1976 Bus - Test for AFM? ** It's still bucking!!**

Carbs & F.I.

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vwlover77
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Post by vwlover77 » Sun Jul 12, 2009 6:26 am

My opinion is that is not the case. When you remove the cap, an air leak is introduced.
Don

---------------------------
78 Westy
71 Super Beetle Convertible Autostick

"When we let our compassion go, we let go of whatever claim we have to the divine." - Bruce Springsteen

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Amskeptic
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Post by Amskeptic » Sun Jul 12, 2009 9:50 am

TrollFromDownBelow wrote:I'm a little confused ..... thought I saw a post on IAC that effectively stated if there is a significant change in idle speed (assuming an increase in idle speed) when you remove the oil filler cap that that was an indication of bad things.... i.e. vacuum leaks and leaky valves.

Is this not the case?
Perhaps a lean condition that drops the idle horribly?
Look it up the questionable post and paste it here. I cannot bear the thought of inconsistent advice confusing our esteemed customer base.
Colin
BobD - 78 Bus . . . 112,730 miles
Chloe - 70 bus . . . 217,593 miles
Naranja - 77 Westy . . . 142,970 miles
Pluck - 1973 Squareback . . . . . . 55,600 miles
Alexus - 91 Lexus LS400 . . . 96,675 miles

vdubyah73
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Post by vdubyah73 » Sun Jul 12, 2009 4:36 pm

my idle dropped and loped when the oil cap was removed. I knew then and now that the idle changed due to the crankcase ventilation being open to the atmosphere with the cap off. But I never made the connection in my own head that the sucked in valve cover gasket was a vacuum leak. So the advice is... when looking for a tuning symptom that resembles a vacuum leak don't forget to look at valve cover gaskets. Really who'd a thunk it, a valve cover gasket can cause a vacuum leak in the early VW FI system. A vc gasket can probably cause problems with modern systems as well, although the modern ones are more adaptive.
1/20/2013 end of an error
never owned a gun. have fired a few.

TrollFromDownBelow
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Post by TrollFromDownBelow » Mon Jul 13, 2009 9:59 am

I have deyxlesic memory ... post below...

viewtopic.php?t=4936&highlight=oil+cap+idle
1976 VW Bus aka tripod
FI ...not leaky, and not so noisy...and she runs awesome!
hambone wrote: There are those out there with no other aim but to bunch panties. It's like arguing with a pretzel.
::troll2::

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chitwnvw
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Post by chitwnvw » Mon Jul 13, 2009 10:54 am

So if I understand correctly a lot of sucking in the case is good sign. As everything wears and air is allowed to past the valve guides, past rings, this sucking will lessen.

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vwlover77
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Post by vwlover77 » Mon Jul 13, 2009 11:08 am

Yup!
Don

---------------------------
78 Westy
71 Super Beetle Convertible Autostick

"When we let our compassion go, we let go of whatever claim we have to the divine." - Bruce Springsteen

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chitwnvw
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Post by chitwnvw » Mon Jul 13, 2009 12:56 pm

Might even be seen as the backwards leak down test.

TrollFromDownBelow
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Post by TrollFromDownBelow » Tue Jul 14, 2009 6:17 am

Just as an update - I think I may have been struggling with a worn AFM - had high speed bucking issues that I may have been confusing with a mixture problem (upper 1/3 of wiper area almost completely worn off). When Colin originally cam out ... 4 years ago he said the AFM was pretty worn, and suggested I buy a new one. Bit the bullet and ordered a rebuilt one from busdepot ... stay tuned...
1976 VW Bus aka tripod
FI ...not leaky, and not so noisy...and she runs awesome!
hambone wrote: There are those out there with no other aim but to bunch panties. It's like arguing with a pretzel.
::troll2::

TrollFromDownBelow
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Post by TrollFromDownBelow » Fri Jul 17, 2009 9:15 pm

installed the AFM. Noticeable improvement in throttle response...but still has the high speed bucking... :pale: It runs fine for about an hour then poof... starts bucking/hesitating unless I drop the speed to 60 or less...seems more noticeable in cooler weather? I also replaced the coil tonight as well ... didn't help.

Other symptoms ... high speed oscillating idle, and noticable miss at idle, and rotten mpg.

everything that I've checked, that's in spec:

valve clearance
dwell
timing
compression
fuel pressure
vacuum leaks at hoses and valve covers...

I'm starting to get a wee bit frustrated .... I'm still thinking it's sucking air..whadya all think?
1976 VW Bus aka tripod
FI ...not leaky, and not so noisy...and she runs awesome!
hambone wrote: There are those out there with no other aim but to bunch panties. It's like arguing with a pretzel.
::troll2::

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vwlover77
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Post by vwlover77 » Sat Jul 18, 2009 5:24 am

I once had what I guess would be called "high speed bucking". It felt like the engine was being switched on and off rapidly at highway speeds. It turned out that the brand-new Bosch plug wire set had a defect in the wire from the coil to the distributor. Another set of wires fixed it.

But that doesn't explain a high-speed oscillating idle.

Have you blocked all of the hoses to the decel valve as a test?

Have you replaced the Temp Sensor II?

Have you very very very carefully examined the S-boot for cracks?
Don

---------------------------
78 Westy
71 Super Beetle Convertible Autostick

"When we let our compassion go, we let go of whatever claim we have to the divine." - Bruce Springsteen

vdubyah73
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Post by vdubyah73 » Sat Jul 18, 2009 1:56 pm

Have you tried a pair of Weber's? :geek: I like mine, runs smooth all day every day and easy to trouble shoot, if you even have to. Dual ICT's make it run like an old reliable industrial engine. Starts everytime and goes.

On the other hand it may not even be FI. It may be electrical, and you've been barkin up the wrong tree.
1/20/2013 end of an error
never owned a gun. have fired a few.

TrollFromDownBelow
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Post by TrollFromDownBelow » Sun Jul 19, 2009 7:31 am

vwlover77 wrote:I once had what I guess would be called "high speed bucking". It felt like the engine was being switched on and off rapidly at highway speeds. It turned out that the brand-new Bosch plug wire set had a defect in the wire from the coil to the distributor. Another set of wires fixed it.

But that doesn't explain a high-speed oscillating idle.

Have you blocked all of the hoses to the decel valve as a test?

Have you replaced the Temp Sensor II?

Have you very very very carefully examined the S-boot for cracks?
New TS II about 2 mos ago, checked the S boot carefully about 1 mos ago when I pulled the Plenum off.

Have not blocked off the Decel valve... yet! But will.

My biggest issue from a driveability aspect is the high speed bucking.
1976 VW Bus aka tripod
FI ...not leaky, and not so noisy...and she runs awesome!
hambone wrote: There are those out there with no other aim but to bunch panties. It's like arguing with a pretzel.
::troll2::

TrollFromDownBelow
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Post by TrollFromDownBelow » Sun Jul 19, 2009 7:25 pm

Replaced points and condensor today - got rid of a miss at idle and improved ease of starting, but still bucking.

I'm wondering if I have a vacuum leak around an injector, or maybe a poorly operating injector. I have a high pitched whistling /squealing sound from the 3/4 bank. .. anyone have any thoughts on this?
1976 VW Bus aka tripod
FI ...not leaky, and not so noisy...and she runs awesome!
hambone wrote: There are those out there with no other aim but to bunch panties. It's like arguing with a pretzel.
::troll2::

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Amskeptic
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Post by Amskeptic » Wed Jul 22, 2009 11:51 am

TrollFromDownBelow wrote:My biggest issue from a driveability aspect is the high speed bucking.
Try these three steps just for fun:

Note black cog position. Go six clicks lean (clockwise).
Test drive. Better? Worse? Same?
keep it there/bring it back/keep it there

Hook up timing light to center coil wire. Rev engine to 4,000 rpm.
Is flash consistent?

Test resistance of rotor (5 kohms) and spark plug connectors (1 kohms)
and check that you have non-resistance plugs (no "R" in part #) and non-resistance wires.

Do we know the flow rate of your pump? It needs a minimum 36psi pressure when engine is off, 28 psi at idle, yes, but it has to maintain that pressure whilst actually flowing at spec (1 liter in 30 seconds?) (that's a 2 liter coke bottle in one minute). I had a pump that gave great pressure readings but the minute it needed to actually flow, the pressure dropped precipitously. That was a nasty restriction.
Colin
BobD - 78 Bus . . . 112,730 miles
Chloe - 70 bus . . . 217,593 miles
Naranja - 77 Westy . . . 142,970 miles
Pluck - 1973 Squareback . . . . . . 55,600 miles
Alexus - 91 Lexus LS400 . . . 96,675 miles

TrollFromDownBelow
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Post by TrollFromDownBelow » Fri Jul 24, 2009 5:34 am

I have my fingers very tentatively crossed.... intake manifolds were ubber loose ... infact one nut had completely vibrated off. All of the nuts were barely finger tight. Took it to work this morning and no bucking at freeway speeds, or under hard acceleration. Doesn't quite have all of the power I think it should have, but need to finish dialing in timing and dwell. Still want to do a flow test.

It will be interesting to see if I get a bump in mpg ... best I've got is 16 mpg.
1976 VW Bus aka tripod
FI ...not leaky, and not so noisy...and she runs awesome!
hambone wrote: There are those out there with no other aim but to bunch panties. It's like arguing with a pretzel.
::troll2::

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