LT1 and knock/detonation - lots of stuff tried, still pings - LS1LT1 Forum : LT1, LS1, Camaro, Firebird, Trans Am, Engine Tech Forums
 
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post #1 of 10 (permalink) Old 07-18-2015, 05:12 PM Thread Starter
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Red face LT1 and knock/detonation - lots of stuff tried, still pings

Running 89 octane in my 95 Caprice LT1 wagon. 114k.

Car's PCM has NOT been tuned. No CEL. No stored codes. Barely passed smog in April due to high NOx.

Here's the list of new parts since 4/2015
ICM, coil, MAF, MAP, Coolant temp sensor, water pump (GM), opti (GM), thermostat (180), plugs NGK TR55s, O2s (Bosch), fuel pump, Taylor wires. New EGR valve (GM).

Put in the EGR last week, and my mileage took a dump. Down 3mpg. 10.8 with 100% in-town stop-and-go driving. New EGR is suspect, but the pinging pre-dated EGR replacement.

The car pings when hot, and under moderate load. Audible. Car shows no other problems. Runs great.

Seafoamed the hell out of it yesterday. No change.

Fuel pressure is 39 PSI @ idle and 45-47 WOT at the rail. After 20 minutes key-off, still has 20+ psi. Vacuum is good and stable - no bouncing needle. Tested EGR (per Shbox) pre-solenoid, post solenoid, and at the valve. It all checks out as it should, but the valve is a little slow to respond to vacuum. (Is that normal?) Pushing it in @ idle causes the car to stumble.

Looking at live-data, the ping happens when the EGR opens and under load, and you can hear it.

Disabling the EGR through the tuner eliminates the ping. Car drives fine in this mode, but can't drive it that way forever.

Turned the EGR back on, disconnected the MAF and took it for a drive - CEL on, but no change. Still pings.

Its a tricky one. Any ideas? I bought the Autozone ICM... suspect? Injectors are original - could they be the culprit? Dont think its the opti because the ping stops when the EGR is switched off.

Your assistance would be sincerely appreciated.
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post #2 of 10 (permalink) Old 07-18-2015, 06:32 PM
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Is your pcm pulling timing when knock happens? Is your knock sensor detecting the knock? Have you thought about going to 93 octane fuel. Even though compression isn't quite as high for iron head engines, you still need 93 octane fuel.
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post #3 of 10 (permalink) Old 07-18-2015, 08:08 PM Thread Starter
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Yes, the PCM is logging it, and pulling timing. As soon as I run this tank out, I'll be going 93.
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post #4 of 10 (permalink) Old 07-19-2015, 08:46 AM
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Try refueling at a different station? There is a Petrocan near me and my mower won't even run on it. Just a thought, I've heard of stranger things. What are your 02's reading, rich or lean?
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post #5 of 10 (permalink) Old 07-19-2015, 09:04 AM
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Another thing people don't think about with detonation is excessive carbon buildup on the head. IDC how much seafoam you use you will never get it out.

Not only does carbon buildup change the compression ratio but it also gets very hot. Hot enough to reach the flash point of gasoline causing detonation, and you say it only happens when its hot right?

Try some 93 octane and see what happens

Disabling the EGR will give you lower combustion chamber temps that's why it wont ping when you disable it.

EDIT: Shoebox, wow I really twisted that around. Yes disabling the EGR gives you a hotter burn and more peak pressure. EGR will give you more overall combustion pressure

1996 Trans Am, blown up, rebuilding

2014 GSXR 750

Last edited by screaminZ; 07-21-2015 at 04:08 PM.
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post #6 of 10 (permalink) Old 07-20-2015, 09:20 AM
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Quote:
Originally Posted by screaminZ View Post
...

Disabling the EGR will give you lower combustion chamber temps that's why it wont ping when you disable it.
It is just the opposite. The EGR function is to reduce combustion temps by displacing oxygen. Don't know what the OP has going on, but maybe his EGR is pulling in air instead of exhaust.
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post #7 of 10 (permalink) Old 07-20-2015, 07:57 PM Thread Starter
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Thanks for the input folks. I've added some octane booster into the crap gas and that has alleviated a lot of the ping, but not all. I used some propane to check for vac leaks and none were found. I used a borescope to look at the piston top of cyl 3 (the easiest plug for me to reach) and its actually in pretty good nick with no extra-ordinary carbon.

I am probably going to pull the bosch 02's and replace them with AC Delcos. Others have said that these things can be a bad mix with these cars.

I'll keep this updated as I try, and discover, new things.
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post #8 of 10 (permalink) Old 07-24-2015, 05:53 AM
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Re-seal all EGR fittings first, could be multiple small leaks drawing air.
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post #9 of 10 (permalink) Old 07-24-2015, 09:03 PM
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Quote:
Originally Posted by cutlassttop View Post
Re-seal all EGR fittings first, could be multiple small leaks drawing air.
Sometimes propane just doesn't work as good as ether.
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post #10 of 10 (permalink) Old 07-25-2015, 04:19 AM
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If you check the scannerdanner troubleshooting videos on youtube, he has one where he uses a spray bottle of water to check for vacuum leaks.

He sprays a heavy stream while engine running. You can actually hear the water gurgling as it's sucked into the engine at the place where the vacuum leak is.
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