Engine knocking under acceleration help please

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crmalcolm

New Member
Posts
5
Hi,

First post, please be gentle - I have had a look around for other threads that seem similar but none are the exact same symptoms to this.

I have a noise from the engine (P38, 4.0SE, Auto) that occurs during acceleration. I have attempted to diagnose as much as possible, and will attempt to describe here:

The noise occurs when accelerating, starts at about 1500rpm and gradually lessens to the point it can't be heard at 2500-3000rpm.

It is worse when cold, and lessens a bit when warm - but never goes away.

If cruising, then the noise can not be heard. If gently accelerating, the rate seems to drop as the gears change so it appears to be in sync with engine speed.

With the window down, from inside the car, it sounds a bit like a blowing exhaust under the car.
With the window up, it sounds a bit harsher from under the passenger footwell.

With the car sat stationary, and engine idling, a faint ticking can be heard. Listening through an extension bar it seems to be from the right-hand bank. From underneath the car, it sounds like it is in the sump.

A week ago, when prepping the car for a long run, the oil was just above the min on the dipstick - this was topped up with 5W/30 Part Synthetic Ford engine oil (as recommended by the store). We did a motorway run of 150miles with no issues, then first noticed the noise after leaving the motorway and pulling away at a roundabout.

Do the symptoms seem immediately obvous to anyone?
Could it be the quality of oil?

Any guidance is greatly appreciated.
 
Thanks for the responses.

Saint.V8 said:
Dropped Liner....???
Click to expand...
Would a dropped liner give any other symptoms?
There is no overheating or loss of power, and would it not be evident throughout the rev range?

kev1nlm said:
I would whip the sump off and check the mains
Click to expand...
With the sump off, would a problem with the mains be obvious or would they need to be removed to see an issue?

odyssey said:
Exhaust manifold gasket leak?
Click to expand...
Saint.V8 said:
Good call, I had a exhaust gasket leak on my 3.9 classic that sounded like a dodgy tappet - a really good ticking sound, drove me made trying to find tha cause....until I was leaning over looking at something at got a face full of exhaust smoke!!!
Click to expand...
If it did turn out to be a gasket leak, could this be felt with the engine running?

As a list of possibles grows, the gasket check would seem to be the quickest to eliminate?
 
wammers said:
Main bearings never knock. They rumble, big ends knock.
Click to expand...

sorry but that's not true, my v8 mains came loose after 15000 miles! started with a small knock 500 miles later a louder knock then 250 after that it went quiet and then 200 after that catastrophic engine failure when the crank ripped out of the block

notice the cracks along the top row of main bolts!

 
Relatively easy to check ex gasket, use a piece of hosepipe or other flexible tube as a stethoscope to get as near as you can to the manifold, take the heatshield off if poss. Another way is to get someone (or 2 people if you have twin tailpipes) to hold their hand over the end of the exhaust, you should hear the blow louder, if indeed that's what it is.

A
 
odyssey said:
Relatively easy to check ex gasket, use a piece of hosepipe or other flexible tube as a stethoscope to get as near as you can to the manifold, take the heatshield off if poss. Another way is to get someone (or 2 people if you have twin tailpipes) to hold their hand over the end of the exhaust, you should hear the blow louder, if indeed that's what it is.

A
Click to expand...

A good way to check for exhaust leaks is pour a bit of brake fluid into the inlet and then run the engine - the fluid will cause mucho smoke which will be clearly visible at any blow-point - don't do this near a main road or in a built up area because brake fluid produces a serious amount of smoke! Enough to completely block the vision of passing drivers. I know - in my idiotic youth I rigged up an electric washer bottle system to squirt brake fluid into the inlet of my car creating my own James Bond like smokescreen generator! Bloody stupid and dangerous! An adaptation of that system triggered by a pressure switch was later used to squirt water into the inlet manifold of my SAAB 900 Turbo to allow me to increase the turbo boost without introducing knock.
 
kev1nlm said:
sorry but that's not true, my v8 mains came loose after 15000 miles! started with a small knock 500 miles later a louder knock then 250 after that it went quiet and then 200 after that catastrophic engine failure when the crank ripped out of the block

notice the cracks along the top row of main bolts!

Click to expand...

I am sorry but it is true main bearing do not knock. They rumble, a cap dropping off is a totally different thing. The fact you were bloody daft enough to drive in that condition until the crank failed says it all. Just to add. If the main came loose oil feed to the two big ends at either side of it would be lost in which case they would knock.
 
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StuckAgainSteve said:
A good way to check for exhaust leaks is pour a bit of brake fluid into the inlet and then run the engine - the fluid will cause mucho smoke which will be clearly visible at any blow-point - don't do this near a main road or in a built up area because brake fluid produces a serious amount of smoke! Enough to completely block the vision of passing drivers. I know - in my idiotic youth I rigged up an electric washer bottle system to squirt brake fluid into the inlet of my car creating my own James Bond like smokescreen generator! Bloody stupid and dangerous! An adaptation of that system triggered by a pressure switch was later used to squirt water into the inlet manifold of my SAAB 900 Turbo to allow me to increase the turbo boost without introducing knock.
Click to expand...
. Not sure that this is the best thing to do for a catalyst engine with O2 sensors. My vote would be on exhaust leak.
 
Might give 'em a good clean! :) The inlet ports, combustion chambers and ringlands of the car I used with my "James Bond" mod were spotless - back in the days when strip-downs for "decokes" were not uncommon!
 
wammers said:
I am sorry but it is true main bearing do not knock. They rumble, a cap dropping off is a totally different thing. The fact you were bloody daft enough to drive in that condition until the crank failed says it all. Just to add. If the main came loose oil feed to the two big ends at either side of it would be lost in which case they would knock.
Click to expand...

lol ok then if you say so. And I was daft enough because we werte on a charity 3000 mile trip around Europe which is completed in 4 days so my options were limited.
 
kev1nlm said:
lol ok then if you say so. And I was daft enough because we werte on a charity 3000 mile trip around Europe which is completed in 4 days so my options were limited.
Click to expand...

Did the charity pay for a new engine? Pistons slap, little ends tap, big ends knock and mainbearings rumble classic diagnostics.:D
 
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Ive been in the job for twenty odd years now and as a rule I would agree but on this occasion it was definitely a knock and a few other v8 owners have said the same the crank falling out was a common problem hence the X bolted engines. Weirdly there was no loss of oil pressure but it was always quite low to start with!
The charity didn't pay for a new engine its all part of the challenge and the engine didn't expire until we hit the M25 so I was pretty happy :)

A couple of pics of "tina"


 
Well, update time.
The noise seemed to lessen over the last couple of weeks, and was almost non-existent once the engine was up to speed.

That was, until today. Started the car up and there was a more tinny sound that resembled stones rattling in a tin can. When stationary, the sound could be heard when the throttle was first pressed, then also appeared for a moment when lifting off. After a bit of trial and error, the sound can now be 'held' if the engine is sat at 1,700rpm.

With this in mind, it was time to get the wife to hold the throttle, while I investigated the source. The sound can be heard from alongside the car, but not from in front with the bonnet up. Inside the engine bay the other engine noises drown out the noise. Underneath though, the sound is clear. Armed with a metal bar, and using this as a stethoscope I traced it to the right-hand downpipe.

It is definitely coming from the front box on the R/H pipe.

Firstly - what is this box?
Secondly - is there likely to be something in this box that could come loose, or could it possible be a part of the O2 sensor that has migrated?
 
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