Random Misfire, No Codes

Mercury

TJ Enthusiast
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Jul 18, 2018
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237
Location
Louisville, KY, USA
Hello, all!

I have a random misfire with no codes, 1999 TJ 4.0 manual. You can hear it, the vehicle shakes a little when it happens, but I don't have, and have never had, a misfire code.

I've changed the crank sensor, TPS, chased down a vacuum leak and repaired it. Throttle body gasket changed. We tried pulling each injector harness with the engine running but there was no difference in performance degradation and the misfire continued. I tested fuel pressure a few weeks ago, and while I can't remember the values off the top of my head I had looked up what it was supposed to be and it was dead-on. Plugs are iridiums, installed about two years ago. Cap and wires as well.

I was driving back from out of town a week ago, it was bucking at high speeds, running a high idle (1200 RPM), though at idle it would sometimes try to stall. I was stuck on a long road trip and had to drive it about three hours that way. Replacing the CPS seems to have fixed the bucking issues, the TPS fixed the high idle.

I've searched a bunch of threads, but most involve a code, or have worse symptoms than mine. You can hear/feel it, but the engine runs and gets me where I'm going.

I'm at a bit of a loss here. Would anyone recommendation a next step? Is it safe at all to drive it like this?
 
Got an odb reader that can read live data?

I think it's safe to drive it. If the computer doesn't think anything is wrong...

How's your timing chain?

What's your gas mileage?

Check for cracked exhaust headers and a plugged cat?

-Mac
 
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Got an odb reader that can read live data?

I think it's safe to drive it. If the computer doesn't think anything is wrong...

How's your timing chain?

What's your gas mileage?

Check for cracked exhaust headers and a plugged cat?

-Mac

I do have a reader with live data. What data am I looking for?

No idea on the timing chain, that's a bit above my current experience level. However, I can learn if I know what to look for and can find a good write-up or YouTube video.

Gas mileage is consistently 15.5 mpg. I've tracked my fuel economy for years and, unless there's a vacuum leak or other problem, this is where I stay.

No cat, and did a smoke test on the exhaust, didn't see any leaks. Doesn't mean there isn't, but I haven't been able to find one.
 
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I was able to check my timing chain when I had my oil pan off replacing my RMS. I did my oil pump for kicks at the same time. I was able to move the chain a ton with my finger and the consensus was it needed changed.

What's your mileage?

When I went after the timing chain I did my water pump and fan clutch at the same time.

As to live data I'd probably start at the top of the list and work your way down and see if you see anything anomalous.

My 97 and I assume your 99 don't have too many data points.

-Mac
 
I was able to check my timing chain when I had my oil pan off replacing my RMS. I did my oil pump for kicks at the same time. I was able to move the chain a ton with my finger and the consensus was it needed changed.

What's your mileage?

When I went after the timing chain I did my water pump and fan clutch at the same time.

As to live data I'd probably start at the top of the list and work your way down and see if you see anything anomalous.

My 97 and I assume your 99 don't have too many data points.

-Mac

Ahhhh, I see. Haven't had the oil pan out, but there's a first time for everything. I'll have to see what I'm looking for when I do it, but I'll make it happen.

160k miles, roughly.

I do need to do my water pump.

There's quite a bit of data, but unfortunately I'm not familiar enough to know what's normal. I keep up with my O2 sensors just because they've been a headache for almost a year and they finally started working. So trim and such.
 
You probably don't want to go through the pan unless you have a reason.

If you're already needing a water pump I'd just pull the timing cover when you're in there and take a peak.

What makes you think you're due for a water pump? Any chance things overheated? Oil pressure good?

-Mac
 
You probably don't want to go through the pan unless you have a reason.

If you're already needing a water pump I'd just pull the timing cover when you're in there and take a peak.

What makes you think you're due for a water pump? Any chance things overheated? Oil pressure good?

-Mac

I didn't realize you could see it through the water pump, that's useful. It overheated a little over the summer at idle in 90 degree weather. 2 years ago my mechanic replaced the pump saying there was a lot of corrosion in the cooling system (previous owner issue), so they did a flush and removed a lot of it. However, they just idle the vehicle for a few hours and flush, not multi-day coolant flushes with different flush kits. I'm sitting on a Prestone one and Thermocure, going to run Prestone first to remove scaling and Thermocure afterwards for the corrosion.

Oil pressure is solid. Replaced my oil pressure sensor a few months ago and no issues.
 
Can't see it through the pump...the timing cover is right next to the water pump...few more bolts and you'd have the cover off.

If you pull the timing cover you'll need a new gasket and oil pan seal repair kit...part of the timing chain cover is against the oil pan...at the least.

I used a Melling timing chain kit.

I also replaced my crank pulley because the rubber was 20 plus years old and cracked, checked.

Having an inch pounds 1/4" torque wrench is really handy.

-Mac
 
Can't see it through the pump...the timing cover is right next to the water pump...few more bolts and you'd have the cover off.

If you pull the timing cover you'll need a new gasket and oil pan seal repair kit...part of the timing chain cover is against the oil pan...at the least.

I used a Melling timing chain kit.

I also replaced my crank pulley because the rubber was 20 plus years old and cracked, checked.

Having an inch pounds 1/4" torque wrench is really handy.

-Mac

Ahhh, okay, so not the easiest access but not terrible.

So it might be the timing chain. How would I test that?
 
Wiggle it and see how much play is in the chain. Problem is by the time you dig your way all the way there you might as well change it.

What brand cap and rotor did you use? Original coil? I put an Accel cap, rotor and coil on my 97 and 8mm (I think) Summit wires. I was chasing a random misfire...codes and all on mine...ended up being the crank sensor.

Have you tried wiggling spark plug cables and injector wires to see if the miss stops or gets worse? Sounds like you did something similar.

Got a laser IR temperature gun? Could measure the heat output on the exhaust header from each cylinder and see if there is a significant delta.

Try moving ground wires away from wires running to the ECU. I put a big 7 cable kit on my Jeep and it rerouted one of the grounds and I swear getting it closer to the ECU harness made it rub rough...might even call that slight miss.

Thinking it's also time for a compression test and based on those results maybe even a leak down test. That's easier to do than the timing chain.

Got a video or audio recording of it running?

-Mac
 
I... I didn't know there was an ignition coil to replace. That's actually fairly cheap, under $20 on Amazon, I'll just buy one to try it.

Not sure of the cap, rotor, or wire brand. They were replaced a couple of years ago with my spark plugs, and the cap I had ordered and sitting around for a few years before I installed it.

We wiggled the spark plugs and actually pulled the injector wire on each one, no change in the misfiring.

No thermometer, but I need one for other things, I may go ahead and order one if I can find it cheap.

I actually replaced all of the power and added grounds a few months ago. Upgraded to bigger, OFC wire. I didn't remove the factory grounds, but I did cut out the corroded ends, put new lugs on them, then added heavier grounds. So wires running near the PCM caused issues? I'll have to check my routing, I may have a power cable running close.

I have a compression tester coming tomorrow morning. Not sure how to use it, but I'll YouTube in the morning. Is a leak down test something where I can rent the tool or would that be something that requires a mechanic?
 
Wiggle it and see how much play is in the chain. Problem is by the time you dig your way all the way there you might as well change it.

What brand cap and rotor did you use? Original coil? I put an Accel cap, rotor and coil on my 97 and 8mm (I think) Summit wires. I was chasing a random misfire...codes and all on mine...ended up being the crank sensor.

Have you tried wiggling spark plug cables and injector wires to see if the miss stops or gets worse? Sounds like you did something similar.

Got a laser IR temperature gun? Could measure the heat output on the exhaust header from each cylinder and see if there is a significant delta.

Try moving ground wires away from wires running to the ECU. I put a big 7 cable kit on my Jeep and it rerouted one of the grounds and I swear getting it closer to the ECU harness made it rub rough...might even call that slight miss.

Thinking it's also time for a compression test and based on those results maybe even a leak down test. That's easier to do than the timing chain.

Got a video or audio recording of it running?

-Mac

No video or audio, but I can get that in the morning. It's not a hard miss, it's soft, it may not even come across in a recording. And it's so random with it, it may run for half a minute without a miss, then several back to back.
 
No video or audio, but I can get that in the morning. It's not a hard miss, it's soft, it may not even come across in a recording. And it's so random with it, it may run for half a minute without a miss, then several back to back.

Yeah the only thing I can think of that would be that inconsistent is either the coil not providing a spark every long once in a while or a really really inconsistent injector.

Maybe grab a screwdriver and stick it to your ear and see if you can identify something.

Personally if it's that inconsistent, there's no codes, your gas mileage is good, it's not overheating I'd drive it all day and night and take it on a 1600 mile road trip to Canada and back on the WABDR like I did mine last summer.

The 4.0 is the AK47 of engines. You can abuse it, drag it through the mud, overheat it and it'll still run forever.

My neighbor has an 04 WJ and her kids overheated it... probably more than once. Fan spray of coolant out the back of the 4.0 as soon as it got up to temperature. Slapped a new head gasket on... didn't bother with any machine shop work... runs like a top.

I bought a set of remanufactured Precision Auto Injector for mine...spent a little more for balanced...was $160. Could try that.

I think you're chasing waterfalls and windmills...but I also understand that a slight miss every once in a while would drive me bat shit crazy.

-Mac
 
Yeah the only thing I can think of that would be that inconsistent is either the coil not providing a spark every long once in a while or a really really inconsistent injector.

Maybe grab a screwdriver and stick it to your ear and see if you can identify something.

Personally if it's that inconsistent, there's no codes, your gas mileage is good, it's not overheating I'd drive it all day and night and take it on a 1600 mile road trip to Canada and back on the WABDR like I did mine last summer.

The 4.0 is the AK47 of engines. You can abuse it, drag it through the mud, overheat it and it'll still run forever.

My neighbor has an 04 WJ and her kids overheated it... probably more than once. Fan spray of coolant out the back of the 4.0 as soon as it got up to temperature. Slapped a new head gasket on... didn't bother with any machine shop work... runs like a top.

I bought a set of remanufactured Precision Auto Injector for mine...spent a little more for balanced...was $160. Could try that.

I think you're chasing waterfalls and windmills...but I also understand that a slight miss every once in a while would drive me bat shit crazy.

-Mac

Honest to God, if it wouldn't hurt the engine (and I just don't know enough to know if it would) I would just drive it. I'll still try to fix it, I don't like my stuff being broken, but I have a six hour road trip tomorrow that I can't take if the Jeep isn't running.

From yours and other recommendations I'll replace the coil, grab some seafoam fuel cleaner, disconnect the clockspring (apparently if it goes bad it can send voltage pulses to the TPS through a shared ground), run a compression test.

I'll grab an audio recording here in a few minutes.
 
I've checked timing chain slop on old school big blocks before by putting it at TDC then having someone watch the rotor while rotating the engine backwards by hand slowly till the rotor begins to move then measure the degrees with a degree wheel. The Ford 460's were famous for eating a timing set in short order and that's how we checked them. Not sure f this is OK to do on the 4.0, I know on some engines like Mercedes diesels it is not a good idea to rotate them backwards as they have timing chain tensioners.
At any rate the engine should be rotated back in the proper direction prior to starting.
 
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I've checked timing chain slop on old school big blocks before by putting it at TDC then having someone watch the rotor while rotating the engine backwards by hand slowly till the rotor begins to move then measure the degrees with a degree wheel. The Ford 460's were famous for eating a timing set in short order and that's how we checked them. Not sure f this is OK to do on the 4.0, I know on some engines like Mercedes diesels it is not a good idea to rotate them backwards as they have timing chain tensioners.
At any rate the engine should be rotated back in the proper direction prior to starting.

Hmmm, I'll have to look into that. It's above my current level (though I'll learn), so it's worth looking into.



I don't hear anything concerning...

It's really mild. It happens a lot, but it's pretty soft.
 
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