Top Knock #5 Cylinder During hard Acceleration

Hey Guys, I have a 95' Safari with a 4.3l Vortec V6 with a ton of miles. 196K to be exact. The engine runs very well and is still very strong. It has no power loss and idles perfectly. However, the problem that has come up is that during hard acceleration meaning taking of a little faster than normal or putting my foot in the gas it will give a top knock. If i start off in normal acceleration it will not be there. I think it is a loose rocker arm or a worn lifter/rod. Remember that it doesn't lose power at all and it runs very good at an idle and at normal acceleration and driving speed. Doesn't knock when going up a hill. Just only when i give it more gas then usual and only on the #5 cylinder or when you are facing the van, the driver side rear cylinder. There are no leaks, coolant or oil loss. Maybe a bit of a exhaust smell from the front of the van.

Any help or suggestions would be greatly appreciated.

Is the SES, (Service Engine Soon) light on? How did you determine it was #5 cylinder?

The service engine soon light is not on. I determined that it was the #5 cylinder by listening and i can also feel it when i touch the valve cover. I could be wrong though. I can hear it from the back driver side of the engine.

.... I think it is a loose rocker arm or a worn lifter/rod....

If that were the case, it would make noise all the time, even at idle. I have seen piston slap and worn wrist pins that only made noise under a heavy load. Sounds like you may have to do some exploratory surgery or wait until it lets go to figure it out.

Thanks a lot for the quick reply.

I would like to give ya a complete scenario of this Van.

It belongs to my in-laws they bought it for $2000. The Van is a good looking vehicle inside in out with great paint and custom rims on it. Not to mention everything works on it including the TV/VCR combo that came with it. Initially not a bad deal.

But, when i first approached the front of the Van when they brought it over to my house it stunk of exhaust gases extremely bad. I mean bad enought to make your eyes water. The back of the van just had the regular exhaust smell, nothing out of the normal there. There was also free flow from the tail pipe.

The Van actually sounded darn good at an idle, no squeaks, no ticks, no rattles sounded almost like my hot rod Impala. I said to my father-in-law,"sounds nice, but the exhaust smell from the front of the van is outrageous..." he then said to me, " It has a knock to it when i rev up the engine..." So i started to investigate.

I put it up on my ramps and climbed under it and used my stethascope and set it all over where i could safely reach without getting burnt. The knock is coming from the back driver side of the engine next to the bell housing and i can also hear it on the same side right on the back of the head. I got to that from inside the van and removed the access cover to get to the engine.

So, basically i thought it was a rod knocking at first. But, it only does it when the engine is revved up and never at an idle or doing the speed limit unless i accellerate hard or basically put my foot in the gas. The engine stays at normal operating temperature and the oil pressure is fine. So that is what made me decide that it could be a hydraulic lifter or rocker arm. I could even go as far an guess that it could be a loose torque converter or torque converter bolt. I will be checking that in the morning, ever since it is relatively easy to get to.

Now that i have explained this a bit further. Would you might have any more suggestions for me to look at.

Oh another thing is that the oil is fine with no metal fragments in it. It doesn't smoke at all.

If it is piston slap then i am going to have to pull the oil pan and take a look and see what i can find. Sometimes in that case there should be some kind of sign at the bottom of the cylinder wall. At least i am hoping that there is some kind of sign so i know which one to pull.

Thanks again for your help on this matter.

The torque converter is a real possibility. It may have a loose bolt, or it might even have a cracked flexplate. Gotta pull the trans or engine to see all of it though. If it's piston slap, dropping the oil pan will tell the story. The exhaust smell in front is a puzzle. When it knocks, can you feel it missfire?

The is no misifre at all. The plugs are relatively new with no carbon build up. The plug wire are is good shape and i found a receipt for a new EGR and the date on it was about a year ago.

Like i said before the Van is in remarkable shape considering the miles that have been put on it.

However i did find that the transmission was about 2 quarts low on fluid and the fluid smells a bit old. I check it at an idle at normal running temp.

I am starting to believe that it may possibly be a loose torque converter. It is just a hunch. There is a little access plate on the bottom of it. But, it is extremely difficult to do anything though it.

I think i am going to drain the transmission and replace the filter, fluid and transmission pan gasket and see how it reacts. It is takes care of the noise, then my problem lies there? if it doesn't? Then i will drop the transmission and do some exploratory surgery on it.

I will give ya heads up and let ya know how this turns out. If i have any questions? I will be sure to get your advice on it.

Thanks again for your time and patience on this matter.