ENR34 - Low-speed detonation/pre-ignition

[AdiR34]

Hey y'all!

In the latest of my un-luck saga, I had been hearing some metal-sliding/rattling noises at lower RPM under engine load/acceleration. Originally thought it might be transmission related since that was where I'd usually shift (I live in a slow neighborhood so lots of lower-rev driving). But, upon having a mechanic hear it, it turns out that I have a detonation issue. Before I got a timing belt service, this was happening in 1500 up to 3K RPM, but post timing belt replacement it's been down to about 1700-2100. I've been doing my damnedest to keep her above 2100 for most driving - little bit more jerky but I'd rather not lose an engine over this.

So far: 

- replaced timing belt, timing retarded slightly as per mechanic who did the work... problem got smaller but not solved (as an aside, getting lots of belt whine and even squeal when I start her up in the mornings)

- Been only using top-tier fuel available locally (91 is premium for my state in USA, since it's high altitude we get f*ed on gas quality. Been using Shell... but does have the "contains up to 10% ethanol" sticker, unsure if that's gonna be an issue and if I should seek out no-ethanol gas)

- Plugs are BRAND NEW via the seller... maybe they did a bad install?

RB25DE, 5MT. Glad to be able to actively control engine speed for an issue like this.

Other than a bad MAF, what else could it be? Could the spark job have been a bust? Compression test came back 180 on all cylinders except one at 160... something questionable there?

[AdiR34]

Sorry - RB25DE Neo, as y'all likely already know.

[AdiR34]

Also - running 5W-40, was recommended by seller. 

[GTSBoy]

Does it do it at light throttle as well as high throttle opening? Only large throttle opening?

If the answer is "No, it doesn't do it at light throttle opening", then (part of) the answer is as it has always been. Don't stamp on the throttle at low revs. Open is with some discretion. Roll into it.

Now, because it's not a turbo, it shouldn't demand the highest octane fuel....but even 91 MON (equivalent to ~95 RON in the metric world) is only so-so for NA Jap engines.

The 10% ethanol won't make any difference forwards or backwards. 10% is just filler. OK, actually, it is a little bit bad. Ethanol adds some (effectively) free octane. It allows the blend to reach the 91 octane rating without the petrol it is mixed with being that high octane to start with. But ethanol needs less air to burn than petrol does. So when a blend has ethanol in it, the fuel-air mix will always be a bit lean (except when running in closed loop cruise where the O2 sensor will add fuel to bring the stoichiometry back to target). When yo stamp on the throttle, you are not in closed loop and so a lean condition may well exist. Lean can (and will) cause pinging.

The low compression cylinder may be a concern. If the rings (and in particular the oil control ring) are poor, and it is ingesting oil into that cylinder, that can cause it to ping. Oil reduces the effective octane rating of the fuel that is squirted into that cylinder, and rattle rattle rattle is the result.

It's unlikely to be a bad MAF.

You should make sure that the harmonic balancer is not in the process of failing. The outer hub can slip around relative to the inner. This makes the timing marks on it wrong. If you set the timing to a target, as read with a timing light from those marks, then the timing can be very wrong. You need to take out #1 spark plug and use a screwdriver or similar rod as an indicator and turn the engine over (which will be easier with all the plugs out) by hand so that you can see the screwdriver rise up, dwell around TDC and start falling again. It is difficult for the uninitiated to accurately find TDC that way, but it will do in a pinch. When you are pretty sure you know where TDC is, you need to check the timing marks. If it is not real bloody close to TDC, then you have to question the health of the balancer.

Another thing that can go wrong is carbon build up on piston crowns or on the combustion chamber face of the head. These can glow hot and cause pre-ignition, and will tend to do it on light throttle as well as heavy.

[joshuaho96]

The "one cylinder is 160 while others is 180" part of the compression test sounds bad. That's 12% lower than the others. I would bet something has happened there.