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Whats the cause of this variable but consistent miss
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timo78
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PostPosted: Sat Mar 15, 2014 12:47 pm    Post subject: Whats the cause of this variable but consistent miss Reply with quote

Howdy,

Intermittent issues are the hardest for me to ID.

I've get this random miss/hesitation. But it's context is always the same. It only occurs when I am gassing it, WOT, high intake draw scenarios. It generally hits then it goes away, it is a brief momentary stumble, but hasn't often shown to be a miss, miss, miss type scenario, only a single drop. Weird and an annoyance.
I'm thinking intake leak so....

Vacuum/Intake tests:
-I hooked up a vacuum gauge to the plenum, and get 17-18 fully warmed up. Good
-Gassing it drops to 2, release swings it to 23ish, then returns to 17-18. Good
-Maybe the EEC. A 'Pull' on that held up to 20. unsure but assume ok?
-Also hooked VG to throttle body, 5-thru-10 when throttling depending on aggressiveness
-Distributor holds up to 10, I went no further not wanting to harm

Fuel tests:
The rail gauge shows 28 at idle, and 38 on WOT. assume ok?

I passed over ignition related stuff because it fires right up like a champ even when coldcold. Runs good at speed, and even after initial little miss it'll still climbs good [albeit slow:] I've got a great testing neighborhood. Lots of hills.

What am I missing? Your suggestions are appreciated.
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Busdriver79
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PostPosted: Sat Mar 15, 2014 1:33 pm    Post subject: Reply with quote

This sounds ignition related...check plug wires for cuts,and grounding...the condenser might be on its way out...check the obvious coil connections, distributor cap and rotor...maybe you should go ahead and tune it up... Rolling Eyes
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PostPosted: Sat Mar 15, 2014 2:30 pm    Post subject: Reply with quote

Points or electronic replacement? This is EXACTLY how my electronic points behaved before it became more frequent misses. The trash can solved that problem.
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timo78
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PostPosted: Sat Mar 15, 2014 3:34 pm    Post subject: Reply with quote

Yeah It's is in proper tune. That's part of the frustration. The other is the randomness of occurrence.

It only happens when gassing it under LOAD. I can't get it to miss in testing parked on the curb. Ignition checks out [7.5 idle 28all-in] . It's got pertronix, and a brand new Bosch blue coil. I've had two coils fail in past, but they just went kaput ..not slowly. I have a spare and can test against that.

The last time I had these general symptoms it was a hairline crack/leak in the plastic coupler/gasket between the plenum >>intake tubes,>> and the head. I did drift propane over the whole engine and couldn't get it to spin up, so I did the other tests above.

Took it out today and tried to get to miss and it wouldn't. Its very random in occurrence.

Any other suggestions?
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PostPosted: Sat Mar 15, 2014 5:08 pm    Post subject: Reply with quote

It would be easy to switch to points to rule that out if there is no other suggestions. Mine would only miss under load on a hot day. Really frustrating.
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PostPosted: Sat Mar 15, 2014 5:55 pm    Post subject: Reply with quote

So interesting you mentioning heat. That was another thing I noticed in the context of when it happens most/worst. Flawless today, but it's cold out. I was TRYING to make it happen. Gassing the crap out of it all over town.
I may just throw some points in there to test. I like the PX setup and that would be a shame, but oh well running on the old school is better than questionable with the high tech. Thanks
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PostPosted: Sat Mar 15, 2014 6:02 pm    Post subject: Reply with quote

timo78 wrote:
So interesting you mentioning heat. That was another thing I noticed in the context of when it happens most/worst. Flawless today, but it's cold out. I was TRYING to make it happen. Gassing the crap out of it all over town.
I may just throw some points in there to test. I like the PX setup and that would be a shame, but oh well running on the old school is better than questionable with the high tech. Thanks


Mine wasn't a pertronix brand, but it was 112* out on the way from Phoenix to Los Angeles and I had a deadline. It stumbled like a single cylinder miss like you described, and I didn't want to deal with it so I replaced points since I had them. Later when I had more time I put the module back in and the miss came back with extended heat and load. It got worse so I trashed the module. It was free as part of a promotion, but I won't buy another module outside of Mallory's system. They're the only ones I've tried that have perfect accuracy between the four cylinders.
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PostPosted: Sat Mar 15, 2014 6:24 pm    Post subject: Reply with quote

asiab3 wrote:
Mine would only miss under load on a hot day. Really frustrating.

But wasn't yours some off the wall oddball brand and how does that relate to a Pertronix? Wasn't it even missing any identification on the box. Whatever brand it was they sure wern't proud of their product. Maybe that particular brand didn't like heat that day but that's sure not the situation with my Pertronix experience of running in some actual real heat enviornment.


I wouldn't say it's impossible for a Pertronix to ever fail but mine have been maybe the most dependable auto engine item Ive used. With a 2 year guarantee if one did go bad it would be good to replace it versus thinking that they are bad in general.
Last year I did have a wierd ignition problem and did try switching in some points. But the problem also continued until finding that it was some unrelated issue.
When hearing of intermittent Pertronix problems I've gotten to where I just don't buy it. Now a total failure is different. Hooked up backwards smoke quickly comes out of the distributor. Laughing
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PostPosted: Sat Mar 15, 2014 10:30 pm    Post subject: Reply with quote

Desertbusman wrote:
asiab3 wrote:
Mine would only miss under load on a hot day. Really frustrating.

But wasn't yours some off the wall oddball brand and how does that relate to a Pertronix? Wasn't it even missing any identification on the box. Whatever brand it was they sure wern't proud of their product. Maybe that particular brand didn't like heat that day but that's sure not the situation with my Pertronix experience of running in some actual real heat enviornment.


Yeah mine definitely wasn't a well-known brand. I also only recommended trying points since nobody else seemed to chime in, it's quick and free assuming he kept the old parts. And I bet the Bosch Fairies didn't switch his wires in the middle of the night to get him to buy points either Twisted Evil
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PostPosted: Sat Mar 15, 2014 10:39 pm    Post subject: Reply with quote

True that, putting the old points and condenser back (temporarily Wink )in is the cheap easy way to see what's up.

OP, I'm really glad that you revised your earlier post Wink
That was absolutely totally out of line.
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PostPosted: Sat Mar 15, 2014 10:44 pm    Post subject: Reply with quote

Desertbusman wrote:
(temporarily Wink )


Yeah, the 009 I got from a trash can "temporarily" lasted over seven thousand miles while I troubleshooted something else. When I got it out on the workbench last week to clean it up I found out it was a good German one!

OP- let us know what you decide to do. I don't personally know how your FI tests check out, but have you checked the condition of your fuel filter? (Does that even apply to FI systems?)
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PostPosted: Sat Mar 15, 2014 10:47 pm    Post subject: Reply with quote

How old is your FPR? I would check that first.
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PostPosted: Sun Mar 16, 2014 10:56 am    Post subject: Reply with quote

Hoody wrote:
How old is your FPR? I would check that first.


It's a few years old, and I have the original in the spares box. It seems to check out with the values noted above.

While I had dismissed the ignition. Being that AsiaB3 had similar circumstance and it was ignition related, I will likely do a swap and see if that corrects it. I have a brand new points/condenser set in the spare box. I've had one PrX fail in the past 20years of ownership, and that was due to leaving the key in the ON position for too long. Even then it still worked, but was "under energized" and would throw a yellow/orange spark. Made it a pain to start when cold, but still 'worked'.

I may do some remote monitoring of the engine with Vac/FuelP/Timing gauges hooked up. An idea I've been pondering using the Android tablet camera>spydroid software>streaming/recording to a laptop the gauge readings in back. Feel the dropout and then review the footage to see which gauge drops out in relation.

Thanks for all of the helpful responses.
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PostPosted: Sun Mar 16, 2014 3:17 pm    Post subject: Reply with quote

Today drove it like I stole it [if thats even possible with a bus]
70mi round trip in the rain. Which I personally love driving in for the extra cooling it applies. No issue ever showed itself. Random Gremlins!
I'll touch this thread back if/when it is corrected to close out. Thanks for all your suggestions.
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AB westy nut
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PostPosted: Tue Mar 18, 2014 2:27 pm    Post subject: Reply with quote

Sounds similar to the issues I documented on my bus last fall. Have you checked your AFM? If you can get your hands on a spare, it's an easy swap just to see if you can replicate the condition. I haven't solved my issue yet as winter came and the bus went into storage, so I share your frustration!
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PostPosted: Tue Jul 29, 2014 10:46 am    Post subject: Reply with quote

Continued:
Hesitation-Missing when "gassing it!!" and poor performance. In this instance appears due to a blown distributor vacuum advance can.
Got some time off work, and went back through the systems again recently. In this round the Vac Can bleeds out, albeit slowly. Weird because the can was holding charge during earlier tests. The part# stamp is 581.
[edit] Thank you Karl. 581 corresponds to 07 060 -the spec vac can. >> http://www.thesamba.com/vw//forum/archive/index.php/o-t--t-177103--.html

Now the big question for the sage experts is: Where can I find a replacement? Bus Depot is out of stock on the Bosch originals, and could not recommend a universal. Although "visually" the universals look just like what I've got. CIP1 has universals, [noted for Beetles]

Any suggestions on where a new OG/reman unit can be obtained for a 78 Type4 application?

Thanks!

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PostPosted: Tue Jul 29, 2014 3:37 pm    Post subject: Whats the cause of this variable but consistent miss Reply with quote

when a vacuum can began to fail on me, it would die at stops and stall out
while reversing, like I didn't know how to drive stick + hard to start, like draw
the battery down hard to re-start. If this is not happening to you, check the
resistors at the end of your wires, the two short ones on cyl 1&3, two long
on cyl 2&4, they can get weird when heated and start dropping spark to that
cylinder. Usually only one is reading weird, the rest will be 4K or 5K ohms.
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PostPosted: Tue Jul 29, 2014 3:45 pm    Post subject: 021 905 205P Reply with quote

http://www.oldvolkshome.com/ignition.htm#B1974MDC7

07 060

http://www.ratwell.com/technical/FAQ/FAQContent.html
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PostPosted: Tue Jul 29, 2014 4:04 pm    Post subject: Reply with quote

timvw7476
It's almost certainly the vac can in this instance. Thanks though. I see you sell stuff. Ya got any vac cans with 581 stamped on the armature that hold a vac charge? Very Happy

TCash
Thanks ! I was reading the OldVolks earlier today and noting how wildly different the cans are in their advance/retard characteristics.
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PostPosted: Tue Jul 29, 2014 4:43 pm    Post subject: Whats the cause of this variable but consistent miss Reply with quote

Sorry, just have a can from '76 that I'm using since the other blew. The
Itinerant air cooled guy was going to try repairs of blown cans with sections of rubber, butyl- stuff if I recall.. don't know if he was successful at it or not,
getting the can apart would destroy the rolled bead that seals it. Plenty of people would like instructions on that repair how-to!
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