I've tried to format this post to make it easier to follow with the history first and my questions at the end
My 1991 M42 will start and idle 100% when totally cold, but after about 30 seconds the revs will decline to about 500rpm, at which point it sounds like it adds a jab of throttle to raise revs to about 1500rpm until revs lumpily decline to 500, and then the cycle repeats.... The car was running well last summer, has been in storage and apparently was not happy to be left alone
If I adjust the throttle butterfly stopper (now I realise I shouldn't have messed with it) to induce a little extra throttle until the hunting is gone, it then sits at 1400ish rpm fine. If left to idle in this state it will have a random misfire as if the ignition is cut for half a second at a time. If I back the butterfly stopper slowly back down it'll idle at about 1200 for a few seconds and it'll start misfiring a lot and eventually hunt again.
Disconnecting the tps (throttle position sensor) kills the engine, but upon restarting with tps disconnected it will hold a higher but better idle. However the random misfire will rear its head if left long enough. I have tested the tps to confirm a smooth resistance change from 3.87 (k?) ohms down to 1.57 (k?) ohms. Harness is supplying 5v as it should.
I am dailying the car now with the tps disconnected and the throttle butterfly valve adjusted open enough to allow idle. It is much more drivable with the tps disconnected, I don't believe the tps itself is it fault, but maybe something it works with in a closed loop system.... but then again the stomp test isn't working, which can be a sign of a bad tps.
1. All vacuum pipes and intake boot, many of which new, have been checked and double checked (even with the carb cleaner method) and all test good. I've added hose clamps on many for good measure but to no avail. Mess under intake has been removed. I also read that with the engine running, removing the oil filler cap should make the engine stutter, which it does, albeit slightly. Only thing left here is the rocker cover gasket.
2. I read the ICV will rarely fail. I have cleaned it, confirmed it moves freely and have tested it with 12v power to ensure its not dead.
Note the car has been mostly restored prior to last summers use, so all electrical earths etc are now 100%.
Both blue and brown temp sensors have been replaced and coolant level is good. Also I just fitted a cop conversion with good afaik coils which made no difference at all to the issues of the car. AFM carbon tracks also look good although I haven't tested it yet as I don't know how
Q1. How can I check if the rocker cover gasket is leaking vacuum‽
Q2. I can blow air (with my mouth) through the ICV with or without 12v power applied, albeit with different restrictions. Is this normal? And how many ohms resistance should I expect across its 2 terminals?
Q3. What should I be looking at next? Online research hasn't yielded anything so far and I dont want to start throwing parts at it for the fun of it.
I'm thinking to try test inlet air temp sensor and pull and clean the fuel injectors
My main suspects were the ICV, TPS, AFM and vacuum system but getting nowhere and its doing my head in now
Please help lol
