vetracer

My '86 coupe has detonation or "pinging" for a few seconds every time I start to accelerate hard. I know my timing is on the mark, just replaced the harmonic balancer and checked TDC on compression stroke. This happens anytime the coolant temp is 200 deg or above. The engine is stock except for a cold air induction, descreened MAF, and no pre-cats. I'm running the stock PROM. It has 3:45 gears and a ZF 6-speed. My Diacom shows knock counts every time I jab the throttle. The knock sensor appears to be doing its job, just one or two pings and it stops. These are pretty hard knocks, though as I can even feel a slight loss of power the instant it occurs. Any ideas what to check next? :confused:

Ramanstud

what kind of gas are you running?

vetracer

It just occurred to me that I didn't mention that :)

I haven't put anything but 91 or higher in any car, boat, motorcycle, or any other piston driven machine I have ever owned. ever. :lol:

vader86

Maybe your knock sensor is going out, or youre not running enough octane.
If that doesnt fix it, I dont know what will, unless something mechanical is on its way out.

But even a stock 86 should run fine on 87 octane. Mine does.

MtlSphere

This reminds me of something I read in the owners manual, page 2-5.

"However, now and then you may notice light spark knock for a short time while accelerating or driving up hills. This is no cause for concern, because you get the greatest fuel economy benefit from the fuel's ocane rating when there is occasional light spark knock. Using fuel with a higher octane than that which allows occasional spark knock is an unnecessary expense."

I guess the question is...define "light/heavy" spark knock.

vetracer

Yeah, maybe you are right and it IS normal, but then there should be some responses from other members that their cars are doing the same thing. I've driven lots of small block powered cars and not had the pinging, so I'm a little puzzled.

As far as the octane if you can run 87 with no detonation that's great. It is true that extra octane can be like throwing money down the drain on a vehicle with computer controlled timing; we have a fleet of trucks in our business and I explain this to anyone whose gas bills show they are using 91. But when it comes to my vehicle, and especially my Corvette, well - that's different and the extra octane coudn't hurt, right? Hypocritical? Probably. :D

Aaron's 87

Carbon build up - top engine cleaner will solve it.

Or, maybe your EGR isn't working well enough. Top engine cleaner cleans that up too. There was a top engine cleaner post yesterday.

vetracer

I've got some of the GM top engine cleaner but have been afraid to use it for fear of gumming up the IAC pintle or something. Where do you spray it in at? in front of the throttle body?

87BlueGoober

i am having the same problem in my 87 but i used a scanner and its telling me to replace the ecm. i will atest to this though, coming back from new york state i stopped at a sunoco and they had 100 octane unleaded so i got some (it costs 3.99 a gallon) it stopped the pinging until i had to get more gas.

so either the knock sensor is going bad or the ecm is.

vetracer

Just read the thread on top end cleaner. I'll try this and be sure to follow the instructions on the can. Sounds like putting it in through a vacuum hose would be better than through the TB though. Interesting how the thread mentions the detonation - hopefully this will help.

BBA

My 87 did the same thing unless I used racing gas.

I cleaned my top end yesterday...but the weather has been rainy all day so I didnt have a chance to tell if it made a difference or not.

VQT88Vette

What about a brand new rebuilt engine? Its couldn't be carbon build up to make the engine knock. My newly rebuilt engine has been giving knock everytime the car is under heavier load than normal cruising.

dgoodhue

Balancer could have spun


[Modified by dgoodhue, 7:22 PM 7/24/2003]

Southern Comfort

If you live in a rainy area you may have some water in your gas if you scrounged gas from the bottom of the tank of the service station.

88_vette

your fuel pump is going bad. It starts off like this you get pinging under accel. then you will start to feel the engine die under heavy throttle in the middle rpm range then... Check your flow rate, and pressure. Remember you must check both.

Bill Curlee

:iagree: Fuel pressure and flow rate and give the topend a good cleaning. The cleaner will also clean the IAC! No worry there! :thumbs:

GlennS87

I agree with 88_vette. If itisn't the fuel pump it could be fuel filter, fuel pressure regulator. Your mods would have a tendency to lean out the mixture but not enough to get the detonation you are experiencing.

johncazin

Also, try switch brands or dealers of gas for awhile- sadly, unscupulous dealers are selling 87 octane as 91- a story in USA Today suggested up to 25% of all gas sold as high octane may be plain old regular. I have a Harley Road King that is pretty sensitive to anything under 93- funny how often it pings after a fill up at a new gas station or two...............

crheinish

I had a similar problem on a TPI car until I switched to a Edelbrock water pump to stop cavitation. ;)

Hib Halverson

I take it, when you open the throttle slowly, you don't get knock, but when you open it quickly, you do. There are a number of reasons why your engine rattles a little when you whack the throttle open.

I'll assume, from the first post, that issues such as ign. timing, timing mark indexing, gasoline quality, cooling and ECM calibration have all been considered.

On higher mileage engines, you sometimes can have a problem with a fair amount of combustion deposits (both hard and soft) on the backs of the intake valves. Ordinarily, the injectors pulse and some of the fuel spray is atomized as it exits the injector and the rest bounces off the valve guide boss and the back of the intake valve and is atomized. Once a lot of deposits build up on the intake valves, because they are somewhat sponge-like, they tend to absorb part of the fuel stream just as the stream's volume is increasing during tip-in. This can make a momentary, slightly lean mixture. This momentary lean mixture can cause transient knock or even misfire. The short-duration performance loss you feel is the knock retard working as a result of the knock input. Stock calibrations feed spark back in very slowly after a KS incident so even though the knock might have been very brief, the performance loss is more long lasting because the ECM slowly returns the spark curve to normal. When I say "slowly" I'm speaking relatively. The knock might last a fraction of a second. The feeding-in of the spark curve due to knock retard might take a second or two.

You may need to pull the heads and clean the combustion chambers, the piston tops and do a valve job but, before you do that, treatment with a strong fuel system cleaner might solve the problem or at least fix it enough that the knock stops. Most injector cleaners or "top engine" treatments are solvent-based and, because they must flow through rubber and/or plastic fuel system parts, are not really strong enough to fix a case of fouled intake valves. It's better to use a detergent-based fuel system cleaner because detergent-based cleaners, since they don't attack rubber or plastic fuel system parts, can be made much stronger and can be used in high cincentrations with no harm to fuel system parts.

You need four bottles of Red Line Complete Fuel System Cleaner, a detergent-based product. Use all four, in sucession, one bottle to a takful of gas.

If that doesn't solve the problem and no other remedy can be found, then I'd suggest pulling the intake manifold and inspecting the backs of the intake valves. If you see a lot of deposits and muck on them, pull the heads and do a valve job.