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Stampedetrail
Joined: 29 May 2005
Posts: 274
Location: Fairbanks, Alaska
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 Posted: Tue May 22, 2007 7:10 pm Post subject: New preignition condition |
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Hello again. Last thursday I was cruising around like normal, and I've recently had the wheels balanced so I thought I'd take the highway and see if that annoying wobble is gone away now. Up over 70 MPH, no real problems, the wobble is still there but not nearly as bad as before. I take it home and think, boy that idle sure is high, almost 2K, maybe crank that down a little. So I do that, go out to get some food and something is wrong. Preignition, that clack-clack in low gear under load, very persistant and appearing quite suddenly.
Following the code if it aint broke don't fix it, I assumed I had caused this condition by messin' with the idle, so I adjusted that back to where it was. No change. I adjusted the mixture a bit richer, no change. I adjusted the timing a bit retarded, no change. I filled 'er full of premium and drover 'er around the neighborhood for half an hour, no change. WTF?! What did I do?
I pulled the plugs, NGK slightly hotter than stock, they looked perfect--light greyish-tan with no spots, pits, or anything, except a little carbon built up around the rim (the threaded part). Replaced them with Bosh standard plugs, no change, and a lot less vroom.
WTF?! What did I do?
I've ordered a fuel filter, but I have very little faith that this will solve my problem. It's a very persistant preignition condition that has just appeared suddenly and is not going away! Help!!
BTW, this is the car
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1977.5 924 "Martini" head / New Old Stock rebuild
1985.5 944 Why? Because I can. |
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Ozzie
Joined: 12 Mar 2005
Posts: 4448
Location: Townsville, Qld. Australia
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| Posted: Tue May 22, 2007 7:45 pm Post subject: |
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My guess.
Your centrifugal advance in the dissy is stuffed which has your timing well advanced to start with and none when you want it.
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Porsche 924 1984 (UK import) NA
Its AUTO and its BLACK
Montego Black on black/red
Engineer of Electro/Mechanical Systems Maintenance |
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Stampedetrail
Joined: 29 May 2005
Posts: 274
Location: Fairbanks, Alaska
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| Posted: Wed May 23, 2007 5:37 pm Post subject: |
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Good idea, that would mean though that in a crisis situation I could retard the ignition a bunch and eliminate the preignition, but so far not. On your comment I just tonight replaced the cap & rotor & examined the dist., did not see any obvious signs of any failure. No change in condition because of that tho. I wonder if it could just be carbon build up in the head. Sticky valve possibly? Just can't quite guess, but it definitely does not sound mechanical. Spose I need to do a compression test & pop the valve cover (sigh)
Though I wonder if its possibly a valve-timing issue? That would be some troubleshooting. Who's going to come and help me rebuild this thing 
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1977.5 924 "Martini" head / New Old Stock rebuild
1985.5 944 Why? Because I can. |
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Ozzie
Joined: 12 Mar 2005
Posts: 4448
Location: Townsville, Qld. Australia
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| Posted: Wed May 23, 2007 7:28 pm Post subject: |
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I f your springs in the cent advance are ok you can turn the rotor approx 20deg and it will spring back.
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Porsche 924 1984 (UK import) NA
Its AUTO and its BLACK
Montego Black on black/red
Engineer of Electro/Mechanical Systems Maintenance |
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Mike924
Joined: 12 Aug 2004
Posts: 2601
Location: IoW UK
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| Posted: Wed May 23, 2007 10:22 pm Post subject: |
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Could also be a vacuum leak in the intake tract or a duff/leaking vac advance on the dizzy.
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1985 Porsche 924 'Lux', Kalahari Beige (my ex)
1993 Porsche 968 Coupe, Midnight Blue, 6 spd
'There is no substitute for a little grease under your fingernails.' - Chrenan, 924board.org |
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Stampedetrail
Joined: 29 May 2005
Posts: 274
Location: Fairbanks, Alaska
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| Posted: Thu May 24, 2007 6:39 pm Post subject: |
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| Ozzie wrote: | | I f your springs in the cent advance are ok you can turn the rotor approx 20deg and it will spring back. |
Ah! It does spring back. So am I to understand that there's a centrifugal as well as vacume advance in the distributor? I've been running it with no advance or retard lines, plugged at the manifold of course, and its been just fine until now. Haynes on transistorized ignition, in theory "no engine which relies on rotating mass as the means of producing power can turn faster than the ignition system can produce spark. Ignition remains "dead on" at all times and under all conditions..."
What does this mean?
| Quote: | | Could also be a vacuum leak in the intake tract or a duff/leaking vac advance on the dizzy. |
The dizzy is not an issue but I'll thoroughly check the vacume on the manifold, though I immediately rebuilt the throttle body seal when this started happening. Wonder about that crankcase air breather?
I dont know, maybe I'm over-analyzing it. Preignition exists when the charge ignites before the spark goes off, so something is causing that to happen. Hmmm.
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1977.5 924 "Martini" head / New Old Stock rebuild
1985.5 944 Why? Because I can. |
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Mike924
Joined: 12 Aug 2004
Posts: 2601
Location: IoW UK
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| Posted: Thu May 24, 2007 6:53 pm Post subject: |
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Reconnect the vacuum advance and see if it makes any difference.
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1985 Porsche 924 'Lux', Kalahari Beige (my ex)
1993 Porsche 968 Coupe, Midnight Blue, 6 spd
'There is no substitute for a little grease under your fingernails.' - Chrenan, 924board.org |
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!tom
Joined: 28 Aug 2006
Posts: 1941
Location: Victoria, BC Canada
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| Posted: Fri May 25, 2007 2:22 am Post subject: |
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| Stampedetrail wrote: | | Preignition exists when the charge ignites before the spark goes off, so something is causing that to happen. Hmmm. |
If this were the case, than your ignition timing wouldn't matter at all. Since clearly it does, this is an over-simplification of what preignition really is.
I'm no expert, and there have been a number of in-depth discussions here about it, so it would probably be a good idea to do a search. There's also a distinction between "knocking" and "preignition", and the term "pinging" is often used to describe both conditions.
One of the above conditions is caused by spontaneous combustion due to very high cylinder pressures, which happen because the charge is ignited sooner than it should, allowing the gasses in the cylinder to expand due to combustion as the volume of the chamber is decreasing more than designed. The remedy to this condition is to retard the ignition timing.
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78 924 NA
5-lug |
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Ozzie
Joined: 12 Mar 2005
Posts: 4448
Location: Townsville, Qld. Australia
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| Posted: Fri May 25, 2007 8:48 am Post subject: |
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In my case I was running a bit lean, riching the mixture made all the diff.
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Porsche 924 1984 (UK import) NA
Its AUTO and its BLACK
Montego Black on black/red
Engineer of Electro/Mechanical Systems Maintenance |
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Stampedetrail
Joined: 29 May 2005
Posts: 274
Location: Fairbanks, Alaska
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| Posted: Sat May 26, 2007 12:54 pm Post subject: |
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| !tom wrote: | | Stampedetrail wrote: | | Preignition exists when the charge ignites before the spark goes off, so something is causing that to happen. Hmmm. |
If this were the case, than your ignition timing wouldn't matter at all. Since clearly it does, this is an over-simplification of what preignition really is.
I'm no expert, and there have been a number of in-depth discussions here about it, so it would probably be a good idea to do a search. There's also a distinction between "knocking" and "preignition", and the term "pinging" is often used to describe both conditions.
One of the above conditions is caused by spontaneous combustion due to very high cylinder pressures, which happen because the charge is ignited sooner than it should, allowing the gasses in the cylinder to expand due to combustion as the volume of the chamber is decreasing more than designed. The remedy to this condition is to retard the ignition timing. |
Thank you for your help here. I did try retarding it, but it will not start well in that condition and we were still experiencing preignition. If I don't find the answer soon, the engine is coming out. No sense replacing the head with a rebuilt one with so many miles on the bottom end.
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1977.5 924 "Martini" head / New Old Stock rebuild
1985.5 944 Why? Because I can. |
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Paul
Joined: 02 Nov 2002
Posts: 9491
Location: Southeast Wisconsin
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| Posted: Sat May 26, 2007 11:27 pm Post subject: |
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Check your rotor, cap, and the electrical connection at the distributor.
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White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy. |
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Stampedetrail
Joined: 29 May 2005
Posts: 274
Location: Fairbanks, Alaska
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| Posted: Sun May 27, 2007 7:06 pm Post subject: |
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Too late I'm afraid. I already replaced the rotor & cap & rebuilt the connection to the distributor. Then I took that all apart & started removing the engine.
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1977.5 924 "Martini" head / New Old Stock rebuild
1985.5 944 Why? Because I can. |
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