98 Frontier misfire in closed loop

[Dirtybob]

Hello all, first post here.
I have a 98 Frontier, ka24de, 5 sp, 2wd that's got me stumped. It's recently started missing when in closed loop (runs perfect when cold). Only code that comes up is cyl 2 miss (po302).
Pretty sure this would be a sensor issue but I've replaced the CTS, MAF, and O2 sensors within the last few weeks. Also threw in a new set of plugs, fuel filter, and regulator. Fuel pressure looked good, injectors sound good (stethoscope) and ohm testing has them all about the same (I forget the numbers but were +/- 0.1 of each other). None were leaking (key on, engine off). Distributor and knock sensor replaced last summer (code & no spark). Not finding any vac leaks, scan tool reports 24kpa idle (my mech. gauge quit working). EGR appears to be functioning correctly.
Engine idles fine hot or cold, but after entering closed loop it misses badly from off idle until the rpms come up (maybe 3k). If I put my foot in it (after closed loop) it has no power and feels like it is missing on multiple cylinders rather than the reported cyl 2 miss (bad coil?).

[Niti QX4]

Several questions:
1. how long have you owned this truck
2. when did this problem start
3. what were the circumstances when the problem occurred
4. what is the brand of the O2 sensors
5. what is the brand of the various other components
6. have you done compression testing
7. have you compression tested the cyl 2
8. what are your pressure readings
9. need numbers for a dry test and a wet test on the compression readings
10. what is the mileage
11. when was the last vehicle inspection where it passed
12. do you have access to a 4 gas analyzer

[Dirtybob]

1. how long have you owned this truck
coming up on 4 years now

2. when did this problem start
3 weeks ago. Leaving it parked until I can get this resolved.

3. what were the circumstances when the problem occurred
just driving to work, nothing out of the ordinary

4. what is the brand of the O2 sensors
one bosch, one denso (I kept the old one)

5. what is the brand of the various other components
IIRC the distributor, knock sensor, MAF, and CTS are generic parts house stuff but I kept the orignials (old dist. and MAF are O.E.).
swapping MAF and O2 sensors does nothing.

6. have you done compression testing
7. have you compression tested the cyl 2
8. what are your pressure readings
9. need numbers for a dry test and a wet test on the compression readings
I ran the car for about 5 mins, yanked the plugs, then did a compression test, all readings were ~150 psi +/- 5
did not write down the numbers

10. what is the mileage
220k miles

11. when was the last vehicle inspection where it passed
no inspections here

12. do you have access to a 4 gas analyzer
no. I don't have a decent scan tool either.

[Niti QX4]

Okay, those compression numbers are balanced so that's good. Not as concerned with high or low but even.

Did you change the plug wires also?

[Dirtybob]

yes, replaced plug wires when I did the distributor.

[Niti QX4]

Okay. Based on that, I would go back to the OEM dizzy and see if the problem moves or goes away.

[Dirtybob]

No joy.

[Niti QX4]

Well that stinks.
Sensors the ECM looks at:

Accelerator Pedal Position Sensor
Air/Fuel Ratio Sensor
Camshaft Position Sensor
Component Locations
Crankshaft Position Sensor
Engine Control Module
Evaporative Emission Control System
Exhaust Gas Recirculation System
Heated Oxygen Sensor
Intake Air Temperature Sensor
Knock Sensor
Malfunction Indicator Light
Mass Air Flow Sensor (Hot Wire)
Positive Crankcase Ventilation System
Relearning Procedures
Throttle Position Sensor
Vehicle Speed Sensor

IIRC, you can stay in open loop by disconnecting the MAF. If the truck will keep running normally that way, there are not many things that will cause your specific issue.

Unless you are specifically getting misfire in cyl3 in closed loop, I can't right now think of any reason the knock sensor would be an issue.

I fear it might be the ECM, but let's see what a good scan tool shows. Without the data, we are back to old school FAST (fuel, air, spark, timing).

Did you move the spark plug from cyl3 to a different cyl just to confirm the plug isn't cracked and having a heating issue?

[Dirtybob]

May have to try running it with the MAF disconnected then, if nothing else as a sanity check.
I do have a loaner scan tool but it is not very impressive (only samples data stream like 1/second). It can talk to the pcm and things appear normal, but when trying to watch O2s switching and such it is too slow to be of any use.
Plugs were replaced with new. No issue with the old plugs though they did look a bit rich if memory serves, unsurprising considering the misfires.
Does the pcm ignore cam/crank sensors when in open loop?

[VStar650CL]

If the plugs all look black, pull the vacuum hose off your fuel pressure regulator and see if there's fuel in it. FPR's can leak internally. On a KA it will blacken all the plugs and give you a negative STFT, but it dumps into the manifold between 2~3 and will make both of those run particularly rich.

On your KA the crank sensor is only used for the tacho and for misfire diagnostics, the ECM doesn't use it for normal operations. It looks at the cam sensor constantly and won't run without it, open or closed.

[Niti QX4]

Okay, here are your base specs:

Airflow Sensor Specifications
Application/Qualifier Measured Value (Min-Max)
@ 2500 RPM 1.9 - 2.3 Volt
@ Idle 0.9 - 1.8 Volt

Compression Pressure Specifications
Pressure (Min-Max) PSI Maximum Pressure Variation (PSI)
149 - 178 14

Coolant Sensor Specifications
Resistance (Min-Max) Ohms Temp ºF (ºC)
7000 - 11400 14ºF (-10ºC)
2100 - 2900 68ºF (20ºC)
680 - 1000 122ºF (50ºC)
300 - 330 176ºF (80ºC)
235 - 260 194ºF (90ºC)
140 - 150 230ºF (110ºC)

Crankshaft Sensor Specifications
Resistance (Min-Max) Ohms
510 - 635

Exhaust Gas Recirculation Temp Sensor Specifications
Resistance (Min-Max) Ohms Temp ºF (ºC)
79000 - 97000 32ºF (0ºC)
5700 - 7000 122ºF (50ºC)
800 - 1000 212ºF (100ºC)
100 - 200 302ºF (150ºC)

Fuel Injector Specifications
Application/Qualifier Resistance (Min-Max) Ohms
2.4L 10 - 14
3.0L 10 - 14

Ignition Coil Specifications
Primary Coil Resistance (Min-Max) Ohms Secondary Coil Resistance (Min-Max) Ohms
1.0 10000
1.0 10000

Ignition Timing Specifications
Set Timing Spec (Min-Max) Crankshaft Degrees
18 - 22
* Disconnect throttle position sensor harness before adjusting

Knock Sensor Specifications
Resistance (Min-Max) Ohms
500000 - 620000

Ignition Coil Specifications
Primary Coil Resistance (Min-Max) Ohms Secondary Coil Resistance (Min-Max) Ohms
1.0 10000
1.0 10000

Spark Plug Specifications
Gap/Clearance (Min-Max) Inch Gap/Clearance (Min-Max) mm
.039 - .043 1.0 - 1.1

Voltage Regulator Specifications
Voltage (Min-Max) Testing (Temp ºF)
14.1 - 14.7 68
============================================================================
These are the trouble codes that indicate closed loop control issues:

Diagnostic Trouble Codes P0125 Insufficient Coolant Temperature For Closed-Loop Fuel Control
Diagnostic Trouble Codes P1148 Closed Loop Control
Diagnostic Trouble Codes P1168 Closed Loop Control
Diagnostic Trouble Codes P0125 Insufficient Coolant Temperature For Closed-Loop Fuel Control
Diagnostic Trouble Codes P0510 Closed Throttle Position Switch
Diagnostic Trouble Codes P0510 Closed Throttle Position Switch
Diagnostic Trouble Codes P1446 EVAP Canister Vent Control Valve Close

===========================================================================

Exhaust Valve Specifications
Exhaust Gap/Clearance (Inch) Exhaust Gap/Clearance (mm)
.013 - .016 .33 - .41

Intake Valve Specifications
Intake Gap/Clearance (Inch) Intake Gap/Clearance (mm)
.012 - .015 .31 - .39
===========================================================================

Last thing I can think of if it's not the ECM is maybe the valves need adjusting. Because your compression numbers are at the low end of the spec, the valves opening late would effectively change your timing somewhat (intakes open later and shut later). Same with the exhaust. I would check that that before condemning the ECM.

Based on how the truck acts with the MAF sensor disconnected will tell you if it it indeed a problem only at closed loop. I'm thinking if misfires continue open or closed loop, I'd check the valves.

[Dirtybob]

Spent some time with it again this morning after picking up a new vacuum gauge.
Warmed it up and manifold vacuum was rock solid at 21" hg. This would also rule out a valve timing/adjustment issue since the gauge would flutter if there was a problem there.
Disconnecting the MAF made it run much worse.
Recalling the 24kpa idle MAP reading from the scan tool I had to look up the equivalent in inches of mercury, turns out that's around 7", hmm.
Checked vacuum at the MAP sensor and it was a solid 21".
Disconnected the MAP sensor plug and drove it. It ran better but not perfect (assuming this puts it in limp mode? plus I never cleared the code from having the MAF unplugged).
Unless my logic is flawed I'm thinking the MAP sensor is toast...

[VStar650CL]

Unless I'm mistaken, I'm pretty sure the MAP doesn't zero for atmosphere. That still wouldn't account for a 7" reading, but it may not be off as wildly as you think. That's also a parameter which scanners often mis-translate.

[Dirtybob]

it may not be off as wildly as you think. That's also a parameter which scanners often mis-translate.

I hear you.
I still remember the rabbit hole that was tuning the old GM 8051 pcms with reverse engineered software (no factory support). Trust nothing until verified. What do you mean that's epc solenoid duty cycles and not transmission line pressure? lol

[VStar650CL]

I hear you.
I still remember the rabbit hole that was tuning the old GM 8051 pcms with reverse engineered software (no factory support). Trust nothing until verified. What do you mean that's epc solenoid duty cycles and not transmission line pressure? lol

Good one. Classic.

[Niti QX4]

Interesting finding on the MAP. I follow your logic on the timing related to vacuum readings, but I would still check those valve clearances just to rule that out. If you've had the truck four years, and you haven't adjusted the valves, it wouldn't hurt just to verify the proper clearances.

Only other thing to check I can think of would be exhaust restriction. Maybe crack the front cat bolts loose and see if the problem seems to be lessened.

Otherwise, I'd find a local tech school and have them use the 4-gas analyzer on it to see what you're getting.

Fuel - flow, pressure
Air - temp, mass, restriction
Spark - intensity
Timing - advanced or retarded

Dang. Hard to troubleshoot without it in front of me. I'm really hoping you find this soon as I'm really curious about what the solution is on this one.

[Dirtybob]

As far as those compression readings being on the low end - I had to remove the band clamp on the business end of the unit to even get it to fit inside the plug well and was not able to thread it in more than finger tight (hose on barb fitting). I guarantee there was some leakage, plus the gauge is quite old and likely doesn't read 100% accurately. Good enough for relativistic readings though. Also if the problem is mechanical then why would it have to warm up to run like poo?
I did look at the valve adjustment procedure and was a little surprised to see that it's a non-adjustable solid lifter setup using shims - ugh. Haven't run solid lifters since the late '80s (drag racing stuff), guess I'm spoiled by adjustable valvetrains with hydraulic lifters...
I may get in there at a much later date and check clearances but that's looking like a back burner project. Feels like a lot of effort to get marginal/no improvement from some parapalegic hamsters and I'm getting lazy in my old age.
This truck is already on my naughty list since I've found a few replacement parts to be unobtainium (steering knuckle and injector cushions so far). Thank goodness for junk yards.
New MAP sensor should arrive later in the week, can't justify spending the extra $100+ to get it locally. That will pay for a flat of ammo and a few boxes of clay pigeons.

[Niti QX4]

....
New MAP sensor should arrive later in the week, can't justify spending the extra $100+ to get it locally. That will pay for a flat of ammo and a few boxes of clay pigeons.

I'm with you brother!

[Dirtybob]

No luck with the MAP sensor. I should've known the sensor reports the difference between atmospheric and manifold pressure/vacuum. 102 kpa (atmo. key on engine off) - 24 kpa = 78 kpa = 23.1 in. hg. - reasonably close to what my vac gauge was reporting...

[Niti QX4]

....
Disconnecting the MAF made it run much worse.

Wait, disconnecting the MAF made it run worse cold? It should have gone into speed density mode and idled and ran okay cold. Shoot, it probably doesn't go into closed loop right away, should take at least 30 seconds or more.

https://automotivetechinfo.com/2021/01/ ... rformance/

So, I'd check the 5 volt supply to the MAF and make sure it's steady. Is there any alternating current on your 12 volt DC measurements (bad rectifier) because that would mess with the ecm, the maf, and possibly the O2 sensor readings as they would not be steady.

Do you still have the original MAF?

[Dirtybob]

Truck was warmed up when I tried disconnecting the MAF. Turned it off, unplugged then started back up. Ran like poo. Yes, I have the original MAF and a junkyard MAF in addition to the parts store one. They all act the same.
Will be a minute before I can get back after this problem.

[Niti QX4]

Please post up what you find.

[Dirtybob]

Haven't had much time/motivation to mess with this but I did go through some checks on the TPS (per the service manual). Of course, everything checked out fine...
According to the service manual, the highest priority items to look at are the tps and ignition timing. Both of which are in spec.
Suppose next on the list to to have a look at the IACV-AAC valve circuit, one the the few things I have not looked at yet.

[Niti QX4]

Thanks for the update!