Early 86 iron head Corvette that I've been bringing back from a sorry state over the last year or so after mostly sitting for the last 20 years and being mighty neglected.

The car runs and drives reliably and pretty well now.
I'm almost positive when I picked up an obd1 scanner awhile back I checked for knock retard under hard accel and there was none.
The other day I thought I heard some sustained knock. Plugged in and observed the knock data today and saw it pulling as much as 15 degrees
on a moderate to hard 3rd or 4th gear pull(manual). Timing is set to 6. Ignition system is new, injectors are new.
This motor does have two cylinders that were kinda low on a leak down but not tragically so, plan to rebuild or replace eventually but it's not smoking or using
any oil and maintains good oil pressure and temps.
I have started running 87 in it instead of premium, I plan to run this tank out and try premium again and see what it does.
No other symptoms, showing rich on hard accel, EGR is repaired and working, just about everything that could be considered a wear item is new, all stock other than no cats.

Is some knock retard normal? Could it be this 158K motor just wants premium? I use non-ethanol fuel, or so the pump claims.

 

That's probably why it is knocking.

Yes. False knock at engine start up and shutdown and sometimes on really hard, bone jarring shifts.

Your engine when brand new wanted 91 octane premium fuel. It could run on a minimum 87 octane regular fuel with reduced performance and you still might get some spark knock. No heavy loads and no lugging the engine in too high of a gear with your manual transmission.

You might be able to get away with running 89 octane mid-grade fuel.

If you have half a tank or less fill it up with some 91 octane premium fuel. If you have more than half a tank fill it up with 93 octane if you can get it in your area. This will get you close to 89/90 octane.


Doesn't matter except for long term storage in cars that sit. If you drive the car on a regular basis then running the corn gas is OK just don't go above E10.

 

Hell I assumed when it said 87 than that was sufficient for a mild compression smog motor.
I'm happy to run premium, think it's 92 here. It'd be nice if that's all it is and not something else going wonky.
I've seen manuals for cars that suggested premium with a note that lower octane could be used while giving lesser
performance but I don't think that's said in the one for this car.

Thanks..

 

As long as everything working right with the engine you just have to see what it likes.

 

A related note regarding fuel is that when recovering a vehicle such as this which has been stored/abandoned/lost for long periods it can actually be beneficial to do initial fuel batches of the lowest octane (highest volatility) fuel available. Depending upon ambient temperature and barometer, that might be 100% gasoline or can include various alcohols. Of course, the same volatility that can help clean the system will also likely contribute to preignition.

I've actually used methanol for some very contaminated fuel systems, understanding that the corrosive nature of that can have detrimental effects for more sensitive materials. It's amazing how well methanol can clean up 60 year old fuel deposits in the bottoms of tanks, and also how it can degrade non-metallic and brass parts.

As others have already stated, once the major problems and issues from abandonment/storage have been overcome, using the appropriate fuel will be necessary for using the engine as it was designed. Until then, stay out of the throttle on a hot engine, pulling loads, etc.

 

Have you checked to be sure the knock sensor is connected? Sounds silly and too simple, I know. But I had a mild knocking issue during the 90s after a shop replaced my starter. They broke the wire to the knock sensor, but I didn't discover that for a couple of years. You might have a look, especially if the starter has been changed.

After the starter change, I'd get a mild knock during acceleration, but nothing that hindered normal performance. This went on for a couple of years. Then I bought a home in northern Los Angeles County, where I had to drive over a mountain range. There was one short stretch where trucks would lose about 30 mph, and my engine would knock so badly that I had to back off, turn on my hazards and crawl with the trucks, only a minute or two. Soon, I needed a new starter again, but this time I changed it myself. And when I got under the car, there was my knock sensor wire, broken and hanging. I repaired the wire, and the car has run perfectly ever since.

 

I put half a tank of 91 in yesterday (about as high as is commonly seen at the pump here for some reason) and watching the knock counts and timing it's certainly better. I'll run this tank out and get it back to fully 91.

 

Yep, sensor and plug are new from awhile back, they were broken and falling apart.

 

When I did my TPI conversion, I read-up on the specifics of TPI. I read somewhere that the ECM will periodically bump timing to induce knock to verify that the knock sensor and associated circuitry is working properly. This happens in a second or two. If the ecm senses that the knock circuitry is not working properly, it will set an error code.

Fred

 

Depending upon the model year, the ECM will attempt to add timing under various conditions to try to maximize economy, but as soon a detonation is detected it backs off in larger steps than it increases. In that way, it is constantly trying to see what it can get away with.

My '86 has HFSM enabled, and it can get really advanced at part-throttle cruise and light loads. As soon as the throttle is hammered there are a couple audible knocks and the timing gets dump[ed back to the table, with no adders.

 

The knocktest is done while in PE mode.