So, two potential issues, which may or may not be related:
1) If my speed is over 60 mph, the car often struggles to accelerate going uphill, even slightly. Like it can just maintain speed (most of the time), but not go any faster. I know these cars are not the most powerful, but is that normal? It is a quarter breed (XK mated to GM tranny), but the transmission seems to shift correctly based on the inputs, so I'm thinking something else. Most of the time, it does not misfire/backfire when it fails to accelerate...I just floor it and it floats along. First question is if this is normal - I just don't know what the baseline is.

2) Bit of a weird one here. Once I get to about the 40 minute point on a drive, the car will sometimes start to backfire. If I put it in park at this point and floor the accelerator, it will jump to about 3k RPM, then backfire through the intake (it's visible with the bonnet open). If I keep pushing it the car will die. If I let it sit for 10-15 minutes, I can rev as high as I want to, and it's fine, and I can continue on my merry way.

I would typically say that this is a fuel issue (sluggish acceleration under load + backfire through intake are both symptoms of a lean fuel mix), but why does it take 40 minutes to develop the backfire issue? The car obviously reaches temp way before that, and the temp holds steady...also, I've tested fuel pressure under multiple circumstances and everything seems within spec...I don't think drive-time or heat would affect that. Only other thing is maybe if the fuel is vaporizing in the fuel rail, perhaps that could cause it, and the 40 minutes is how long it takes the rail to reach that temperature. I have replaced the GM module inside the ignition amplifier because I've heard that can get overheated.

Your thoughts?

Thanks!
Benjamin

 

Before getting deep into the weeds.......

Have you replaced the fuel filter? It so, did you empty the old one into a clear jar? What did you see?

Cheers
DD