bbunn1

Sorry for any duplication from an earlier post. My son's '97 Blazer 4x4 (4.3) is experiencing a "spark knock" at heavy acceleration. The thing runs very well when idling or at low speeds and frankly, runs fine at high speeds but does have a noticeable rattle as if the timing is off. I know the timing is preset and not adjustable. How does one overcome this spark knock if that is the case?

Thanks!

BlazerLT

First off, get the coolant changed and the cooling system flushed and reverse flushed.

Install a new rad cap. NOT Ac Delco.

Chances are the engine is not keeping cool enough.

Also go get a bottle of Redline Complete Fuel System Cleaner and dump a whole bottle in when filling up the tank.

This will take the carbon off of the valves which soak up fuel which should be going into the cylinder.

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1995 Factory Blazer Service Manual for sale, PM if interested.

old_master

Does the "Service Engine Soon" light work properly? It should come on with the key on and the engine off. If it does, is the light on while the engine is running? If so, what codes are stored? Autozone or Advance Auto will retrieve the codes at no charge. Write them down and post them here and we'll help you with the diagnosis.

Spark knock is a result of combustion temperatures being too high. Several things can cause that to happen such as: Poor fuel quality, (low octane), inoperative EGR valve, restricted EGR passages, faulty knock sensor, base ignition timing too far advanced, incorrect camshaft timing, coolant temperature too high, (as LT suggested), the PCM is unable to retard timing as/when necessary, and excessive carbon build up in the combustion chambers. The two most common causes of spark knock are coolant temperature being too high, and insufficient EGR flow. Hope this helps.

BlazerLT

one of the causes can be the coolant being worn out or the cap is not holding the 16psi pressure needed.

Seeing the stock AC Delco cap is flawed, I would go out when the engine is stone cold and take the cap off and see if the level of the coolant is right up to the top of the rad. It should be just under the cap and anything below is bad. Top up the coolant with Dexcool ONLY and install the new cap.

Also check to see if there is lots off coolant in the overflow resevoir, the level should be 3/4 and fill it to about 3/4 and above the hot level even when cold.

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1995 Factory Blazer Service Manual for sale, PM if interested.

bbunn1

I thought he had already flushed and refilled his cooling system but I find out that is not the case. So, that will be the first thing he does in addition to replacing the cap.

old_master: No SES

Thanks, we'll let you know the results.