Misfire

Hi,

Have you hooked up a fuel pressure gauge and monitored it while it was misfiring? You can just tape it to the windshield so you can see it while you are driving it. It just hooks up to the schrader valve on the fuel rail.

Also, when you say it is misfiring, what gives you this impression? Does it run rough and vibrate or can you hear a noise during this time? The reason I ask is because this sounds just like detonation or the engine is pinging. Seems to do it at cruise or slight acceleration and goes away when you give it gas.

Even if you think it is not detonation, I would recommend just putting about $10-15 of high test fuel and then adding an injector cleaner/octane booster in with that high test. If this is gone when running on real high test fuel then we may have carbon build up or one of the other reasons for detonation. If not, we can rule it out for good.

The only reason, I suggest this even if you don't think this is detonation is because I got burned on one for about a week that I was chasing a misfire and this is what it was. That matches your explanation EXACTLY.

If not, let's get that fuel pressure because I agree that this sounds like fuel related. It is possibly a MAF or MAP sensor issue. However, it is odd that it would only be at these certain speeds. Clearly, I don't think the speed has anything to do with it, but it is the right engine load and RPM that it has this issue.

Let me know about this and we can go from there. Thanks

Okay. 50 PSI sounds good but just make sure that is when this issue is happening. Should be the same but I have seen some of these fuel pumps drop out. However, because it is so consistent, I doubt this is the issue.

At the same time, the reason it is so consistent is why I suspect detonation. If you don't turn anything else up, just putting some high test (93 octane or higher) in it to test it is worth trying. I am not suggesting you need to pay for that expensive fuel all the time, just looking to see if this is the issue then we can take corrective actions.

Keep me posted and we can go from there. Thanks

Have you confirmed this is actually a misfire by using a scan tool and looking at the misfire counter when this is happening? Or looked at the o2 sensor information, specifically the short term fuel trims?

If not, I would suggest doing that before you replace anything else. The reason is, the transmission is not causing a misfire but the reason you feel it in that gear is because that is the load and RPM range that the misfire develops. Again, if you have not put a bottle of octane booster in the fuel and put about 5 gallons of 93 or higher, then please do that. If this goes away then it is not a misfire, but spark knock.

Once you monitor the systems and if this is not misfiring, and it is coming from the transmission. I would suspect you have a torque converter clutch that is slipping. You can confirm this by monitoring the torque converter clutch slip in the scan tool.

It just seems like we have gotten to the point of guessing and based on what you are stating, I am not convinced it is a misfire. If it is, I would not suspect anything in the transmission being the issue.

Okay. Just to clarify. I am not suggesting the o2 sensor is an issue but the scan tool will allow you to see what the o2 sensor is reporting. Meaning is there too much or not enough fuel in the exhaust? This will point us in the direction of if we have a misfire and if it is a dead mis causing unburned fuel to go through the exhaust? Or is a lean misfire where there is not enough fuel in combustion chamber at the time of ignition causing a weak combustion?

Please keep me updated and we can go from there. Thank

Just post it to this site. That way all who view this in the future can identify if your issue is the same as theirs.

Have you done any of the following?

1. Added a higher octane fuel and octane booster to see if this goes away as it could be detonation
2. Monitored the o2 sensor data (short term, long term fuel adaptives) to see if it is actually misfiring
3. Monitor the torque converter slip in the module?

I would hate to see you guess at something like taking out the trans and hoping to see something or just changing the ECU.

HAHA. Well we all have ones that just kick our butt. At least the owner on this one isn't going to yell to much at you. Even if he does, just walk away from the mirror.

Watch the short term fuel trim when it is doing this. It sounds just like a lean misfire at cruise because when you step on the gas, it goes away because you are adding more fuel. This way we can at least confirm it is a lean misfire. Then we can start looking at what causes lean conditions like unmetered air from a leak past the MAF.

Send me the short term readings and we can go from there.

We just need something to go on. We need that first thread to start pulling and it will lead us to it.