Stumped........ :(

[tolusina]

Hi All,

I'm stumped, I'll take my crow deep fried with biscuits and gravy please.
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The car is a 92 Cabriolet equipped with California spec Digifant I.

While getting on the freeway, accelerating through the gears, as the engine reached approximately 4000 rpm in third gear it suddenly lost power as though it had hit the rev limiter, I backed off the throttle and was able to continue briefly with reduced power until the engine finally stalled. Since then, all I've heard from the engine is a light puff in the intake manifold, not even a pop.
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The crankshaft, camshaft and distributor were all properly synced and remain so, there are no indications of a fargin' spun crank gear.

The symptom felt very much like that caused by a split intake duct between the MAF and the throttle body, that was the next thing checked and it's fine.

There's adequate spark at the plug ends of the wires, the tach needle wiggles while cranking, I've jumpered 15 to 30 at the ignition switch just to be sure.

There is injector pulse signal present at the injector loom measuring around 10% duty cycle while cranking.

I replaced the fuel filter, the old one was fine.

The in tank pump pumps approximately a quart in 30 seconds as per spec, I replaced the short damper piece at the outlet of the fuel pump in the tank just to be sure.

Fuel pressure at the fuel rail measures around 43 psi, dropping to about 38 psi when vacuum is applied to the fuel pressure regulator. The fuel pressure regulator is not leaking fuel past it's diaphragm.

I replaced the ignition distributor rotor to be sure.

Exhaust back pressure at the upstream sample port while cranking shows approximately one quarter psi, intake manifold vacuum while cranking is approximately 5 inches. To make absolutely certain plugged exhaust is not the issue I loosened the header pipe directly at the exhaust manifold outlet.

I tried a junkyard ECU from a '91 German Jetta, DigiFant I.

I popped the cover off the MAF, the arm oscillates with intake pulses while cranking, 'helping' the arm move or not while cranking makes no change, voltage readings when the arm is operated manually are all fine.

CTS resistance is fine.

Unplugging CTS and/or MAF make no difference.

Couldn't stand looking at the corroded battery terminals and wondering, also thinking that if corrosion was the problem, I'd be really embarrassed, fixed all that, new terminals, soldered eyelets and so forth.

On the off chance the car had been vandalized by some mystery potion poured in the tank, today I pumped out 8 gallons to empty, flushed a half gallon of fresh through, put in 2 gallons of fresh 91 octane.
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Nothing I've checked or done has shown me anything wrong, yet the car still refuses to start.

Any and all ideas and suggestions are welcome, snickering may or may not be, depends on how well it's done. What have I overlooked?

TIA,
Ron

(cross posted on the 'tex....... http://forums.vwvortex.com/zerothread?id=3481055 )

[Calimus]

All I can think of right now off the top of my head is pull the cam gear. Make sure the keyway didn't bend rather then shear. If it just bent, then you are prollie just far enough out of time on the cam to cause the issue. Check the same on the crank just to be sure. There have been issues in these two places before.

Hrm....thinking..... It sounds like you have checked the fuel system out pretty well, but have you actually checked to see if the injectors are spraying any fuel?

Have you don't a compression check also to make sure you didn't blow the head gasket? Lack of compressionw ould certainly result in lack of fire.

These are all I can think of right now. Hope this helps somehow.

[kamzcab86]

Engine swap time?

[tolusina]

All I can think of right now off the top of my head is pull the cam gear. Make sure the keyway didn't bend rather then shear. If it just bent, then you are prollie just far enough out of time on the cam to cause the issue. Check the same on the crank just to be sure. There have been issues in these two places before.

Hrm....thinking..... It sounds like you have checked the fuel system out pretty well, but have you actually checked to see if the injectors are spraying any fuel?

Have you don't a compression check also to make sure you didn't blow the head gasket? Lack of compressionw ould certainly result in lack of fire.

These are all I can think of right now. Hope this helps somehow.

Since the cam, flywheel and distributor marks were and are all spot on, the fargin' crank gear keyway must be OK. Cam keyway is another story, checking it is on the todo list.
I've got to get my 'better' grade fuel pressure gauge from storage, I can then manually (electrically) operate each injector and watch for pressure drops. Also, I posted above that I've checked injector pulse, I neglected to check all 4. Both tests are pretty easy with DigiFant I, not so easy to isolate individual injectors with DigiFant II. Todo.

I did have the plugs out the other day, remote starter connected, compression gauge ready in the tool kit, I did not follow through, I should have and you are about the 5th to slap my wrist about it. Definitely another on the todo list. Work just ran me out of daylight today.

[tolusina]

Engine swap time?

Umm, I'm driving the ABA donor car these days, how did you know?

[kamzcab86]

Hmmm, I must be more psychic than I thought!

[SoCal_Cabby]

can we get the Clif note version....i dont feel like reading that much, let alone comprehending it all

[Briano1234]

Bumpage for an update

[tolusina]

I didn't see anything about testing with starter fluid (ether)......
.....That's the simplest way I know to rule out/in fuel issues.

Not something I've ever tried before. Back in my old days, ether was used to help a cranky, worn motor start and that was about all it was used for. It was generally considered lame to have to resort to ether for any reason at all.
I'd honestly never given any thought to using ether to rule on fuel issues, on your recomendation, I gave it a go. Stuff was cheap enough, about $2.39 a can.
So I sprayed into the intake a couple of ways, certainly made a point of spraying while cranking. Got nothing. Hmm, starting to look like a spark issue. But I've got spark.
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....popping (or maybe in your severe case, puffing) in the intake could mean a broken exhaust valve.....

.....sounds like a floated valve or a broken cam...or woodruff key.....have you done a pressure test on the cyl's.......

.......The only thing I can suggest would be a compression test..

...see if the injectors are spraying any fuel?...Have you don't a compression check...

...is fuel leaving the injectors? theyre staggered and if one lead breaks then you loose 2 or all of them. with only 2 i dont see the motor firing, and digi injector harnesses are notorious for getting brittle and breaking....

...pulling the fuel rail with the injectors in it and watching them fire into a bucket you've found fuel... the only think you're missing in the mighty three part equation of the internal combustion engine is compression....

....The engine does crank with a proper rhythm, I'll be surprised if compression testing shows anything, I'ma do it anyway...

Ok, ok, I had to check compression, I got too many responses plus I should know better. Besides, it led directly to the next step while the plugs were out.
Readings were 175, 175, 180, 190. No problems indicated, I'm passing on looking at the camshaft and valves for now.

Next up was re-connecting the fuel pressure gauge, I'm going to use it to test for injector spray without pulling the injectors. If they do spray, the harness is ok, the ECU is telling them to work. Spark plugs are still out at this point.
Gauge connected, cycled the key a few times to build pressure, pulled the fuel pump fuse (pulling the relay killed the ECU, not what I needed). Cranked the engine with the remote starter button (everyone should have, get or make one of these!!), watched the pressure drop on the gauge. Hmm, are all four spraying? Yup! Fuel sprays from all four cylinders while cranking. I had planned manually triggering each, no need at this point.

It's looking more and more like a spark problem, but I've got spark, I've checked it several times.

Swapped coils with my A3, A3 won't start, Cabby fired immediately, loud too, the exhaust is still open. fargin' Cabby coil is only about 2 1/2 years old and it does make spark, just not enough.
I really dislike swapping parts to diagnose a problem but sometimes the only sure diagnosis is testing with a known good unit, in this case, a known marginal unit.
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Thanks again for all the suggestions, they really did help me get to the conclusion.

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can we get the Clif note version....i dont feel like reading that much, let alone comprehending it all

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Ron

[Briano1234]

Cool thanks for the update. I spect that I need to acquire a spare coil.... I don't think I have ever had any bad coils on any of my cars ever....

[tolusina]

Cool thanks for the update. I spect that I need to acquire a spare coil.... I don't think I have ever had any bad coils on any of my cars ever....

Yours are probably DigiFant II, with a conventional looking round coil and a separate power stage.
I'd not think that my experiences with the DigiFant I transformer looking thing with integral power stage are any indication of what you can expect.

I'm a bit miffed with my car though. It's let me down only twice in 80,000 miles, both times it was the coil. The 1st time, the power stage died, took about 5 minutes to figure out and confirm, a buddy came to my rescue with a replacement, I was back driving about 2 hours later.
This time, the fargin' thing still made spark, just not enough to make bang in the cylinders.
My A3 Jetta GL with Motronic uses the same coil. That one still works, just barely, it tracks and arcs badly whenever the load increases. I did catch a pretty cool pic of the arcing.
So, I've got two Dubs, both need the same $70 coil/power stage. I'm thinking junkyard parts to convert both to CIS style round coil with separate igniter/power stage/Hall control. The plan today is to see if the Motronic ECU output (power stage input) signal is speed variable dwell or steady dwell, is the coil #1 dwell speed variable? If I find the power stage input is steady dwell and the output variable (just like CIS), I'm converting both cars.

[fargin' = sumthin' simple or basic (in concept) broken, cain't see it for shoot.]