I have had my 89 vette for about two years and it has ALWAYS spark knocked under heavy loads (not a constant knock, more of a random knock).... okay, so about 5,000 miles ago I had a head gasket failure and pulled the heads, had a valve job done, and put everything together and the spark knock was GONE. Well, I thought it was the carbon build up that my mechanic had been trying to convince me was the source. So I drive the vette to Florida (home in Virginia- round trip about 1900 miles) and the car ran GREAT, but when I got home the spark knock was back! I originally thought is was a tank of bad gas, but have filled up since then with no change (at the place I always buy gas). I called my mechanic and he said that carbon should not be the problem with only 5000 miles since the heads were done, and that I should check the timing, look for vacuum leaks, and check the EGR system. Timing was dead on 6, no vacuum leaks, and the EGR system is okay... I just finished the EGR diagnostic charts in the service manual. He has always told me that the occasional knock it not a crisis but I CANNOT stand hearing it. I am about to retard the timing a little more to get rid of it, but wanted to ask here for any ideas. I have the stock plugs, and thermostat in the vehicle. Would a different heat range plug help? I am not going to put a colder thermostat in because this should work with the stock thermostat installed. :confused: Oh, I already have replaced the knock sensor.

Suggestions?

As always, THANKS

Skip

 

How thick was the new head gasket compared to the old one? A thicker gasket can reduce the compression ratio, slightly, but it can also kill any good quench you may have had.

Your timing is right on. But are your timing marks right on?

Lastly, with a properly operating knock sensor, you should almost never hear spark knock. Are you sure it is pinging you are hearing?

Just a few thoughts. Good luck, and...

RACE ON!!!

 

I feel you pain, I had a similar problem with my 88. I'll I can do is tell you the two things that seemed to help mine. One was the connection inside the fuel tank for the fuel pump the terminals were really corroded. The other thing and this made the biggest difference, was the back half of my main cat was loose and rattling around. I broke it up and removed it just last week, now the car is running like it never has before. Good luck I know how frustrating it is.

Ethan

 

Okay... as far as the timing is concerned, it was right where I left it following the head gasket replacement. So for about 5,000 miles the engine was happy with the base timing and the head gasket replacement. Now, since it has started knocking (or pinging... what is the difference?) it is doing it A LOT. I pretty much can hear it everytime I accellerate moderately to hard... sometimes even under light load but low rpm. I checked the fuel pressure... it is good, I replaced the O2 sensor and no change. I am thinking about a colder range spark plug, but the annoying thing is that is was fine for awhile. Why is is back? Also, I can tell the computer is doing something because my engine performance in nowhere close to was it was before the knocking started. I suppose that this the timing being adjusted to stop the knock. I am waiting for the UPS man to deliver the cable to connect the car to my laptop so I can see what is going on in there.... but would appreciate any ideas.

Skip

 

There are several of us here who have experienced mysterious knock.
I have a 91 L98 that gets knock retard right around 2200 RPM.

I can't HEAR mine but I'm getting knock retard right around 2200 RPM.
Enough to get timing retard and as you said, there goes the performance.

I've changed knock sensors, put teflon around the threads, retarded the timing, advanced the timing, richened the mixture a LOT in the RPM range I'm getting knock, adjusted the VE tables until I'm tired of doing it.

In MY case it MAY not be spark knock. I'm thinking I may have a wrist pin or piston slap the knock sensor is picking up. I just don't know.
What I HAVE done is adjust the KNOCK ATTACK RATE to lessen the knock retard just in the RPM range of interest AND adjusted the KNOCK DECAY RATE so that if I DO get knock retard, the ECM pulls the retard out quicker.
This seems to have helped performance.

IF I KNEW what was causing it, I would fix THAT. What I have done is a band-aid.

Good luck with yours.

 

Could be a lifter going bad and when you removed the head and readjusted the valves it fix itself for a while and now its back again. Also did you check your fuel pressure while driving the car under a load? another thought, if your car is running a little rich(maybe on just 1 cylinder: sticking injector) you could have built up enough carbon in your combustion chambers to knock a little.


[Modified by chriswtx, 9:24 PM 4/7/2004]


[Modified by chriswtx, 9:26 PM 4/7/2004]

 

I believe it is pinging and not a lifter. All the injectors are new, so it is not that. The local chavy mechanic thought is was carbon before I removed the heads, but does not think it is now with only 5,000 miles since head removal. But, the carbon theory is the only one that makes sense... since it went away following head removal and is back now. I read about others finding exhaust restrictions, but that would not dissapear for awhile like mine did. I just don't want to believe that carbon is the cause after 5,000 miles... because if that is the cause, what do I do about it? I do have a fuel pressure gage and I did not check it under load, other than having my wife hold the brake and give it gas in Drive while I monitored pressure. It went up as expected. Also, the gage has a bleed valve and the fuel pump kept up pressure with the bleed valve open.

:confused:

Skip

 

:cheers:

I had same problem in my 88. Did all the checks you did, Finally I replaced the knock sensor AND the Electronic Spark Control Module with a module from a different year/model.

Gorden (Guru) Killebrew suggests this change.

look for posts in this section by 65Z01 and go to his web page, he has lots to say and the recommended part # I have the box at my shop. I bought it from NAPA and it is an Echlin product.

Hope to meet you at one of our events in the near future. Check out Southeast events section and look for RATPACK

:seeya

:steering:

Jack

 

I agree that you should NOT "hear" ping, the ECM should pull timing when it first detects knock.

After you get some scans of the ECM during an acceleration on the highway, if you are getting knock counts and knock retard test the knock sensor by rapping on the block near the sensor at idle to see if a knock was registered by the ECM. If not, replace the knock sensor and replace the ESC module, use the upgrade GM P/N on my site.

Also if you see false knock counts & retard and there is no metal-to-matal cause (like rattle in the cat, loose lifters, etc), replace the ESC module with that upgraded part.

If you are hearing ping under a light steady engine load it is likely real knock. When I was getting false knock it was at high RPM during a WOT run; the new ESC module cured it.

If your knock sensor & ESC are ok try a low temp stat & fan switch to drop engine temps, always run premium fuel and verify base timing is at 6deg BTDC. Put a fuel pressure gauge onto the Schraeder valve, lay it under a wiper and verify that fuel pressure is adequate during a knock test drive. Pull the spark plugs and inspect for a faulty cylinder condition. Also, note the total ignition timing during an ECM scan on the road when you hear ping.

Good luck with finding the problem cause.

 

Seems like it wouldn't be carbon after 5000 miles, but $5 of top engine cleaner is worth a shot. Whenever I hear mine knock, I run a can through. Never knocks afterwards. How's your fuel filter?

 

Check your knock sensor circuit. I had a similar problem whith my 89 and it turned out that 12 volts got applied to the circuit, ruining the knock sensor. Ran a new wire and replaced knock sensor according to service manual and all was well again. I know how frustrating this can be, it took me 3 Sundays to diagnose and correct the problem. :cheers:

 

Plugs ;)

 

Just a comment related to the knock sensor...when you re-installed it, did you use a torque wrench? If it's grossly over tightened, it's likely ruined. If it's just slightly over torqued, it's super sensitive and the motor won't EVER run correctly. I learned this the hard $$$ way after the engine shop cranked my brand new KS to ~ 40-50 lbs/ft! I specfically gave them the torque specs and the page in the factory manual that stated: 14 lbs/ft ! #$%holes!
my motor felt like it was alternately running on three and a half cylinders. Popping, banging, you name it. I located it by running an EASE software program and emailed it to a fellow on the west coast who found that gross knock retard even when the engine was cold and fat. Hope this helps!