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Misfire at WOT

Wiser

Wiser

NAXJA Forum User
Location
92345
1991 AW4 NP231

I'm having a bad misfire between 2700-4000 rpm while idling and under load. I've been trying to diagnose it for 2 weeks now and have ran out of some ideas. No Check Engine Light. I've searched and used google search for NAXJA.

Symptoms
-No power during acceleration
-Bucking/misfire during WOT

Parts Replaced
-Bosch O2 sensor
-Aftermarket CPS
-Aftermarket TPS
-ValuCraft Coolant Temp Sensor
-New Cat when smogged 5 months ago
-Autolite Double Platinum Plugs .035 Gap
-Duralast Wire Set
-Fuel filter
-cap and rotor changed 5 months ago

Diagnostics
- MAP sensor Prong B is 1.6v with Key in on position
-Prong B with motor on is .5-.6v

-removed flanged cat to rule out plugged exhaust. Still misfire at idle.
- was told the champion plugs I had before the new Autolites were fouled (white in color)
-sprayed the intake manifold and had a small leak on the TB base gasket but has since been replaced. still misfire.
-Put fuel rail on a gauge and its 40psi idle, 41-43 at WOT with no load. It does fluctuate between 41-43, is this normal?
-found a little oil in the distributor but cleaned it out
Thanks in advance.
 
I don't know about the rest but these 4.0's hate non NTK O2 sensors and non champion plugs. The plugs were my misfire issue at idle but I never even drove it after firing it up the first time on NGK plugs. Went to the store and got champions and issue went away. Your gas mileage will likely suck with that Bosch O2 sensor. Replace that one under warranty if you can and get an NTK.
 
It sounds like a coil problem to me, they tend to breakdown at higher RPMs first, bogging down when you step on the gas. Other usual suspect is fuel pressure but your readings are good for 91.
 
Last edited:
ehall said:
It sounds like a coil problem to me, they tend to breakdown at higher RPMs first, bogging down when you step on the gas. Other usual suspect is fuel pressure but your readings are good for 91.
Click to expand...

Wouldn't the coil be hot to the point one couldn't touch it with burning your hand?
 
dan1977p said:
I don't know about the rest but these 4.0's hate non NTK O2 sensors and non champion plugs. The plugs were my misfire issue at idle but I never even drove it after firing it up the first time on NGK plugs. Went to the store and got champions and issue went away. Your gas mileage will likely suck with that Bosch O2 sensor. Replace that one under warranty if you can and get an NTK.
Click to expand...

Changing out the O2 sensor is worth a shot
 
dan1977p said:
But the plugs are your main issue. ;) Spend $8 there and you'll see the misfire go away. Getting the right O2 sensor will also help in the long run. Do them one at a time and you'll see the difference.
Click to expand...

I Had the cheapest champions in before the Autolites, they were white/ gray
 
They may have been over-gapped and running too hot. RC9YC should be the right part number. If the O2 sensor isn't working right, it could be telling the computer it's too right and leaning it out causing this as well. So far in the two 91HO's I've had my hands on, replacing the platnums with regular old coppers fixed the rough idle. I would expect your white plug issue is from somewhere else and this is just a symptom.

Have you done a compression test? Vacuum test? Ignition test?

Ehall mentioned the coil too and if it's sending too much juice it could burn your plugs too.
 
This is a bit confusing "
I'm having a bad misfire between 2700-4000 rpm while idling and under load"

Is it misfiring at idle at 750 rpm, or in park at 2700 + rpm or both?

Leaking vacuum line at the throttle body that runs to the MAP sensor???

If misfiring at 750 rpm at idle, check the plug wires and the injector wires for a criss-crossed pair!!!

Try non platinum plugs.
 
Ecomike said:
This is a bit confusing "
I'm having a bad misfire between 2700-4000 rpm while idling and under load"

Is it misfiring at idle at 750 rpm, or in park at 2700 + rpm or both?

Leaking vacuum line at the throttle body that runs to the MAP sensor???

If misfiring at 750 rpm at idle, check the plug wires and the injector wires for a criss-crossed pair!!!

Try non platinum plugs.
Click to expand...

Runs pretty smoothe at idle but if I were to rev it up (in park) to 3000+ I get the misfire cut out.

I get the misfire/cutout during a hard acceleration while driving.

So no matter if I'm parked or driving, if I rev the motor to 3000+ I get the misfire/cutout

I'm going to put the champion plugs I just pulled out back on.
 
Last edited:
I recently had sputtering/breaking up issue past 3/4 throttle. Yesterday I removed all grounds and positives, removed coil and coil bracket and wire brushed all contacts and tightened them back up. All is good. I had an Accel coil go bad on me in less than a year and it wouldn't rev past 3Krpm. . .
 
gradon said:
I recently had sputtering/breaking up issue past 3/4 throttle. Yesterday I removed all grounds and positives, removed coil and coil bracket and wire brushed all contacts and tightened them back up. All is good. I had an Accel coil go bad on me in less than a year and it wouldn't rev past 3Krpm. . .
Click to expand...

I'm going to try that. Thanks
 
does anyone know if the 1998 and 1991 flex plate are the same? When I first got the jeep and swapped out the original broken flex plate with one from my old 98 Xj. Maybe a shot in the dark but if they're different, that could be messing with my CPS/ timing
 
Last edited:
If you had the wrong flex plate, it wouldn't run. So that rules that out.

There are three pins on the MAP sensor you showed the voltage for pin B. What was the voltage on the other two referenced from the negative battery terminal?

I would need to look up the specs for that year's MAP, but that would not be a good number for a Renix.
 
Wiser said:
1991 AW4 NP231
Diagnostics
- MAP sensor Prong B is 1.6v with Key in on position
-Prong B with motor on is .5-.6v
Click to expand...

From FSM:
(2) Test the sensor output voltage at the sensor connector between terminals A and B as marked on the sensor body (Fig. 21). This is done with the ignition switch ON and the engine OFF. Output voltage should be 4-to-5 volts. The voltage should drop to 1.5-to-2.1 volts with a hot, neutral idle speed condition

Looks like your MAP sensor may need replacement or at least some checking of inputs and grounds. The vacuum line to the sensor may be bad too but that wouldn't cause the low voltage with the engine off.
 
If you put in a used flex plate it can cause weird issues like this, and yes it may still start if the flex plate is wrong/damaged. Have you checked fuel pressure under load? You may also have a crank sensor with crap output at high rpm, causing an erratic signal to the pcm.

And fwiw, my 92 has a Bosch o2 for +100k that works fine.
 
old_man said:
If you had the wrong flex plate, it wouldn't run. So that rules that out.

There are three pins on the MAP sensor you showed the voltage for pin B. What was the voltage on the other two referenced from the negative battery terminal?

I would need to look up the specs for that year's MAP, but that would not be a good number for a Renix.
Click to expand...

dan1977p said:
From FSM:
(2) Test the sensor output voltage at the sensor connector between terminals A and B as marked on the sensor body (Fig. 21). This is done with the ignition switch ON and the engine OFF. Output voltage should be 4-to-5 volts. The voltage should drop to 1.5-to-2.1 volts with a hot, neutral idle speed condition

Looks like your MAP sensor may need replacement or at least some checking of inputs and grounds. The vacuum line to the sensor may be bad too but that wouldn't cause the low voltage with the engine off.
Click to expand...

I'll check the voltage when I get home from work. If the map sensor was faulty wouldn't it general throw a Check Engine Light?
 
A lot of operating problems fail to throw a code or light the check engine light. That is when the old school mechanics debugging gets fun :)
 
Wiser said:
I'll check the voltage when I get home from work. If the map sensor was faulty wouldn't it general throw a Check Engine Light?
Click to expand...

Probably not.
 
dan1977p said:
From FSM:
(2) Test the sensor output voltage at the sensor connector between terminals A and B as marked on the sensor body (Fig. 21). This is done with the ignition switch ON and the engine OFF. Output voltage should be 4-to-5 volts. The voltage should drop to 1.5-to-2.1 volts with a hot, neutral idle speed condition

Looks like your MAP sensor may need replacement or at least some checking of inputs and grounds. The vacuum line to the sensor may be bad too but that wouldn't cause the low voltage with the engine off.
Click to expand...

Prong A (not running) 5.05 volts
prong B (not running) 4.34 volts
Prong C (not running) 0.02 volts

A (running) 5.06 volts
b (running) 1.63 volts
C (running) 0.04 volts
 
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