Need help with diagnosis for throttle response

[JonsE30]

Some back story: I got milkshake oil so I rebuilt the engine, albeit for the first time, so I’m sure it’s not as good as it could have been, but I did the mess under the intake following that diagram of which hoses to add and delete. I changed spark plugs and fuel filter but I didn’t change the fuel injectors, o2 sensor nor the catalytic converter and it passed smog on the first try.

When i give it some gas there is some hesitation sometimes. When i let off to let it idle, it will drop low, but catch itself, but not all the time.

At first I thought this was vacuum leak so I checked the end points: valve cover nipple to the bottom of the throttle body, throttle body to charcoal canister, back of head to icv, icv to bottom of intake boot, bottom of cylinder head to brake booster, fuel pressure regulator to throttle body, oil filler cap. Is there any other places to check for vacuum leak?

Eerything changes idle when those are tampered with is a good signal right?

Is there an issue with my throttle body or throttle cable?

Is there is an issue with my o2 sensor? I don’t have any check engine light though.

Is there an issue with my catalytic converter? Was I supposed to change it I had a bad head gasket because I was burning a lot of coolant?

What else is there to check?

[bmwman91]

A fairly common cause is a sticky ICV. Did you clean yours out with some carb cleaner?

Can you describe the "hesitation" behavior a little more? Does the engine lose power as you open the throttle, or does it just feel like there is somehow a delay between applying the throttle and the engine revving up?

[JonsE30]

Crap, I cleaned it with brake cleaner not carb cleaner. I thought it was similar. Is it still OK? The hesitation sounds like both of those. I noticed it really picks up around 4000 rpms. I am unsure if this is just norma, but I will take a video tomorrow over the engine bay and from the driver seat and post a link. Would it matter if the engine is cold or hot? Sent from my iPhone using Tapatalk

[bmwman91]

Brake cleaner is probably fine. I think it is mostly acetone.

Well, vac leaks really only affect stuff around idle, so if you are feeling hesitation above 2000RPM then it is probably something else. Engine temperature is a significant factor for how the ECU calculates fuel and ignition advance, so it would not be surprising if the car behaved differently when cold vs hot.

Once you get some videos up maybe it'll be easier to tell if there an issue of some sort.

[JonsE30]

Here’s a video clip. Earbuds or headphones recommended. I hope this helps. https://youtu.be/HgzYehO8T4w

[bmwman91]

Hmm, well it has been so many years since I drove a stock M42 that it is a little hard to say how abnormal the response to the throttle blips is or is not. Those stock flywheels are heavy as shit. Assuming you have had a chance to drive it, does it feel like it lacks power at lower RPMs compared to before the rebuild? Once it revs to a certain point does it feel mostly normal? I do recall the M42 not being super duper responsive to fast throttle blips at idle. Even with a built 2.1L and a 7.4lb flywheel, if I just tap the throttle like that it isn't revving itself off the tach or anything.

Whatever idle instability there was did not seem all that terrible, or at least not what I could hear. Maybe it could be better, but I've seen a number of M42's nearly stall coming off the throttle when there is a serious vacuum leak or stuck ICV.

[JonsE30]

That makes sense. When it gets going, it feels normal where I don’t feel it will stall. I know these engines are not fast to begin with. I just wanted to be sure it was not a major issue to be addressed immediately.

[bmwman91]

Does it feel particularly weak when trying to get going from a stop? If you give it a large throttle input, either in neutral or when releasing the clutch, does it seem to lose power?

It is normal for the M42 to feel like basically nothing happens after you give it more than like 25% throttle below about 2500RPM, but it should not feel like it is losing power.