I have a '95 with a 302 and 210,000 on the truck I have no idea if its ever been rebuilt but I doubt it. It sometimes knocks when I accelerate from a stop. There are about 4-5 knocks with a nasty puff of black smoke then its gone. It also knocks at part throttle around 2100 rpm. I have cleaned the throttle body with no improvement. I have also checked/cleaned the EGR, its fine, and I have run a can of seafoam through the engine. What next? Thanks for your help.

 

It almost sounds like there is some carbon build up in the cylinders somwhere (piston tops, combustion chamber, etc). When you get the knock, the fuel is pre-igniting...in other words, the fuel ignites before the spark plug fires, which causes a HUGE load on the piston/head gasket/etc...basically an uncontrolled burn and that burn is the noise you hear. When it happens, the combustion temps are extremely high....enough to melt aluminum and head gaskets, so many times when you do hear the knocking, the carbon in the cylinder is being burned off...hence the black smoke.

Preignition can also be caused by improper ignition timing, stale/bad fuel and an excessively lean fuel mixture. Most common problem is improper timing....does the check engine light come on? Have you had it scanned for codes yet? You can scan it yourself at home if you've got some mechanical know-how. It's easy once you learn how to do it...and best of all no scan tool required. I had 3 '95's and a couple '94's...so I've been down this road a few times, so to speak. Last one I had was a 95 with a 302/M5OD mazda 5 speed that had a tick under 300,000 miles on it...and it still drove great. Engine was a little weak, but it drove fine. Believe it or not, it still had the original trans in it too! It was on it's last leg though....

 

The check engine light does not come on. I haven't scanned the codes, would I still get a code even though the check engine light doesn't come on? Would you elaborate on how to scan the codes without a scanner? Someone else had mentioned timing as a possible issue, I am planning to check the base timing as soon as I can get my hands on a timing light.

 

with that many miles i add a tune up to things to do if you havent done one already. while you do runn a compression test and see what you get.