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Old 10-25-2019, 08:32 AM   #1
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Default Detonation due to Carbon Buildup

My 1999 SL2 tends to get a lot of carbon buildup inside everything. as a result, I'm always overdue for a throttle body cleaning and the EGR valve throws a fit every few months.

But I've got a bigger concern -- when I load up the engine under 2000 RPMs, it rattles and sounds like it's pinging until I let off the gas pedal.
Other than that, it sounds like a totally normal engine anytime I'm at idle, over 2,000 RPM, or giving it very light throttle.

In the early morning when the engine hasn't warmed up, I don't hear it as often. But I don't know if it means the pinging is related to engine temp or if I just drive gentler in the morning.

As a result, I don't have a very usable cruising band -- it likes to burn oil over 2500/3000, and it will make "detonation" noises under 2000.

And no, it does not throw a SES light!

I tried to find something on the car that could be loose and rattling at certain engine loads and frequencies -- but no luck yet.


I've seen tons and tons of posts on this forum about carbon buildup, throttle body cleaning, and EGR issues -- but I never see any about detonation. I would have expected the knock sensor to make detonation all but obselete. Has any other member experienced detonation with or without excessive carboning?
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Old 10-25-2019, 09:00 AM   #2
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Default Re: Detonation due to Carbon Buildup

Run a few tanks of high octane, you might see results with the first one but more likely it'll take 2-3. Bonus points if you can find 100% gas instead of the far more common 90%.

Knock sensor failure is pretty uncommon, but IIRC it's as cheap as the ECTS and simpler to swap out (no fluid loss).

It's possible the serp belt tensioner is rattling just right and fooling the PCM into thinking it's spark knock.

Would be useful to see fuel trims and ignition advance from a live OBD-II scanner.
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Old 10-25-2019, 09:16 AM   #3
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Default Re: Detonation due to Carbon Buildup

So what type of gas mileage are you getting and what does the temp gauge read when at full operating temperature? I'm thinking along the lines of solving the root cause of the excessive carbon build up. I think the other issues will resolve themselves if you take care of this.

If you are showing low operating temperatures the PCM is compensating in fuel management by running the engine rich. This causes excessive carbonization and poor fuel mileage. The operating temperature for your 99 SL2, which has a GEN2 gauge calibration, is at or just below 1/3 or if you have an OBDC reader in and around 193 degrees Fahrenheit.

If low temperatures suspect a bad ETCS and/or its connector and/or a leaky thermostat.
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Old 10-25-2019, 09:47 AM   #4
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Default Re: Detonation due to Carbon Buildup

Switch to higher octane fuel. Spark knock is bad for an engine and burnt valves is a risk you run with lean conditions.

You don't hear it as much with cold starts because the engine is in open loop and runs a richer fuel mixture.

Also pull and check the spark plugs. They may be fouled.
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Old 10-25-2019, 10:37 AM   #5
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Default Re: Detonation due to Carbon Buildup

Very good points -- thank you for the replies!

@ fetchitfido:
I ran almost a full tank of 89 octane a few weeks ago - did not notice a difference, however it was only one tank. Typically I run 87.

I was thinking about testing the knock sensor -- I just have to get to it with a voltmeter. I don't think it's gone bad (would it throw a code?)

And I was thinking about running the engine without belts for a few seconds to see if anything sounds different. Still working up the courage to deal with the belt again.


@ trottida:
I get anywhere between 29 and 45 miles to the gallon -- for pure city or highway driving respectively. From what I've heard this is pretty good for an SL2 - but it makes me think it could be running lean at times.

The standard operating temperature seems to be about 1/3 or 3/8 of the gauge. When I sit in traffic, it often gets to 1/2 but then the fan kicks in and it quickly goes back down.

@ Saturn Night:
The cold start thing makes sense. I bet it is related to the rich trim more than anything.

I had spark knock on my old, carbureted truck due to a fault with the vacuum advance and had to run no advance in order to temporarily "fix" it -- detonation is no joke!
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Old 10-25-2019, 12:21 PM   #6
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Default Re: Detonation due to Carbon Buildup

45 mpg on a DHOC, unlikely unless you coast down lots of mountains and experience a 100 mph tail wind.
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Old 10-25-2019, 01:26 PM   #7
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Default Re: Detonation due to Carbon Buildup

I don't think I've ever been over 44 with my SL2. But, that is close enough. Lots of old entries in the logbook over 40.
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Old 10-25-2019, 03:47 PM   #8
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Default Re: Detonation due to Carbon Buildup

Quote:
Originally Posted by can0fspam View Post
And I was thinking about running the engine without belts for a few seconds to see if anything sounds different. Still working up the courage to deal with the belt again.
I would think that if it was the belt tensioner, you'd still hear it at 2k rpm with no load.

If the fuel filter has over 120k miles or so, I'd check fuel pressure.

Might be worth checking MAP sensor operation, too.

Keeping rpms below 2500, and always shifting between 2k and 2,5k, will net you some decent mpgs
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Old 10-25-2019, 08:37 PM   #9
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Default Re: Detonation due to Carbon Buildup

Quote:
Originally Posted by can0fspam View Post
V
@ trottida:
I get anywhere between 29 and 45 miles to the gallon -- for pure city or highway driving respectively. From what I've heard this is pretty good for an SL2 - but it makes me think it could be running lean at times.

The standard operating temperature seems to be about 1/3 or 3/8 of the gauge. When I sit in traffic, it often gets to 1/2 but then the fan kicks in and it quickly goes back down.
The best US MPG I've managed in the 20K mi I've owned my current SL2 is 41.3. In my SL1 it was 45.2. You're doing pretty good if you're getting 45!
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2012 Ford Focus SEL HB MT
2011 Suburban LT

Past Saturns
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1993 SW2 AT (10y)
2001 LW200 MT (3.5y)
1992 SL2 MT (5y)

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Old 10-26-2019, 12:28 AM   #10
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Default Re: Detonation due to Carbon Buildup

I suppose I forgot to clarify -- this particular DOHC has the SOHC transmission with 2 overdrive gears. The real magic happens when you reach highway speeds

Thanks chaos, I'll look into the fuel pressure and the MAP. This car has been through a lot and had several owners so I don't know if the fuel filter ever has been changed.

I'm not familiar with high pressure EFI systems. Is there a way to properly test pressure with the high psi of an electric pump?
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Old 10-26-2019, 01:14 AM   #11
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Default Re: Detonation due to Carbon Buildup

There is a test port for a fuel pressure test gauge under the hood in the fuel line going to the fuel rail. You should be able to borrow one from Autozone or equivalent.
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Old 10-31-2019, 10:08 PM   #12
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Default Re: Detonation due to Carbon Buildup

My 96 SL1 had knock from 1/8 of carbon buildup on top of the Pistons from oil burning. No amount of chemicals ever removed it. Ive done everything from combustion chamber cleaner to a 50cc nozzle and water methanol injection. The only thing that did it was manually scraping it off when I replaced the head. Some of that is in my 1996SL11.9L thread. On the 96 I still had detonation with 91 octane.

You can also try a colder heat range 6 spark plug. A few of us are having great results with a dual electrode.

NGK: BKR6EKB-11 there hard to get but a platinum version is available on Amazon.

Denso: K20TR11 can be bought at advanced auto.

The colder plug really quieted down the active knock retard on my current 2002 SL1. I watch the knock retard on my ScanGauge.
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Old 12-20-2019, 06:01 PM   #13
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Default Re: Detonation due to Carbon Buildup

Here's an update: I replaced my belt tensioner with a new Gates model from Rockauto.com. The noise in question is gone.
I think the old tensioner may have been original with 188,000 miles. It looked very worn and had surface rust in places.
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