97 Jeep ZJ 4.0 running rough at start

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AmishFury

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only does it after the vehicle has sat for a bit and evens out after a few moments

when i start up it runs rough kinda feels like a misfire in the past it would occasionally do it but a little tap of the gas pedal fixed it... now the tap of the gas pedal just makes it run rough at a higher rpm until it evens out

if it sits for a few minutes after running it starts up fine... but an hour or more it will run rough when i start it again
 
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doesn't seem like common tps symptoms... i've experienced a bad tps on this (day after i bought it the tps went full derp)

an issue with one (or more) of the injectors has been suggested as a possibility by one of my coworkers
 
Another thing is on these... they are notorious in the last few years' of production for cracking cylinder heads.. .may have a hairline crack letting in coolant into a chamber... goes away after a bit of cylinder heating... Not saying yours "for sure" has this issue - just throwing it out there for something to ck/think about....
See if you can localize to a particular cylinder ( tends to happen in center 3 of head ) and remove plugs and crank over with a shop towel over the holes to see if any "debris" (coolant, fuel, whatever) comes out of the holes - after it has "set" the required amout of time to duplicate the concern....

BurrWinder
 
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A ZJ wouldn't have a cracked head unless it's repeatedly overheated. The cracked head issue started on the 99s.

I'd put it up to [censored] injectors. Mine will idle a bit rough on cold start unless I drive it really hard or put a lot of miles on it in a day
 
I did have a bad TPS that caused a rough idle, but it also threw a code. Have you cleaned the throttle body and IAC? Checked for vacuum leaks? It may well be injectors, but check the easy stuff first.
 
My '93 was bad about building up carbon around the injectors. Two things helped: a high dose of fuel injector cleaner and a high-rpm/high load run to blow out the carbon. The valves would rattle a lot as I got over 4000 rpm and then there would be a poof out the tailpipe and the valves would quiet down. I would do that about once a month.
 
Originally Posted By: svhanc
My '93 was bad about building up carbon around the injectors. Two things helped: a high dose of fuel injector cleaner and a high-rpm/high load run to blow out the carbon. The valves would rattle a lot as I got over 4000 rpm and then there would be a poof out the tailpipe and the valves would quiet down. I would do that about once a month.


I have a 2000! I think if I were to hit 4000RPM, it would deposit all of the piston skirts in the oil pan for me!
 
Leaky fuel pump check valve, leaky injector. Both will be evident if you have a fuel pressure gauge.

Possible intake valve deposits. Every once in awhile you run across engines with lots of carbon buildup on the back of the intake valves. When starting, the deposits soak up the fuel like a sponge causing a lean misfire for a few seconds until the deposits get saturated. Borescope is useful for inspecting that.
 
97s aren't prone to head cracks. Any 96+ can have the piston skirt issue, but it's pretty rare.

The actual issue is likely injectors, although a flaky cam or crank sensor can cause misfires as well. Bad plug, plug wire or cap/rotor could cause issues too (things change as they start to warm up, so a little heat could make an issue go away).

As far as carbon, 4.0s do like to be run hard. They carbon up pretty badly if you baby them. When my mother still had hers, she always commented on how it idled better after I drove it (she did a lot of short trips and NEVER took it above 3k rpm, I made a point to wind it out at WOT a few times every time I drove it).
 
Originally Posted By: rslifkin
97s aren't prone to head cracks. Any 96+ can have the piston skirt issue, but it's pretty rare.

The actual issue is likely injectors, although a flaky cam or crank sensor can cause misfires as well. Bad plug, plug wire or cap/rotor could cause issues too (things change as they start to warm up, so a little heat could make an issue go away).

As far as carbon, 4.0s do like to be run hard. They carbon up pretty badly if you baby them. When my mother still had hers, she always commented on how it idled better after I drove it (she did a lot of short trips and NEVER took it above 3k rpm, I made a point to wind it out at WOT a few times every time I drove it).


Mine always runs better after an off roading event or jeep club run.
 
the cracking head thing is 99-02 and they crack under the valvecover so pulling plugs wont tell you anything as far as that issue goes.make sure that the insulation thats around the injectors is there.if its missing the 2 center injectors can heat soak which basically boils the gas out of them
 
I would start by cleaning your IAC, the idle air control valve on JEEPs is something that should be cleaned once a year or so. Also a good tune up using CHAMPION COPPER plugs could be in order, and the addition of MMO to your fuel diet.

I've owned MANY JEEP's over the years and any time I have had a rough idle issue it was related to the IAC and or plugs.
 
Originally Posted By: wsar10
I would start by cleaning your IAC, the idle air control valve on JEEPs is something that should be cleaned once a year or so. Also a good tune up using CHAMPION COPPER plugs could be in order, and the addition of MMO to your fuel diet.

I've owned MANY JEEP's over the years and any time I have had a rough idle issue it was related to the IAC and or plugs.


Agreed; plugs, cap and rotor can clear up many rough idle problems...since things get much worse when it cools off. After the tune up, please post back and let us know if the problem persists or not.
 
Originally Posted By: wsar10
I would start by cleaning your IAC, the idle air control valve on JEEPs is something that should be cleaned once a year or so. Also a good tune up using CHAMPION COPPER plugs could be in order, and the addition of MMO to your fuel diet.

I've owned MANY JEEP's over the years and any time I have had a rough idle issue it was related to the IAC and or plugs.


IAC and throttle body were thoroughly cleaned back in march while trying to diagnose an issue where it would stall at stops (turned out to be related to a failing water pump)

plugs/wires/cap/rotor all replaced a year before that
 
Originally Posted By: AmishFury
Originally Posted By: wsar10
I would start by cleaning your IAC, the idle air control valve on JEEPs is something that should be cleaned once a year or so. Also a good tune up using CHAMPION COPPER plugs could be in order, and the addition of MMO to your fuel diet.

I've owned MANY JEEP's over the years and any time I have had a rough idle issue it was related to the IAC and or plugs.


IAC and throttle body were thoroughly cleaned back in march while trying to diagnose an issue where it would stall at stops (turned out to be related to a failing water pump)

plugs/wires/cap/rotor all replaced a year before that

are you plugs OE champions ?
If not start there.......
 
Have you recently dis-connected your batt. ?

DO you have any stored codes (by key trick NOT scanner), IIRC the key trick works on the 2nd gen ZJ's. Either displays on odometer or the CEL blinks a specific number of times that dictates a DTC.
 
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