Mazda 5 Diesel knocking after injector replacement

[Luke Hall]

Hi,

I'm a bit confused... My local garage has just replaced No.1 diesel injector after the engine entered a permanent state of "limp home mode". Their diagnosis was that of a "low to high short in injector No.1".

Following the replacement of the said injector, they charged me nearly £500 and gave the vehicle back. The engine is running and pulling as it used to but there is a very noticeable loud knocking/rattling sound now which wasn't there before. It's most noticeable under acceleration and at mid revs. Prior to the original problem, the engine ran very smoothly with now noticeable knocking or the usual diesel clattering.

The knocking/clattering sound is very definitely just one cylinder and not all of them, and is apparent from cold to fully warmed up. No faults are being generated or stored and the garage bloke who did the work appears to be completely stumped. A quick Google of the symptoms suggests that it is possibly the result of "injector knock", but why this would start happening with a new injector is a mystery to me.

Could this be caused if a reconditioned injector was used instead of a new one? I have my suspicions that that may be what they fitted.

Any knowledge or sage advice would be very much appreciated!

Best wishes,

Luke

[GreenV8S]

The injector itself can be quite loud but if you're hearing it while you're driving that suggests you're getting engine knock.

A leaky injector is one possible cause.

A worn piston can produce a similar noise, but that's not likely to affect a single cylinder and not likely to happen just as you change an injector.

I haven't tried any diesel injector cleaners but if I had that problem I'd give it a go.

[mickthemechanic]

Hi

When you say the garage are stumped have they done any tests to find the source of the knock? Also its possible but unlikely that the new injector is faulty or that some dirt has found its way in to the new or one of the other injectors during repair. Blank them off one at a time starting with new one and see if the knock stops. Is there any smoke?

[Luke Hall]

Thanks very much for your replies.
When I say they're stumped, they said they had no idea what was causing it. All he did at the time was plug it in to the diagnostics computer again, which showed no faults. I can't see any smoke but the honest answer is that I hadn't checked properly yet. He did try unplugging the new injector but to be honest, the engine runs so roughly with one unplugged, it's difficult to determine what you're hearing.

I was wandering if perhaps the ECU needs some additional tweaking after fitting a new injector? My experience with this Mazda engine in the last few years suggests to me that it works in strange and mysterious ways...!

[stevieturbo]

It would be common for most cars, that the injector does need coded into the ecu.


And it seems it's also common, that places selling recon etc injectors....often do not sell what they're claiming to sell, which caused me severe agro a few months ago when I changed my set for "proper" Denso injectors...which were anything but. Denso sticker on the box was about the only Denso part !!

[Wakojacko]

Hi there I have the same knocking noise I am going to change all 4 injectors reconditioned ones tho did you sort the problem out

[Escy]

Luke, did this sort itself out with a bit of driving or did you need to take action?

[Luke Hall]

Escy said:

Luke, did this sort itself out with a bit of driving or did you need to take action?

I've long since not had the vehicle but I can tell you that the problem was caused by the garage not coding the the ECU with the new injector's details. Therefore, the knocking heard was classic pre-ignition caused by faulty injector timing. Shortly after my post, the injector actually blasted its way out of the cylinder head because the garage had damaged the thread for the retaining bolt. By this time I'd had enough... Got it repaired by a main dealer and sold it!

[Escy]

Thanks for the follow up.