Clattery when cold
Join Date: Feb 2012
Location: United Kingdom
County: Greater London
JTDM Bosch 'injector knock'
Do we believe in diesel injector knock, with the Alfa common rail diesels?
I'm fairly convinced I have it. It happens on idling after a cold start, a tappet-like rattle on #3. As the car warms up it vanishes completely.
The theory seems to be that either the spray pattern is disrupted by deposits inside the nozzle, or the pintel and body acquire deposits or wear that cause minute sticking and over-size squirts, most noticeable when cold and tolerances are tightest.
The car starts, idles and runs perfectly, hot or cold, with no smoke, and will cheerfully blat up to 120mph+ in not long at all. I don't mind the usual clatter when cold, but the slight knock worries me if it's detonation.
At cold on idle #3 and #4 injectors give a loud knocking noise via screwdriver stethoscope, much different to #1 and #2.
There's some history that may be relevant. The fuel filter was definitely neglected for a few years prior to being changed by the previous owner about 5k ago. He said the old one was in an appalling state. Then he had (what turned out to be) a swirl flap detach and fall into the combustion chamber. He continued to use the car for another 4k mls, during which time he had the injectors professionally cleaned to try and cure the lumpy running, poor starting and MPG, all of which turned out to be bent valves/compression loss on #3.
I've read a ton of contradictory guff all over the place, with some Bosch specialists claiming that this is the most common failure mode of this type of Bosch injector. Also that stripping and cleaning buggers them up, as the exact coded calibration is lost. They need re-coding, but nobody seems to do that. That reconditioning never works properly because Bosch won't reveal specs nor sell parts except for nozzles, and the only solutions are used s/h (that may be as bad or worse), or brand new/reconditioned by Bosch - at vast expense, and (of course) that you should always replace them as a set.
I suppose the way to check is to pull the injectors and get them tested, but there seem to be few testers (and even fewer reconditioners) who don't just dunk them in ultrasonic snake oil. I'll do that if I have to, but it looks like an expensive can of worms.
Finally a question then: if it is knock, and it is caused by a sticky pintel or whatever, has anyone had any luck using Archoil 6400CR or BG225 cleaner additives - ie has had knock disappear after using?
[EDIT: forgot to mention, I don't think it is a hydraulic lifter because I cleaned and freed them all off when I replaced the head with a reconditioned one back in March. Also the knock disappears on the overrun if I blip the throttle.]
Last edited by halftone; 30-04-16 at 15:50.
Reason: Add info