Spark knock?

[Russ]

Girlfriend's '98 Mazda 626 (2.5L V6, 110,000 miles) has a slight knocking sound coming from the engine. I cannot seem to narrow it down to any particular area. I have never had to deal with an engine that had a spark knock, piston slap, or even pinging before, so I'm not all that familiar with the nature of these sounds. This is one of those "premium is preferred, but lower grades may be used also" type of engines. She keeps putting regular in it. After I first noticed the sound, I told her to switch to premium. She did, and the sound almost went away. Now she's back to using regular (can't "afford" premium - a whole $3 extra per tank, however, there's $120 per month readily available for cigarettes

). Now, I keep telling her that she'll literally blow up the engine if she keeps doing this. Of course, that is only a bunch of BS, designed to "scare her into doing the right thing". What I was wondering about, is what kind of consequences can actually be expected in this situation (if it is in fact spark knock and she keeps using regular) and what would be the timeline.
Can this even be spark knock? Wouldn't knock sensor detect it and the timing would be retarded? There are no apparent driveability issues (aside from things like less than perfectly smooth idle and such, but the way I see it, that should be expected of an engine of this age and mileage).
Should I perform Seafoam treatment, put FP in the tank, etc, or am I way off and this is a different problem altogether?

 

[Jelly]

quote:

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Originally posted by Russ:
This is one of those "premium is preferred, but lower grades may be used also" type of engines. She keeps putting regular in it. After I first noticed the sound, I told her to switch to premium. She did, and the sound almost went away.

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This one's easy!

There's no punishment if you don't listen to me, but I'd strongly suggest going over to Wal-Mart and picking up some Gumout Regane.

Bet it'll cure the problem (deposit buildup).

Just to add on what I said before...what you're experencing is preignition, where the carbon deposits I mentioned are igniting the lower octane fuel. The noise you're hearing is a "ping", or mild knock that isn't nearly as harmful as a detonation knock...detonation begins after the start of normal combustion, whereas preigntion begins before. Prolonged preignition can produce detonation though, so like I mentioned, go pick up some Regane.

The symptoms you describe don't sound like spark knock (ignition timing problem).

[ October 20, 2004, 02:30 PM: Message edited by: Jelly ]

 

[Matt_S]

Two things. First, make sure she's got good, OE-type plugs in the engine. The owner's manual usually gives a part number. A lot of times, people wind up with hotter plugs than stock (especially with Bosch Platinums), which can cause pre-ignition.

Second, you should do something to get rid of carbon, whether it be the Regane, Techron, Fuel Power (a board favorite), Seafoam or by using water to steam treat the carbon off of there (there's a thread in this section on this). I'd tend toward the water treatment to clean it initially, then switch to a good tank additive to keep it that way.

 

[BlazerLT]

Maybe needs a fuel filter.

 

[mechtech]

If the Mfr. recommends premium, the spark/fuel map in the car's computer is optimized for it. They say regular can be used, because the knock sensor system will pull timing back under loads.
You will lose efficiency this way.
Whether you call it pre-ignition or detonation, the effect is the same - uncontrolled reaction in the combustion chamber. Not good.
The way I figure it, your girlfriend should smoke I less cigarette for every day and a half time period, to compensate for the correct fuel costs. Try explaining it to her that way.