Help with GN low load misfire...please.

Ajrothm

New Member
Hey Guys, can I get some help on diagnosing a low load misfire condition.? I know this will be a stretch but, here is what I have.

New GN I just bought, I’ve put 100 miles on it. I don’t think I noticed this condition originally, however it has to be at a certain throttle position/load range to do it.

The misfire will start to occur at around 1.04-1.22 TPS voltage on Scan Master. It seems to be the worst most prevalent in the 1.12-1.16 TPS volt range. It will do this when accelerating normally, keeping up with traffic.... 2nd gear around 35-40 mph and 3rd around 45 mph. Steady throttle this does not occur, less throttle input, this does not occur. Heavy throttle this does not occur.. but If you catch right in that 35-40 mph range at 1.16 TPS, it will steadily buck/surge until you speed up slow down.

AFR is in the 14.0-14.2 range when this phenomenon occurs and doesn’t seem to change much...(no lean spikes or anything).

Today I changed the spark plugs, ohmed out the spark plug wires (all were less then 100 ohms). All vacuum hoses appear to be on. Just installed new fuel pumps, drained the tank completely so the gas is fresh.

Engine is 4 years old, 3k or so miles. Everything was new at that point...(cam sensor, crank sensor, ignition module, coil pack, MAF). Pretty much EVERYTHING was new and adjusted by very good Grand National tech.

Car idles good, revs smooth and clean in park. It’s only under light load/normal acceleration that this happens

I know this is a stretch but... any tips as to what to watch for, look for? There is no MALs on the SM, is there other channels on SM I should look at when this occurs to give me a clue? MV were all over the place as usual at that point.

Thanks for any help!

Just a new guy trying to get a new car lined out and it’s my first Turbo Buick. Thanks again!
 

Ajrothm

New Member
The coil pack, module, cam and crank sensors were all replaced with AC Delco units back in 2016....has about 3-4K miles on everything. I changed plugs and ohmed out the spark plug wires yesterday and all were under 100 ohms. I suppose it could be a coil or module, but I would think it would break up under boost/high load then? Mine is fine in boost.

It starts misfiring around 25-30% throttle, 1.04-1.32 TPS voltage... it’s still in vacuum at that point, maybe about -8” vacuum...at around -4” vac it will studder pretty hard. Once it goes 0-positive, it smooths right out.
 

Ajrothm

New Member
At the time the misfire occurs, the AFR is steady in the 13.9-14.3 range, no spikes or rich dips. Definitely does not appear to be fueling related. “Feels” like an ignition misfire. Almost like when you develop a crack in a plug or get a slightly burned/arcing wire... just quick, abrupt/sharp shudders. It doesn’t pop or anything. It just feels like a quick ignition cutout.... milliseconds.
 

Danster

Senior Member
What Turbo and intercooler do you have? You mention bucking/surge, that's different from a misfire.
 

captndave737

Well-Known Member
To me it sounds like the TCC is locking and unlocking as it goes into 3rd gear. Have you tried driving with the shifter left in second gear? This will help find out what's going on. Mine used to do that right at that speed and throttle setting. It used to drive me nuts so I installed a manual switch. I also didn't like the fact it locked in 3rd gear at all.

A quick check would be to unplug the 4 pin connector on the side of the transmission and go for a ride. If all is smooth you found the problem.
 
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Pronto

Chaos is a ladder.
Do you have a Powerlogger? A log would be nice to see. You could be having a MAF issue. It could be "dropping out" at the TPS range. On the SM check the AF readings at the point of the issue. I had a MAF that would have a dropouts that were easy to spot on the Powerlogger log.
 

Ajrothm

New Member
To me it sounds like the TCC is locking and unlocking as it goes into 3rd gear. Have you tried driving with the shifter left in second gear? This will help find out what's going on. Mine used to do that right at that speed and throttle setting. It used to drive me nuts so I installed a manual switch. I also didn't like the fact it locked in 3rd gear at all.

A quick check would be to unplug the 4 pin connector on the side of the transmission and go for a ride. If all is smooth you found the problem.
I haven’t tried driving it with it manually in 2nd but....I can get it to misfire in 1st gear at the same load range too so... my converter only locks in 4th at about 50 mph. It’s very distinct when it locks.... it locks hard. (5 disk Vigilante).

This misfire is definitely “load” related... it’s not really rpm based, it’s not gear based. It’s about hitting a certain load at light throttle.. in the 1.04-1.32 TPS voltage range.

Today I tested the TPS voltage sweep with a DVM and it was perfect. Also ohm tested the coils and all were around 11,950 ohms. Also took the module to OReilys to test it on their ignition tester and it tested good.

Next up will be the cam sensor “disconnect while running” test and see if it changes anything. But I’m running out of ideas....

To recap:

1) New Plugs, gapped at .030”
2) Wires ohm’d out at less then 100 ohm
3) Coils ohm’d out at 11,950 ohm
4) Module tested good on a tester
5) TPS sweep test is good
6) wide band O2 does not change (14.0) during the misfire/studder. (No lean spikes etc)
 

Ajrothm

New Member
Do you have a Powerlogger? A log would be nice to see. You could be having a MAF issue. It could be "dropping out" at the TPS range. On the SM check the AF readings at the point of the issue. I had a MAF that would have a dropouts that were easy to spot on the Powerlogger log.
I don’t have a powerlogger (yet). I would think If the MAF signal wasn’t dropping out, I would see a lean spike or something on the wideband? When this misfire/studder occurs, the afr is 13.9-14.3 range and cruising in closed loop(I believe). It runs good in this afr range while normal driving, EXCEPT in that particular load range.

The MAF is a 3.5” late model style LS MAF. It’s all plastic. It’s 4 years old and has 3-4K miles on it. It “appears” to be clean across the wires...
 

Ajrothm

New Member
What Turbo and intercooler do you have? You mention bucking/surge, that's different from a misfire.
Turbo is a 6266 and a FMIC. This misfire occurs at low load, way before it starts to build boost. I’m around -8” vacuum when I start to feel it misfire/studder.
 

Ajrothm

New Member
So it’s something with the cam sensor. I can disconnect the cam sensor and drive it and it doesn’t misfire at all...as soon as I have plug the sensor back in and start driving again, it instantly starts the misfire. I tested this 3 times on my test loop and never even shut the car off.

so now to learn about cam sensors...
 

Ajrothm

New Member
CD5BA922-9479-4360-B432-CBA245018D0C.jpeg

Cam sensor cap has a groove cut in it from the “rotor”. Not sure if that’s normal but I doubt it. I cleaned the cap and reinstalled it. Car instantly misfired again. Pull over, disconnect the sensor and the car runs fine.

The “rotor”/shaft have no noticeable play in any direction.

Would this point to a faulty cap. I haven’t tried to move the base of the sensor as I don’t want to get the timing out of adjustment...
 

Ajrothm

New Member
36411F80-CC9D-4108-BAA3-AB86F42C4CF9.jpeg

Here is the base of the sensor. This was new in 2016. Has maybe 3-4K miles on it. Grabbing the “rotor” and trying to move it, it’s very tight. No play at all.

Wondering if I should try to move the base a little in either direction and see if it changes anything....?

or just order a new cap and go from there?

I don’t have the timing light tool to set the timing so I guess I need that too.
 

No disintegrations

Active Member
Mark the base so you can put it back in just in case. But yes move the base a bit either way and try it. It won't change spark timing. It affects the fuel injection somehow. The engine uses it mostly for finding number one cylinder compression stroke on start up. After starting it is used for injection sequence.
 

Ajrothm

New Member
Mark the base so you can put it back in just in case. But yes move the base a bit either way and try it. It won't change spark timing. It affects the fuel injection somehow. The engine uses it mostly for finding number one cylinder compression stroke on start up. After starting it is used for injection sequence.
Man...you called it. Let me post up my findings for the group.
 

Ajrothm

New Member
13625373-29C0-4EFE-AA92-3D5C6F5F15ED.jpeg

First I marked the sensor base/rotor so I could put it back in the same spot without the light tool.

73EC8B10-BA96-4E2E-B81F-90F5E699439A.jpeg


Next I pulled the base out and spun the shaft and noticed the rotor thing was WAY out of shape...



64492DB4-6CB1-4C60-9663-C1081DFA4EAB.jpeg

9A6938DC-F237-43B7-8656-82E962506545.jpeg

0B6A0EF4-2292-4F37-BD71-0957020A408D.jpeg


I removed the rotor, “straightened” it and reinstalled, spun it etc. Reinstalled it back in the motor, lined up my marks, after about 20 tries, I got it in place. Car fired right up and the test drive was successful, no misfiring at all. Went for about a 10 mile drive and it ran great.

Now the question, how/why did the rotor get bent? Is the oil pump shaft pushing up on the sensor under WOT and causing contact with the rotor to the sensor cap, thus bending the rotor?

who knows, I’m just glad I found the problem after 2 days of slaving/stressing over it.
 

TurboBob

Resident mad scientist
Staff member
looks like someone tried to balance the rotor. looks like they took out a lot of material. (they are not that way from the factory)

in any case, sounds like you've solved it.

Bob
 
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