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  #1  
Old October 6th, 2005, 11:40 AM
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Pinging/detonation

I'm getting alot of pinging/detonation when accelerating under load. I never noticed this before - it may have started after I had the heads/head gasket redone - maybe the shop has the timing adjusted improperly.

Questions:
1) shouldn't the knock sensor be retarding the spark to compensate for this? If I hear detonation, should I automatically think there is something wrong in the knock sensor circuit? (possible damage during head rebuild)

2) it could be my perception, but incredibly the overheating problem I have been struggling with seems worse now since the heads were redone (which did in fact fix the blown head gasket/coolant leak). Could this be a problem of the timing being advanced too much, and could it really cause that much overheating? The engine just shouldn't be that sensitive to overheating (providing all other cooling components are working properly)
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  #2  
Old October 6th, 2005, 11:53 AM
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Well no knock sensors on OBD1 ECUs (pre96) only on OBD2s (96 and newer) so that is not the problem I would check the timming and makes sure the gas in your tank is good.
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Old October 6th, 2005, 12:08 PM
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Really? I thought I saw a sensor on the left side of the block with a wire coming from it - looked like a classic knock sensor.
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Old October 6th, 2005, 12:50 PM
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hmm, take a pic of it but on 94's there were not any knock sensors. Unless you have a 97 engine? Do you have a distributer?
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Old October 6th, 2005, 01:13 PM
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It'd a '94 engine, in all it's distributored- and v-belted glory I will take another look at the thing - when I saw a wire coming from the side of the block I assumed it was a knock sensor - it could be something else...

I'll check the timing, too. Could this really affect the overheating problem I've been having noticeably?
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Old October 6th, 2005, 02:47 PM
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I cut my pinging by 90% by rolling back a degree or so off the timing. Just loosened the dizzy and turned it clockwise til the rpm dropped a bit. I'm thinking that disconnecting the vacuum advance will totally handle the issue.

Doug W.
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Old October 6th, 2005, 02:58 PM
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It was set wrong at the shop - it was near TDC - I backed it off to about 5BDC. If this doesn't solve the pinging problem, I'll back it down a couple more degrees.

If you remove the vac advance your acceleration will suffer, right?

PS the wire I thought went to a knock sensor was actually some small-gauge ground wire - odd. From up top it looked like a knock sensor lead.
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Old October 6th, 2005, 03:08 PM
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The small black wire is the ground for the distributer Knock sensors normaly have 2 conductors and a shield.
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Old October 6th, 2005, 04:56 PM
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So now even with the timing set properly I am getting alot of detonation - at least, I HOPE that's what it is. It sounds like it - the more load on the engine, the more noise... and it gets pretty voilently loud with alot of load - loud enough that I'm hoping it's not something far worse.

High RPMs and low speed, no noise. Medium speed, light load, no noise. Low speed, moderate-heavy load, some noise. High speed, moderate-to-light load, alot of noise. I'm getting a little spooked.

In hopes that it's bad gas, I filled up (tank was 3/5 full) with premium. We'll see how it drives home. 100K on the engine.
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Old October 6th, 2005, 06:01 PM
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Try disconnecting the vacuum advance temporarily (there's still centrifical advance) and plugging the needle hole in the plenum that feeds it. See if it still pings...

Doug W.
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Old October 7th, 2005, 03:45 AM
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I disconnected the vac advance, and this (and also the premium fuel mix??) improved the situation *alot*, but I can still get it to ping under heavy load... not nearly as bad as it was.

I added some octane booster and fuel system dryer/cleaner to the tank - I hope this was just one aweful tank of gas...
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Old October 7th, 2005, 12:49 PM
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I understand that intake valve guide seals can cause detonation as oil ends up in the fuel/air mix and causes a dieseling effect. Did they replace these when they did the heads? Just a thought. Even though, still does not explain the overheating. Perhaps there is blockage in the block (or better yet, head) causing a hot spot which also could lead to detonation and overheating?
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Old October 7th, 2005, 01:16 PM
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I hope they replaced the seals - they tanked the heads & decked them, and did a valve job. I don't think there could still be a blockage in the heads after that. The block, on the other hand... ??
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Old October 7th, 2005, 02:02 PM
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What gaskets did you use when you did the head job? Shim steel or composite? When they shaved the heads, it would have increased the compression ratio a little bit. If you used the steel gaskets again, then you may have too high of a compression ratio for pump gas.

Also, don't trust the timing marks on your balancer.... mine were 5 degrees off. AI assume you remembered to disconnect the distributor vacuum line when adjusting the timing. OR, here is another method I've used that works quite well. Instead of a timing light, you can also adjust timing with a vacuum gauge. Find the timing spot where the vacuum is highest, then back off 2 or 3 degrees from there.

You might also be sufferering from cross-induction. How are the plug wires, and the distibutor cap?

-Hans
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Old October 10th, 2005, 06:56 PM
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Quote:
Originally Posted by Hans
What gaskets did you use when you did the head job? Shim steel or composite? When they shaved the heads, it would have increased the compression ratio a little bit. If you used the steel gaskets again, then you may have too high of a compression ratio for pump gas.

Also, don't trust the timing marks on your balancer.... mine were 5 degrees off. AI assume you remembered to disconnect the distributor vacuum line when adjusting the timing. OR, here is another method I've used that works quite well. Instead of a timing light, you can also adjust timing with a vacuum gauge. Find the timing spot where the vacuum is highest, then back off 2 or 3 degrees from there.

You might also be sufferering from cross-induction. How are the plug wires, and the distibutor cap?

-Hans
Well, if the timing marks are really that far off, then that could be the problem - I'd never imagine that was possible. Yup, the vac adv is off and plugged.

Cap and rotor are pretty new (9 mos). Plug wires look good - no visual signs of cross-sparking (eg run engine at night, watch for fireworks). I have a new set which I never installed.

My truck had the steel gaskets, so that's what went back on. But, mind you, I didn't notice this detonation until a couple weeks after I had the truck back, and at about 7/8 full tank after a fillup (I think thinking it was just about enough to get bad gas into the system).

I was out of town this past weekend - I'm going to drive it a little bit more and see if the octane booster/fuel system dryer have any effect.
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  #16  
Old October 11th, 2005, 10:35 PM
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OK, the pinging is definitely diminished ALOT, but I can still cause it to occur under heavy load at highway speeds. I think I will push the timing back a bit more.

While doing this I'll also have a vaccuum gauge fitted as suggested - see how well the timing light agrees with the vaccuum gauge...
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