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Trouble shoot 4.3: swap into TJ, warm run problem.

Trashman

Trashman

Trainwreck
Joined
Oct 13, 2021
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4472
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I have a Jeep TJ running a late model 4.3l (not the "LS" gen)

The prior owner ran an aftermarket electric pump that is doing wonky things. The tank venting for one isn't working worth a shit and the pump is apparently loosing PSI over an hour or so worth of driving which makes the Jeep misfire when attempting to accelerate.

I would be more than happy to run the 4.3l off the factory TJ stuff in the tank.

Research however shows an approximately 5 psi difference. The Jeep pump seems to put out approx 49 psi, the 4.3l seems to want 52+.
 
20241027_173812.jpg
 
Isn't the TJ returnless and the 4.3 return style?
 
Skinny said:
Isn't the TJ returnless and the 4.3 return style?
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May be. I didn't put the Jeep together, I am trying to work out the bugs.

When I open the gas cap it usually seems like it sucks in a ton of air.

The Jeep ran a bit smoother loosing the gas cap after it was stumbling after the hour long drive I did.
 
That sounds like an unrelated issue. If you have that much suction then your EVAP system isn't working correctly. There should be a fresh air solenoid that allows outside air in when purging. If it's stuck, broken, clogged then you would have that symptom.

That would also make your pump pretty angry because it's fighting a vacuum in the tank to pull fuel in.

I bet if you drove with the cap loose it wouldn't die based on what you are describing.
 
I agree with you.

I am guessing, the Evap system is gone and capped off. I haven't dropped the tank to verify.
 
I don't know what type of cap that factory TJ is. If it's not vented that's a problem.

I typically take the cap off, split it, then drill a pee hole inside the cap to make it vented.

Or buy a vented cap.

Of course if you live in Commie-fornia this is illegal and makes Greta angry. But I take joy in modifying emissions equipment and chant fuck the EPA while doing so for good measure.
 
Skinny said:
I don't know what type of cap that factory TJ is. If it's not vented that's a problem.

I typically take the cap off, split it, then drill a pee hole inside the cap to make it vented.

Or buy a vented cap.

Of course if you live in Commie-fornia this is illegal and makes Greta angry. But I take joy in morning emissions equipment and chant fuck the EPA while doing so for good measure.
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I considered that as well. It seems to run worse the longer I drive it.

Today I ran from Prescott to the Verde Valley, no issues (1 hr approx) but, I filled up halfway.

Drove the entire hour home without opening the tank. I made it about 7/8 the way home before it started sputtering leaving roundabouts/stop signs.

I made it home and the Jeep idled terribly. Popped the cap, it helped but still died st idle and missed when giving it throttle in neutral.

Waiting about an hour. Runs fine again.
 
Trashman said:
I considered that as well. It seems to run worse the longer I drive it.

Today I ran from Prescott to the Verde Valley, no issues (1 hr approx) but, I filled up halfway.

Drove the entire hour home without opening the tank. I made it about 7/8 the way home before it started sputtering leaving roundabouts/stop signs.

I made it home and the Jeep idled terribly. Popped the cap, it helped but still died st idle and missed when giving it throttle in neutral.

Waiting about an hour. Runs fine again.
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Probably got the pump hot pulling that vacuum, once it cooled and you equalized the tank it ran ok. sounds like your killing the pump working twice as hard.
 
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Likely will figure out how to vent the tank (gas cap or otherwise) and throw an extra pump in the glove box :laughing:.

Still curious If I could run TJ stuff and put it back to "stock" which hopefully would spell more reliability.
 
Stingray said:
try a long ( 1 hr or more ) with the cap loose. see how it goes, sounds as simple as a tank vent but Ca won't like that:laughing:
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Luckily, I am not in CA :laughing:.

This was it's first long trip since I got it back together.

Originally it was 4.3l (205hp) with an AX5....

That didn't fair so well, as expected. I had already bought a NP265/241 to swap in before the AX grenaded.

It is coming along well, just have to work out a few things.

The prior owner built it weird. They swapped in the 4.3, a rear 44 with ARB, 4.56 on both axles, chromo shafts, roll bar, bumpers winch, lift ect. But kept the AX5?
 
TJ has a weird fuel module with a regulator in the top. Not sure how hard it would be but I'm sure it's been done before.

Yeah I agree that pump is probably just about melting down working against the tank vacuum.
 
I think if you do a little reading on the AA or Novak sites, you'll find that the stock TJ pump will feed that 4.3L no problem.
 
Drive in this morning.

Stumbles off stops, and when coming out of roundabouts, per yesterday but wide open acceleration is fine.

Gas cap is one turn on, as the Jeep has a gas door on it to hold it in in case it gets loose.
 
Bebop said:
100%

People use them to feed LS engines as well.
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On the systems with a return line, the engine does the pressure regulating. Not sure what year TJ you're working on. If it's an older one with a return line, throw a stock tank with a stock pump in it and run it.
 
I may give that a try. Mines a 98. I will likely drop the tank tonight to see what is still there.

I am not sure if they are drawing off/through a pump in the tank or what (if that is possible) who knows at this point.
 
I am curious if the prior owner tried the stock pump, it didn't work, so they did the electric. Or the stock pump died, and this is the cheapie replacement or what.

This should be the last wonky part to work through.
 
The Jeep acts the same as my 94 S10 did when it needed plugs/wires now though. Will see if this is fuel/electric I guess. Maybe both.
 
My yj had a 350 tbi with that pump, It's currently really happy with the LS infront of it too.

Have you cleaned the fuel filter or tank? It might be sucking up dirt.
 
I helped a buddy due a V8 swap years ago in a YJ. The rig would run great for an hour or so and then start to stumble and eventually wouldn't run. We chased it down to vapor lock. It was a 90 YJ, so carb'ed OEM. We installed a tank from an injected YJ with a factory pump and no more problems.
 
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