/MediaLibrary/Indmar_96-06FuelpsiRef.pdf

Looks like yours was running a little low, usually low means fuel pump and high means fuel pressure regulator. Since you were not far off and the fuel pump being so expensive, I am not sure what I would try, either pump or crossover lines. Maybe someone else that had the fuel pump replaced will chime in, mine was high so all I needed was a new regulator.

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Flat spot dissappearing with fuel pressure guage installed is an indication you have Fuel Hammer. Easy fix. I think you simply have to replace the rear fuel crossover line. Search this site for Fuel Hammer and you shoul dget plenty of information.

Edited by RTS
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  • 4 weeks later...

Here is what it is supposed to be running: http://bakesonline.com/images/MediaLibrary/Indmar_96-06FuelpsiRef.pdf

Looks like yours was running a little low, usually low means fuel pump and high means fuel pressure regulator. Since you were not far off and the fuel pump being so expensive, I am not sure what I would try, either pump or crossover lines. Maybe someone else that had the fuel pump replaced will chime in, mine was high so all I needed was a new regulator.

If you need the cross over lines, they're Indmar Part # S786129.

I've seen them on ebay, if you do a search for that part number you'll likely find them at a good price. You need two.

http://www.ebay.com/itm/Indmar-Fuel-Hose-Cross-Over-S786129-/151104872090?pt=Boat_Parts_Accessories_Gear&hash=item232e8d5e9a&vxp=mtr

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The fuel pressure you found is correct, and normal. 35 psi, max 40 at WOT.

The hammer issue is caused typically by older injectors creating a "fuel hammer" effect at certain rpms. Much like a water hammer repercussion in your house when shutting off water flow, an injector closing will send a shock wave back in the fuel system that will affect the fuel flow to other injectors. At the right rpm this create a pulse where the fuel will not flow through an open injector, and create the "dead" spot in rpm.

- Do not purchase the cross over lines in the ebay link above, as they are solid cross over lines, and will not help your issue.

- If you have solid cross over lines currently, running between the fuel rails front and back over the intake manifold, then you can purchase the Indmar cross over line as listed above (S786129) or look at getting aftermarket criss-cross lines. The non-metallic line is what you need, to "absorb" the injector pulse. This is what your fuel pressure gauge hose is doing while connected.

- For serious cases, criss-cross lines (front left to back right, and front right to back left) are required to eliminate it completely.

Peter

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  • 4 years later...

This fuel line S786129 fixed the rpm gap problem. After running my boat with the rpm gap for 3 years this 15 minute fix of replacing this fuel line that has a one inch hose built into it was amazing. Apparently as the engine ages the opening and closing of the valves creates a hammering effect like when you turn off a faucet quickly in your home and you hear a knock. That knocking affects the computer and prevents the engine from running at certain rpm’s. In my case I could not run from 2700 to 3100 rpms. The original cross over fuel line is solid aluminum so the new part S786129 has a little hose built into it and that absorbs the knocking or hammering and now the engine runs smooth through all rpms.  Dealer will sell you the part for $140 (in 2018) but I found one on eBay for $59. No need for the dealer to do this, anyone can replace it as it is just two nuts at the ends of the fuel line. There are two cross over fuel lines but replacing only one does the fix.  Now we can run at perfect wakeboard speeds again. 

Edited by Duski2
Sp
  • Like 2
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