My Suzuki Grand Vitara 2.5 v6 bogs down on acceleration and won’t pull itself past 40 mph. I just tuned it up 2 weeks before this started happening and it idles smoothly and starts fine. When you accelerate it sounds like it is getting to much air like old four barrel carburetor when the secondaries come in to soon.
When I’m under the hood and rev it up it makes a spitting noise at rear of engine around throttle body it also makes the same noise when you try to accelerate.
I think I would check the exhaust back pressure at the tailpipe. When you mash the gas pedal to the floor there should be a lot of exhaust coming out tailpipe. A restriction in the converter or muffler could be causing the noise under the hood as the exhaust gases are being pushed back to the engine.
Here is a video showing you how you can test your exhaust back pressure
Second I would rent/borrow a fuel pressure gauge from your local auto parts store and manually check fuel pressure at the engine. You might have a weak fuel pump issue, which could be “working” but not sending enough fuel pressure and fuel volume to the engine to perform under heavy load.
Watch as they test for fuel pressure in the video
The last thing I would look at would be ignition spark timing, you might have spark to each spark plug but did the timing belt jump a tooth or something else internal to the engine change cause the timing to be “retarted” which reduces engine power.
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