Predetonation Q

[danzo]

About two weeks ago my car developed a weird condition that I can't seem to trace. The engine knocks 'n' pings when warm and under light load. It also idles too low after it's warm. If I accelerate at more than 1/8 throttle or I'm going down the highway it won't predetonate. Initially I thought it was a vacuum leak but haven't found any. The distributor advance unit seems to be OK (does hold vacuum when suction is applied) and the ignition timing is set at exactly 10* as per the FSM. Running the car on 93 octane makes no difference.

Is there some vacuum actuated valve somewhere that helps determine ignition timing? I can't find anything in the FSM. Since I finally found my timing light I will retard the timing tomorrow just to see if the knock goes away.

[tercelwill]

the FSM says the timing should be set at 5 degrees BTDC with the vacuum advance disabled and 13 degrees when enabled.

[blade]

Sorry for the dum question but what is vacuum advance and what dose it do???

Calvin

[takza]

With the OEM setup...the terc is pretty sensitive to vac advance...the 10* is probably the issue.

[danzo]

My bad - I don't know why I thought the ignition timing was supposed to be at 10*. My timing was at about 14* with everything hooked up, i.e. just slapping a timing light on a warm engine. I retarded it to 10*, drove it and it still knocked fairly significantly. So I set it to 5* with everything hooked up and it has an almost imperceptible ping under light load. For now I'm going to leave it at this setting and mix in some 93 octane, though I don't know why my car is acting like this - I just don't want it to ping and potentially cause damage.

Calvin, the distributor's vacuum advance unit is the 'biscuit' shaped metal thing on the bottom and firewall side of the distributor. It advances the ignition timing based on engine load via the amount of vacuum in the intake. When your engine is under high load (hard acceleration or close to top speed) the vacuum advance will make the distributor spark the plugs earlier than usual, sometimes as much as about 40* before top dead center (BTDC).

[blade]

Oh ok thanks sorry for the dum question

[Petros]

I run 10 deg advance (vac line off) on both my Tercels, and have no ping. If the EGR is malfunctioning it can cause ping, or you could be running too lean for some reason. It should not ping under light loads with 10 deg advance on regular gas.

If the ping just started recently I would look for something that changed, broke, wore out, etc. could be a vac leak, even an internal one from one of the vac operated componets. I would keep looking.

The '85 I drove to your place, and back to WA I set at 10 deg adv. and it never pinged, even on regular gasoline.

Good luck.

[takza]

My bad - I don't know why I thought the ignition timing was supposed to be at 10*. My timing was at about 14* with everything hooked up, i.e. just slapping a timing light on a warm engine. I retarded it to 10*, drove it and it still knocked fairly significantly. So I set it to 5* with everything hooked up and it has an almost imperceptible ping under light load. For now I'm going to leave it at this setting and mix in some 93 octane, though I don't know why my car is acting like this - I just don't want it to ping and potentially cause damage.

A good description as to how my car runs with the vac advance hooked up (leaks a little). Now run with no vac advance and probavbly have it at 10-12* BTDC.

[sdoan]

The engine knocks 'n' pings when warm and under light load. It also idles too low after it's warm. If I accelerate at more than 1/8 throttle or I'm going down the highway it won't predetonate.

Danzo,
This is exactly what my '83 did for a couple years. It was accompanied by poorer and poorer gas mileage. Finally the #3 exhaust valve burned. When I replaced the engine I found that one of the two vacuum advance diaphragms was leaking. I know you've checked but it might be worth another check of the vacuum diaphragms.

BTW I was able to drive about 350 miles home on 3 cylinders.

Let us know what you find.

[danzo]

sdoan how did you know your diaphrams were leaking? When I hooked up my vacuum pump only one of the three ports (the uppermost one by itself) held vacuum. I think this is this normal since my spare dizzy does the same. I don't think my car is running lean as Petros suggested (where are the trip pics BTW?) since I'm getting slightly less than 30mpg.

I did drive to Houston and back (550 mile trip) about a year ago when my car was running on three cylinders due to an f'ed carb. I'm a little worried now that I'm also hearing a slight knocking from the motor. It sounds like a high-pitched rod knock but at the tempo of valve knock, and it's very faint. Maybe I'll do a compression check soon. Thanks for any help.

[sdoan]

Danzo,

I checked the diaphragms the old fashioned, unsanitary way - I connected a long piece of vacuum hose to the vacuum can, stuck the other end in my mouth and sucked on it.

You can generate enough suction to pull the diaphragm and then plug the end of the hose with your tongue. If it leaks you either will not be able to generate suction or it will disappear in a few seconds. On my distributors one vacuum can leaked and the other didn't. On most of the distributors one vac can has two vacuum ports and the other can, one. You, of course, have to plug the second port on the can that has two ports or you won't be able to generate any vacuum. Provided you did that, your description makes me think you have a leaking lower diaphragm.

BTW - the sparkplug color was nearly white while the car was pinging at mid-throttle.

Here's some pics - the first shows pulling the old engine and the second shows the next trip at a stop at Crater Lake on the way down to Mount Lassen.

[4wdchico]

Have you checked for a vac leak at the EGR pintle? Way more common with high mileage on an EGR valve for the engine to pull air that way than have a leaky vac diaphragm.

[sdoan]

Have you checked for a vac leak at the EGR pintle? Way more common with high mileage on an EGR valve for the engine to pull air that way than have a leaky vac diaphragm.

By EGR pintle do you mean where the actuating rod goes through the vacuum diaphragm inside the EGR valve?

[4wdchico]

Have you checked for a vac leak at the EGR pintle? Way more common with high mileage on an EGR valve for the engine to pull air that way than have a leaky vac diaphragm.

By EGR pintle do you mean where the actuating rod goes through the vacuum diaphragm inside the EGR valve?

You are on the right track with your thinking: The pintle shaft on an EGR valve is very similar to the stem on a valve. When, with high mileage on the EGR valve, an excessive clearance develops between the pintle shaft and the EGR body you get a direct leak from atmospheric air into the intake manifold. You can easily check this, on an EGR that has already been proven not to have a diaphragm leak, by applying raw propane to the holes in the bottom of the EGR diaphragm housing, if the Idle changes you have a leak there.

This pintle shaft vac leak is a virtual certainty with long enough use on an EGR valve. In contrast, a leaky diaphragm in an EGR valve may never happen no matter how many miles you put on it

[danzo]

Sorry for the delay - I think I do have an EGR leak since the idle changes when carb cleaner is sprayed on it. Not sure how to get Propane down there since you can't hold a Propane bottle upside down, right?

Anyway I was wondering if I could override the EGR and basically keep everything else stock? Would the car run OK? This may not be possible, but new EGRs are fuh-reakin' $185 and I'm not sure how fruitful it would be to get a j-yard one esp since I would most likely need to get it from a different year/model. Maybe someone on the forum has a good one they don't need? Thanks for any advice/help.