Fuel Flow

[RyanL]

Alright gents wanted to scratch an itch here,

 

I have recently completed the 99' conversion on my 89' MJ. I used the 22.5 gallon Dakota tank with a 98' Dakota fuel sender, wired in to operate with the 99' layout. (The 98' sender is the only type of sender that has the correct polarity to read on the XJ fuel gauge correctly besides the XJ tank and sender). So, saying all of that, is this how the fuel is supposed to flow?

 

 

 

The tank had be to drained to install the Dakota heat shield, amongst other things, and the tank is a lot easier to handle, mostly empty of course. The fuel pump relay was jumpered with a wire to induce constant fuel flow to drain the tank before removal. The fuel rail reads a solid 47 P.S.I. pressurized, so I was just curious if this is a indicator of a bad/spotty fuel sender, or if the fuel flow is staggered for the injectors or something? :dunno:  We have been trying to chase down an intermittent engine "hiccup" and could this be the cause?

 

Thanks in advance,

 

Ryan

 

Edit: The line was clamped BEFORE the fuel filter.

[Minuit]

Are you sure the pump is in full working order? That would be the first thing I would check.

[RyanL]

That's the thing, it seems to be working fine (energizes on key to RUN, runs when the car does, etc.) but the flow is what I am worried about. Is the "skipping" normal?

[HOrnbrod]

One of the tests for a fuel pump is a timed test that measures the volume of fuel pumped out of the Schrader valve on the rail. The minimum volume is given in the FSM. I've done this test a few times when troubleshooting fuel flow on various vehicles, watched the fuel flow into an open container, and never saw a pulsing flow like that. It looks like the pump isn't completely submerged and it's sucking air.

 

How did the timing light test go?

[Minuit]

I doubt it's normal. I imagine that 'skip' would cause the coughing you're seeing. Maybe a dying/improperly wired pump but make sure to get some other opinions first. I've been up since 5 this morning and my fundamental car knowledge may be slipping a bit :cheers:

[RyanL]

That would be a possibility, part of the reason the tank had to be drained was for a repair to a small hole (could be introducing air to the sender), but wouldn't it pulse all the time with a low fuel level if the sender wasn't submerged completely? In the beginning it had a solid flow.

But with the hole in the upper corner, it is a valid possibility. When the tank is repaired it should give us a better picture.

And the timing test went as you said it should.

After more research my father and I discovered that the O2 sensor behind the cat prefers a regular 2% oxygen to run idle correctly. It sends the information to the PCM and the computer adjusts the idle from there. If the sensor gets more oxygen than it wants through say a bunch of clamped fittings on the exhaust pipe some people had to splice together on a 97+ conversion (ahem :doh: ) it will forced feed the engine fuel, not knowing where the extra oxygen came from, reading the engine as running lean. The connections were professionally welded today, but with the hole in the gas tank, we cannot be sure if it was the problem or if it is another thing because it is not safe to run the truck.

But it could also be a bad sender. When the tank is repaired and the sender replaced it will hopefully lead to some answers! If the fuel skipping and engine performance are related it will answer a lot of questions! 

One of the tests for a fuel pump is a timed test that measures the volume of fuel pumped out of the Schrader valve on the rail. The minimum volume is given in the FSM. I've done this test a few times when troubleshooting fuel flow on various vehicles, watched the fuel flow into an open container, and never saw a pulsing flow like that. It looks like the pump isn't completely submerged and it's sucking air.

How did the timing light test go?

[RyanL]

Any help is good help! Even if it is tired help haha! I honestly hope it is the problem, because we have been chasing this problem for weeks. I am pulling the tank down tomorrow, and repairing the hole I mentioned above, and if that solves the skipping, sweet! If not I will replace the sender.

 

I am almost convinced now the skipping is not normal. As you say, seeking other opinions is a must. These confirm my suspicions!

I doubt it's normal. I imagine that 'skip' would cause the coughing you're seeing. Maybe a dying/improperly wired pump but make sure to get some other opinions first. I've been up since 5 this morning and my fundamental car knowledge may be slipping a bit :cheers:

[Alexia]

The fuel pressure should be 49.2 PSI +/- 5 PSI. 47 PSI is fairly typical on those fuel pumps. The stuttering looks like a pump issue. Mine flows just fine when powered on.

[Akula69]

As part of the idle problems Ryan was having we thought the sender was bad, so I bit the bullet and purchased a new Airtex sender for 221.00 ( :thwak: ).

 

We submerged the new sending unit in a fresh bucket of gasoline and hooked it up, only to find it behaved exactly like the old one. Needless to say, it was removed, dried and returned, but as closure to this thread the lesson is: the Dakota sending units / fuel pumps will pump solid stream for a period of time, and if line pressure does not cause an auto shut-off they will "sputter" while running.

[HOrnbrod]

As part of the idle problems Ryan was having we thought the sender was bad, so I bit the bullet and purchased a new Airtex sender for 221.00 ( :thwak: ).

 

We submerged the new sending unit in a fresh bucket of gasoline and hooked it up, only to find it behaved exactly like the old one. Needless to say, it was removed, dried and returned, but as closure to this thread the lesson is: the Dakota sending units / fuel pumps will pump solid stream for a period of time, and if line pressure does not cause an auto shut-off they will "sputter" while running.

 

Guess that's somewhat of a safety feature? You can do a time vs. volume fuel capacity test with the MJ pump for a minute and it flows steady for the full minute. Anyhow, glad you found out it's normal for the Dak unit. Probably the modular fuel units used in the late model XJs are the same way.  :thumbsup: