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Old 10-26-2021, 12:58 AM   #1
EpiglottisDynasty
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Default 1994 NA 940 detonation

I picked up a 1994 940 a few months ago, and it was audibly detonating every time I got on the gas, even on 91 octane. I replaced a dead in-tank pump, all vacuum lines and the stuck pre-heat valve, cleaned the MAF, put in colder spark plugs (two heat ranges colder) and an 88 degree stat, and replaced the fuel filter. It's less severe, but it still audibly detonates, even on premium, and even well before the kickdown detent. No detonation the first couple of minutes of driving, but after getting up to temperature (middle of the gauge) it is nearly impossible to accelerate without getting some.

I've probed the MAF, knock sensor and the Lambda with a meter... voltage on the MAF at idle is right where it should be, knock sensor isn't an open circuit, lambda ticks nicely back and forth at idle... and actually swings rich when opening the throttle. I don't have a tool to pressure test the fuel line, but there is no sign of leakage at the FPR vacuum line, and removing and plugging the vacuum hose shows the expected rich condition at the lambda. Checked the timing, and it is right on, at least by the markings (checked on the t-belt gear, not the dampener.) No obvious signs of vacuum leaks, unless there is one in some unexpected place. Tried blowing smoke up a vacuum line, didn't see any coming out anywhere. If I open the throttle quickly, I'll get a couple of quick louder clacks, and then they stop, so the knock sensor must be picking up something, but some small random crackles persist after. It has a Bosch injection system.

I'm kind of at a loss here. It isn't incredibly severe, and you can't feel it, but it is audibly there, and it is definitely more noticeable than my LH2.2 240, which, I was under the impression, is more knock prone.

Last edited by EpiglottisDynasty; 10-26-2021 at 01:09 AM..
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Old 10-26-2021, 02:46 AM   #2
Mr. V
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Focus on the knock sensor: logical cause of a detonation issue.

Consider replacing it.

Make sure the knock sensor is set to the correct torque (low value).
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Old 10-26-2021, 01:00 PM   #3
culberro
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Check the cam timing, and the roll-pin in the cam. Verify the cam is timed correctly to the crank.
Similar symptoms. Low power, detonation with any fuel type. Stock M cam will idle just fine when it’s a tooth off.
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Old 10-26-2021, 07:02 PM   #4
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Quote:
Originally Posted by Mr. V View Post
Focus on the knock sensor: logical cause of a detonation issue.

Consider replacing it.

Make sure the knock sensor is set to the correct torque (low value).
I will try that, I did actually buy a new knock sensor last time I ordered some parts, but haven't put it in yet. The sensor is definitely doing something though.

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Check the cam timing, and the roll-pin in the cam. Verify the cam is timed correctly to the crank.
Similar symptoms. Low power, detonation with any fuel type. Stock M cam will idle just fine when it’s a tooth off.
Cam gear is definitely timed correctly to the crank. I didn't take off the gear and check the roll pin though, so I'll check that. Am I looking for it just being bent or something? It doesn't seem down on power or anything.
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Old 10-28-2021, 11:06 AM   #5
dl242gt
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Possibly a lot of carbon built up in the cylinders? Maybe time to try the water torture or sea foam treatment? Get some of the carbon out of there. How is the exhaust manifold to engine gaskets? You can have a leak at them and it'll tick like it's a valve clacking or detonation.
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Old 11-06-2021, 03:02 AM   #6
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I replaced the knock sensor... little if any improvement. Is some amount of detonation normal for these engines? Maybe I'm chasing a nonexistent problem? Opening the throttle quickly always produced a few solid knocks, and even just holding it at the detent will get me intermittent random clacks. Should I be chasing a hard to see/hear leak or a fueling problem next?

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Originally Posted by dl242gt View Post
Possibly a lot of carbon built up in the cylinders? Maybe time to try the water torture or sea foam treatment? Get some of the carbon out of there. How is the exhaust manifold to engine gaskets? You can have a leak at them and it'll tick like it's a valve clacking or detonation.
Maybe worth a try. I did that once to my 240, but it always makes me nervous. Should you unhook the cat before doing that?
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Old 11-07-2021, 05:30 PM   #7
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No need to disconnect any exhaust stuff for the sea foam or water torture cleaning.
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