Knocking while maintaining speed

mazda
protege

#1

Hi,



I’ve got a 99 Protege with approximately 138k on it with the 1.6L. As of late, I’ve noticed that at just about any speed, if I give it just enough gas to maintain speed, the car develops a slight knock.



At idle or when accelerating there is no discernible problem. I’m not sure, but it also seems like I can feel the knock through the car. Maybe it’s psychological but when I get a valve rattle you can just hear it, this I’d swear I can feel it too.



Just had the car in for service, they checked plugs and it all looked good. Yes unfortunately I use cheap gas.



Before I take it back in I was hoping to have a couple things they could check instead of a generic noise complaint.



Thanks.


#2

As long as you’re using 87 octane gas the engine should be fine, since that’s what it’s designed to run on. It shouldn’t require higher octane.

Having said that, try a tank or two of “mid-grade” and see if it makes any difference.


#3

Many people say that they hear an engine “knock” when the correct term might be “ping”. However, since we cannot hear or examine your car, nobody knows for sure.

In any event, if the engine is “pinging” when you give it more gas, there is a strong possibility that your EGR valve is gummed up and is sticking. Have your mechanic check the EGR for proper operation.

Another possibility is that the engine is running too hot.
I would suggest that you first check the level of the coolant in the radiator (when it is totally cooled down). If it is low, replenish it with the correct type of coolant and water (50/50 mix). If the coolant level is normal, then try to assess whether the cooling fan(s) are operating normally. If this is a situation of a too-hot engine, a bad thermostat could also be the source of the problem.

If your spark plugs are in good condition, and if you are using gasoline of the correct octane, then a stuck EGR valve and/or a hot-running engine are the two main possibilities for pinging.


#4

[quote]Many people say that they hear an engine “knock” when the correct term might be “ping”. However, since we cannot hear or examine your car, nobody knows for sure. [/quote)

Well, strictly speaking sound wise, a ping would have more of a high end component to it vs. a knock. In car terms, is there a discernible difference?

I’ll check coolant levels, thanks. Gas is the correct octane.


#5

Are you saying you’re hearing a bearing knock? That’s a whole other kettle of fish.

“Pinging” is in the upper end, as you say. It occurs in the combustion chamber, and sort of sounds like a rattle.

A deeper knocking, lower in the engine, is more likely to be bearings, and you don’t want to hear that.

So, what are you hearing?


#6

Yeah, it’s a knock. I hesitated using the word thump because to me that gave an indication of greater force being felt through the frame.

I can’t get this to happen except when I’m in drive. I have yet to reproduce it in park or neutral and I’m not going to attempt it in reverse… :smiley:


#7

Well, it does sound like pinging (aka spark knock) is not the issue here, so you can probably rule out EGR/engine heat problems as the source of the noise.

As Mcparadise stated, this noise could well be from bearings–either main bearings or perhaps the pilot bearing. Unfortunately, repairing these issues could exceed the book value of the car. I think that you really need to have a skilled mechanic listen to it to determine exactly what is going on.

If it turns out to be worn bearings, I would suggest that you just keep driving it until the engine gives out.


#8

If you want a cheap solution, that might work, try Marvel Mystery Oil. Stuff is great with knocks and valve train noise. It wont fix anything, but it could buy you some time.


#9

The circumstances that you described which cause the knock are the text book indicators of a ROD KNOCK… The only other knock that is sensitive to “coasting” is slack at the flywheel. Broken flex plates, loose torque converter bolts, etc.


#10

Worn halfshaft maybe and one can feel a halfshaft knock through the car.


#11

Try a tank of mid-grade gas. If it reduces then it’s spark knock.
Spark knock can also be caused by carbon deposits in the combustion chambers, possibly caused by a lot of low speed, gentle driving


#12

The circumstances that you described which cause the knock are the text book indicators of a ROD KNOCK… The only other knock that is sensitive to “coasting” is slack at the flywheel. Broken flex plates, loose torque converter bolts, etc.

I might not have been clear so let me clarify based on the middle part of your response. This isn’t happening while coasting. If I’m on a level surface and accelerate, I don’t hear it. If I give it just enough gas to maintain my current speed, that’s when I hear it. If I let off the gas, the noise goes away.


#13

As previously suggested, try mid or even higher octane gas (91 octane). If it reduces your problem, then your problem is spark knock, or pre-ignition or detonation, where the flame front is occurring prior to where it is supposed to during the combustion process. This can be caused by carbon build up, bad egr, or more likely, a timing issue, such as a bad knock sensor. Perhaps the knock sensor wire/plug fell off? Or maybe it quit working. I’d look in that area…if…your problem is spark knock, and not rod knock.


#14

If, as you drive along, you begin to slowly allow the accelerator to rise and when the pedal is at the position where the engine is no longer adding to nor subtracting from the movement of the car then you would be coasting. And as I said, it is a text book case for a “Rod Knox.” Of course, there are other possible causes. But pinging,detonation,valve rattling, pre-ignition are unlikely when “coasting.” All those terms refer to the same problem and hot acceleration is the text book case for it.


#15

Sorry, my definition of coasting doesn’t involve giving any gas. That’s why I clarified.


#16

Okay so the verdict is in. We were all wrong. lol

The issue was both the front axles were shot. The boots were either gone or torn, all the grease in the left axle was gone, most of it in the right and they were hammering around.

New axles, all is just fine.


#17

Maybe it’s psychological but when I get a valve rattle you can just hear it, this I’d swear I can feel it too.

This is the clue none of us caught.


#18

Ahem We were not all wrong. :slight_smile:


#19

Kudos and a tip of the hat. I did notice that you were the only one that got this one.