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Random inconsistent miss issues

How's the wiring look where it transitions between the engine to the firewall? It's not uncommon for the wiring to become chaffed or frayed inside, particularly if the motor mounts are bad. Often it shows up only when the engine moves during acceleration. I'd wiggle and shake that harness and see if it stumbles at idle.

Was one of the first things I checked. Harness looks fine. No chafing
 
Ok so here’s what I’ve checked so far. Wiring is good from pcm to injectors. Did notice a chafed spot on the pigtail connector on the number two injector but it’s not causing an issue. I will replace it after I fix the current issue. Clean IAC and replaced TPS sensor. I noticed an improvement in driving with the new TPS but the miss and noise is still there. I’m starting to think something internal. Possibly lifters or valves.. Or I’ve heard of head gasket issues causing misfires. Will be checking fuel pressure and cylinder pressure this weekend. Maybe that will tell me something. Any tips or ideas are welcome. Thanks
 
You keep saying the harness is good, but what have used to verify this? I don't see any values posted so........
 
You keep saying the harness is good, but what have used to verify this? I don't see any values posted so........

I checked wires for chafing on the harness from pcm to injector plugs all was good except for the number 2 injector plug had some chaffing near the plug. I don’t think that would cause a cylinder one miss and I plan on fixing that after I resolve the current issue. I checked positive side of injector for 14v constant power and I got a little over 14v. Used noid light to check pulse ground side, and noid light was pulsing as it should. To me this would tell me that the harness is fine. If there is anything else I should check on the harness please let me know.
 
First you were not checking for wire "integrity", second the individual "triggers" are negative (the important ones), the injectors share a common positive.
 
First you were not checking for wire "integrity", second the individual "triggers" are negative (the important ones), the injectors share a common positive.

Not sure what you mean by “integrity” but I checked wires for continuity and resistance between pcm plug and injector plug. All had continuity and less than 0.5 ohms resistance. Not sure if that’s what you meant. And yes I said I checked that in the previous reply. I check positive side of injectors for 14v and I checked negative side with a noid light. The noid light was pulsing, like it should, so the grounding was working correctly from what I can tell. If this isn’t what you meant please explain. Thanks
 
This weekend we ran several tests but got nowhere
First we ran fuel pressure test and got a steady reading from idle to wot of 53psi
Next did a spark test on the coil pack and had good spark at the number one plug.
Next we ran compressions test and got these values.

#1. 155
#2. 166
#3. 159
#4. 174
#5. 166
#6. 156

So compressions seems to be good

Lastly we ran a scope into the cylinder and everything looked great for an engine with 227k miles cylinder walls were clean piston was clean.
 
Did you resolve this issue?

I finally realized that my 2000 XJ occasionally runs poorly under load. 98% of the time, it runs great under load, but every once in a while when I put it under load it has no power and will miss if I put more throttle into it. No issue at idle though. It seems to happen not on a momentary basis, but instead, the poor running will last for minutes at a time. So far, it seems totally random for when it will happen. It doesn't correlate to engine temperature or outside temperature.

I haven't looked in detail at the injector wiring yet, but the motor mounts are in near-perfect condition. Coil pack is a new NGK installed last summer. Plugs are Bosch double iridium installed last summer. I gapped them a bit on the larger side, but the miss issue is not consistent, so I don't think this is a spark plug issue. O2 sensors are new NTK installed last summer. CPS is NAPA supreme or whatever their better model is. I tend to feel like it could be the CPS because so many people have issues with non-OEM CPS.
 
kinda sounds like my old stroker motor ended up being multiple bad lifters 2 on exhaust side 1 on intake . remove valve cover and put a straight edge on the rockers look for anything not consistent . I hope you find the issue and this helps . good luck
 
.......98% of the time, it runs great under load, but every once in a while when I put it under load it has no power and will miss if I put more throttle into it. No issue at idle though.........
Might be the CPS; however that condition is indicative of spark failure in
the cylinder. Cylinder pressure is greater under load and will blow out the
spark if it is marginal to start with. You mentioned gapping the plugs wider
than normal. That can contribute to a miss under load. Marginal plug wires
and ignition coil can also cause this....

I'd start with the simple things first, and regap the plugs. I always gap on
the narrow side to compensate for future wear.
 
kinda sounds like my old stroker motor ended up being multiple bad lifters 2 on exhaust side 1 on intake . remove valve cover and put a straight edge on the rockers look for anything not consistent . I hope you find the issue and this helps . good luck

That thought crossed my mind too. Its pretty noisy on cold start.

Might be the CPS; however that condition is indicative of spark failure in
the cylinder. Cylinder pressure is greater under load and will blow out the
spark if it is marginal to start with. You mentioned gapping the plugs wider
than normal. That can contribute to a miss under load. Marginal plug wires
and ignition coil can also cause this....

I'd start with the simple things first, and regap the plugs. I always gap on
the narrow side to compensate for future wear.

Thanks. Yeah, I agree its indicative of spark failure, but just odd that it doesn't always happen. This perception may be incorrect too. As I think about what the vehicle does, it may be that the miss happens at ~2000 rpm, but when the motor gets above 2500 rpm, it may not miss.

Anyhow, I guess I'll try gapping the plugs down first. I think that's a bit easier than popping off the valve cover.
 
I'm sure you've read the other posting about missing/rough running,
in which the solution was a new upstream O2 sensor. You've replaced
yours already, but might be worthwhile to spray the connectors with
WD-40 or electrical cleaner (if re-gapping plugs doesn't help) or the
bad lifters idea doesn't pan out. I'm thinking corrosion in the connector
might be increasing resistance and giving erratic O2 readings.
 
Gapping down the plugs tomorrow. I'm pretty sure that I sprayed both O2 sensor plugs with electronics cleaner, but I'll check again if the plugs don't take care of the issue.
 
Gapping down the plugs tomorrow. I'm pretty sure that I sprayed both O2 sensor plugs with electronics cleaner, but I'll check again if the plugs don't take care of the issue.

Look them over carefully that the iridium pad isn't cracked or missing. Normally they come with the proper gap and you don't need to mess with them.
 
Look them over carefully that the iridium pad isn't cracked or missing. Normally they come with the proper gap and you don't need to mess with them.

The plugs are gapped at 0.040", so a bit larger than the 0.035" spec, but not bad. Plugs looked fine. The spec value for the Bosch double iridiums isn't listed, and its possible that 0.040" was the gap they came with. I ran out of time today to regap. I may try that this weekend.
 
The plugs are gapped at 0.040", so a bit larger than the 0.035" spec, but not bad. Plugs looked fine. The spec value for the Bosch double iridiums isn't listed, and its possible that 0.040" was the gap they came with. I ran out of time today to regap. I may try that this weekend.

I gapped all the plugs down to slightly less than 0.040", probably sitting at 0.038". And I installed the MSD high performance COP for the 2000/2001. The vehicle is definitely more peppy. Shifts now have more of a hit to them, there is more power, and throttle response is better. But I can't definitively say that the perceived miss at 1800-2200 rpm at higher loads has gone away. I'll have a better idea after my upcoming road trip.

On a side note, I'm not sure why people complain that much about servicing the plugs with the COP setup. It took me 10 minutes to get the COP assembly off the vehicle, but I suppose that's still too long compared to 10 seconds to pull off a plug boot.
 
On a side note, I'm not sure why people complain that much about servicing the plugs with the COP setup. It took me 10 minutes to get the COP assembly off the vehicle, but I suppose that's still too long compared to 10 seconds to pull off a plug boot.

Because it's a major PITA if you are trying to troubleshoot/tune your system and need to read/test individual plug firing.
 
That's about the same cost as my Screamin Demon conversion.
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