rb30 warm engine misfire + shocking low rev response problem

[Cheww]

ight so ive been in a pickle for while but i think im closer to pin pointing it, but i need help.

so a while back i started getting this warm engine misfiring, where once the engine reached running temp it would like cough, puff and basically shit the bed with shit response until it was undrivable, however the symptoms were far less noticeable when given a decent amount of open throttle and high revs funny enuff. (and when it was cold) 

 

- since it shat the bed ive taken out the plugs twice, first time it smelt like fuel then the second time a month after the ends where white and crispy (got pics) probably worth noting the car wasn't seeing a whole lot of action the 2nd time the plugs were removed.  

- ive found fuel in the intake manifold gasket when i took it off, and worth noting the inside of the plenum/mainifold was filthy AS HELL with a layer of oil/grease/37year old filth lining it.

- when i took off the rocker cover the shit is filthy as hell, good layer of grime on every thing, but the grim gets blacker the closer it gets to cylinder 6th, apart from needing an oil flush (dont think the last owner did one, after resurrecting it from a 10year sleep) it looks all good ish. i remember seeing a black spec on a cam lobe but it looked like it was ment to be there, but ive never seen a warn down cam so idk really.  

ran a compression test and it pulled through great, all cylinder with 5% difference between each of each other

things ive replaced/checked:

- replaced AFM, ECU, leads, plugs and refurbished the distributor/CAS/ignition coil

- had the Throttle position sensor checked, injectors cleaned

did a fuel pressure test with my hand on the fuel lines and that seemed fine, cant rule this one out for certain tho

- gonna change the some of the connection to the dizzy, afm, and maybe injectors, but i dont think its electrical due to the nature of it only happing when the engines warm.

 

               ight so ive got a little hypothesis based on some research ive been doing 

so in regard to this being based around the fact it misfires when its warm im thinking about blaming it on poor valve timing. Because from what i understand with an engine that require lash adjustment (rb30 doesnt i know) that a lower lash increases valve lift and duration, good for high end power at the sacrifice of low end torque. I also understand that the lash/gap is indented to compensate for metal expansion under heat. I know im going out on a limb but IF the hydraulic lifters 'seized up' and stared to act as a solid lifter with zero lash the symptoms would most prominent when warm, and with zero lash the open valve duration may be pushed to an extreme (opening to early and closing too late) MAYBE IF the intake valves closed late enuff they might still be partly open at the beginning of the compression stroke forcing puffs of air up the intake, bringing filth, oil and drying the spark plugs up.

( i know some of this seems far fetched(especially the second half), but ive replaced too many things), guess im just asking if theres someone with a bigger brain to correct me or give me some direction with this

cheers boys 

 

[Ben C34]

That's definitely far fetched, and no way are you on the right track. It is physically impossible for that to happen.

 

Have you replaced the coolant temp sensor?  Or at least checked it?

[robbo_rb180]

^^^  or coil, bad grounds, alternator output voltage.

Its an electrical issue, resistance increases with heat. Get a multimeter and start testing. 

Is this on a factory ecu? 

[Cheww]

1 hour ago, Ben C34 said:

That's definitely far fetched, and no way are you on the right track. It is physically impossible for that to happen.

 

Have you replaced the coolant temp sensor?  Or at least checked it?

yeah i replaced that too, i think it actually does misfire a lot less or not at all when unplugged. ive forgot, tested that one a while ago

Edited June 23, 2021 by Cheww

[Cheww]

15 minutes ago, robbo_rb180 said:

^^^  or coil, bad grounds, alternator output voltage.

Its an electrical issue, resistance increases with heat. Get a multimeter and start testing. 

Is this on a factory ecu? 

yeah factory ecu. i dont have a ground on my extractors anymore surely it cant be that, so what do you recommend? checking for voltage on the ground wires?  not too familiar with electrical side of things but down to do my homework on it   

Edited June 23, 2021 by Cheww