My LB9/700R4 T/A is bone factory stock, and otherwise in good repair. Recently I’ve been noticing a bit of engine knock on moderate to heavy accelerations, either from a full stop or like when merging onto a freeway. I hear it briefly at the beginning of the acceleration then it stops. I also sometimes hear it briefly again right after the upshifts. It’s a little worse going up grades. I always use 93 octane fuel, and I use quality brands (Sunoco, Mobil) from different locations, so it’s probably not a batch of bad gas. Also, the outside air temps are currently in the 50s, so it’s not hot. It was not doing this over the summer. I’m scratching my head. Other than this the engine performs as expected and seems healthy. Any ideas what might be causing this?

 

knock or a misfire? There is a difference.


I can not imagine a relatively stock 3rd gen would be detonating on premium unleaded especially since all but the TPI cars were engineered to run strictly on regular 87.

AND even on the TPI cars, Premium was "recommended" for optimum performance. so the TPI cars will run on 89 or even 87 without knocking.

The stock 3rd gens also have a knock sensor.

Cooler temps also make a car LESS prone to detonation... I would expect an engine to detonate with HOT SUMMER temps not COOL FALL Temps.

You are probably confusing a mis-fire under load as detonation.

Start by checking the condition of the cap, rotor, and spark plug wires, also inspect the spark plugs.

Test the spark plug wires... no continuity or resistance means a wire is damaged and needs to be replaced.

If you have never done this as part of your tune-up.. now would be the time. Especially if this car is "BONE FACTORY STOCK" which to me means the ignition system has not been touched since 1987...

 

It sounds like your garden-variety engine knock, but I’m open minded to other possibilities. Did not consider misfire.

I figured the knock sensor was catching it, and that’s why it was so brief in each instance. I never felt any loss of performance.

The car was thoroughly inspected by two Buick dealers and given a clean bill of health when I bought it last May, but I was also told to drive it and see what shakes loose considering the previous owner only drove it 1,200 miles a year on average for almost 20 years, and it had not been on an open highway in at least that long. He did maintain the car regularly, but I cannot say when the last tune up was performed, so I will take your suggestion. Thanks!

A friend suggested I decarbonize the intake. Thoughts?

 

it sounds like a low milage car.

I would not worry about carbon build up on car that was engineered to pass emissions testing. Carbon build up is caused by running too rich... these car run stoich / lean rich at WOT and it sounds like the car needs ingition system checked and driven.

check the fuel filter as well.

 

66,700 miles as of last night. I guess that's low for 26 years. It has the wear of a five year old or so car.

Close to prepping and putting it away for winter storage. I'll plan for performing a tune up/major service in the Spring and see how how it behaves after. Thanks again!

 

with that low mileage I would image the spark plug wires are factory original, if so they are 26 years old and should be replaced.

Just because they "look" good does not mean they are good... small fractures in the insulation can be arcing the spark, causing a miss-fire under load.

Age is more important that mileage for "soft" parts; tires, hoses, spark plug wires are the 3 things affected more by time than mileage.

 

That all makes absolute sense. I do not doubt the plug wires/plugs are factory. I know some of the hoses are original. The tires are not original, but they are old and I was planning to buy a new set in the Spring even though they look perfect. So, I will add replacing all the aforementioned soft parts you suggest to my list. Great advice, thank you.

 

Had a similar problem with my friends 305 in his monte ss. Turned out to be a collapsed lifter on number 8. Would be quiet at idle then would make noise as the rpms went up.

 

If a tune up does not solve, will keep that in mind. Thanks!

 

if any one can help. My 92 rs lo3 tbi has spark knock at low rpms. new gm goodwrench moter,esc module, knock senser, new egr valve, egr solenoid. ses comes on but no stored codes

 

Could all this be from cheap gas? 91 octane will fix all this over time maybe not just a tank. Be consistent and love your car.

 

this spark knock is why I put new engine in 3500$ later still doing same thing. I don't know what else to do . ses light comes on but no codes

 

What is your base timing set at?

 

Excuse me for asking this but what did you spend 3500$ if you still have an LO3? Spark knock can only be from the gas you put in and the spark you put out. It really is that simple.

 

The only thing i can guess is there mixing water in the gas down there.

 

just went back with the stock motor . to keep things simple. base timing is at 0

 

it may not be spark knock, but at low rpm (light throttle ) it rattles . ses comes on after five min of driving , but no codes motor has less then 1000 miles

 

you mean 350$ not 3500$ Lo3 Replacement and on top of that you still have spark knock.

 

why are you so concerned about how much money I spent, that is not the problem. who cares if I went back stock, just was thinking you might be able to help me since your a senior member. but all you care about is if its an lo3 or if I spent 3500$ are not. let me break it down for you long block 2300$, headers, 300 , chrome kit 350$ new hoses, new water pump, starter, plugs, wires, sensors, don't tell me there is no way I got the receipts.

 

I'm sorry, i didn't mean to offend you or anything man, ok you spent 3,500 on a stockish 305. I get all that, and good for you.

 

just wanted to let you know I replaced the o2 sensor today problem solved. the first one must of been a defect .

 

Last year during the summer of 2021, after four years of storage without a single start, I took my T/A to a Chevy dealership and had it serviced and inspected. After a new set of Goodrich Radial T/A tires, a new battery, fresh fluids, filters, belts and hoses it was given it's usual clean bill of health. I drove it around the Hudson River Valley of NY, North NJ and Long Island for a few weeks to shake it down and nothing came loose, so I proceeded to drive it from Long Island to my home in Tempe, Arizona, almost 3,000 miles from my storage space in NY to my AZ garage. The then 34 years old, bone stock 68K mile car not only made the trip without a hiccup, but it appears to have done the car a world of unexpected good. The detonation knock I had when I started this thread years ago is now gone. Beyond that, this car now accelerates significantly better than it had prior. In the past the longest distances I'd driven it in a short amount of time was 200 miles in a day to 400 miles over a weekend. That cross country trip cleaned something out because I can really tell the difference in performance. It suddenly has mid-range torque! This car now pulls and wants to keep climbing in speed on the freeway. It never ran like this back east. It clearly wanted to be driven.

I've got to say, for a now 35 year old car built during an era of GM that's notorious for poor quality, this car has been a joy to own and weekend drive over the past 9.5 years. Proves that if you treated these cars right, they were solid and stood the test of time.

 

Engines like to be exercised and operated under load on a fairly regular basis. I typically throw a can of B12 or SeaFoam into the gas before a long highway trip. After you have it good and hot from driving, give it an italian tune up getting on the highway somewhere. You would be amazed how much soot and carbon blows out of the exhaust when the throttle is at or near wide open.

I know in 2015 I took my Express van on a 2,200 mile trip. My fuel mileage jumped up on the 2nd and 3rd tank of fuel. The heat and pressure having the engine loaded for hundreds of miles operating at and near stoich continuously cleaned everything. Would not surprise me if even the cats received a good cleaning.