1.9L engine miss at 2800 rpm?

[jlrftype7]

[TomInAlaska]

An update after yesterday and today's tinkering session: the miss at ~2800 rpm is still there. Today I cleaned up the very corroded socket on the (what I assume is the original) coil, installed a "correct" distributor cap (late style with pins to match the new wire set), changed the terminal on the coil end of that wire to work with the early style coil so everything actually clicks together, and installed a new ground strap from the starter mounting bolt to the body for good measure. There is still a distinct miss / hesitation every time I rev up through 2800-3000 rpms. My next step is going to be to pull the throttle body and see exactly how badly worn it is and try to do something about it if its bad which I suspect is the case.

In my research into the L-Jetronic system, I've found some evidence that in at least some applications, the signal from AFM maxes out at about 3k and then the ECU runs strictly off the internal map. So, if that's the case with our engines, it almost makes sense that a very worn throttle body (and/or vacuum leaks) could cause a very abrupt change in how much fuel the ECU is sending when the revs are passing that changeover point, right? In my mind, if the map in the ECU thinks that the engine is only getting the amount of air that it did from the factory when everything was tight and new but now with all the wear and tear there's actually more air going flowing in than the map anticipates, the ECU will sharply cut the fuel to the ideal factory settings as soon as the signal from the AFM maxes out causing an abrupt miss or hesitation. Or am I grasping at internet straws?
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'73 Super Beetle (resto-mod/rally look, 1776)

'84 GL adventure van project (1.9L, 4speed) https://www.thesamba.com/vw/forum/viewtopic.php?t=755844&highlight=

[Wildthings]

[TomInAlaska]

I guess my question is, will the amount of air flowing through a significantly worn throttle body be different enough at the ~3000 rpm transition point when the ECU starts metering fuel based on its internal fuel map instead of the signal from the AFM (if that’s even a thing that’s happening) to cause an abrupt decrease in the fuel delivery and cause the miss or hesitation that I and at least a few others are seeing?

My first thought is, almost certainly not. But…?
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'73 Super Beetle (resto-mod/rally look, 1776)

'84 GL adventure van project (1.9L, 4speed) https://www.thesamba.com/vw/forum/viewtopic.php?t=755844&highlight=

[DanHoug]

Tom- were this mine, i'd want to pin down whether the miss is fuel or spark related. to do that, attach a timing light to the coil lead and either rev the engine stationary if that induces the miss or bring the leads out from the engine cover, tape the timing trigger down so it is always on, and then go for a drive watching the light in the rearview mirror.

if the light goes black or fumbles for flash at 2800, you have an ignition/ECU problem.

the same can be done with a noid light between your injector and harness if the miss occurs while just revving the engine. would have to make an extension harness to watch it while going for a drive or have a helper look at it from the backseat with the engine cover off.

these two tests, 10 minutes in total, will give you a very specific place to start troubleshooting because right now you don't know if the miss is from the spark dropping out, the signal to the fuel injector dropping out, or if those two parameters are good, something entirely different.
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-dan
60% of what you find on the internet is wrong, including this post.

'87 Westy & '89 Westy both 2.1 4spd

Past projects can be found at--
www.thefixitworkshop.com

[djkeev]

Tom, I'm no expert at tracing misses, but you said you clean out the corrosion from your coil, you allude to it being old, possibly original.

If it we I, that corroded coil would be replaced already.

It may not be your 2800 rpm miss, but it for sure is an item needing replacement anyway.

Dave
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Stop Dead Photo Links how to post photos

Ghia
http://www.thesamba.com/vw/forum/viewtopic.php?t=392473

Vanagon
http://www.thesamba.com/vw/forum/viewtopic.php?p=6315537#6315537

Beetle
https://www.thesamba.com/vw/forum/viewtopic.php?t=482968&highlight=74+super+vert

[TomInAlaska]

Dan,
This afternoon I'll definitely put my timing light on and see if there's a spark issue and, if I can put together a tester for the injectors, that too.

Dave,
A replacement Bosch coil should be here later this week. Sadly, I couldn't find one (or even an aftermarket replacement) here in town.
_________________
'73 Super Beetle (resto-mod/rally look, 1776)

'84 GL adventure van project (1.9L, 4speed) https://www.thesamba.com/vw/forum/viewtopic.php?t=755844&highlight=

[Wildthings]

If you haven't done so already, try unplugging the wires from the TPS (Throttle Position Switch)

[4Gears4Tires]

Sounds like the only thing you haven't replaced is the ECU.

I would have originally guessed timing data loss, but you replaced the distributor and that includes the hall sensor. My next guess after the distributor would be the ECU.
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'87 Syncro
Ferric Oxyhydroxide Superleggera Edition

[TomInAlaska]

[djkeev]

TPS are clicking or did you verify electrical connection with a meter?

Dave
_________________
Stop Dead Photo Links how to post photos

Ghia
http://www.thesamba.com/vw/forum/viewtopic.php?t=392473

Vanagon
http://www.thesamba.com/vw/forum/viewtopic.php?p=6315537#6315537

Beetle
https://www.thesamba.com/vw/forum/viewtopic.php?t=482968&highlight=74+super+vert

[4Gears4Tires]

[Abscate]

Have you isolated the miss to a cylinder or is it a complete loss of power?

Does she stumble at 2800 or jerk occasionally?
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.ssS!

[TomInAlaska]

Dave,
TPSes were verified with a multimeter at the switches and at the ECU plug.

Abscate,
Its a very distinct miss/hesitation/stumble/momentary loss of power at exactly the same rpm or very small range of rpm every time I rev it through ~2800rpm. 100% repeatable and predictable and happens when stationary or driving and under all load and throttle conditions (well, all the throttle conditions that would make the engine rev past 3k anyway)

More information to add to the puzzle: looking at other topics on timing and distributors and misses/hesitations led me to check the wear in my distributor and I found that while it is very tight radially, I have about 5mm of axial/end play. I then put my timing light on and watching the flash as I revved the engine but could not make out any change in the frequency of the flashes during the miss/stumble event. I tested the light on the coil wire and on each cylinder wire for multiple rev cycles. For good measure and since I had the timing light hooked up, I checked the actual timing itself and found it to be a little bit off of the "~35degree all in / Tencentlife method" so I made a fresh mark at 2" CW from the 'U' mark on the second pulley ridge and set the timing there or a touch advanced when all the advance was in. This had the idle timing near the 'U' after I reset the idle speed to about 850rpm using the big screw on the throttle body. The engine seemed to idle smoother and have a little less rpm fluctuations up and down like it had previously and seemed to rev faster and smoother after all these tests and changes just now. However, the miss/hesitation/stumble is STILL there, but it does seem to be improving (might just be my imagination and/or wishful thinking). I'll try to actually drive the van in the next day or so and see how it does.
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'73 Super Beetle (resto-mod/rally look, 1776)

'84 GL adventure van project (1.9L, 4speed) https://www.thesamba.com/vw/forum/viewtopic.php?t=755844&highlight=

[Abscate]

That’s pointing away from fuel delivery and filter issues.

If you are comfortable, crack the top and inspect the AFM TRACK per the post above
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.ssS!

[TomInAlaska]

[Abscate]

Yes, it should be smooth. I also don’t like that cap wiring very much
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.ssS!

[TomInAlaska]

[Abscate]

Waiting on the coil replacement. Ignition wires age, history too?
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.ssS!

[TomInAlaska]