Stalling/bucking at mid-throttle and rpms

[BDorsey]

Hi all,

Hope no one minds a Vanagon in the Bay forum. I'm double posting since I have an air-cooled type 4 engine and am hoping some later Bay folks might have some advice.

1982 California, 2.0L air-cooled FI, newly rebuilt engine with 100 miles on it.

The problem is that when it gets to mid to high rpms (like just before a shift) and I give it a bit more gas, the engine stalls and/or bucks badly. I usually can't get through the stall by giving more gas. At lower rpms, I can press the pedal all the way with no stalling.

I have done every test in the Bently and the AFC service document for fuel delivery and ignition except the O2 sensor b/c I don't have a CO meter, or the spray pattern test. All tests are within specs except the full throttle speed limiter (not the WOT switch, the "relay" that controls the switch on Cali vehicles).

I rewired the FI loom, replacing almost all connections and all wires that showed any wear or damage. I have new injectors, a rebuilt distributor by SparksWerks, new plugs and wires. I've inspected the tracks and tested the voltage across the sweep of the AFM. I've tried a new coil and installed a new ignition control module (I thought the original failed the test but a second test showed it was ok). Timing is set correctly for Cali vehicle as is the idle. I have checked for vacuum leaks visually and with propane but found none. I have adjusted the valves properly for hydraulic lifters.

I have learned a ton about the AFC FI system so that is a positive, but I am stumped and frustrated. The only thing I can think of now is the ECU. But, I would really appreciate any ideas before I go that route.

Thanks much for any help.

Cheers,
Brian
_________________
1982 2.0L Aircooled Country Homes Camper

[timvw7476]

I'm going to assume the ignition is electronic. So that leaves the resistor in the rotor, embedded in epoxy, the carbon button inside the distributor cap. and all four 2k resistors built into the wire ends. (they unscrew ) Also leaves the wires, possible breaks in them, & the plugs themselves.
The one thing outside ignition gear would be fuel pressure regulator. If there's fuel leaking from the spigot end where vacuum line goes, it is bad.
Oh, classic ground corrosion can do weird stuff, as can less than ideal battery voltage. Erratic operation troubleshooting is always a fun one!
oh, do check the connections to the blades on the coil, I've seen a tired solderless connector or two cause bad stuff. Same for carbon-insulated sparkplug wires, start falling apart, dropping carbon in sparkplug wire wells in the dist. cap, misfire city.

[aeromech]

I’m impressed

This is going to be a tough one because you’ve changed so many parts and with only 100 miles you’re basically still doing a test run.

I’m my experience, you are headed down a very common trail. That means, you have figured out in your mind that it must be something either difficult or expensive. I’ve been guilty of falling into that trap myself many times.

My advice is to work the problem by heading down one possibility at a time until you’ve completely eliminated it. For instance, fuel delivery. I’d hook up a fuel pressure gauge and go for a drive. What’s the pressure when the engine cuts out?

I’d do a smoke test and do it completely. If you don’t get that test right you can miss leaks.

I’d try and barrow an ECU. I have a couple but they are for water cooled. I have an ECU test box but that’s also for later vanagons. Keep in mind that it could be something as simple as a bad rotor. I had that happen once. A five dollar part I assumed was good because it was 3 months old.

Try plugging the brake booster vacuum line. Do all the cheap, easy, things first before jumping into things like an ECU. If you rebuilt your harness I’d check for recessed pins. Check it closely.

Usually when problems happen you can go back in your mind and say well it ran right before. The problem started after I did a tune up or changed the fuel pump. In this case you’ll need to be methodical and work the problem until you win the battle and it sounds like you have been. Good luck
_________________
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Boeing 727,737-200-300-400,757,767
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[aeromech]

Bill does great work on distributors. He has a SUN machine but I don’t think he can test an electronic distributor like you have. I’ve seem broken wires on the connector before. Other than giving it a visual I’d think the only way you can really test your distributor would be to install it into a known good Vanagon. So, all you can do to buy another Vanagon. Just kidding
_________________
Lead Mechanic: San Diego Air and Space Museum
Licensed Airframe and Powerplant Mechanic
Licensed Pilot (Single engine Land)
Boeing 727,737-200-300-400,757,767
Airbus A319,320,321
DC9/MD80
BAe146
Fokker F28/F100
VW type 1 1962,63,65,69,72
VW Type 2 1971 (3 ea.) 1978, 1969
VW Jetta
VW Passat
Capable of leaping tall buildings in a single bound

[Abscate]

Is it a consistent breakup at a set rpm or a range/ inconsistent stutter?

Shoot a video of it showing the tach and audio.
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[Wildthings]

If you do not have a working vacuum retard and your timing specs require such, then your timing is likely way way off. Time your engine at 28° BTDC @3800+ rpm, hose(s) off and plugged.

[raygreenwood]

This is a Vanagon ....so is it running Digi-jet or Digi-fant injection?

If it is running digi-fant....there are a couple of items that can cause that e act symptom.

1. Digifant...just like L-jet and D-jet....had a central critical ground point. It is usually one one of the bolts between transmission and engine or may be on the top of the case somewhere (I am more familiR with inline watercooled engines with digifant).
However in digifant, it's not a large cluster of wires like in D and L.....but the effect of poor ground on digifant is MUCH more dramatic. It causes bucking and cutting out at shift points or large throttle movement points because those "rock" the drive train causing the connection bolt to squirm.

2. This is a lesser issue but can cause the same symptom. If your system (some digifant had this some did not) has the two micro switches on the TB.....if they are out of adjustment or if they are in adjustment but your throttle cable is out of adjustment.....it can cause bucking when you move off and on throttle at shift points or during cruise at very small throttle openings.

3. If yours it watercooled and has a knock sensor and it's cracked....it can cause this.

4. If yours has electronic distributor and the module is poorly grounded....it can cause this.

Ray

[SGKent]

is the distributor electronic or points? If points replace the condenser with a known good one.

Does the engine run correctly as soon as the RPM falls, or does it take a moment to recover?
_________________
"Most people don't know what they're doing, and a lot of them are really good at it." - George Carlin

[BDorsey]

Man, I so appreciate all the folks jumping in to help. Thanks all for the input.

I'll reply to each suggestion here to keep things organized.

timvw7476 - Electronic ignition, so no points or capacitor.
Cap and rotor are new and the rotor resistance checks out.
Plug wires are new, and I've tested an older set with the same result.

Aeromech - Fuel pressure and delivery rate are within specs but I haven't tested while driving. I will do that. I checked all ground connections. Cleaned and secured top of the case near distributor, between engine and transaxle, tin to firewall near coil. Checked continuity on all loom grounds.

There is one thing that nags me about the grounds but I can't imagine it matters. Within the loom, there are three separate ground wires that should connect to the main ground connection. I spliced two of them a few inches before the ring terminal just to reduce the number of connectors at the ground bolt. Does that sound problematic?

Smoke test is a good idea. I've been considering it but just haven't done it. Maybe there is a vacuum leak that only occurs under load and particular throttle opening.

A smoke test would find an exhaust leak too, correct? If there was one, would that cause a misfire?

Under what circumstance should I plug the brake booster line? Could you describe that test a little more?

Abscate - I don't have a tachometer (except on my tune up kit) so I can't easily do the video. But, the stall is consistent in the sense that I can predict when it will happen: Pre-shift RPMs (I would estimate 3-3.5K) and give it a bit more gas = stall/buck. Letting off the gas brings it right back, more pedal does nothing to help. I can hold the pedal just before the stall and know that just a little more will cause it.

Wildthings - Bill replaced the vacuum can with a working one. It is not the same part number but the vacuum curve was very close, he said, and should work well. So, I've set timing at 5* ATDC. I've checked it through the RPM range and it advances as it should - I think about 40* max advance. Does this seem adequate?

Ray - FI is L-Jet. Main ground is next to the distributor and I believe all connections are good, it's a clean case and new wires/terminals. I can always check them again though. Only one microswitch on the TB for WOT. I have adjusted this per the AFC doc. Just to be clear, the trouble does not occur only at WOT, but rather at more or less mid throttle. Ground for the Ignition Control Module would be through the loom, correct? I've checked all those wires but, again, that could be repeated to confirm.

SGKent - as soon as I let up on the pedal, normal operation. It seems like there is a certain combination of RPM and throttle position (or minimum throttle position) that results in stalling. I keep trying to imagine all the parameter values of the engine at that point to determine why that combination causes a stall but it is beyond me.


So, right now I am thinking that the two best leads are: measure fuel pressure when producing the stall and smoke test for vacuum leaks. I will get to those this week and report back but I'll take any other advice in the mean time. I think I will also write to Bill at SparksWerks to see if he has any reason to suspect the distributor.

Cheers,
Brian
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1982 2.0L Aircooled Country Homes Camper

[obieoberstar]

How did it run before you rebuilt the engine? And why did you rebuild the engine?

What condition is your exhaust system in? Catalytic converter and muffler?

Just brainstorming here...

[BDorsey]

Hi all,

Ok, I checked a couple things. My son watched the fuel pressure while we drove and we observed that there was no change in pressure immediately preceding the stall. Fuel pressure does change with RPMs but there was no drop associated with stall. Pressure was always between 28 and 38 PSI.

Second, we connected my tach and the faithful assistant monitored RPM while we produced the stall. It consistently occurs between 3 and 3.5k. But, I think it is also dependent on throttle position because in first gear we got to 4k before the stall and it only occurred when I got to mid-throttle position. It does have to be greater than 3K RPM though because in 3rd at lower RPM I could press the pedal all through its range without stalling.

Haven't been able to do a smoke test yet. I am putting together my own smoke tester. Will report when I've done this.

Thanks for any thoughts.

Brian
_________________
1982 2.0L Aircooled Country Homes Camper

[Wildthings]

[timvw7476]

nice work.
do check the fuel pressure regulator. it is threaded into the front engine tin, upper right side near cylinder one. If there's any fuel in that vacuum hose, the FPR is throwing off your A/F ratio.

[BDorsey]

Hi - a couple more test results.

Wildthings - I understand compensating for worn or incorrect parts with respect to timing and have followed your method in the past. But, all components are in good working order on this engine (well, except whatever gremlin is causing the stall/miss). With the idle timing set to specs (5* BTDC) the timing hits all the proper advance degrees as I run up the RPMs. In fact, setting the timing to 28* at 3500 rpm as you suggest (hoses blocked) caused a great loss in power and did not solve the stalling problem.

With this result, is there any other reason to ignore the vacuum advance and only use the centrifugal mechanism? Thanks.

timvw7476 - I pulled the vac line off the fuel pressure regulator after running the engine and found no fuel or smell of fuel so I think it is working well.

Ok, back to fashioning a smoke tester...

Brian
_________________
1982 2.0L Aircooled Country Homes Camper

[Wildthings]

Something needs to be off for the 28° BTDC spec to nhot work, are you sure the key in the fan hub has not sheared? Does your timing light have an advance feature? If so you need to read to the "0" mark on the timing scale and not any of the other marks. You do have a stock timing scale don't you?

[raygreenwood]

[SGKent]

does this engine have a catalytic converter, and if so how old is it?
_________________
"Most people don't know what they're doing, and a lot of them are really good at it." - George Carlin

[BDorsey]

Wildthings - I just installed the fan when installing the rebuilt motor and all was good then. I do have the stock, plastic timing scale. My timing light is an old one my father gave me years ago - simple induction lead around the plug wire and power from the battery.

Can you help me understand the thinking behind the 28* without vacuum advance? According to Bentley, idle should be 5* ATDC, mechanical advance at 3400 rpm should add 21-25*, and then vacuum should add 9-12*. So, idle + centrifugal = 16-21* BTDC at 3400 and adding vacuum get us to 25-36* BTDC. It seems that if vacuum advance is working then setting mechanical at 28 would take it to 37-40* with vacuum, which is above what we just calculated for spec total advance. I think I am missing something and would really like to understand. Thanks.

Ray - I've removed, cleaned, and tightened the transmission to chassis ground. I also have an additional ground strap from the starter motor to the chassis. I can triple check those once the rain stops in a day or two.

It doesn't look like I can access the ground wire in the distributor without disassembly, correct? It was just inspected/rebuilt by SparxWerks so I hope everything is connected.

Cap and rotor are new and I don't see any significant carbon build up.

SGKent - it does have a catalytic converter and the records from the PO show it was installed new in 2004, 54K miles ago. I had it inspected several (maybe five) years ago when we were diagnosing another problem and it wasn't clogged then. Doesn't mean it can't be now...

A few other bits of info - sorry I neglected to mention these.

1. The Idle Stabilizer is removed and the plugs connected because it wouldn't start with it in and I concluded it was not functioning. I haven't replaced it yet.
2. The idle tends to creep up sometimes to maybe 2k rpm, often when coming to a stop, but not when just starting it up. When it does race, I can get it to settle down by hitting the gas just a bit - like you would with a stuck carb choke.
3. When I remove the vacuum retard hose and plug it the idle will often shoot up to 2500 or so.

Not sure what is going on with the idle or if these are related to the misfire.

Brian
_________________
1982 2.0L Aircooled Country Homes Camper

[raygreenwood]

[SGKent]

what Ray said. ^^^

It sounds like one issue is that the vacuum advance, mechanical advance or both are sticking. Might want to oil the wick under the rotor also.