Is this just the nature of the vane style AFM?

[Sagaris]

My car runs excellent. I get between 26-29 mpg, It accelerates well and sounds teriffic but when I am idling and blip the throttle, it doesnt seem to respond well at all. It revs just fine when I give it more throttle , but when its just a quick blip, not even to 1000rpm then it bogs and cuts out a bit. I am thinking that this is just the downfall of having a vane style AFM and that the flap inside the AFM cant respond accurately when it has something so sudden happening.

Again, car runs fine but I wonder if this is typical or not

Any thoughts?

[SpecM]

yea, that's just an AFM for ya

[jlevie]

That can be the result of an intake leak. Both my Spec E30 and my 87 325is, that I know to be in good condition with no intake leaks, respond as expected to throttle blips. So I submit that this car has some problem with the engine management system. A smoke test of the intake would be the first action to take.

[AnimalE30]

My car has the exact same problem. Never did a smoke test, did spray carb cleaner in every nook and cranny to find any leaks. Found a couple, fixed them, but the car still doesn't respond to blips too well.

[jlevie]

You probably still have intake leaks. A number of the places where an intake leak can occure won't respond to a spray of carb cleaner. Either because you can't saturate the area or because it is too far from the intake ports. Only a smoke test will find leaks at any of the possible locations.

Well, that or you can replace every hose, gasket, and seal that could result in an intake leak. The complete list of possible causes of an intake leak is:

Intake boot
Throttle body gasket
ICV hoses & connections
Brake booster, hoses, and connections
Crank case breather hose
Evaporative control hose and valve
Fuel pressure regulator & hose
Injector seals
Valve cover gaskets & bungs
Oil filler cap
Dip stick o-rings
Oil return tube o-rings

[Sagaris]

It is about a half second hesitation. I can blip the throttle, take my foot off the pedal, it cuts out and then it responds.

I have replaced the plugs, wires, cap, rotor, timing belt, O2 sensor (OEM) coolant temp sensor (OEM) , air filter (K&N panel filter) cleaned the ground wires, and done a visual inspection for vac leaks.

Again, it runs perfectly besides this minor response issue off-idle.

Im going to attempt to make my own smoke test apparatus this week and see if I can find anything.

[nando]

yea, that's just an AFM for ya

no, it's not. it's a result of something wrong with the engine, and partially from no acceleration enrichment. the movement of the flapper door is probably 2% of the total equation.

half a second is not normal. it should be maybe a 1/10th or so off idle.

[caliGreen]

You probably still have intake leaks. A number of the places where an intake leak can occure won't respond to a spray of carb cleaner. Either because you can't saturate the area or because it is too far from the intake ports. Only a smoke test will find leaks at any of the possible locations.

Well, that or you can replace every hose, gasket, and seal that could result in an intake leak. The complete list of possible causes of an intake leak is:

Intake boot
Throttle body gasket
ICV hoses & connections
Brake booster, hoses, and connections
Crank case breather hose
Evaporative control hose and valve
Fuel pressure regulator & hose
Injector seals
Valve cover gaskets & bungs
Oil filler cap
Dip stick o-rings
Oil return tube o-rings

good info.. my thoughts were the same as the thread starter.. good power but when i tap the gas makes a boging? just dont go Vroommm but tap to half throttle makes the vroom..

[Sagaris]

I checked my AFM resistor track and it has some pretty deep grooves. I attempted to move the wiper to an ungrooved part of the track but it didnt move hardly at all so I am not going to mess with it anymore.

I guess I will bust out the carb cleaner tomorrow until I can come up with a way to do a smoke test. Those intake manifold gaskets are going to be hard to spray.

[AnimalE30]

Could injectors be a slight cause of this? gunked up not being able to respond to quick inputs?

[jlevie]

Could injectors be a slight cause of this? gunked up not being able to respond to quick inputs?

No.

[LowR3V'in]

It shouldn't bog, but if you floor the thing as fast as you can
it's normal for there to be nothing going on for 1/2 a sec.

no acceleration enrichment.

The 1980s was a high tech era for bmw.

[Sagaris]

I blasted through a full can of carb cleaner while checking for vac. leaks

I had no real revelations except for a popping sound when I sprayed it down around the intake manifold, which was difficult to do even with the straw. The idle did not change when I heard the popping sound though.
A few months ago when I replaced my injectors, I noted that one of the bolts on the intake manifold was loose enough to turn with my fingers so I wouldnt be suprised if more intake manifold nuts were a bit loose so I took a look at that today. I took the airbox and intake boot off and tried to snug all the intake manifold nuts down, put it back together, and took it for a drive. The hesitation during sudden throttle blips off idle is somewhat improved and what I assumed was valve tick, is now reduced.

I know that when exhaust manifold gaskets are bad, you can hear the distinct ticking coming from them (See also: any early 90's Nissan V6)
but do bad intake manifold gaskets tick too???

Unless the evap. canister has some sort of leak in it, I am pretty sure my intake manifold gaskets need replacment. I think I will wait on it until I do a head rebuild.

[jlevie]

The use of carb cleaner doesn't mean that you don't have intake leaks. It only means that any leaks that are present didn't respond to to that test or you din't hit the leak with the spray. Find a shop that can smoke test the intake. That is the only sure way of finding any and all intake leaks.

[Sagaris]

I picked up a vacuum gauge and connected it to the vac. line on the FPR.
The gauge moved between 15.0-15.5 inHg every 2-3 seconds.

The Bently has a decent diagnostic chart for vac. at idle

According to the Bently, my readings indicate:
15-22 inHg:
Normal performance Engine in good condition

10-15 inHg:
a) incorrect ignition timing
b)incorrect valve timing
c)low compression
d)Leaking intake manifold gasket or fuel injector seals


since mine is 15 I am on the fence.
I do not see how a and b are relevant unless engine parts are mechanically damaged but c is possible (190k miles) and d is likely as well. The slight fluctuation throws me off. when the engine is revved the reading stays steady and doesnt jump around.

For anyone else that want to look into vac. gauge readings I came across these sites that may or may not be helpful.

http://autospeed.com/cms/A_2393/article.html

http://www.classic-car-magazine.co.uk/articles/fault-diagnostics-vacuum-gauge.html

just an update for anyone following or interested until I can do further testing