Random Misfire, No Codes

Mercury

New member
Hello, all!

I have a weird situation with my 99 TJ 4.0. I've had a random misfire for a while. Sometimes it's just a quick pop every several seconds, sometimes it's several in a row. There are no fault codes. Both O2 sensors are replaced (NGK), TPS, MAP, IAC; all of the big engine grounds have been replaced, all of the major wiring has been replaced.

Compression test showed all cylinders within 10%. Smoke test showed a tiny leak near the muffler. All vacuum leaks repaired. Fuel pressure held steady at 39 PSI.

The only thing we found was a broken ignition coil. That was replaced with the contact point to the engine cleaned up and wire wheeled for a better ground connection. This helped the misfire but it isn't gone.

It seems like when I first start the Jeep and for the first few minutes it's fine, then the misfire starts. But there have been a couple of drives where I barely heard it at all.

I'm at a bit of a loss. The only thing we can think of would be the fuel injectors. My mechanic is waiting on a tester for fuel injectors, won't be in until next. But is there anything I can look at in the meantime?
 

It's frustrating that there are no codes, that would make things much easier.

How are the spark plugs and wires?
 
It's frustrating that there are no codes, that would make things much easier.

How are the spark plugs and wires?

Right? At least with a code I would have a direction.

Shoot, forgot to mention those, thanks. Changed plugs (I had the Champion iridium's) for the recommended coppers and the wires are new. I also changed that piece (name escapes me) that spins in the distributor cap. There was no oil or gasoline buildup when they were pulled, and my mechanic ran a bore scope in the holes and everything looked good.
 

Since the replaced coil helped, it sounds like it might be related to that. Did you use a Mopar coil? Did you go with the recommended spark plugs correctly gapped and torqued?

Next time you get the miss, step into the throttle at little to see if that will force the miss fire to drop a code.

Worst case, take it to an ignition shop and have it scoped.

Good luck...these can be a challenge to diagnose.
 
Fuel pressure should be 49 +/-5 psi

What spark plug brand and part number.
What are they gapped to?

How do the plugs look? Pics please


Never look down on anyone unless you are helping them up - Jesse Jackson
 

Fuel pressure should be 49 +/-5 psi

What spark plug brand and part number.
What are they gapped to?

How do the plugs look? Pics please


Never look down on anyone unless you are helping them up - Jesse Jackson
Sorry, though I replied to this.

It's the Champion single copper they recommended in the forums. I was using the iridiums, but I switched them out as part of these diagnostics.

They looked good when I pulled them. Not fouled.

I just replaced my CPS with a Mopar, no change.
 
Plugs should be Champion 7034, Autolite APP985,

Single platinum plugs or stock NGK

Copper will wear quick due to the high energy ignition.

You could run double platinum or iridium plugs just don’t expect them to be better. They may last longer. That’s it.


Fuel pressure. Verify fuel pressure at idle. At 2000 rpm and at 3500 rpm. It should all be the same. 49 lbs +/-5

Consider dirty intake, throttle body, IAC or dirty valves.
Cleaning these every 30K mi will keep you running nice. Might as well shine up battery and ground connections.



Never look down on anyone unless you are helping them up - Jesse Jackson
 
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Plugs should be Champion 7034, Autolite APP985,

Single platinum plugs or stock NGK

Copper will wear quick due to the high energy ignition.

You could run double platinum or iridium plugs just font expect them tone better. They may last longer. That’s it.


Fuel pressure. Verify fuel pressure at idle. At 2000 rpm and at 3500 rpm. It should all be the same. 49 lbs +/-5

Consider dirty intake, throttle body, IAC or dirty valves.
Cleaning these every 30K mi will keep you running nice. Might as well shine up battery and ground connections.
These are the Champions recommended for the 4.0, but I'll have to find my receipt to be sure on the exact one. I switched out the Champion iridiums I had and there was no change.

I'm going to rent the fuel pressure tester again. When I tested it last it was fine at idle, but I didn't test at higher RPMs or with the vehicle under load. If this turned out to be something in the fuel system I would freaking tapdance.

I've cleaned the throttle body, replaced the IAC. I've cleaned up ground points and replaced ground and power wires/connections.
 
Fuel pressure 48 at idle, momentary dip to 47 when I press the gas, held at 44 a few minutes after it was shut off.

Tested vacuum again. Running it held at 20, but as soon as the engine was off it dropped to nothing. Using my hand vacuum pump I couldn't get it to hold any vacuum. Decided to run smoke through the intake, bam, throttle bottle leaking where the throttle shaft goes through on the spring side.

A video shared with me on another forum showed how to rebuild that portion using a set of picks and some o rings from harbor freight. Took a little over an hour, but there are fresh o rings, the bearing has been shifted over a bit so it's riding on fresh metal. Liberally greased.

Hooked it all back together, the miss is way quieter now!

I'm either dealing with another vacuum leak or the vacuum leak was making whatever was causing the miss worse. I'll be testing and diagnosing, but at the moment I'm happy to take the win.

Funny thing, now that the TB is all lubed up my gas pedal is super sensitive. I'll have to grab a beefier return spring tomorrow before I go out for my drive.
 

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.
 
Usually vac leaks cause high idle. >750rpm. Bad leak near 2k rpm. Smaller 900-1000rpm

Non OEM IAC,TPS, CpK and CpS even O2 will fail right out of the box. Never throw the Originals away until you prove it was bad.

Never look down on anyone unless you are helping them up - Jesse Jackson
 

I still have my original OEM IAC, TPS, and MAP, they were all replaced recently. The TPS is the only I'm sure failed. I still have the original IAC and MAP, I might just throw those back on tomorrow. The CPS was recently replaced with Mopar. O2s are NGK.

I've noticed a few times that it will hang around 900 - 1100, but it's mostly sitting at 750 - 800.
 
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