I have a 2000 Dakota 3.9L V6. The engine knocks/rattles really bad under load (climbing a hill/incline) but doesn’t while idling. What could possibly be the problem. Please help. Thank you.

 

Spark knock. Could be a blown plenum gasket, fouled plugs, or carbon buildup.....

 

Thanks! I’ll look into it.

 

Check for exhaust leaks too ..

 

Does it sound like a bunch of rocks or marbles in a tin can? That is usually, but not always, spark knock from timing being a bit off. Try a little higher grade of gasoline and see if it changes. 91 octane will stop a worn engine from knocking because it needs a tune up. You might want to pull a plug and see what shape it's in. How many miles on the engine? If it's over 120,000 to 150,000, your timing chain set might be worn.

I'd go simple at first. Try checking the plugs and maybe go with higher octane fuel. If all you need is new plugs, it'll be cheaper than more involved repairs. After checking the plugs, get a vacuum gauge and see what vacuum your engine is pulling. Especially if you live near fields. If rodents build a nest in your exhaust, it can cause a lack of power and knocking. Those little cheese thieves can really cause problems.

 

Thanks for all your suggestions. But I’m gonna try replacing the catalytic converter first because it sounds like a bunch of marbles in a tin can.

 

Replacing the cat is not cheap. If the substrate is falling apart inside it, you'll hear it rattle if you bang on it with the side of your fist.

 

Your cat. or the muffler might have collapsed inside. This will restrict the exhaust under load.. A vacuum gauge will tell the tale and do it a lot cheaper than throwing money at a problem. With it cold, crawl under the truck and smack ( <-------fancy technical term) the exhaust with your fist. You'll be able to hear the rust rattling inside. Your truck is definitely old enough for this to be the issue. I'm just a fan of figuring out the problem before undoing the locks and chains and getting a pry bar to get my wallet open.

 

Thanks for the tip!