Mystery Knock

rayfast · 06-30-2009, 03:21 PM

I have a 1986 Toyota MR2 4AGE NA. I replaced the plugs, wires, distributor cap, and rotor button a few weeks ago and, until yesterday, have been enjoying excellent performance and gas mileage.

Yesterday afternoon near the end of a 25-mile drive, I heard a knock coming from the engine. It sounded a little like a rod bearing, but it only occurred when I loaded the engine (accelerating or going up hill). The sound disappeared at idle or while coasting. I also noted a corresponding significant loss of power.

Attempting to isolate the problem to a cylinder, I disconnected the plug wires from the distributor one at a time. Disconnecting any wire resulted in the car stalling, except when I disconnected #2, which ceased the knocking and brought a slight but perceptible increase in power.

I have spark to the #2 wire and, though I haven't checked the plug itself yet, I suspect it's firing because disconnecting it causes a change in sound and performance. So that narrows it down to fuel (bad injector?) or compression (bad valve?). The knocking (some kind of internal backfire?) makes me suspect a valve problem. Does that make sense to anyone else? Anyone seen this before?

YELLOW_MR2 · 06-30-2009, 03:51 PM

It could be the rod, well most likely, You might want to fix that quick becuase if your running it like that. It will mess up your rod. I had the same thing on my MKI but the previous owner kept driving it and messed the rod up..

Jackstand Queen · 06-30-2009, 10:44 PM

I recently went through very similar engine problems. Ok, does your engine has a knock sensor, maybe an 88 to 89? That would explain the performance loss, perhaps, the knock sensor could be causing the ECU to seriously retard the timing.

Two things can help you diagnose your issue more precisely. A thin walled metal tube. With it close to your ear, touch various areas of the engine and listen for the loudest return. Be careful of fanbelts. Ouch. The other thing is the frequency of the sound. At 3000 rpms do your hear the tapping, or knocking about 3 times a second? If so, I'd say you have a rod bearing going south. Fix it, before it throws a rod, while its a cheaper fix that is.

My engine is in a million pieces right now. It just needed a rod bearing by the way.

Papo J · 07-01-2009, 12:33 PM

None of the NA MK1s have a knock sensor. Make sure that the ignition timing is right and that the firing order is correct, 1,3,4,2 looking at the distributor cap clockwise from the top right.

tjmr2 · 07-01-2009, 06:44 PM

I'd start by pulling out all the plugs and looking at them. #2 might have some carbon on it that is causing detonation under load. Torque for the plugs is only 13 ft. lbs. Don't over do it, this is an aluminum head.

If all plugs look the same, or #2 is carbon fouled I'd look at the wire, distributor cap and rotor. If the cap and rotor look OK and you still have your old wires try one of them.

Did you change the timing? Timing that is too far advanced will cause detonation in a hurry which will destroy the engine.

Did you change gas octane? Or did someone else fill the tank for you and put in lower octane gas?

06-30-2009, 03:21 PM	#1 (permalink)
rayfast No Skills Join Date: Oct 2007 Posts: 11 Thanks: 0 Thanked 0 Times in 0 Posts iTrader Rating: (0/0% )	Mystery Knock I have a 1986 Toyota MR2 4AGE NA. I replaced the plugs, wires, distributor cap, and rotor button a few weeks ago and, until yesterday, have been enjoying excellent performance and gas mileage. Yesterday afternoon near the end of a 25-mile drive, I heard a knock coming from the engine. It sounded a little like a rod bearing, but it only occurred when I loaded the engine (accelerating or going up hill). The sound disappeared at idle or while coasting. I also noted a corresponding significant loss of power. Attempting to isolate the problem to a cylinder, I disconnected the plug wires from the distributor one at a time. Disconnecting any wire resulted in the car stalling, except when I disconnected #2, which ceased the knocking and brought a slight but perceptible increase in power. I have spark to the #2 wire and, though I haven't checked the plug itself yet, I suspect it's firing because disconnecting it causes a change in sound and performance. So that narrows it down to fuel (bad injector?) or compression (bad valve?). The knocking (some kind of internal backfire?) makes me suspect a valve problem. Does that make sense to anyone else? Anyone seen this before?

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06-30-2009, 03:51 PM	#2 (permalink)
YELLOW_MR2 5SFTE Powered! Join Date: Nov 2008 Location: Washington St Posts: 1,088 Thanks: 94 Thanked 23 Times in 21 Posts iTrader Rating: (4/100% )	It could be the rod, well most likely, You might want to fix that quick becuase if your running it like that. It will mess up your rod. I had the same thing on my MKI but the previous owner kept driving it and messed the rod up..

06-30-2009, 10:44 PM	#3 (permalink)
Jackstand Queen Cage Fighter Join Date: May 2008 Posts: 677 Thanks: 52 Thanked 56 Times in 53 Posts iTrader Rating: (1/100% )	I recently went through very similar engine problems. Ok, does your engine has a knock sensor, maybe an 88 to 89? That would explain the performance loss, perhaps, the knock sensor could be causing the ECU to seriously retard the timing. Two things can help you diagnose your issue more precisely. A thin walled metal tube. With it close to your ear, touch various areas of the engine and listen for the loudest return. Be careful of fanbelts. Ouch. The other thing is the frequency of the sound. At 3000 rpms do your hear the tapping, or knocking about 3 times a second? If so, I'd say you have a rod bearing going south. Fix it, before it throws a rod, while its a cheaper fix that is. My engine is in a million pieces right now. It just needed a rod bearing by the way.

07-01-2009, 12:33 PM	#4 (permalink)
Papo J 6SPD GTE Join Date: Sep 2006 Posts: 189 Thanks: 1 Thanked 19 Times in 18 Posts iTrader Rating: (0/0% )	None of the NA MK1s have a knock sensor. Make sure that the ignition timing is right and that the firing order is correct, 1,3,4,2 looking at the distributor cap clockwise from the top right.

07-01-2009, 06:44 PM	#5 (permalink)
tjmr2 Cage Fighter Join Date: Jan 2008 Location: Colorado Posts: 310 Thanks: 0 Thanked 49 Times in 46 Posts My Google Map iTrader Rating: (0/0% )	I'd start by pulling out all the plugs and looking at them. #2 might have some carbon on it that is causing detonation under load. Torque for the plugs is only 13 ft. lbs. Don't over do it, this is an aluminum head. If all plugs look the same, or #2 is carbon fouled I'd look at the wire, distributor cap and rotor. If the cap and rotor look OK and you still have your old wires try one of them. Did you change the timing? Timing that is too far advanced will cause detonation in a hurry which will destroy the engine. Did you change gas octane? Or did someone else fill the tank for you and put in lower octane gas?