timing belt?

With that many miles change the belt, I dought anybody here recomended high octane when your issue is simply timing. But with a lack of power you really need some work done to it, if compression is good you should never get less than 20-24

Was the mileage drop gradual?

Timing belt, cam, crank, oil pump seals, water pump and drive belts should be changed every 60K miles (although a few might strech the pulleys and pump to 120K miles, I wouldn't). Timing belt stretches some and retards spark timing and leads to poor performance, but not sure about causing the engine to knock. Does it knock at highway speeds or low speeds only?

There could be other reasons for the drop in MPG. Have the spark plugs been replaced at all? I'd use NGK's Iridium IX BCPR5EIX-11 or at least the cheaper NGK G-Power Platinum BCPR5EGP. Are the plug wires in good shape? Is the oxygen sensor still good? Is the EGR system working properly? Was the fuel filter ever changed? Is there a vacuum leak (check PCV grommet and valve cover gasket)? Is the engine using oil?

How's the temperature gauge doing? Is it flucturating as the load increases/decreases? I hope it's not the head gasket.

J.Lef wrote:

The knocking or pre-ignition noise and the drop in mileage if they are connected, could be due to too far advanced ignition which has moved outside the engine management's ability to correct. Why that has happened could include belt stretching. On the otherhand, engines which have a build-up of carbon in their combustion chambers, can suffer "engine knock". In that case the engine would be consuming oil.

Jason

...

Yes. Over the space of about two years the mileage started dropping, and acelleration diminished, even though it is still peppy, not what it used to be. I had a complete tuneup on it, and change my filters and oil on a timely basis. The car usually does not need any oil, between changes(every four thousand miles), sometimes maybee I have to add one. The knocking was only occurring on brisk acellerations. If I got off the pedal a little and took it easy, the knocking would stop. It would happen going away from a stoplight if I needed to move, or getting on a freeway, or accellerating to the passing lane. My son drives the car moderately, so it is not abused. Upon switching to premium, 95 pecent of the knocking stops, so thats what we have been using for the past two years. The temp gauge, stays rock solid where it always has been, and barely moves once up to temp. This is a great running car, and my son was thinking about throwing a new set of tires on for another 30,000 miles of driving, but is unsure now, because of the gas mpg problem. It hardly useds any prestone either. What do you think I should ask our mechanic to do?

Thanks to all that have responded

Sounds like there may be a combination of two different things here.

The reduced performance and mileage is likley because of the aged, streched timing belt retarding timing. Be prepared to change the oil seals, water pump, pulleys at the same time at increased cost. I'd prefer Gates Timing Kit and drive belts

and would stay awayfrom the Bando belts they now sell at the dealer. Do use Toyota coolantand only distilled water if needed. My two cents on the knocking part may be (1) mixture lean: false air entry, low fuel pressure (actaully lots of causes of lean mixture); (2) insufficient EGR flow: manifests under moderate acceleration but should not pose as much of a problem moving from standing still as you experienced.

The throttle body ports controlling the EGR system can be partially blocked. I helped one guy clear it up after he cleaned the throttle body and the ports got plugged up and then the engine knocked on acceleration. Also, replacing the fuel filter can help rid the start-up knocking in a 5S-FE. The lower flare nut can be a bear to remove and the filter is supposed to be lifetime. Was your fuel filter ever replaced? Iridium plugs that can burn leaner mixtures can also help. But I am not sure if yours is the same problem, maybe others can chime in.

It's probably not interal oil leaks (piston rings or valve stems) because you are not using oil. Is there blue smoke on cold starts? Oil reduces the octane count and contributes to detonation.

J.Lef wrote:

A good mech knows what to do, check everything. But knocking nornaly is the timing is to advanced or pre-ignition, the fuel is igniting well before the piston reaches top center, although to far advanced you loose power. the oposite is a stretched belt, it retards timing and reduces power economy. It could be set to advanced, you can buy a 15$ timing light and see yourself, there are 2 wires to jump T & E1 on the terminal block, if you want to do it yourself its a easy 5 minute job. But lack of power-milage, look at a blocked Cat converter, bad o2 sensor. draging brakes or bearings. Have you ever checked or changed the differential oil, it is seperate from the trans and can only be done from a lift with special wrench. There is also an engine knock sensor, but first try a timing light, fix the knock it might be your whole issue. One idea go see if the distributor was left loose, go and try to turn it, a mechanic might have forgot, this could be your whole problem.

He said high octane stopped the knock, so its pre ignition, not a lean mix, its timing related.

Octane is the anti-knock index. Higher octane gas can better resist detonation. But an engine that calls for regular gas should be running on regular gas. High octane gas is just a temporary fix. Lean mixture, as well as advanced timing, can lead to increased combustion temperatures and detonation.

Preignition is caused by excessively hot cylinder components, a condition unlike detonation, exists typically in only a small number of cylinders. Hot valves, carbon particles, or even a spark plug not able to properly dissipate heat can be the causes.

m Ransley wrote:

jongdole I agree with you in your paragraph one but not that preignition is caused by excessivly hot components, it can an does happen with cold motors running crap gas like I used to get in Europe when we brought over in 67 a 67 390 Ford with relativly high compression, It was designed for high octane. Most gas just could not handle the 390s compression, pouring in a gallon of alcohol per tank was our only cure for many years, alcohol raises the flash point. Preignition is in simple terms gas igniting by itself prematurely, an excessively hot condition does not need to exist with poor crap gas. But im sure the OP has good gas so it is possible he is hot but more likely his timing is just way off.

OP said the car is an "old lady" car. I'll bet there's plenty of carbon built up on the pistons and everywhere else. They need to try some of that top cylinder cleaner GM sells (I assume they still sell that stuff) and to clean the EGR and passages (as was already suggested).

Or they could try an Italian tune up - take it out onto the highway and floor it. Repeat until exhaust is not thick black smoke. (I had to do this with my mother's 1996 Buick LeSabre. Took about 3 runs at high rpm. Car ran much better afterwards.)

We are getting off the original subject. But I'll just point to the following sites on the similarities and differences of detonation and preignition (with nice pictures) without discussing further:

PREIGNITI> jongdole I agree with you in your paragraph one but not that preignition