Engine "thunk" when engine is started

[Fasckira]

Got a weird one that started yesterday morning. When I first got into the car, I turned the engine on and heard a "thunk", not too loud but it sounded like something had shifted into place somewhere in the car. I turned the engine off and turned it back on, but it didn't happen again.
Later that afternoon when I was leaving work, I jumped into the car and the same thing. I hear this thunk just as the engine sparks into life. If turn the engine off and repeat, I dont hear it again.
Now for the 3rd time, its happened again this morning. The engine still runs smoothly, and inspecting the engine everything seems to be connected correctly.

Has anyone got any ideas?

 

[mrcinquecento]

The only thing that immediatley springs to mind as a possibility without really hearing it is the starter motor either having a loose or stretched sercuring bolt or maybe an en engne mount just starting to go in no more.

if its not doing it whilst being driven at all though, it would point to the starter.

 

[Stuart]

yeah the first thing that springs to my mind would be an engine mount or torque mount (the one that stops engine rocking back and forth)

 

[Gregry254]

At the risk of sounding stupid (like most of my helpful posts) you did a weird airbox mod with more tubing right? could it be anything to do with that?

 

[Fasckira]

At the risk of sounding stupid (like most of my helpful posts) you did a weird airbox mod with more tubing right? could it be anything to do with that?

See I though that, but that doesnt explain how it only happens when the engine starts after its been sitting a while. The aircone is currently hanging by a bracket off one of the engine mounts and thought it may have just been the initial movement of the engine was bouncing it off the engine bay but then like I said when the engine has been warmed up you dont hear it on start and you dont hear it rattle about when driving.

I didn't loosen any of the bolts on the engine mount either, literally just got another bolt and put it over the top.

 

[Stuart]

At the risk of sounding stupid (like most of my helpful posts) you did a weird airbox mod with more tubing right? could it be anything to do with that?

Greg I must say.........












































I think you are right, when the car has been sitting a while, the induction kit 'assumes'a natural position, when the car is started there is a vaccum created in the piping which tightens everything up, thus when left a little while with the engine off - not long enough to let it naturally settle.

if that makes sense

 

[mrcinquecento]

Greg I must say.........

I think you are right, when the car has been sitting a while, the induction kit 'assumes'a natural position, when the car is started there is a vaccum created in the piping which tightens everything up, thus when left a little while with the engine off - not long enough to let it naturally settle.

if that makes sense

yup, right enough, i forgot about that mod!

it would also explain why if the engine is turned off and restarted, it doesnt do it again!, as its not had time to "relax" into its natural position with no suction.

iirc, the pipe was flexi tubing, which is far, far more suceptible to changing and assuming its own natural position.

think the problem has been nailed

well remembered Greg!

 

[Stuart]

yup, right enough, i forgot about that mod!

it would also explain why if the engine is turned off and restarted, it doesnt do it again!, as its not had time to "relax" into its natural position with no suction.

iirc, the pipe was flexi tubing, which is far, far more suceptible to changing and assuming its own natural position.

think the problem has been nailed

well remembered Greg!

yeah, i forgot about it too:surprised:

 

[Gregry254]

Oh noooooo withdrawing from a serious drug addiction and I solved a problem I would never of thought of before haha

 

[mrcinquecento]

Oh noooooo withdrawing from a serious drug addiction and I solved a problem I would never of thought of before haha

must be a side-effect

 

[Fasckira]

Thats awesome news, Im glad its something as simple as this!

Ill be changing that cone to a more fixed place soon and the tube will be fixed in suspended spot between the airbox and the cone, so that should go away.

Thanks again guys.

 

[jms700]

i have a similar problem, when i start my car in the morning for the first time it sounds as though something in the engine is sticking. it only happens when i first start the car and i have no modifications to the engine so it cant be that. wonderign if any one can help. thanks

 

[Gregry254]

Dirt/leaves stuck in the air filter?

I remember reading a post somewhere about someone complaining of the same thing, went through a load of hassle then he realised it was just some leaves that got sucked into the filter!

 

[jms700]

great , thanks, will have a look tomorro