High knock - over 120

[r33cruiser]

Hi Brains Trust,

I've searched the forum and the net and couldn't really find a similar issue.

Here are the details:

Semi-stored vehicle which gets driven bi- monthly to quarterly (new house, new child get in the way, you know what its like). I've previously never had any issues, but after blowing out the cobwebs 2 months ago I got a MASSIVE knock reading of 130 on my PFC. I thought it was bad fuel, I took the car out yesterday with fresh plugs & fresh fuel after draining the tank and I had the same problem. When I putt around off boost the max knock is 14 which is strange, but as soon as you give it some boost the engine light flashes and the knock goes through the roof.

Details of engine:

Fully built rb25det with all the fruit (big cams, pistons, rods, gt35r etc etc)

It has had the same tune after the initial run in tune since about 2012 (Matty Spry). The highest knock I ever had previously was maybe 15 with limiter bashing, its been gravy.

I've checked all vacuum lines, all intake gaskets seem to be okay. There is a very slight leak in a gasket in the exhaust after the cat converter. There is a very slight seep on the water pump, its been like that for the last 6 months and hasn't worsened (lose maybe 1ml per drive)- I'll be replacing the whole thing.

Before I drop a bomb and take it to a mechanic, just wondering if anyone else has had the same problem?

[GTSBoy]

Many people will have had the same problem.  Either the knock is real, or it is imaginary.  If imaginary, then it is the knock sensor or the PFC having delusions.  If real, it will be caused by one of a whole range of possible causes and no-one out here will be able to tell you which;

• Faulty/dirty injector causing a lean cylinder,
• Faulty coil/igniter firing early.  Possible, but not real likely.
• Oil blowby coming around in the intake and ruining the octane rating of the fuel.
• Something hot/damaged/sharp in a cylinder causing a detonation initiation point.
• Adding to the list after remember that I forgot to put some stuff on....
  • CAS maladjustment
  • Belt skipped a tooth
  • Something weird with the belt tensioner

Edited March 31, 2019 by GTSBoy

[r33cruiser]

Hi GTSBoy,

 

Cheers for the reply. I'll have a closer look later this week- including your suggestions.

I have a clear timing belt cover, all looked to be okay but I'll get in there and have a closer look.

Nothing on the car has changed for the past 5 or so years. To go from a perfectly working, well-serviced machine to this is a mystery ?.

[GTSBoy]

The most likely on an infrequently used car is the injectors.  But realistically, anything on my list can start out of the blue.

My diagnostic process would be to put it on a dyno.  Check fuel pressure and mixtures & listen for real knock in a nice controlled environment.  Better than spending hours and hours staring at it and fiddling with shit and never getting anywhere.

[Lithium]

5 hours ago, r33cruiser said:

When I putt around off boost the max knock is 14 which is strange, but as soon as you give it some boost the engine light flashes and the knock goes through the roof.

I'm guessing if the highest you'd ever seen before was 15 driving it like an animal, and 14 is strange - it used to be more like "0" just putting around? (I'm not really a PFC man so not sure what is 'normal' here).  Even with dodgy fuel or injectors I wouldn't expect the knock sensor to pick up any rowdiness at light load, it'd be more likely to do a misfire which the knock sensor is not interested in.

I am not saying that it is this, but whenever I see an engine which is noisy even at light load and REAL noisy under load I start wondering about bearings - maybe get someone to give it a gentle rev while you have the bonnet is up and listen out for any cyclic noisiness that you wouldn't normally expect... ie, knock that isn't det.   If it is something like that and you can catch it early then you may save yourself from fixing more than you might if you keep driving it with an unhappy bearing.

Good luck!

Edited April 1, 2019 by Lithium

[r33cruiser]

Thanks Lithium!

Funny thing is it doesn't make any untoward noise's while being driven, performance is still good except when I back off as soon as the PFC shows ping getting too high (no audible pinging). It might not be important, but now that I think of it, the idle has been slightly high floating around 1100-1200rpm. Usually the idle will settle to 950-1000rpm and with the cams, the idle will start to BRAP. When taking off from a stop, it'll bog down like it will stall (very slightly) - perhaps a vacuum leak as well?

Cheers for the head up re misfire and possible bearing issue. I'll check this out, but on idle & start-up there are no tell tale bearing noises. Just some minor head noise from cams and solid conversion.

Edited April 1, 2019 by r33cruiser
more info

[r33cruiser]

No audible signs of bearing issues. Coils look good.

Spark plugs on the other hand- 4 of the 6 had white tips indicating its running lean. The other 2 were brown as per normal.

Will replace fuel filter, check injectors and check fuel pressure.

Not my pic, but they were in similar condition (attaches)

[Guilt-Toy]

Are you in a position to pull the injectors and get them cleaned and flow tested?

[r33cruiser]

Hi Mate,

 

Absolutely, its on my to-do list. I figure if two cylinders are okay but the other four are leaning out more than likely its an injector issue.

 

[r33cruiser]

Wasup

Long time no see. For anyone interested this issue has been resolved - yes its been this long, you know when you keep putting things off? Yeah, this was one of them.

I'll save you all the frustrating details of checking over everything from compression checks, injector inspection to knock sensor checks to know the answer:

- a nipple on the throttlebody  (used for charcoal canister) was open causing a big vac leak. New blocked off hose put on and I'm back in business. Until i did a DIY pressure test, I never would have found this in a blacked out engine bay.

 

 

[mosquitocoils]

9 hours ago, r33cruiser said:

Wasup

Long time no see. For anyone interested this issue has been resolved - yes its been this long, you know when you keep putting things off? Yeah, this was one of them.

I'll save you all the frustrating details of checking over everything from compression checks, injector inspection to knock sensor checks to know the answer:

- a nipple on the throttlebody  (used for charcoal canister) was open causing a big vac leak. New blocked off hose put on and I'm back in business. Until i did a DIY pressure test, I never would have found this in a blacked out engine bay.

 

 

That's amazing and thank you for coming back and posting the end result!