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Annoying intermittent misfire
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IffyG
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PostPosted: Sun Jun 05, 2011 12:30 am    Post subject:  Annoying intermittent misfire Reply with quote

So after getting the trans fixed, I have come to the realization that some of what was going on is the result of an intermittent misfire. I have confirmed this on my Aeroforce gauge misfire monitor. I had to drive 400 miles round trip today, so I had some time to monitor a bunch of things and observe what the car was doing. Unfortunately, it's not throwing any codes, but I'm 100% certain there is a random misfire below the threshold for the CEL to flash.

First, the car does absolutely fine when cruising on level ground, but about a third of the time when starting to go uphill and the engine loads up it will misfire on a few random cylinders. The Aeroforce gauge usually reads 2 per cylinder when this happens, whatever that means, but I can certainly feel and hear that it is misfiring. The car also misfires occasionally when downshifting and also during part throttle acceleration.

I have also noticed that the issue is amplified once the car warms up to operating temperature, but I haven't had it miss when the car is still cold.

I will be pulling and inspecting the plugs tomorrow to see if there is an issue with them. I'll also take a look at the coil packs. Aside from that I'm inclined to think there is a problem with the fuel pressure regulator, ignition control module or possibly the EGR. Does anyone have any other input that might help me figure this out?

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Killr68
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PostPosted: Sun Jun 05, 2011 1:17 am    Post subject:   Reply with quote

Keep us posted. I think I'm experiencing a similar problem. All I know for sure is that I have no codes thrown, but she seems to drive rough periodically once she warms up. Sad

Oddly enough, I have a new FPR and Ignition Control. Might just be me.
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98ss
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PostPosted: Sun Jun 05, 2011 12:34 pm    Post subject:   Reply with quote

What would happen if there was a flat spot in the TPS sensor? It probably doesn't fit the bill when it you are upshifting and downshifting, but could make sense when the car is under load.
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IffyG
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PostPosted: Sun Jun 05, 2011 2:04 pm    Post subject:   Reply with quote

The car started missing as soon as I started it this morning. Does anyone think this might be related to my recent trans rebuild in any way?

Also, does anyone know what the criteria for setting a MIL for misfiring is? I'm dumbfounded that it isn't throwing a code or flashing at me.

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VictoryRed09
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PostPosted: Mon Jun 06, 2011 12:21 am    Post subject:   Reply with quote

one possible place to inspect by looking close.
Did the shop take the trans out and leave the engine in the car?
If so, they had to suspend the engine with an engine support bar. The Spark plug wires may have been pinched causing a small break in the wire .. as time goes by the small break in the wire will enlarge. Also check all wiring harnesses for smash marks. Also check that the engine is grounded to the body. ( missing ground strap.)
if the engine and trans were taken out as a unit. check to make sure all connectiones ( electrical and fuel) are correct.

Second possible
another possiblity, could be a slight lean fuel mixture ( not sure if you had compensated for the CAI and releaved Exhaust mods), that now, with a firm hooking transmission, places the engine at a higher load and makes the slight lean condition now show as a misfire.

just a couple of things to think about as diagnosis is really hard accros the internet..
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IffyG
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PostPosted: Mon Jun 06, 2011 12:41 am    Post subject:   Reply with quote

Thanks for the input, I know trying to diagnose a misfire over the internet is impossible, but without a steady problem or a code thrown I'm pretty much flying blind and wanted to get as many suggestions as I can get for things to look at.

I know the engine was supported from above and they dropped the transmission out the bottom of the car, so the first thing I checked was all the ground and wiring. Everything appeared to be in it's proper place. Today I checked that all the spark plug wires were seated properly and pulled the front three plugs to see if they are collecting deposits. The wires and plugs looked alright to me (certainly better than they did when I changed the plugs at 105k). So, I've pretty much ruled them out as being the culprit. Combined with the fact that my Aeroforce is showing the misfires occuring on all cylinders makes me hesitant to believe that all 6 plug wires were damaged and I have a global issue (ICM, fuel pressure, MAF, etc).

I haven't done any tuning to compensate for the mods, but I had been having all sorts of issues I had attributed to the transmission being jacked up for the last few years that turned out to be this misfire. Honestly, the only reason I noticed it was a misfire at first was because I had to pull my stereo out of the car so the shop could get at the fuel pump for another issue that cropped up last week. They tested everything on the fuel system and as far as they could tell, it was all in good working order with proper fuel pressure and operation. So I've ruled out an issue with fuel pressure or the fuel pressure regulator for the time being.

Today as a last effort for the weekend, I unplugged the MAF and it seemed to eliminate the misfire. I want to do a bit more driving with the MAF unplugged to be sure, but while testing it today it also eliminated the stumble/stalling issue I've been having when the engine is returning to idle so I'm hopeful that its the cause of the strangeness I'm experiencing now.

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PostPosted: Tue Jun 07, 2011 9:32 pm    Post subject:  Intermittent misfire, 2001 with a 3400 Reply with quote

Feels like I am reading my own message. Same issue with the intermittent misfire. I see it under slight load or slow acceleration, and it is getting worse. I have also noticed that it occurs in hotter, more humid weather (today was in the 90's). I went South for Easter and the problem started to show. I came back up North and the problem went away until the hot, humid weather arrived up here. Given this clue, it sounds like a MAP or MAF problem to me. I clean the MAF regularly after learning the K&N filter lesson, so I am ruling out contaminents at this point. Don't know what to do about checking or cleaning the MAP???

But then again, suppose it is related to the AC running, which is now being used with the hotter, more humid weather Shocked ????? The plot thickens.
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PostPosted: Mon Jun 13, 2011 5:58 pm    Post subject: Question Reconnected MAF and the engine stalled. Thoughts ??? Reply with quote

I have carefully cleaned the MAF and it feels like it is performing better, but the weather has also cooled significantly, so the conditions are not the same. I tested the sensor by tapping on it after the engine was warm with no change in RPM. Then I disconnected the sensor, and again, noticed no change in RPM. However,... when I reconnected the MAF, the engine stalled immediately. It restarted with a little stumbling, but quickly returned to a smooth idle. Does this mean anything definitive to anyone?
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IffyG
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PostPosted: Thu Jun 23, 2011 10:35 pm    Post subject: Question Re: Reconnected MAF and the engine stalled. Thoughts ??? Reply with quote

Did some more driving in the Monte today and have pretty much determined that the issue only happens under light load or cruising at highway speeds, never when I'm driving it hard or around town. It almost reminds me of what happened when my MAF was on it's way out 3 years ago. I want to see if I can rule out the MAF sensor (since I replaced the factory one with a used one from Morad). Does anyone happen to know what typical MAF sensor values are under different conditions?

thfro wrote:
Then I disconnected the sensor, and again, noticed no change in RPM. However,... when I reconnected the MAF, the engine stalled immediately. It restarted with a little stumbling, but quickly returned to a smooth idle. Does this mean anything definitive to anyone?


Doesn't mean anything. I'm pretty sure whenever you plug or unplug the MAF when the engine is running it will kill it.

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PostPosted: Sun Jun 26, 2011 12:13 am    Post subject:   Reply with quote

Took the car out again today and bumped up the data rate on my Aeroforce. Some of the time during the misfiring the MAF frequency and lb/min values drop to zero. I interpret this as something being wrong with the MAF sensor, the wiring or the PCM. Any ideas on how I can determine which of the components is the causing the problem?

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PostPosted: Thu Jun 30, 2011 1:16 pm    Post subject:   Reply with quote

Does anyone have any input on the MAF values dropping to zero during prolonged misfires? I'm thinking this would be the cause, but I don't know enough about how the PCM functions to know for sure.

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PostPosted: Tue Jul 12, 2011 6:41 pm    Post subject:   Reply with quote

I replaced the MAF last night with a used MAF and the problem has gone way. I've put about 60 miles on the car without a single incidence of the misfiring/bucking/backfiring it was doing before. I'm hesitant to say the problem is fixed for good, but I'm leanings towards thinking it is.

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PostPosted: Tue Jul 12, 2011 7:51 pm    Post subject:   Reply with quote

Glad you managed to figure it out.

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PostPosted: Wed Jul 13, 2011 12:29 am    Post subject:   Reply with quote

Not as glad as I am. It's been 100* for the last few days in Nashville and until today I had been tooling around in my Intrigue that doesn't have working A/C and the front two windows don't go down.

In other news, I've got some nasty popping now that I am assuming is due to loose cradle bolts from the transmission repair. Hopefully I can get those tightened up this weekend and get the Monte Carlo back in proper operating condition.

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PostPosted: Wed Jul 13, 2011 2:00 pm    Post subject:   Reply with quote

IffyG wrote:
Not as glad as I am. It's been 100* for the last few days in Nashville and until today I had been tooling around in my Intrigue that doesn't have working A/C and the front two windows don't go down.

In other news, I've got some nasty popping now that I am assuming is due to loose cradle bolts from the transmission repair. Hopefully I can get those tightened up this weekend and get the Monte Carlo back in proper operating condition.


I've got a box full of maintenance parts to install on the MC. I've been busy since spring started and haven't had a chance to work on the car. My JMB should show up on friday so I'm hoping that will motivate me to work on the car.

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