Throttle tip-in issue/stumble (3.0 5-speed)

[Masospaghetti]

I used QD electronic cleaner. What's special about the IAC and carb cleaner?

[80stech]

Carb cleaner will ruin any rubber o-rings.

[Masospaghetti]

I see. The IAC grommet seemed okay, it was still very pliable but maybe I should have replaced it.


I also bit the bullet and replaced the MAF (VDO/Continental), coils (Bosch), and O2 sensors (Denso). It seems somewhat better but it's not completely eliminated (it's also possible it's a placebo effect).


If I had a vacuum leak, wouldn't that show up in the long term fuel trims?


Is it plausible that a slow or lazy IAC would cause tip-in problems?


Does the electronic throttle body get slow or lazy with age?

[80stech]

Others have said that the IAC can make a difference but it really shouldn't and I haven't seen that, maybe they could chime in more on that. I've never heard the throttle body causing that issue but you never know, maybe more likely than the IAC motor ?? What happens when you clear the DME adaptations ??

If you don't know the history it is possible that someone has used carb cleaner to clean the throttle body which could for sure cause it to react slower as the o-rings swell and drag. I would eliminate as much as you can first though.

[Masospaghetti]

I tried clearing the DME adaptations but did not notice any difference. I also went ahead and checked my fuel pressure, 54 psi when pump is running, 48 psi when pump shuts down. I understand 54 psi is the specification so this checks out.


Based on my understanding of the idle air system, the IAC allows air into the manifold up until it reaches 100% duty cycle, then the throttle begins to open.
Total speculation but it feels like this handoff is maybe not happening in a smooth manner. Like you said, maybe the throttle is a bit lazy or slow or there's something weird going on with the IAC. Unfortunately these are both expensive parts and I would be buying OE grade parts (the throttle body is $217 at Rockauto).


I wish I knew what this setup was supposed to feel like when new. There are a lot of complaints of similar symptoms for other M54 powered vehicles on the 3-series and 5-series forums.

[80stech]

I don't think the IAC motor works like that but you could check that pretty quick looking at the live data. If it does then IAC motor is definitely on the table. If resetting the adaptations or running with the MAF unplugged doesn't help then it's way less likely to be a vacuum leak.

[Masospaghetti]

Before splurging on a new IAC I wanted to take it out again and make sure it's clean, I also saw this thread about lubricating it which I will try. https://www.e46fanatics.com/threads/...n-icv.1043323/


Throttle body looks really clean.


I pulled the spark plugs because one of the other threads mentioned excess carbon buildup causing similar problems. This looks a bit excessive to me but curious to get thoughts. All cylinders look about the same. I'm considering adding some cleaner (such as throttle body cleaner) directly into the cylinders to try and get some of the crap out.

[80stech]

The second pic doesn't look great but I'm not used to looking at broscope pics so hopefully more chime in. The intake valves count too, maybe more. The more carbon that can absorb fuel the worse the problem can be but it's not an exact science. Chrysler had an excellent foam spray (meant to spray in the carb/throttle body with engine running) for carbon back in the day but I don't know if it's still available. I think some of the pro-type (into the injector rail) cleaners work pretty good for carbon too. Your biggest friend is PREMIUM gas.

Just to make sure you are saying more of a tip in slight stumble or hesitation , not bog, right?
Did you verify how the IAC motor works off idle ??

[X5chemist]

I've seen carbon buildup like that on a few old engines. My advice, drive it "harder"!

[workingonit]

Originally Posted by X5chemist

I've seen carbon buildup like that on a few old engines. My advice, drive it "harder"!

Back in the day, circa late '60's to early 70's, I was entrusted to do the upkeep on my parent's and grandparents cars. Still at home, in college, and having trained at a GM Tech school, I was the person given the job; my grandparents lived two houses down, and had an available garage (our house had one with a utility/washing room + storage roon, with no room for cars), so I at least had a place to work, sometimes.

P.S. I also did my younger brothers' cars, too, and also got the job of getting license plates and tags for everyone, all at once, one day a year (I often had to spend up to eight hours in line). And, I also dropped everyones' tax returns off at the post office drop-point (open 'til midnite). I was the "good son", and so it went.

My parents and grandad (grandmother hadn't driven since 1935) had helium feet (as opposed to my lead foot), so their engines woul get carbon-ed up, a lot. It didn't help that my grandad was a big Champion Sparkplug fan, and bought them in bulk for the family...I switched to better brands...A/C, NGK, Bosch later on), and all the cars had points ignitions, so no hot spark, at that time.

So, the first thing I'd do, is drive a car down to the freeway, do several full-throttle runs during a 10 mile loop, and then see whether it ran better. If not (though the "italian Tune-up" usually helped a lot) then I'd repeat the run, using Marvel Mystery Oil (in both carb and tank, killing mosquitoes with the resultant oil fog). After that, I'd do points & plugs. And change the oil (this maintenance cycle was done every 3k miles). Rinse/repeat.

I liked the use of additives from that early date, and have continued to use them ever since. My personal cars never seemed to get carboned-up (lead-foot, long commutes, drag racing, you know), but I'm sure modern ignition parts, and better gas and oil, all help a lot.