Detonation...

[P72Ford]

This afternoon I was coming home from my brother's house, and I noticed that my car was pinging alot. It wasn't occuring under hard acceleration; it started at about .25 throttle, and got worse the harder I stepped on it. I drove it home easy, but I'm wondering what is casuing this. I've never had it happen before. The timing is set right at 12 * BTDC, and I am running 87 octane. Bad gas? Let me know what you think. Thanks guys.


Nathan

[GilroyRacing]

Its probably the junk gas they are selling us at outragous prices!!

[CheeseSteakJim]

IIRC it happens because the combustion is happening at too high a temperature. I think one of the more common causes is bad gas, a lean condition, or too hot a spark plug. I've never had it happen either but I do know that it's very VERY bad to continue driving the car like that. You can melt through a piston in no time. Figure it out before using the car for daily driving again.

[91waggin]

This happened to me awhile back, but mind you I'm running a stock EFI 302. Anyway, IIRC, something was wrong with the EGR and (I think, again this was 6 or 7 years ago) it was letting too much exhaust gas into the a/f mix, which raised the combustion temps too high. Of course, my dad, who was driving it at the time, thought it was hot carbon deposits in the cylinders, which was raising the combustion temps. So he started running all kinds of carbon removers through the gas tank, which clogged the cats. So, while the EGR solved some of the problem, it kept pinging because the cats were clogged.

So, make sure your EGR is working properly, and make sure your cats (if you're running them) are ok.

I could be completely off-base with this stuff, it was awhile back, but I'm pretty sure that's what it ended up being, because my EGR valve and my cats are both shinier and newer looking than the rest of the parts around them.

[CheeseSteakJim]

Now that you mention EGRs 91waggin, P72Ford weren't you doing something or planning to do something involving the EGR? I seem to remember you talking in great detail about it a while back, but I can't remember what it involved.

[P72Ford]

I have EGR on the car, but it is disconnected for the time being... That could be an issue.

Also, my dad's '99 P71 was pinging the other night as well, which makes me think that maybe it is indeed bad gas. At any rate, I won't be driving it until I figure it out.

[Archangel]

Disconnected EGR should not be an issue, when the ECM figures out the EGR valve ain't opening it will retard the timing and fatten up the mixture a bit to avoid detonation, I've had my EGR off for a while now and I have no issues whatsoever even when hauling ass like my life depends on it.

[91waggin]

I have EGR on the car, but it is disconnected for the time being... That could be an issue.

Also, my dad's '99 P71 was pinging the other night as well, which makes me think that maybe it is indeed bad gas. At any rate, I won't be driving it until I figure it out.

You guys aren't both running some similar kind of additive in your gas are you? Did you guys get gas at the same place?

[P72Ford]

The gas is not from the same station, and it isn't even the same brand. We don't use additives.

I definitely think it is the fuel. Today, I noticed that the pinging was intermediate, and starts when the vacuum drops to just above 10" Hg. But, it doesn't happen every time the engine starts to develop a load. Sometimes I can go WOT and there is nothing. Other times, it starts detonating like crazy, and I'm hardly even pressing the pedal. I retarded the timing this morning, after I drove to school, without a light (not sure what its at). It didn't ping on the way home. My plan is to leave the timing where it is, and use up this gas. Then I'm going to fill it with a different brand gas, and move the timing back to 12* BTDC. That should tell me if it is indeed the gas.

[gadget73]

Bad gas happens, especially if you both filled up at the same station around the same time.

[1987cp]

What's your current calibration on the 1406?

[P72Ford]

I haven't changed anything within the carb; so I guess I'm at the default location on the chart. If it was a carb calibration issue, wouldn't it have manifested earlier though?

It is slightly (and I mean slightly) lean under light throttle; I think one step richer on the primary rods should fix that. But, its barely noticeable. I'll get a kit when I get the opportunity.

[1987cp]

Not necessarily; I ran into a nasty popping issue just a month or two ago that hadn't seemed to be a problem for a while. A rod change to position "2" on the calibration grid (think it's running a jet size smaller than stock) solved the pop/stumble issue and gave a nice surge of power that even allowed more aggressive initial timing (I'm at maybe 16 or even 18 initial right now ... can't remember which), even with a slower stepup spring. It responds better because I don't have to dip into the "power mode metering" every time I want to accelerate. And yeah, this is all on 87 octane since my compression is slightly under 9:1 and I'm too cheap to pay for fancy gas.

[Tiggie]

They are probably changing over to the summer blend of gas, which might be contributing to why this hasn't happened before.

[P72Ford]

Well, today I set the timing with a light to 10*BTDC. Then I went down the street and filled the car with Mobil 87. No pinging on the way home. I will see if it comes back tomorrow. If not, I will advance the timing back to 12*, and see what happens. If it pings, I set it back to 10.

I noticed I am getting hideous mileage now as well; so, I'm starting to think that the dog piss in the tank was to blame. We shall see though.

Also, a good point made by Tiggie. I believe they have just started to swticth over. Less enthanol now means lower octane, correct?