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miss fire when warm but not when cold?

poomero

poomero

NAXJA Forum User
Location
Tigard
I have a 90 inline 4 and it has a bad miss fire. When the jeep is cold it runs like a champ. when it is warm and i stop and turn it off then back on it has a super bad miss fire. i thought it might be a plug or something so i changed the plugs, wires, cap and rotor. i am at a loss and its killing me. if anyone has any ideas please let me know what you are thinking. oh i also checked all the vacuum lines and i have no vacuum leaks. please help if you have any idea. thanks for the help guys.
 
First, determine if it is one particular cylinder or random.

Engine off, pull all four plug wires off, then barely seat them on the top of each plug. Start the engine and with insulated pliers or a rag remove the wires one at a time looking for the misfire, which will be indicated by no change in the idle, i.e., removing a plug wire from a cylinder that isn't firing will make no change.

If you can identify the problem cylinder you have reduced the possible trouble spots by 3/4. If the the engine dies just reattach that plug wire and restart the engine and try a different plug.
 
If it doesn't miss when cold, it probably means it runs fine until it warms up and the computer senses the engine is warm and quits running a preprogrammed collection of settings. Once it tries to start using inputs from the sensors, it gets bogus info from one of the sensors and it starts running crappy.

You can fake it out and prove this by disconnecting the temp sensor on the block on the drivers side down by the oil pan. There are two sensors, a ping sensor right at the block and the temp sensor a few inches higher.

You didn't say how many miles but my guess is that you have a bad O2 sensor. Less likely are the MAP or the Air Charge Temp Sensor.
 
any luck? i'm having the same problem. my jeep was sending a mapp sensor code so i got a new one, didn't help. also replaced the cps just to make sure it wasn't that. mine's a '94 4.0. haven't had much time to mess with it this week but i'm going to see what i can do to it tonight! (unless it rains)
 
I have had no luck tracking this miss fire down. I have noticed that when the jeep is miss firing when warm if i put it in park for maybe 5 min. it will start to run right. This is really messing me up cuz i have no idea now what it can be. I dont know why puttingit in park would help but it does. if anyone has any ideas let me know.
 
Just for giggles, let everything cool off.

Lift the hood, and then heat up the ignition module with a hair dryer (not stupid hot - but you shouldn't be able to hold your fingers on it for more than five seconds. Should get you about operating temperature.)

Then, start the rig and see how it runs. I've found precisely one RENIX ICM that had gone west, and it was a heat-related failure. Selectively heating electronics is something I've found useful for troubleshooting over the years...
 
The plugs were a ll the same color when i changed them and the cap and rotor were in good shape still. I will try the blow dryer trick this weekend and see if that helps.
 
poomero said:
i changed the cap, rotor, plugs and wires all at the same time. everything is new.
Click to expand...

Ok, get yourself a spray bottle and fill it with tap water. Tonight, after it gets good and dark, start the engine and let it fully warm up until it starts missfiring.

Now, start spraying the water on the cap, plugwires, and even on the coil and ICM (little thingy under the coil). The object here is to check for firing to ground (path of least resistance), so "hosing" it down isn't needed, just a good misting and you can watch for a light show.
 
"When the jeep is cold it runs like a champ. when it is warm and i stop and turn it off then back on it has a super bad miss fire."

didn't say when it starts running good again.....like in a few blocks maybe?
Is it heat soak maybe?
 
BobDog said:
"When the jeep is cold it runs like a champ. when it is warm and i stop and turn it off then back on it has a super bad miss fire."

didn't say when it starts running good again.....like in a few blocks maybe?
Is it heat soak maybe?
Click to expand...

Good thought, maybe one weak injector. The Renix isn't really that susceptible to the heat soak problem, the Renix manifold setup sheds heat better than the later models.
 
it will only start to run good again if it is put in to park for a few min after it is on when warm.

if it cools all the way down and i restart it it will run like normal.
 
poomero said:
it will only start to run good again if it is put in to park for a few min after it is on when warm.

if it cools all the way down and i restart it it will run like normal.
Click to expand...

Still sounds like something's getting heat-soaked that doesn't like it.

Try heating the ICM and checking first. If that doesn't work, try letting the engine cool and heating the injectors one at a time (these shouldn't need to get as hot to fail - so you really don't want to push it.)
 
if something is getting heat-soaked why would it start to run right after it is in park for a few min? when the car is miss firing and then i put it in park while it is miss firing it will start to run right. im at a loss.
 
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