It seems the likely cause of misfire under load is ignition related, not engine hard parts like valves or camshaft. It takes more voltage to jump the spark plug gap when cylinder pressure is higher (air is an electrical insulator.)

Fuel delivery would probably cause different cylinders to misfire

Ignition misfire is caused by coil failing, plug boot arcing through, wrong /defective spark plugs, weak power supply (bad connection, broken wire, etc.). I suggest carefully examining the power supply wiring and grounding connections. Also check alternator voltage output before again replacing the coil pack. IF you can, have that bank tested on engine with an oscilloscope measuring the current draw to that bank, triggered by the end coil, and compare to see if all 12 coils on that bank are drawing equal current. while it is misfiring. Then if still 'no joy', pull the coil pack and carefully examine for spark leakage through the rubber plug insulators, including the joints--which will leave carbon tracks or pinholes. Carefully examine the boot ends for 'flash-over' .

 

All the rubber inserts are new inside the coil packs. I didn't use any dielectric grease on the boots or ceramic portion of the plugs only because Clark told me it was unnecessary, which I was a bit skeptical of at first. But the passenger bank is fine with no misfires, so I don't think the lack of grease is the issue here.

The car never does any hard misfires under load. Before I changed the coil packs and plugs, the car would do a hard misfire when pushing it beyond 4k rpm where the car would start to shake violently, and I would need to pull over and restart the car in order for it to run normally again. As long as I didn't drive it aggressively and push it past that limit, it was fine. I've long known this to be a symptom of a bad coil pack. After I changed the coil packs and plugs, it no longer did this. I can push the car as hard as I want and it never does a hard misfire like before.

However, as I mentioned previously, I do still feel vibrations while the car is idling even after changing the coil packs and plugs, which we can now confirm are the cylinder 8,9,10 misfires. But I just don't see how or why. Assuming it could only be from the coil pack, I guess I have to pull it out again and inspect it. I'll also try cleaning the grounds for both packs and voltage transformer and see if it does anything. It also wouldn't be a bad idea to test every pin in the coil pack connector to see if they are all working properly and getting proper voltage.

But all this still doesn't necessarily explain the power cutting off/fluctuation issue. It doesn't feel like a misfire but rather it feels like the engine is stalling very briefly, losing all of its power momentarily before it gets back going again, and then repeats itself. You can see this in the original video I provided. The power just comes on and off very intermittently, and you can really feel it "cutting off" as you try to accelerate. It's not like any misfire symptoms I've ever had or seen before, it feels like something else is holding it back.

Sometimes when I floor it, the engine has a massive burst of power (like a v12tt should) before it cuts off completely at 4k rpm, then it briefly has another massive surge of power afterwards, and then cuts back off again. And under certain circumstances I can get it to run perfectly fine without having the power fluctuation issue. For example, when I hold it in first gear manually and give it gas from a stop, the power doesn't cut off at all until it gets to second gear (as also seen in the video). The fact that the issue is very intermittent and doesn't have any fault codes stored in the computer other than misfires makes it almost impossible to deal with.

 

Just hung out with Alex for a few hours to troubleshoot just now. Swapped ignition box with mine and didn't solve the issue. Knock sensor goes off when issues come up. Car might have a tune on it. Crappy fuel in California may be the culprit.

Suggested he try to fill up with race fuel to see if that helps. Also will have to track down from previous owner / shop what tune was installed (car came from Florida with higher octane fuel).

 

Met up with Robert today and he was nice enough to help me diagnose this issue! We went on a test drive with the computer plugged in so we can use live data to monitor any faults that show up. Every time the problem occurred, a fault for knock sensors came up, explaining the sudden cut off and reduction of power. The computer is pulling back timing and fuel delivery intermittently throughout every drive session via the knock sensors.

With this knowledge in mind, it is safe to assume something may be wrong with the ECU map on the car. The car has a tune recorded in its history back in 2010 via the previous owner. It was tuned in Florida, which leads me to assume that the ECU expects a higher quality of fuel (such as Florida fuel) but instead is getting crappy California fuel, and cuts the timing because of it to prevent knock, hence the knock sensor doing its job.

I assume the car never throws any codes because the knock sensors are only doing their job by preventing the engine from making too much power on crappy 91 California fuel when the ECU expects higher octane fuel. Does this sound reasonable to anybody else? I will test this theory out very soon by putting in some 100 octane fuel, and see if the knock sensors stop acting up.

 

Get the ECU remapped to "standard", because nobody seems to know whats in it.
Good luck!

 

Well, it turns out the ECU, tune, or octane rating has nothing to do with the problem. I filled up 100 octane race gas, and the car still has the same problem.

In fact, the issue became much more prevalent because I felt like since there was more added power, the "cut-off" point of power was even more noticeable. The knock sensors kick in and don't allow the engine to make full power past 4k rpm, you can definitely feel it distinctly cut down its power drastically like it's trying to protect the engine. This is in addition to the constant surging under normal driving conditions.

This issue has been beating me up. The last thing I can think of at this point is to remove the coil packs and inspect the spark plugs again. I didn't clean the threads where the spark plug screws into the block when I did the whole ignition system. Assuming if the small amount of oil in that area is somehow causing an issue, then oil contamination could be the culprit here.

It does feel like the car is cutting power due to insufficient spark, causing surging and knock sensors kicking in to protect the engine at higher revs. Another thing to try out.

 

Well, did you? You never answered this!
Disconnect and stick a camera into it.
Mine were partially blocked, and the code was TWC DAMAGE CYL # xxx.
Search my nick here and "there" for my posts on this.
Good luck!

 

How do you know it was/is the Knock Sensor cutting power, etc.? That it DID develop more power with higher octane fuel, but yet still failed at the same RPM in the same manner, says it is not the Knock Sensor/timing adjustment SYSTEM, but does not rule out a corrupted 'MAP'. or defective ignition coils. Faulty fuel injectors are not as probable as their failure points seldom are that repeatable and consistent.


The altered ECU 'MAP' is a huge unknown. That it seems to be only a few cylinders miss-firing, and always the same ones, tends to reduce probability of a 'common mode' failure. However, since EACH CYLINDER ignition timing and fuel delivery are Individually Controlled, the 'MAP' coded in the memory chip could be scrambled and causing the problem.

The repeatability and consistency of the failure of only select cylinders strongly reinforces electronic source probability. A 'mechanical part' failure would not be probable unless the affected cylinders are adjacent and share a mechanical feature, such as all were on the same cam and cam adjustment was going bad due to ELECTRONIC signal corruption.. Mechanical failure of the cam adjusters would not exhibit the failure patterns you have.


The precision and repeatability of the failures strongly argues for ECU map corruption. An ignition coil failing would not remain consistent but would deteriorate more and quickly.The power transistors would continue to deteriorate.. I can not think of any failure mechanism, other than ECU 'MAP' memory corruption.

?

 

I checked the cats using the old rubber mallet trick and reading the o2 sensors to determine if they were bad. According to these two tests, they checked out fine. Is it possible for there to be a very small blockage that isn't detectable unless looking into it directly? And would it be enough to cause my symptoms? There were no TWC damage cyl codes.

We know it was the knock sensor cutting power because it flashed up during our live data stream test drive every time the car hesitated. The car "seems" to make more power with the high octane fuel, which is highly probable but could also be a placebo effect. Regardless, it didn't fix our problem and confirmed that the ECU map may not be the culprit.

The reason why I think the ECU map may not be the issue here is because this problem became noticeable after I changed the spark plugs, coil packs, and had the transmission valve body serviced. These were done back-to-back within a span of two days.

The valve body service was done to replace the torque converter lockup solenoid because it was causing very harsh jerking TC engagement during calm driving. Along with that, the transmission fluid, conductor plate, and connector were all changed. As of right now, the transmission module shows two errors - P2600 The voltage supply of circuit 87 has undervoltage - and - P2501 Engine overevving has occurred.

So I could see 2 possibilities based off this information. 1, the transmission could be acting up, causing the surging and power cutting off. But I find this unlikely, as I have never heard of a transmission making a car act like this. And 2, the spark plugs could be fouled from residual oil left in the wells.

This is more likely, as there was some residual oil left in the spark plug wells that I did not clean out prior to installing the plugs and coils, because I thought it wasn't necessary and wouldn't impact the car in any way. Turns out that just may not be the case.

 

It’s very easy to disconnect the cats and run an open exhaust , and change the plugs

do both of those

 

I think that a very few minutes of running would clear any minor amounts of oil from the plugs, so I doubt very seriously if that is your problem.

I have no experience with the V-12 engines, but the usual detonation setup is two detectors bolted in the vee of the engine. These sensors are crystals that are tuned to the particular ringing frequencies caused by detonation. When they are hit by these frequencies, they generate a tiny voltage, and send it to the computer. The computer can calculated time-of-flight for the ringing to determine which cylinders are in detonation, and then retard the timing for these cylinders. You will rarely be able to detect this operation. If detonation becomes severe and general, the computer will take more drastic actions to save the engine.

I would still be very suspicious of the unknown tune by unknown people. It seems possible, but unlikely, that aging, wear or a mounting problem would cause the sensors to become overly sensitive. I don't recall ever hearing of such a problem on any vehicle, but Google is your friend.

 

Although I have no evidence for the following, I will give my 2 cents.

1. Since the problem emerged just after the transmission repair, I am sure it is related to this, one or the other way. It might be a faulty or incorrect installed sensor, a damaged or malfunctioning controller board, etc.
2. P2600 Undervoltage indicates that a circuit does not get sufficient voltage to operate correctly. This can lead to malfunction and incorrect communication.
3. P2501 Engine is overreving, is crucial when transmitted to the ECU. If this signal is send on/off, it will trigger exactly the behavior you experience. It can possibly be linked to the Low Voltage problem.
4. Both P2600 and P2501 are important errors, that can not be ignored, but have to be eliminated through repair.

 

I can almost definitely confirm that I have noticed this problem right after I got the car back from the transmission shop. However, I just changed the spark plugs and coil packs the day before and did not drive the car too much in between. So the problem could have happened right after the plug change, but I only noticed it after I got the car back from the transmission shop the following day.

Then again, it could be just a coincidence that the problem developed simultaneously after the plug and trans service. Correlation doesn't always mean causation.

The transmission shifts smooth. No harsh shifts or torque converter engagement. I highly doubt the trans could be causing these issues, even though they sprung up right after the service. But then again, these cars are full of surprises. It could very well be the transmission for whatever reason.

Could the knock sensors themselves be a culprit? I have never heard of these failing on any M275 engine. It seems like a big job to change them too, so I hope this isn't the case. I also have a feeling a bad battery may cause this, since my battery is 4 years old at this point but still holds its charge properly. I don't know.

This is driving me nuts!

 

Wait a second , if it was fine before you changed these parts why did you change the coils and spark plugs in the first place ?

Cjearly you must must have been having a problem which is why you changed them . No one spends the money to change those if they are working ok

 

I changed the coil packs because they were bad. They caused a hard misfire when pushing the car under load, which required pulling over and restarting the car to return it back to normal. These have long been known to be symptoms of bad coil packs. I changed the plugs too while I was in there to go with the fresh new packs.

Changing the coil packs and spark plugs fixed that hard misfire issue. Minus the annoying fluctuating and intermittent power cut off, I can push the car as hard as I want now and it never hard misfires. This tells me the coil packs are good, nothing wrong with them. I never doubt Clark's products. It could be the spark plugs causing this issue if anything.

 

If you can hit it, and it takes off like a dragster until you hit 4500rpm, then it is not the coils or sparkplugs.

There would be no logic in that.

 

I don't see how knock sensors can trigger that behavior, even retarding the ignition, and also, that all four should fail at the same time, I doubt that. If one where faulty, you would get an error message.

I still believe that it has to do with the transmission repair. I suggest you take the car back to the repair shop, describe the problems, tell them about the error codes you get, that you did not have these before the repair, and ask them to take the car for a spin. When they get back, ask them to fix the tranny, that the error codes disappear, and take it from there.

 

To add to this discussion: I had the car hooked up to my laptop running AutoEnginuity to keep an eye on various sensors. I did a live view of the "Knock Control Enable" parameter during the drive. This was the parameter that would consistently flip on whenever the issue was felt.

 

Do you actually hear any knocking ?
Do you see all 4 sensors, or only a single combined reaction ?
Do you have Xentry ? If so, you can try disable the knock sensors, and take an easy drive, see if it helps.

 

Just the combined reaction. I had never used OBD tools to track down such issues so didn't think to set it up to look at each individual knock sensor.
I have Xentry, but there is either something wrong with my unit or cable. I have a new cable on order to rule out faulty unit (circuit 30/31 voltage issue).

 

I've just discovered that the problem is severely worse when the car is in C mode and driving as calmly as possible. I always drive in S mode so I never noticed this. This may help diagnose the issue better.

It just hesitates to move very badly. If I keep my foot on the same point in the gas pedal for a while, you can feel the engine suddenly surge in power once in a while before cutting off again. Could the torque converter lockup be bad? I have a feeling this was a botched transmission job, but I have never heard of these symptoms before.

 

It turns out the transmission most likely isn't the issue here.

The issue could very well be a vacuum leak. The symptoms get much worse when I purposely disconnect a vacuum line. I even swapped the map sensor for a broken one in order to purposely cut off all boost, and the same problem was present except was severely amplified. The less power the engine makes, the more noticeable the issue is.

This tells me that there is a small vacuum leak somewhere in the engine bay, and it is almost impossible to detect. I checked all the vacuum lines that are immediately noticeable in the engine bay and didn't see any breaks/leaks. Are there any vacuum lines elsewhere in the engine bay that are hidden from plain sight?

 

You’re getting knock sensor errors so change the knock sensors

 

In my opinion, as long as you have not cleared the transmission error codes, permanently, you can't be sure.

 

I highly doubt the knock sensors are bad. Their triggering is most likely a symptom of the problem, not the problem itself. Just like the misfires. Taking apart both exhaust manifolds to change some sensors only to find out it didn't fix the problem would be quite unfortunate.

The transmission codes were cleared, and I'm not sure about this, but I believe they could be caused from low battery voltage.

The car reads 11.7 volts when ignition is in the ON position. Headlights turn on and off, instrument cluster screens flicker, head unit turns on/off, SRS light flashes on screen, and other electrical glitches happen every time the car is left in key position 1 or 2. I suspect this is due to low battery voltage from a bad battery, and could be the reason why it trips off voltage related codes such as the transmission undervoltage code I got. Everything goes back to normal when car is started. Fires right up too.

Battery voltage reads 13.8/13.9 at idle, and 14.0-14.2 when driving. Is this normal? The voltage never fluctuates when the engine surges, indicating that it is not a battery/alternator related problem. I get these voltage readings from the secret menu in the cluster.

Regardless, new battery will be installed soon, and I will start going over every vacuum related thing in the engine afterwards.