My 86 dodge van with the 318 engine recently acted up shortly after starting. This is a carburetted engine with an automatic transmission.
I was letting it sit at idle to bring the engine up to temperature, when suddenly the idle speed spiked way high, no tach so I can't guess the RPM, but it sounded like someone was pressing the accelerator, I actually went and checked the pedal.
Along with the RPM spike, there was a low thumping sound or thudding sound from the engine, it didn't sound like anything moving, to me it seemed ignition related, the idle was pretty rough and there was a bit of excess vibration. I noticed the sound most at the tailpipe.
After a minute or two the idle slowed closer to normal, but was still fast, and the thudding continued while at idle. I drove it about 200km home (expressway driving) without issue, didn't notice anything once I was in gear.
Not sure if this is related, but I did experience some hesitation/shuddering while accelerating a day later, this occurred when the engine was hot, pulling out after a short (30 to 45 minute) stop. I have experienced the shuddering/hesitation in the past but only rarely and in similar circumstances.
I have an uneducated guess about the low thumping/fast idle issue but I'm wondering what might cause that/where I should look first.