Knocking sound during hard brake


Dadof3Kids
03-23-2005, 12:20 PM
When I have to brake hard on my 1990 Honda Accord LX, I hear a knocking sound (1 knock) coming from somewhere in the engine compartment. It appears to me like it comes from the driver side, behind the pedals. The same sound comes off when I take my foot of the brake and drive off. If I don't have to brake hard, I don't hear the sound. Someone told me that it could be the rear engine mount.

My friend and I jacked the car up, turned the ignition on, put the gear on drive and pressed the gas pedal, spinning the front wheels. When the brake pedal is pressed, the engine moved up then down. Is this normal? Does the bad engine mount theory still hold?

Thanks for your help/suggestions...

mysatilac
03-23-2005, 02:54 PM
That was as dangerous as anything you could do...I wouldn't have tried that if I was using someone elses car or even if I was someone else, you're lucky to be able to type...but that being said

I would also check the brakes on the front driver's side for a loose caliper bolt or something.

Did it give you the same knocking noise when you attempted this?

If not then it reinforces the motor mount theory...

the reason it would knock is the engine slipping off of its mount hitting the top of the mount (because of the inertia of the engine, wanting to keep moving forward), then falling back into place, which is not normal and probably wouldn't have happened while the car was sitting still

the lurching is semi-normal The reason that it lurched as you did this was because you put so much internal force on the engine (slowing it down), you torqued it, the same should happen as you high rev it from idle (even in nuetral). It should shift slightly...

Dadof3Kids
04-11-2005, 07:41 AM
Welll, had the rear engine mount replaced but the thud still comes on during and after a hard brake. Good thing though, the vibration is almost gone.

Dadof3Kids
04-12-2005, 02:17 PM
Replaced the Engine Torque Rod (aka front engine mount) as well which took out the vibration, but the noise still comes on during hard braking. Oh, well....

Igovert500
04-12-2005, 06:20 PM
Did the same thing happen when the car was on the jackstands. I know this is not really helpful, but perhaps it is simply something loose that is sliding back and forth with momentum. I know on my car, I had to remove the battery tie-down...now the battery has a tendency to slide back and forth under heavy braking/acceleration.

Dadof3Kids
04-12-2005, 07:13 PM
I was not the one who tested it on the jackstand. My friend said the noise was there but not as loud. The sound comes from under the pedals, behind the firewall.

phantastic1
04-13-2005, 12:17 AM
check your steering and suspension in that area.

-strut mount
-ball joints
-bushings
-improper seating of brake pads

just put the car back on jack stands...tug on the front tires to see if there's any play in the upper control arm, knuckle, and tie rod ball joints. if those are ok...pop off the front tires and check for the rest.

Dadof3Kids
04-14-2005, 11:42 AM
A mechanic hoisted the car and after checking the brakes, etc, dicovered a missing front sub-frame bolt. He thinks this might be causing the noise since the engine is not properly mounted. He said that probably the last guy who did a timing belt replacement forgot to put the bolt back. Does one need to remove the sub-frame bolt (there's 2 on each side) when doing a timing belt replacement?

jeffcoslacker
04-14-2005, 10:02 PM
A mechanic hoisted the car and after checking the brakes, etc, dicovered a missing front sub-frame bolt. He thinks this might be causing the noise since the engine is not properly mounted. He said that probably the last guy who did a timing belt replacement forgot to put the bolt back. Does one need to remove the sub-frame bolt (there's 2 on each side) when doing a timing belt replacement?

I'd think it would give you easier access to some parts, yes. I used to do the older Ford Escorts like that.

jeffcoslacker
04-15-2005, 04:50 AM
Man I let you down. I'm glad you found it. From my first read of your post, something was tugging at the back of my memory, but I just couldn't bring it up.

After sleeping on it, I remember running across that same syptom on the early Ford Taurus/Sable/Continental cars. They had a problem with engine cradle (subframe) rear mount bolts breaking, and that was sometimes the first symptom. I ran across that by accident before the problem was well-known, but it's been so many years, it kinda slipped my mind. I was a state vehicle safety and emissions inspector from '88-'00, and I remember having a Lincoln on the lift doing an inspection and noticing a gap between the body and the cradle mount on one side. Bolt sheared off.

I'm glad you found it, if the other side broke from strain, it could have caused a very bad situation. On the cars I mentioned when the opposite bolt finally broke from the strain, the entire cradle would drop down, hanging from the front, and would try to push sideways with steering input, making the car uncontrollable. It felt as though the steering was connected with rubber bands. Very scary. Fortunately, most would break suddenly at low speed, like going over railroad tracks or speed bumps.

ProMan
04-15-2005, 08:34 PM
Did you find the cause of the noise? Is the missing bolt the problem? It doesn't feel right that a missing mounting bolt will cause knocking sound during hard brake.

It sounds to me that non of the post pinpoint the real problem. If the symptom is still there, let me know. I may have some suggestions.

jeffcoslacker
04-16-2005, 03:05 AM
Did you find the cause of the noise? Is the missing bolt the problem? It doesn't feel right that a missing mounting bolt will cause knocking sound during hard brake.

It sounds to me that non of the post pinpoint the real problem. If the symptom is still there, let me know. I may have some suggestions.

He's feeling the subframe knocking against the unibody when it is torqued suddenly, like hard braking or acceleration. There is considerable flex in those cradles when a mount is compromised. Any road surface imperfections will make it chatter against the body under strain. I bet if he'd run it over some railroad tracks or something, he'd feel it then too.

scubawrstlr
04-17-2005, 02:57 PM
If the subframe isn't the problem, then I would check the cv axle. Check to see if the boots are torn, if one is torn then you need to replace that side of the driveshaft (it is too late to simply replace the boot)

Dadof3Kids
04-19-2005, 01:05 PM
Okay, thanks for the pointers. I will post the results here. I still have to receive the bolt that I ordered thru the Internet. Looks like they have difficulty sourcing it. I may have to go to a junkyard for the replacement bolt.

Hondaman56
04-19-2005, 01:48 PM
That would be the ABS system working. Has nothing to do with engine mounts.

mysatilac
04-19-2005, 02:21 PM
In that case he shouldn't hear it when he accelerates again,

But yeah ABS does make a knocking sound, which is the pads being lifted off of the rotors for split seconds to release the gasses that are built up with the extreme friction,
Could have been a combination of the two
In my experience ABS only really engages during really hard braking, but it very well could be the ABS, In which case when you replace that bolt, which you still should, the noise will continue...

jeffcoslacker
04-19-2005, 05:55 PM
In that case he shouldn't hear it when he accelerates again,

But yeah ABS does make a knocking sound, which is the pads being lifted off of the rotors for split seconds to release the gasses that are built up with the extreme friction,
Could have been a combination of the two
In my experience ABS only really engages during really hard braking, but it very well could be the ABS, In which case when you replace that bolt, which you still should, the noise will continue...

ABS operation feels more like a muted rattle that outright heavy knocking in the floorboard.

The squeeze and release is to prevent wheel lock, you have to really heat a set of brakes for some distance to get them to outgas and fade.

Dadof3Kids
04-19-2005, 06:23 PM
This car has great features, but it does not have ABS brakes. The weren't available on the 1990 Accord.

Dadof3Kids
05-01-2005, 08:06 PM
Finally got the bolt. Knocking sound has not been heard since installation. Thanks to everyone who shared their ideas.

scubawrstlr
05-02-2005, 12:14 AM
Glad to hear that your problems were solved and all is well. Good luck for the miles a head.

ngc604
05-06-2005, 08:02 PM
does the steering wheel shimmy back and forth when you brake hard. my 93 use to do the same thing yours is doing but after a while the knocking and shimming happens when i do any braking. so i got mine sitting till i have money to get it figured out and fixed

jeffcoslacker
05-07-2005, 07:58 PM
does the steering wheel shimmy back and forth when you brake hard. my 93 use to do the same thing yours is doing but after a while the knocking and shimming happens when i do any braking. so i got mine sitting till i have money to get it figured out and fixed

Yours' sounds like a different problem, or a combination of this and something else. Usually when you get the shimmy, and it increases in intensity the longer you drive it, you have a disc brake not fully releasing, and it heats the rotor more and more from constant friction the farther you go.

Most common causes are a bad caliper or slides that are corroded enough that the caliper cannot "float". Less common is a degenerated brake hose that is holding fluid pressure internally. The way I determine the difference is, after driving it and getting the brake heated, open the bleeder screw. If a fairly strong shot of fluid arcs out, a hydraulic restriction is happening. Replace the hose. If you get a lazy trickle or none, the problem is failure to release due to a mechanical issue, bad caliper or slides. In general, if this is suspected, you can get some idea of what is happening by looking at the pads. If the inside (piston side) pad is worn more, the caliper is the problem. If the outside pad is worse, the slides are the problem. If the pads are worn more on one end than the other (beveled), the caliper support hardware is bad and allowing the caliper to cock at an odd angle, not true to the rotor. This will cause the caliper to knock also. Occaisionally, the rotor is cracked due to defect or fatigue, this too will cause the shimmy, and possible noises.

Sometimes with a very warped rotor, and a caliper that is being violently shaken by it, there will be a knocking sound associated with it. One thing's for sure, you need to replace the rotors after finding the cause of the heat. If you just have them machined, they will warp again right away. Once seriously overheated, the metal's integrity has been compromised, they will never work right.

If any of this is occuring with a missing subframe bolt, it is going to be much more violent when it occurs. But I have felt what that feels like, you can actually feel the frame hitting the body right under your feet, it is unmistakeable. Feels like someone tapping a hammer on the floor under the car.

Dadof3Kids
05-08-2005, 11:58 AM
No, there was no shimmying at all. Good luck.

Add your comment to this topic!