YJ 4L - hestitation and stumble

[Sprockett]

Ok, done some searches on many forums and also already did some work.

Background
- due to the jeep not starting one afternoon 2 weeks ago, it has a brand new coil, cap, rotor
- saturday, I was out in some hub high water and first felt the stumble in 4LO,  2000rpm jerking up a hill
- also saturday, when driving home later, at a complete stop all of a sudden there was no way to get it into 1st, 2nd, 3rd, 4th gears...discovered I could grind it into 5th or reverse then it would go into first. Got her home. Probably leaking slave, etc...Next day this problem was mysteriously gone however. Probably unrelated but thought I'd mention it.


So, the hesitation has been here for the past few days now. It's not terrible...like not a huge bucking undrivable mess but it's definitely there and more pronounced in 4LO obviously. Yesterday me and a buddy (S-Fly) did the following

- check cap and rotor, still look new
- changed the CPS ($105) it didn't fix the issue. From reading I had high hopes this would fix it
- cleaned out of the TB thoroughly...it actually looked ok...didn't fix issue
- put in known working coil, it didn't fix the issue
- found a broken hard vac line to the MAP sensor...wiggled it...it would whistle and blow and heep dies. Fixed vac line...it did not fix the issue.


Looking for suggestions now?

[Bnine]

No engine light?

Hard start at all?

Idle dip when you let off the throttle quickly?

Water test it for further vacum leaks?

Plugs new and gapped properly?

Run rich at all?

Air filter clean?

Test values on the TPS? (reference haynes manual for that)

Check over a bunch of that stuff and see what you come up with. Could be a lot of things. Map could be sticking, TPS might almost be gone, crack plug wire,  small vacum leak, bad plugs, faulty injector, etc etc.

Pretty much you are stuck diagnosing one thing at a time.

Try some of those and report back with what you find.

Good time to by a voltmeter if you dont have one.

[Sprockett]

No engine light?
- No

Hard start at all?
- Starts great. New starter 2 months ago

Idle dip when you let off the throttle quickly?
- No

Water test it for further vacum leaks?
- blew into a line but couldn't find any further leaks, gotta read how to water test?

Plugs new and gapped properly?
- not new but looked good 2 weeks ago when trouble shooting won't start...gotta pull again

Run rich at all?
- Gotta pull plugs

Air filter clean?
- K&N and clean

Didn't check TPS yet...need to find time.

Thanks Bill

[Bnine]

No worries Ryan.

To check for vacum leaks, take a large juice jug or similar contair and pour water generously all around the intake, throttle body, and vacum ports. Any sign of a stumble will indicate a vacum leak.

[Sprockett]

Poured away, no change in idle. Pulled the plugs, they looked ok...I apparently have no way to check the gap but nothing seemed out of the ordinary. I can't check the tps but wouldn't that kind of thing throw a code? The cel light works and nothing has been thrown. I'll have to wait for a buddy to check with a volt meter.

Am in stuck just blindly replacing all the plugs and wires?

[Sprockett]

Replaced all the plugs (gapped) and wires. Also replaced the fuel filter. Checked the TPS with a volt meter and it checked out as per the Haynes. What next? What else could cause similar symptoms?

One week til McLean opens

[hps4evr]

what year of YJ is it?

[esi]

Long shot, but check your battery terminals and related connections, maybe the coil isn't getting sufficient voltage

[Sprockett]

It's a 93 YJ.

The battery terminals are clean and corrosion free but maybe I should check the voltage to the coil. The behavior is not a drastic jerking, just a subtle miss when on the street...but it's quite pronounced climbing when a hill in 4Lo 2nd gear

Thanks

[hps4evr]

does this only happen in 4wd? cause if so i think yer chasing the wrong goose...

[Sprockett]

No not only in 4Lo, it's just easier to feel it when the motor is making torque and under load. It's thoughout the rev range but you feel it more climbing or over 2000rpm. I drive this daily and I first noticed it on the street. I've been chasing it for over a week now.

[hps4evr]

do you have any way of checking fuel pressure? or checking for a strong spark?