Bad stumbling below 25% throttle.....

[PhenixCityGN]

Hey guys, been a while since I've been here, but I have a question:

87GN, running an open loop TT chip see other mods in sig, the car runs like absolute crap below 25% throttle, feels like at least 3 cylinders are cutting out every second. Almost like the car is turning off and on real quick, shakes the whole car. I give it some throttle, and it clears right up, car runs like a champ. 800 on the 02's and no KR @ WOT.

Replaced the fuel filter, thinking it was clogging up, no change.
Replaced the spark plug wires, with new ACDELCO and no change.

Pulled the wires individually from the coil pack, and the one on the drivers side in the front row was firing very weakly, compared to the other 5.

Would this indicate a bad coil pack? Or maybe something else?

Hate to throw money at a problem until I can at least narrow it down.

 

[GN owner]

Testing for spark

What i did was to install very short pieces (approximately 1 - 2 inches) of vacuum hose line between the coil terminal and the spark plug wires for all six of them. Start the car and get a shorting probe and place the probe close to the vacuum hoses. You should see the spark jump from the vacuum hose to the shorting probe. Compare all six terminals to see if there is one that is weak. Keep in mind that the condition of the spark plugs (deteriorated electrode, oil soak plug, etc.) will affect the results.

 

[PhenixCityGN]

Ok, will try the vacuum line test, and I'm going to pull a few plugs and look at them too.

 

[PhenixCityGN]

Hey guys, been a while since I've been here, but I have a question:

87GN, running an open loop TT chip see other mods in sig, the car runs like absolute crap below 25% throttle, feels like at least 3 cylinders are cutting out every second. Almost like the car is turning off and on real quick, shakes the whole car. I give it some throttle, and it clears right up, car runs like a champ. 800 on the 02's and no KR @ WOT.

Replaced the fuel filter, thinking it was clogging up, no change.
Replaced the spark plug wires, with new ACDELCO and no change.

Pulled the wires individually from the coil pack, and the one on the drivers side in the front row was firing very weakly, compared to the other 5.

Would this indicate a bad coil pack? Or maybe something else?

Hate to throw money at a problem until I can at least narrow it down.

New plugs, wires and fuel filter made no improvement.
Any other thoughts guys before I tackle the coil?

 

[Evans Ward]

How about some scanmaster values at idle since I see from your sig you have one? You can also ohm the coil pack. Do you have a spare coil pack to put on? Did you check air gaps at all three places for crank sensor too? Might be MAF also. All grounds secure? Post up those scanmaster values for us please. One thing at a time as you are doing too to keep it simple.

 

[neat]

I had a very similar problem a few weeks ago. I did coil pack, module, EGR valve, fuel pump, filter, and a couple other things. Turns out a screen had fallen out of the MAF sensor.

 

[XLR8]

I had a similar problem - stumble at light throttle. After the car is running try unplugging the cam sensor and see if the problem goes away. A cheap and easy test.

 

[PhenixCityGN]

Ok scan master values at idle all seem normal, O2 numbers bounce between 200 and 600, TPS at .44 etc etc if you want a specific reading.


Ok so far have done these things one at a time:
Fuel filter- no change
plug wires- no change
plugs- no change
new coil pack- no change
Unplugged MAF- no change, threw a code 34

How do I check the air gaps in the crank sensor? I could try that.

I also discovered that it misses heavily from 1900rpm to 2300 rpm, regardless of gear or throttle position. Meaning if you put it in 2nd gear and coast down, it misses from 2300rpm to 1900 rpm then smooths out below that, even with NO throttle.

Cam sensor? Wiped cam lobe?

I'm totally stumped at this point.

 

[turbojd]

I would try another maf, if theres somebody by you with a GN I would swap them and see how it runs.

 

[PhenixCityGN]

Ok:

Put my new in box ac delco MAF sensor on, and no change whatsoever.


still missing bad from 1900 to 2300 rpm.

Fuel filter, plugs, wires, coil pack, MAF, all been replaced one at a time, all made no difference at all.


I'm so retarded, I feel like an ape staring at a nuclear reactor!

 

[TurboBuRick]

Try this

Unplug the cam sensor while the car is running. Take it for a ride and see if the miss is gone.

 

[PhenixCityGN]

Unplug the cam sensor while the car is running. Take it for a ride and see if the miss is gone.

Ok. This seems dangerous for some reason.

If it goes away, does that mean the cam sensor is bad? I have a new one I can put in, but it seems like a chore, so I was hoping it wasn't the problem.

 

[theCATman]

Not dangerous at all. Once the computer has found no.6 upon cranking & starting, the cam sensor does nothing. The injectors will go into batch mode with the sensor unhooked, but the car will run fine.

HOWEVER, it will not restart with the sensor unplugged.

 

[PhenixCityGN]

OK, update time:


Just got back from testing the car with the cam sensor unplugged. Results:

The car buck and pops like Obama when you ask to see his birth certificate at anything past 30% throttle, or above 3200rpm or so. I mean like a damn rodeo bull fighting.

Got a code 41 when I unplugged the sensor, also the MAF I put on seems to be giving me a 34 now, I tried to clear it by unplugging the comp a few times, but it keeps coming back, should I swap back to the old one which I know was good?


So do these results with the cam sensor unplugged mean my crank sensor is bad? or something else? You guys have been so helpful so far, and I am learning fast!

Jeremy

 

[Junior Samples]

Set your SM to your TPS and watch it while depressing throttle within that 25%

 

[PhenixCityGN]

Set your SM to your TPS and watch it while depressing throttle within that 25%

TPS is at .44 at idle, and goes up nice and smooth with throttle operation, and pegs at 4.5 at WOT.

 

[Junior Samples]

Hmmm how bout your EGR? is it hooked up? try unhooking it and test, maybe leaking

Whats your IAC @ idle and also your INT? Sounds like its a lean stumble...just need to figgur out where the lean is comming from

 

[TurboBuRick]

Maf maybe taking a crap?

 

[PhenixCityGN]

Bump

 

[TurboBuRick]

Try unpluging the MAF then start the car and see how it runs.