Random Misfire, No Codes

Seems you have both vacuum and fuel system issues.

Vacuum problem classic symptoms would be the system defaulting to defrost mode, AC controls being dead and the rough idle you have experienced. You have a leak somewhere. Inspect and retest the lines, including the T and Y connectors as well, and don't forget the vacuum reservoir and lines under the battery tray. Give them all a good shake while doing it.

Fuel system issue is evident from the hard starting which suggests loss of fuel pressure. Typical cause is bad fuel regulator (which is a mechanical device and not controlled by the computer) , injectors leaking down, failing fuel pump, leak somewhere in the fuel lines or a combination of some or all of these. The low fuel level warning, which you say is inaccurate, is triggered by a signal the computer is receiving from the fuel pump wiring. Have you checked the wiring harness and other fuel connections on top of the fuel tank? And did you recheck the fuel pressure as freedom in 4 low suggested? It should be steady in the high 40's while engine is running and while the engine is off it should remain there as well for hours on end.

Other things to consider is an intake manifold leak, leaking throttle body , or IAC as has been suggested earlier.
 
Mac57 said:
Seems you have both vacuum and fuel system issues.

Vacuum problem classic symptoms would be the system defaulting to defrost mode, AC controls being dead and the rough idle you have experienced. You have a leak somewhere. Inspect and retest the lines, including the T and Y connectors as well, and don't forget the vacuum reservoir and lines under the battery tray. Give them all a good shake while doing it.

Fuel system issue is evident from the hard starting which suggests loss of fuel pressure. Typical cause is bad fuel regulator (which is a mechanical device and not controlled by the computer) , injectors leaking down, failing fuel pump, leak somewhere in the fuel lines or a combination of some or all of these. The low fuel level warning, which you say is inaccurate, is triggered by a signal the computer is receiving from the fuel pump wiring. Have you checked the wiring harness and other fuel connections on top of the fuel tank? And did you recheck the fuel pressure as freedom in 4 low suggested? It should be steady in the high 40's while engine is running and while the engine is off it should remain there as well for hours on end.

Other things to consider is an intake manifold leak, leaking throttle body , or IAC as has been suggested earlier.
Click to expand...

I started thinking along those same lines. I just went out and tested my vacuum. While the Jeep was running it hit 20, but as soon as it was off the vacuum bled off. When I tried using my hand pump I couldn't get the vacuum over 7. I think it may be the line that runs from the intake to the reservoir. The reservoir itself held vacuum, but the line was iffy. I'm going to the auto parts shop in a moment to get some hose and I'll run it right from the intake to the reservoir and see what happens.

Haven't tested my fuel pressure yet, but I may be able to pick up a tester today as well.
 
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Fuel pressure holds at 48 with the engine running, dips a hair to 47 for a moment when I press the gas, sitting at 44 currently with the engine off.

Replaced the section of hose that was questionable, no change.
 
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Update: I found a vacuum leak. Blew smoke into the intake, saw some trickle out through the throttle body at the IAC and, I think, a little from the spring. It's strange, it didn't look like it came through the IAC where it meets the throttle body, it looked like it was coming through the plug at the connections, and part of the wiring plug itself is broken, so I wonder if air can somehow pass through that plug.

I pulled the throttle body, cleaned the heck out of it, noticed the IAC wasn't perfectly seated so I removed reinstalled it. Tomorrow I'm going to glue/epoxy the plug and put it all back together with a new gasket. Fingers crossed.
 
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Reinstalled the throttle body, still leaking. Looks like it's from where the rod on which the throttle pivots on the spring side and from the IAC plug (not where it needs the TB, but from the electrical connection). Is this something that can be repaired or do I need to start pricing new TBs?


20240213_172807-2-jpg.jpg
 
I partially rebuilt that throttle body and the miss is quieter! Looks like I'm either chasing multiple vacuum leaks or the miss was being exacerbated by the vacuum leak. Running better now.

I'm going to do some more testing soon and see if this issue can be stamped out for good.
 
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macleanflood said:
Found this video on another throttle body thread:


-Mac
Click to expand...

That's one of the ones I followed! It took longer than I expected, but I got to sit in my living room and tinker so I'm not complaining. So far, it's working great! Just need to get a bigger return spring for the throttle. Now that this one is greased up I barely have to press the pedal.
 
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So, here's a thing: my throttle body is "hanging" at various points. When I've held my foot on the accelerator for more than a few seconds and let off it'll coast at the same RPM until I tap the pedal, then it'll drop. I'm putting a beefier return spring on, but does this sound like anything else? Just wondering if that TPS I installed was a dud.
 
Mercury said:
So, here's a thing: my throttle body is "hanging" at various points. When I've held my foot on the accelerator for more than a few seconds and let off it'll coast at the same RPM until I tap the pedal, then it'll drop. I'm putting a beefier return spring on, but does this sound like anything else? Just wondering if that TPS I installed was a dud.
Click to expand...

Have you sprayed lube down the throttle cable?

No cruise control?

-Mac
 
macleanflood said:
Have you sprayed lube down the throttle cable?

No cruise control?

-Mac
Click to expand...

Lube down the throttle cable? Like down where it goes into the sheath? Nope, I haven't, but I'll certainly try it.

What do you use?

I have cruise, but I tested it both on and off and no difference. It was getting annoying; whenever I wanted to upshift the RPMs would stay high and throw off my shifting.
 
The stethoscope says all injectors are clicking, and at the same rate. Tried with the probe against pretty much every surface of that engine that I could reach and I can't hear the miss from anywhere, but it's there.

Discovered that one of the ears that the distro cap bolts to on the distributor is broken and the cap has been loose. I reattached with a longer bolt and a nut, still missing.

I tried to get an audio recording but the forum won't let me post them here. Seeing what I can do.

I'm not sure what else to do at this moment other than getting my fuel injectors cleaned.
 
Okay, took it to a guy who rebuilds 4.0s, as well as race cars and vintage cars. He listened, said it doesn't sound like it's from the engine failing in any way, he said it sounds electrical or ignition. He suggested replacing the distributor, which is something I probably need to do anyways because it's stock.

I'm going to order one, though I'm nervous as all heck about doing it. Such a finnicky replacement, and the idea of messing with timing worries me.
 
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Ordered an automotive smoke machine. I hooked my vacuum tester to the intake the other day and the vacuum leak was back, and I'm done chasing it with starter fluid. I found the throttle body leak is still present. Ordered a new one, it'll be in Friday. I'll install that first, and if the miss goes away I'll return the distributor; if not, then I may as well throw that in, too.
 
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Replaced the throttle body today, confirmed with a smoke tester that there were no leaks. When I started it, it ran great, no miss. Then slowly the miss came back, then the idle got rough, then it started trying to die, and when I ran my scanner I have both upstream and downstream O2 high voltage codes and upstream heater circuit fail.

I used the same sensors, just replaced the TB itself.
 
Reinstalled the old TB, everything went back to the way it was. At least I'm no worse off. But it's strange that the miss went away for a short time.
 
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