924Board.org

Smoothie · Posted: Tue Jan 25, 2005 6:06 am Post subject:

The gear teeth at the bottom of the distributor are at an angle as are the drive teeth on the camshaft, so when you turn it that extra 2°, the distributor is riding up at the angle of those gears. When the engine's running and the cam gear is driving the distributor gear, the direction of spin and angle of gears cause the distributor shaft to be held down.

"ROW" cars without cats could and did use a rev limiting distributor rotor. 931s that originally came with catalytic converters have a special fuel pump relay with rev limiting built-in instead, because cats don't do well with unburnt fuel. (Rev-limiting rotors cut the spark and allow unburnt fuel to pass through. Rev-limiting fuel pump relays cut the fuel off.) As far as I know, only cat-equipped 931s got the rev limiting relay, but I don't recall how cat-equipped NA's handled the problem - ..maybe they got them too?
Post the part number from your relay.
_________________
"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

rennbod924 · Joined: 23 Jan 2005 Posts: 21

OK... So I just dug "the big red books" out of the loft (the factory workshop manuals that came with the car)..

Thought I would test the coil resistance... its 0.7 ohms between the two screw on connectors (primary?), and 5.7kohms secondary...... ran back into the house, checked the specs and for my ignition system (I have the big metal ignition box in the engine bay, not the little ignition amp) and the big red books say it should be 1.0ohms to 1.35ohms... with the secondary being 5.5kohms to 8.0kohms..

Meaning that the coil not up to spec on primary resistance..

The coil I have is a Bosch item, number 1-220-522-006... the big red book says my ignition system should have 211-905-115-B...

A bit more digging in the big red books finds that the TCI-H ignition system coil should be around 0.7 ohms on the primary side.. which kinda points to this car having a coil for a TCI-H ignition system, when its actually not fitted with that system... but I cant find anywhere in the big red books that says what coil the TCI-H system should have, I suspect its the one I have got.

So, would the coil being the wrong type, cause my missfire/hessitation at 4000 rpm with full throttle, which clears eventually as the revs rise by about 500 rpm??

Also... is the shaft in the distributor supposed to be able to be lifted up a bit, or should it lift as its twisted?

Smoothie · Posted: Tue Jan 25, 2005 6:35 am Post subject:

Exactly how much straight vertical rotor lift are you talking?
I measure 1/32" straight vertical lift and a total of 1/16" with the twist on mine.

A bad coil (weak spark) should show most while under extreme load as in when starting out from a stop in second gear or going too slow up a hill in a too-high numbered gear at full throttle. If these are the types of situation where you have trouble, then I'd try a new coil, but going just by the readings you got... slightly out of spec? yes, but enough to cause your problem? maybe, maybe not..
_________________
"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

rennbod924 · Joined: 23 Jan 2005 Posts: 21

Hi Smoothie, thanks for the post...

I will check a bit later (in the morning here in the UK), but from memory it seems almost like you can pull the distributor rotor arm straight up by about between 1/4 and 1/8 of an inch without it twisting at all..

When twisting it through a few degrees it can twist without lifting (springing on the advance mechanism I think), but when its turned the last couple of degrees it hits a lot of resistance and pops up about 1/8th of an inch.

Also, there seems to be some rotory scoring in the distributor cap, along with some on the tip of the rotor arm, but I cant remember if it was like this when I brought the cap, and therefore machining marks, or if its the rotor arm rubbing when it lifts, if it does indeed lift during operation.

I am not even sure if this problem is an ignition fault, as it only happens when its getting full throttle and at 4000 rpm, so its tricky to use a timing light or spark tester to see whats happening.. it could also be a fuel problem, but again, its hard to view the exhaust gas readings from a gas analyser while the car is at 4000 rpm and at full throttle...

I am at a bit of a loss... it does seem that the coil is either wrong or out of spec... and I am no distributor expert, but it does seem like the distributor should not be doing, or being able to do, what its doing... but it could just be something that I have never seen a distributor doing before..

The other, possibly unrelated thing about this car... the voltage guage rarely goes above about 12.5volts... it could be a weak guage, a weak alternator, or nothing at all... but could this be anything to do with it all.

Also... I have noticed that this 924 does not run the fuel pump until you start cranking the engine or its running.. where as I have been reading that the fuel pump should run for a second when you first switch the ignition on... so again, not sure if it means anything, and if it could have anything to do with the full throttle fault.

rennbod924 · Joined: 23 Jan 2005 Posts: 21

One olther thing Smoothie, It pulls away just fine, if anything it has a bit more low end grunt than other 924s I test drove when looking to buy one.

In fact it seems to run just fine except for the power loss at 4k rpm at full throttle...

Its really odd

Smoothie · Posted: Tue Jan 25, 2005 12:26 pm Post subject:

I just realised we're pretty much comparing apples to oranges as far as the amount of play in the distributor's concerned - because I have the DITC ignition system which uses a "solid" distributor (no centrifugal spring advance and no vacuum advance - the center shaft is solid from the gear at bottom to rotor at top). -So you'd need to hear from someone with a "normal" distributor to compare the freeplay with since some of it could be due to the centrifugal & vacuum advance features. 1/4" does seem excessive, but I really don't know how much movement you should be seeing. Even if the vertical movement were part of the problem, there's still the question of why it would decide to become apparent at only 4000 rpm. There's nothing I know of to make it suddenly jump upward at 4k rpm - then again when things break you sometimes get "unpredictable results".

As far as the voltage reading's concerned - unless you're seeing something weird happen right at 4k rpm, I don't think the problem's with the charging system. I'm pretty sure it's not a coil problem either because if it were, you'd see it at speeds other than 4k rpm as well.

If you had this problem with a 931, I'd have some suggestions (specifically to check for a leak in the charge tube), but with an NA, my best (and only) guess would be a problem in the distributor with the centifugal advance - that's the only rpm dependent part that I can think of that might give you trouble consistently at the same rpm all the time.
_________________
"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

rennbod924 · Joined: 23 Jan 2005 Posts: 21

OK.... here goes...

Testing the mechanical advance and the vacuum advance.

I have found that the distributor is advancing the ignition (beyond the timing I set of 8 degrees) by the following amounts.

Centrifugal advance

2000rpm - should be 13 to 18 degrees - actually at 19 degrees

3000rpm - should be 18 to 24 degrees - actually at 31 degrees

4000rpm - should be 26 to 30 degrees - actually at 31 degrees (still)

So it would seem, that my mechanical advance is happening 1000 RPM too early, and too far by one degree.

But, by the time my problem happens, its only 1 degree above maximum spec.

The vacuum advance, seems to be dead on spec (albeit at the maxium side of the range).

At 1000 rpm, adding 200mbar (20,000Pa or 20KPa) I am getting 12 degrees of advance. The specs in the factory manual state it should be between 8 and 12 degrees at 200mbar.

Now here is the clincher, the workshop manuals are working in degrees measured at the distributor shaft, and rpm measured at the distributor shaft. So I have been doubling the degrees in the manual, and doubling the RPM in the manual, to get what I should be seeing at the flywheel.. is this correct?

Also, does anyone think that this could be my problem?

I am going to check dwell now.

rennbod924 · Joined: 23 Jan 2005 Posts: 21

Hmmm...

Dwell as measured at the ignition coil, with a multimeter, I am getting the following readings..

1500 RPM - 27.7 degrees of dwel

5500 RPM - 17 degrees of dwel

The interesting thing is... between 1500 and 3500 rpm, the dwel stays at about 27.something degrees... but as soon as you pass 3500 rpm the dwell gets smaller and smaller... at about 20 degrees by the time your at 4000 rpm and dropping with every sligh increase in engine speed.

Even more interesting, is the the factory manauals state it should be at the following spec.

1500 rpm - 52 to 70 degrees
5500 rpm - 42 to 58 degrees

The thing is... could this be caused by the distributor, or by the ignition control unit.

This is begining to look like the cause of the problem..

rennbod924 · Joined: 23 Jan 2005 Posts: 21

Just been running some more tests as shown in the workshop manual, and as the sensor coil in the distibutor tests as within range (resistance), as the dwell is way down on spec, it mentions changing the ignition control unit.

Has anyone else had an ignition box (the silver one in the engine bay next to the air box) fail in a way that causes a failure like mine?

Has anyone had one of these boxes fail, and what were the symptoms?

Smoothie · Posted: Wed Jan 26, 2005 2:40 am Post subject:

Those centrifugal advance test results you got are consistent with what you'd see with aged, slightly weakened springs, but again as you know that still wouldn't explain a problem only at 4k rpm.

Nice dwell troubleshooting - I'd thought of the ignition trigger box and ruled it out because it shouldn't affect timing, but it's totally plausible that a dwell malfunction could be causing your problem. I haven't had a problem with mine, but it looks like you're on the right track to consider replacing it.
_________________
"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

rennbod924 · Joined: 23 Jan 2005 Posts: 21

Well, there is more confusing aspects now.

I used a multimeter for testing the dwell, a automotive one that has dwell and other car testing settings... it gave me the results I just posted.

But a friend just came round with a BluePoint automotive hand held oscilloscope, and we just found that with his scope, nothing unusual seems to be going on with the trace of the coil low tension side (waveform wise).. and it reports the dwell as being around 68' and 72' throughout the rev range..

So I am now not sure what to test, but I have another ignition control unit comming anyway.

But I am moving on.. just in case its not the distributor, coil or ignition control unit, I have moved onto checking out the injection system.

I have a fuel pressure gauge which looks a bit like the JCWhitney (or whatever one) I have seen links to in other posts.

It came with a CIS adapter, which looks like a T peice. But I can not see any way of plumbing it in without cutting the fuel lines and putting it in series... So I am considering going off to a friend who has a scrap old golf with CIS injection, and taking some fuel lines from the CIS unit to the warm up regulator, to cut up to make a propper removable non destructive test adapter.

I also now have a 4 gas analyser here (again from a friend), and have run the car with that.

The CO was at about 4.0% CO... hydrocarbons at about 250 parts per million, so I wound the mixture screw so that the CO was about 2.5% under the recommendation of the friend who is letting me borrow the 4 gas machine.

Now the problem only occures when I am under load, at full throttle, and the engine is at 4000 RPM.. so unless I get a generator for mains power, and put the 4 gas in the back of the car, I can not see what the emissions are doing while driving... but I did think I would see what the mixture was at, at different rpm ranges while the car is out of gear and parked.

I was expecting the CO to stay at about 2.5% or within a couple of % as I slowley raised the engine rpm... but no.. as I moved off idle the CO went higher and higher, the higher I revved the engine... hitting about 5% at 2000 rpm... 7% at 3000 rpm and about 9% at 4000 rpm!

Is this right? I would expect that if its doing this with the throttle cracked open just a little to bring the revs up to those limits without any load... its going to be even richer when the car is under load and the throttle all the way open at those RPM levels.

So what does everyone think to that??? is this right?

Until I make up a fuel pressure adapter, I am not going to be able to run pressure tests.. but these emissions levels are making me think that it could be flooding when the problem occures?!?!???

Just in case I do not manage to be successfull in making up a test pipe for the fuel pressure... what does everyone think would cause the mixture to get richer and richer as the rpm level increases??

HELP!!

rennbod924 · Joined: 23 Jan 2005 Posts: 21

When I say that we could not see anything going on with the trace/waveform, of course neither of us have any idea what we should be seeing in any detail... but we didnt see anything suddenly going strange like the waveform vanishing or anything REALLY strange.

Smoothie · Posted: Wed Jan 26, 2005 3:56 am Post subject:

I was left wondering as I looked at the gauge and its' fittings for the first time, but did find a way to hook it up by just using the included fittings and not cutting anything. If you can hold out, I'll take a look at the kit when I get home and let you know which fittings were used.
Did your kit come with a reference guide that identifies each fitting?
_________________
"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

rennbod924 · Joined: 23 Jan 2005 Posts: 21

No paperwork came with it at all... just the dial, lots of pipes (which I have learned at for different connections..) but it seems the T-piece will only screw directly onto the bottom of the gauge.

I have used it on my wifes 944 (had to have a special fitting made) and it helped diagnose a stuck fuel pressure regulator on her car. So its already been usefull... but it came from a chap in the USA on ebay.. so didnt come in any packaging or anything. I was hunting for one over here for ages, but the cheapest ones I found in the UK were ones designed for garages, so were about $300 US dollars (about £150 GP Pounds) but did come with fittings to hook them up to just about anything.

Smoothie · Posted: Wed Jan 26, 2005 4:14 am Post subject:

Ok - for now, here's a link to a previous discussion of CIS testers - http://www.924board.org/viewtopic.php?t=10219
My second post to that thread includes photos of my JC Whitney CIS tester. When I get home, I'll scan in the instruction sheet with fitting descriptions.
_________________
"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox