84' Fuel injected 318i Bucks Under any acceleration

[YoungEnthusiast]

Just recently purchased an 84 318i. The vehicle is extremely clean inside and out and runs very well other than a slightly lumpy idle and hard bucking under acceleration (back and forth movements of drivetrain under accel)


I noticed rips in the hose that goes from ICV to intake manifold and I noticed that my throttle cable has a LOT of slack and has about three threads still connecting. Injectors look original, but the bucking seems to decrease the more gas I give it. It idles at 500-800.

I want to make sure this isnt anything drivetrain related and that this may be a fuel/vacuum leak issue. transmission shifts well and it is unknown when the diff and manual transmission fluid has been changed. (Getrag 240).


I have owned a few e30s in the past but never an M10 motor.

Any guidance would be appreciated as Id like to drive this outstanding condition of a 38 year old burgundrot metallic coupe!

[e30davie]

The rips in any hoses would probably be a direct cause of your issues. The engine has an Air flow meter, so any air that goes through the air flow meter must go into the engine. if there are any extra places where air can get in (or out) then the ecu wont know how much air is actually going into the engine. bucking indicates lean condition to me, which suggests that more air is getting into the engine than the ECU thinks.

If it were me here is how i would attack a brand new car (to me) with your described issues.

-tighten the throttle cable.
-clean the ICV if it looks a bit shit
- Make sure there are ZERO leaks in ANY air pipes in the engine. there must be zero. and im talking about looking at every single pipe end, removing the pipes, cleaning things up, potentially cutting back the pipe slightly so there's a new end, making sure that you have a good correctly sized hose clamp and it tightens up very nicely. When i was playing with my m20 years ago i had a dodgy idle and just tightening an already tight hose clamp was enough to stop an air leak and smooth an idle. these air flow meter engines will not put up with air leaks.
-spark plugs - remove them - if they look reasonable hit them with a wire brush and regap. (they are cheap to replace, but for diagnostic purposes you can get some life out of them for free with a scratch brush)
-make sure the ignition leads look alright - measure the resistance and see if they are all the same.
-inspect the dizzy cap and rotor - if a bit shit looking give them a scratch up with a wire brush - if worn out you prolly should get new ones.
-make sure the ignition timing is in the ball park. im not entirely familiar with the m10 but id guess its somewhere around 10deg BTDC at idle. should be a sticker in the engine bay somewhere.

By now you should have a pretty good running engine. if you dont then its time to dig a little deaper

-if you suspect one cylinder always gives you trouble - look at the injectors. Ive had some level of success in DIY cleaning them with a can of cleaner stuff - and if you find you have sucess then best to get them professionally cleaned
-fuel supply issues - fuel pumps, fuel filter (replace it if you dont know the history), fuel pressure reg (unlikely, but possible - make sure the fuel pressure reg vacuum line is in perfect condition)

Ultimately thats all there is too it UNLESS you have bigger problems like a busted valve, or cam timing is out. if you get to this stage and it sitll runs poorly, compression test and leak down test i guess.

[Whit3 E30]

^^^ perfect, the only thing I can add is, I bought a 84 318i back n 2014 from a buddy of mine, had the same thing going on plus a small ticking noise from the engine n idle surge (up and down idle). The idle was easy it was just the 1984 idle control computer which was $20 on ebay (I have one extra that works that I don't need if you need it). For the ticking I adjusted the valves which helped out the ticking but it was still there. So I tore down the engine and found the timing chain was stretched an intake valves were just barley smacking the pistons (its a interference engine). 2 of 4 intake valves were bent a little so I had little misfires going on also, so this is the extreme scenario but if what he said^^ all checks out go from there. But leaking air hoses will definitely be a major cause and I would fix.