Hey everyone
i have a 91 fc vert (US model), 75k miles Orig engine.. compression on all side and rotors are in the mid 90s. no DTCs . At idle engine revs smoothly no studder. does have a small idle bounce (prob a vacuum leak). timing is correct, spark plugs and wires recently replaced. before this the engine was a screamer tons of power and go never an issue. pulls strong starts up easily, all emissions are still on the car and it did pass smog last year. Alt was replaced it went bad and replaced it with a FD alternator but was already doing the knocking. Also put a new battery on it cause of the alternator.

Under load will get a knocking noise coming from Ignition side of engine. Feels like it is in front of my feet on the other side of the firewall. starting around 2k and is def down on power but does not die. revving higher doesn't make it worse or better.. i had a backfire problem recently and found the TPS died.. TPS is replaced the rich running engine and backfire is gone.. but still have the knocking.. i checked the leading side coil and secondary side and they were fine as well.. my AFM was working fine but i still swapped it with another working AFM and it still did the same. i just did the clutch a few months ago but there was no usual wear on the flywheel "from say a faulty starter sticking out and hitting the flywheel.." also takes a bit more to get it started with w blip of throttle to get it going. which is also new..

at this point im running out of ideas.. the only thing i can think is maybe its a fuel injector giving out underload. i forget under what conditions do the secondary injectors start contributing. The engine is mostly original equipment.. But im about to throw my hands up and just take it to a shop. but i would rather do it myself, but im just running out of ideas... the only thing i can think of is fuel injectors

 

Good example of why it’s good to have a wideband oxygen sensor. I understand it’s not needed for stock non turbo but does help identify or at least rule out fuel system problems.

maybe even a cheap narrowband oxygen sensor can help you identify a lean condition.

 

the plan was to 6 port turbo swap the car.. im still collecting all the parts.. its just a shame cause we have to move states. so i guess ill have to trailer it back to California. i wouldnt be suprised if its coming from the vacumm nest or along those lines i just dont have the time or the space to get into it..

 

symptoms could also be a plugged/collapsing exhaust