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Flat spot on FI automatic
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t-royR
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PostPosted: Sun Mar 03, 2024 11:16 am    Post subject: Flat spot on FI automatic Reply with quote

New post to spin off of my old post that was poorly titled.
Here is that post:
https://www.thesamba.com/vw/forum/viewtopic.php?t=...e4ed3a28b7

Good news is I am working my way through found issues. I am still scratching head over some idle adjustment issues and this flat spot I hit when I get to 1900 rpms. I found that the hose from the pressure sensor in the engine bay, to the plenum drivers side, (that makes its way through the plenum, out the otherside and onto the vacuum unit on the tranny), was not connected. My major bad on that when I reinstalled the plenum last December. Hooked it up and I do have better control on setting the idle. However, to be noted, I can't get the idle to go up past 1200 rpms, no matter how far I open the idle adjustment screw. But, I can get the idle to drop way down, (below 500) when turning the idle screw in.

I need a higher idle rpm so when this automatic in gear at a stop light, the engine runs high enough to move the oil around. If I set the idle lower, the engine slows down too much when in gear at a light, and risk overheating.

BIG question I still want to answer is what is causing this flat spot when I get to 1900 rpms. It is consistently at this rpm that this drop off happens. If I cruze by 1900, no problem. it will sit nicely at 2100 without issue. But again, if I creep up to 1900, when it gets there, with my foot no changing position on the pedal, the rpms drop to around 400, then bounce back to 1900, then back down to 400, and on and on, until I change the position of the pedal.

Could it be the vacuum unit/module on the tranny?
Could it be the mechanical vacuum advance in the dizzy acting funny?
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PostPosted: Sun Mar 03, 2024 12:27 pm    Post subject: Re: Flat spot on FI automatic Reply with quote

It's a head scratcher for sure. For possibly irrational reasons I am suspecting the throttle valve switch, even though this is an A/B model with the simple switch. Over the decades I have seen these cause a couple weird running issues that they "shouldn't". Get the car started and warmed up, then disconnect the TVS and test.

The transmission modulator valve and the vacuum advance are easy enough to test by removing the vacuum lines and plugging them or pinching them and see if you get different results on an idle and 1900 RPM test.

Another possible suspect is your manifold pressure sensor. If you have a spare, swap it out.

Remember the process of elimination, though. Establishing what's good is just as useful as pinpointing what isn't- plot it out on paper and make notes- don't jump around and get confused. If you need parts PM me.
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PostPosted: Sun Mar 03, 2024 2:46 pm    Post subject: Re: Flat spot on FI automatic Reply with quote

Thanks Tram. You nailed it!!
I warmed it up, observed the same issue with the drop of at 1900. Then, disconnected the Throttle valve switch. Issue went away. And, the engine idles much better. When I revved it up to 3000, and then removed foot from pedal, it came down to idle nice and calmly. Before disconnecting the valve switch, it would drop off quick and low, before climbing back to idle.
Thanks again!!!!

Now, do I replace it? I have another one.
Troy
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PostPosted: Sun Mar 03, 2024 2:59 pm    Post subject: Re: Flat spot on FI automatic Reply with quote

I want to mention that I reconnected the valve switch again and observed the issue again when hitting 1900. Kind of confirms the cause to be that valve switch.
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PostPosted: Sun Mar 03, 2024 3:09 pm    Post subject: Re: Flat spot on FI automatic Reply with quote

There is a fuel cut-off feature when the throttle is closed, which hits at around 1800-1900 rpm.
The simple throttle switch tells the ECU when the throttle is closed. The switch may be broken or miswired.
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PostPosted: Sun Mar 03, 2024 4:03 pm    Post subject: Re: Flat spot on FI automatic Reply with quote

t-royR wrote:
New post to spin off of my old post that was poorly titled.
Here is that post:
https://www.thesamba.com/vw/forum/viewtopic.php?t=...e4ed3a28b7

Good news is I am working my way through found issues. I am still scratching head over some idle adjustment issues and this flat spot I hit when I get to 1900 rpms. I found that the hose from the pressure sensor in the engine bay, to the plenum drivers side, (that makes its way through the plenum, out the otherside and onto the vacuum unit on the tranny), was not connected. My major bad on that when I reinstalled the plenum last December. Hooked it up and I do have better control on setting the idle. However, to be noted, I can't get the idle to go up past 1200 rpms, no matter how far I open the idle adjustment screw. But, I can get the idle to drop way down, (below 500) when turning the idle screw in.

I need a higher idle rpm so when this automatic in gear at a stop light, the engine runs high enough to move the oil around. If I set the idle lower, the engine slows down too much when in gear at a light, and risk overheating.

BIG question I still want to answer is what is causing this flat spot when I get to 1900 rpms. It is consistently at this rpm that this drop off happens. If I cruze by 1900, no problem. it will sit nicely at 2100 without issue. But again, if I creep up to 1900, when it gets there, with my foot no changing position on the pedal, the rpms drop to around 400, then bounce back to 1900, then back down to 400, and on and on, until I change the position of the pedal.

Could it be the vacuum unit/module on the tranny?
Could it be the mechanical vacuum advance in the dizzy acting funny?



No.

You do not need an idle up at 1200 rpm. I am on the road and have none of my books with me but the automatics should idle at ~900-ish by memory.

If the idle is dropping too far when you are in gear...other things are out of adjustment.

The idle screw is just a SMALL air bypass. Yes, the vacuum unit could either be the wrong one or out of adjustment. In fact, I pretty much tell everyone with a new to them 003 transmission to adjust the bands and put the gauges on the main pressuer anddo the driving and adjusting tests in the Bentley.

For most 003...they have never been readjusted since they were built and now have a zillion miles and years on them and usually a totally different engine than what they came with. But for the moment you want to make sure its just not leaking like Tram noted.

So you have the simple to wire TVS? It can commonly with age be out of adjustment. I am also assuming you have the B MPS. Make sure it does not leak vacuum for the moment. Does it still have the anti-tamper cover on the adjusting screw? If not....its likely out of adjustment.

If you do have the simple B type MPS....do you also have the full load switch under the intake runners?

Ray
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PostPosted: Sun Mar 03, 2024 5:46 pm    Post subject: Re: Flat spot on FI automatic Reply with quote

Hey.
Before I read your response Ray, I went ahead and swapped out the switch in the TVS (I opened it up and removed the switch and put the other in). Closed it up, plugged in the MPS and ran the car. At 1900 rpm, behaved just like when the other switch was in. Not sure what to make of that. I had adjusted that first switch a few times over the years as I worked on this car. Just to get that switch to click at the right number of degrees per the manual. And, both switches when in the car give an nice audible click when I open the throttle a bit. This is all just for information. don't know what to make of it all.

I used to have the idle set a bit lower, but last summer (it was really hot out), I was getting the engine light when I sat at any stop light for a length of time while in gear.

I'll move onto to checking transmission modulator valve and the vacuum advance.
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PostPosted: Sun Mar 03, 2024 5:47 pm    Post subject: Re: Flat spot on FI automatic Reply with quote

and when it was really hot and I got the engine light, the car was idling at around 500 rpms while in gear.
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PostPosted: Sun Mar 03, 2024 5:51 pm    Post subject: Re: Flat spot on FI automatic Reply with quote

the manual gives 950 += rpms for the automatic idle setting
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PostPosted: Sun Mar 03, 2024 5:51 pm    Post subject: Re: Flat spot on FI automatic Reply with quote

950 += 50, that is
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PostPosted: Sun Mar 03, 2024 6:01 pm    Post subject: Re: Flat spot on FI automatic Reply with quote

Ray, I am not following this from your earlier response. I'll read it several times till I get it. I'll post a picture as well which might answer some of the questions.

Here is what confused me.....
So you have the simple to wire TVS? It can commonly with age be out of adjustment. I am also assuming you have the B MPS. Make sure it does not leak vacuum for the moment. Does it still have the anti-tamper cover on the adjusting screw? If not....its likely out of adjustment.

If you do have the simple B type MPS....do you also have the full load switch under the intake runners?

Also, I don't understand that part about the throttle closing at 1900 rpms. I have the throttle pretty well open to get it to 1900. So, not sure what you man by that.
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PostPosted: Sun Mar 03, 2024 6:10 pm    Post subject: Re: Flat spot on FI automatic Reply with quote

Image may have been reduced in size. Click image to view fullscreen.

Image may have been reduced in size. Click image to view fullscreen.

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PostPosted: Sun Mar 03, 2024 7:16 pm    Post subject: Re: Flat spot on FI automatic Reply with quote

Ahhh. MPS. I had a brain-fart. This pic shows mine.
Image may have been reduced in size. Click image to view fullscreen.

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PostPosted: Sun Mar 03, 2024 7:16 pm    Post subject: Re: Flat spot on FI automatic Reply with quote

t-royR wrote:
Thanks Tram. You nailed it!!
I warmed it up, observed the same issue with the drop of at 1900. Then, disconnected the Throttle valve switch. Issue went away. And, the engine idles much better. When I revved it up to 3000, and then removed foot from pedal, it came down to idle nice and calmly. Before disconnecting the valve switch, it would drop off quick and low, before climbing back to idle.
Thanks again!!!!

Now, do I replace it? I have another one.
Troy


First do the adjustment procedure per Bentley. If the problem persists, try the other one and be sure to adjust it also.
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PostPosted: Sun Mar 03, 2024 7:54 pm    Post subject: Re: Flat spot on FI automatic Reply with quote

Here are a couple pics of my backup MPS. I checked vacuum on it and it holds. I'll check the one on the car to see if the same.
I don't have the Bentley. I have the Bentley but I don't find a section on adjusting the MPS. Please point me to the page if you know it.
Image may have been reduced in size. Click image to view fullscreen.


Image may have been reduced in size. Click image to view fullscreen.

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PostPosted: Sun Mar 03, 2024 8:18 pm    Post subject: Re: Flat spot on FI automatic Reply with quote

t-royR wrote:
Here are a couple pics of my backup MPS. I checked vacuum on it and it holds. I'll check the one on the car to see if the same.
I don't have the Bentley. I have the Bentley but I don't find a section on adjusting the MPS. Please point me to the page if you know it.
Image may have been reduced in size. Click image to view fullscreen.


Image may have been reduced in size. Click image to view fullscreen.


You're confusing the MPS and the TVS. The TVS, which you unplugged to solve the issue, is what we are discussing. Wink
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PostPosted: Mon Mar 04, 2024 3:51 pm    Post subject: Re: Flat spot on FI automatic Reply with quote

Thanks. I can adjust the TVS per the Bentley (done so before).

I was reading this from Ray and was wondering if there is an adjustment to do to the MPS.

From Ray...."I am also assuming you have the B MPS. Make sure it does not leak vacuum for the moment. Does it still have the anti-tamper cover on the adjusting screw? If not....its likely out of adjustment."
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PostPosted: Mon Mar 04, 2024 3:55 pm    Post subject: Re: Flat spot on FI automatic Reply with quote

I'm also a bit confused on this point/questions of RAy's
"If you do have the simple B type MPS....do you also have the full load switch under the intake runners?"

I am not familiar with the "full load switch" referred to. I am familiar with most of the FI parts, but haven't seen such reference. Is there another name for this switch that the Bentley would use?
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PostPosted: Mon Mar 04, 2024 4:27 pm    Post subject: Re: Flat spot on FI automatic Reply with quote

t-royR wrote:
I'm also a bit confused on this point/questions of RAy's
"If you do have the simple B type MPS....do you also have the full load switch under the intake runners?"

I am not familiar with the "full load switch" referred to. I am familiar with most of the FI parts, but haven't seen such reference. Is there another name for this switch that the Bentley would use?


It's a small square box with a small diameter vacuum hose attached to it. It should also have 2 wires attached as well. It was only used in 68 and 69 on type 3s, and for 70 got built into the MPS.
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PostPosted: Mon Mar 04, 2024 4:28 pm    Post subject: Re: Flat spot on FI automatic Reply with quote

First generation Type 3 FI used a separate switch to detect full throttle (full load). It's sometimes called the "pressure switch", not to be confused with the manifold pressure "sensor".
See pages 11, 21, 23, 28 here:
https://www.vwtypethree.com/downloads/pdf/Glenn-FI-Type3.pdf

And look for #85 on page 26.
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