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Lacking power

GordoSmasho

GordoSmasho

Nevada Raisins
NAXJA Member
Location
Virginia City, NV
So I just did a motor swap. 92 4.0 with 152k into a 97. The 97 motor was wounded and will be building a stroker out of it for the other jeep. The motor I pulled out felt very under powered. I have had many jeeps of various years and drive trains all 4.0s. I know about what it should feel like as far as power goes.

This 97 is an auto and is completely stock a far as drivetrain and tires. It's a two door and had 235k on it.

So after doing the motor swap it fired right up and sounded great. No codes and the trans is shifting normally and firm. But I am still really down on power. It feels like I'm driving around with a trailer hooked up. Like very labored. I tried swapping ecms with my other 97 to no avail.

Where should I look.
 
I should clarify. The original motor was low on power and had good compression. Now the new motor is also low on power and has good compression. Pulled the exhaust after the down pipe( so no cat) and it is the same. Just revving it in neutral it isn't as quick up the rpms as my other jeep 97. Just need some direction on things to start checking. It has new injectors, cps, map, pickup sensor, plugs, wires, cap, rotor, ox sensors.
 
Did you reuse ALL the accessories from the 97 motor(intake exhast maniflods, sensors, etc)? Or did you use the ones that may have still been on the 92 motor?
 
Intake is a 99' and the exhaust manifold is an aftermarket tube header. Bored throttlebody. All of the other accessories were swapped over. It feels as though at about 80% throttle it loses power. It idles smooth as glass and even when you rev it it's still smooth just slower than normal.

Also the fuel mileage sucks. Like 12mpg on an otherwise stock jeep.
 
I would start testing sensors at this point... that low of fuel mileage makes me think O2 sensor.... Recheck ALL your plugs for your sensors(physically unplug then and plug them back in) and there relays if they have them and check ALL your grounds. With the grounds actually take them off and clean the area and reinstall. You should have atleast 3 majors I can think of.... battery to fender, battery to block, rear of head to firewall.
 
TPS, TV cable, etc
 
So I ran some tests tonight. Fuel pressure is 40psi at idle and dips to 30psi at wide open throttle. Don't know if that's ok or not. TPS tested good. Don't know how to test iat sensor but when I plug in my scanner it seems to read properly in the live data screen. I tried to test the map sensor but I couldn't confirm a ground at the plug. The wire that had the least voltage was black and it always had about 40-60 m volts and fluctuated. Is this ok. The ground for the TPS was a good ground. The voltage did fluctuate properly from the other wire.

Oh and the tv cable is adjusted properly. Rechecked tonight.

I cleaned and checked all the grounds and sprayed some contact cleaner in a few of the plugs.

Thanks for the help guys
 
Last edited:
Did some reading on fuel pressure tonight and from what I'm reading it is quite low. Can someone confirm that or shoot me link on testing 97 fuel pressure. I found a lot about renix pressures. Im going to check my other jeep in the morning. From what I could find it should be 49psi solid and I'm seeing 40 at idle and dropping to 30 under throttle and wot.
 
Sounds like a fuel pump issue, but I would replace the fuel filter first to see if that changes anything. #1 they are cheap, #2 they are easier to replace than dropping the tank to replace the pump.
 
That price was wrong. Apparently. And they actually didn't have it in stock. Found one at a local shop for 170. Swapping it in this weekend. Hopefully this will be the end of engine work for a while.
 
Havent gotten to drive it yet as I'm doing some lift work in pieces this week but I swapped the pump out. Dropped the skid and tank with 12 gallons of fuel still inside. Swapped the pump and had it back in and running in about an hour and a half. It fired right up. Pressure was where it was supposed to be 50 psi about. It revved much quicker and sounds as it should. A lot stronger. I'm going to reset the PCM when I get it on all fours again. I'll post up when it's driving.
 
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