Disclaimer: The engine is from a ZJ but in a YJ so I figured I could still post this here?
Ok so I'm mostly copying and pasting what I posted on another forum. Hoping to find answers, thanks!
I swapped a 5.2l into my YJ a few months ago and its been running very rich to the point where smoke is coming out the exhaust at idle and whenever I stomp the gas while driving small black clouds come out the exhaust.
I've changed o2, map, IAC, IAT sensors. Swapped PCM. Changed spark plugs. Injectors sound fine through a stethoscope (maybe one is constantly leaking???) and many other things. More detailed info on what all I've done and checked can be found in my build thread.
I changed the oil a few days ago because you could smell a lot of gas in it and I didn't want to ruin the bearings.
I've also done a compression test on all the cylinders. The lowest one was 140 and all the others were 150-170. Also did an engine block test and it passed. So I'm pretty sure the head gaskets are good.
it also starts to misfire quite a bit once its warmed up
link to the build thread with pictures and much more detailed information of what I've gone through and swapped on the engine so far.
Fuel injectors should only fire with the engine rotating, it should take input from the cam/crank position sensor (don't remember which) to tell the computer which injector to fire. Your test with the key on while disconnecting the battery might cause it do do odd things.
The rule of thumb on a compression test is 10% difference from highest to lowest pressure is the limit. That would be 17# since that was your highest pressure. Your motor is at over 17% difference between highest and lowest compression.
Did you: 1)Remove all of the spark plugs before starting the test?
2)Hold the throttle at wide open during the test?
3)Do both a dry and wet test so you know if the low compression is valves or rings?
If not you might want to take another shot at the compression test so you know what is going on.
Is your 5.2 out of a ZJ? And would guess it's a 93-95 OBDI since you have a PCM with only one main port/connector.
Did you connect the 2 long ground straps on the back of the engine to your frame and the PCM ground to the engine block? Only ask this since the PCM grounds all the sensors and will create real havoc w/o good/clean grounds. And the crank sensor is one of the first sensors to fail w/o a good ground.
You said you were getting an o2 sensor code but it went away. Hopefully you're using a NTK/NGK OE o2 sensor and not a brand like Bosch. They aren't very expensive if you need to change it out since it was malfunctioning before.
Someone in your other thread said you may install dual exhaust and evidently you just went with a single exhaust. Hopefully you don't have any exhaust leaks before the upstream o2 sensor. If you do they will also suck air in, the o2 sensor will sense too lean of a fuel condition with more oxygen, and the PCM will dump excess fuel in causing a rich condition. Good luck.
I have one of the grounds on the frame, and the other from the engine block to the firewall by using one of the stock ground wires. I'm actually using a Bosch o2 from O'Reillys. I still have the receipt and may be able to replace it with a ntk/ngk. I'm fairly certain that there are no leaks upstream in the exhaust, however I am going to double check because I remember the first time I installed the down pipe it didn't seat correctly against the exhaust manifold and leaked a little bit.
When your piston descends on the intake stroke and the throttle is closed the volume of air is restricted so there is less air to compress. By having the throttle wide open you give the intake stroke a larger volume of air so the compression stroke will create higher and more consistent compression.
To be clear on the wet and dry compression tests, if you test a cylinder dry, (adding no oil to the cylinder) you will get a reading on the overall ability to create compression. Record all the numbers for the dry test and then add a couple of squirts of oil in each cylinder and spinning the motor for a couple of seconds. At this point do another compression test and if the compression goes up significantly your rings are bad. IF not the evidence points to a valve problem since added oil will seal rings but won't seal bad valves.
GW
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