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
Monday, May 6th, 2019 AT 1:48 PM