ZombiePopper

Hey guys,
I have another weird one for you.

For the past couple of weeks this ZJ has been running flawlessly, and being driven on a daily basis (1hr to work and an hour back home, plus any extra driving mileage/time.). REALLY, running like a sewing machine!

About 3 days ago,
I filled up (full tank from a 1/4 tank) at Casey's gas station and this ZJ has developed a "bog" or stumble on the first drive of the day. My initial thought was bad gas.

But the reason I'm doubting my thought process is because;
-Once the ZJ warms up a little and up to operating temp, it doesn't do it.
-plus, if I drive it to work and park it, all day, when I come out after work and drive it, it doesn't do it, even after being parked all day long.
-it only does it on the very first drive of the day, I can drive it, then park it all day long (many/several hours) and it won't do it again
-it doesn't matter if it's 40 or 80 degrees outside, it still does it on the first drive of the day.
-no codes/errors/lights and no odd noises, just a bog or stumble a few times then it's done as it warms up.
-it will stumble at low speed a couple times while accelerating and/or at constant highway speed.
-it starts/idles perfectly fine, it only stumbles/bogs a few times when actually driving on the first drive of the day.
-it's not getting any worse and it doesn't seem to really be getting any better.

I've ran the Casey's gas out and have since filled up again (from 1/4 to full tank with Shell 89) and I've ran that tank out as well and I've filled-up yet again today (from 1/4 to full tank, again).
-I've recently changed the fuel filter
-checked fuel pressure acouple weeks ago (all good)
-replaced/tested pretty much everything in the process of getting this ZJ running/driving well,
it's easier to tell you what I HAVENT replaced;
cam sensor, coil, injectors, that's about it lol.

The only thing that changed between running very well and this stumble/bog is the Casey's fuel.

Anyone have any thoughts/ideas?
(Should I just keep driving it and running fuel thru it, in hopes it stops?)
Thanks

dave1123

You could try some injector cleaner, I'm no saying it will work, but it's a cheap fix, Maybe.

ZombiePopper

Thanks Dave,
Great minds think alike.
I added a bottle of Royal Purple fuel system treatment around the same time as I put Casey's gas in it. And a couple tanks before that, I added some Lucas fuel inj cleaner. (Prior to the Casey's fuel.)
if it was fuel related it should bog/stumble more than just the first drive of the day, I would think....plus 2 full tanks of petrol should clean it out (?)....
What would be faulty, that, when heated or used, would be corrected?

dave1123

Maybe the idle air control is sticking. I'm just grabbing at straws, mind you.

ZombiePopper

Hey, I appreciate your input Dave!
I've recently cleaned out the TB and just replaced the IAC. It starts up/idles perfect, it will only stumble/bog when it's driven for the first drive of the day, I should've specified that, sorry.

dave1123

That was my whole point. The new piston may be sticking in the bore until it warms up. Try a little injector cleaner in the bore.

ZombiePopper

Ah gotcha, Thanks Dave.
I can eliminate fuel and the iac now. (4th tank of fuel BP 92 this time, and I messed with the iac and swapped it out with a good spare but no difference.)
The search continues.
Since I haven't replaced my cam sensor and coil I guess I can replace those one at a time and see if there's a change....
Maybe my TV cable is adjusted to tight?

dave1123

This is kinda off the wall, but the IAC's throttle body bore may be worn oversize since it's got lots of miles on it. Can you try swapping the whole TB with a "younger" one?

ZombiePopper

Heck I'll give it a try! It'll be free for me to try it.
I happen to have a good 99 5.9 that had 150k miles on it, this 94 ZJ 5.2 has 175k miles.
I'll swap everything over to my 5.9 TB and install on this 5.2 ZJ to see if there's a change...

ZombiePopper

Update;
The stumbling/bogging is getting worse and lasting longer. Still only does it on the first drive of the day when it's below operating temp.
I swapped TB units and there was no change.
And still no CEL or warning lights.
I would think if it was the cam sensor I'd get a CEL (?)

dave1123

IDK about the cam sensor, but a bad CPS won't set a code or CEL.

ZombiePopper

Agree 100% with ya on the no CEL on a bad crank sensor, I went there a few weeks ago lol. I thought I read somewhere that a bad cam sensor will throw a CEL, but I'm not certain...
I unhooked the TV cable to see if there's any change and swapped in a different/spare TPS and reset the PCM again.
I'll update if there's any kind of change...

ZombiePopper

Welp swapping the TPS, unhooking the tv cable and resetting the ECU did nothing.
I'm going to swap in a new coil and see if there's any change...

(I recall reading, years ago when they mounted coils on the intakes of some engines, the heat could cause the oil in the coil to swell, and killing spark. Perhaps something similar, but reversed, is going on with the coil in this ZJ???)

dave1123

It's possible, but why would it run better after it gets hot? I remember my '97 started this crap just before I got rid of it. It bogged and stuttered on the first acceleration after backing out of my driveway in the morning, but was fine any time after that. I put it down to a sticking IAC but didn't do anything about it, and it didn't do it every morning either. IDK what to tell you, we've covered about everything I can think of.

ZombiePopper

I hear ya Dave and I appreciate your input bud! I'm in no way discounting it, just trying to figure out what's going on with this darn ZJ.
It's annoying after all the time/money I put in to it, to be left scratching my head and grasping at straws. I got a good 2-3weeks of daily driving out of it before it started this, out of the blue.
The only things I haven't replaced are;
-Water pump (not the issue)
-T-stat (not the issue)
-windshield washer level sensor VIC (not the issue)
-Oil sending unit (not the issue)
-fuel pump (not the issue)
-alt (not the issue)
-ps pump (not the issue)
-AC comp (not the issue)
-Cam sensor (possible)
-Coil (possible)
I figure with the above parts, I'll start with the coil then try the cam sensor since they're the most likely culprits(?)
But as I said, I'm just grasping at straws....