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This seemed weird to me. My daughter called and said her boyfriend's Ford Ranger was "vibrating and misfiring" and when they checked the engine oil level was very low (off the dipstick), and could I bring them some oil. I did, but warned that this likely would not fix the problem. However after pouring nearly 3 quarts of oil into the engine to bring the level up to MAX - it was fine!

Any thoughts on what was going on? I wondered about the tappets, I know some US motors have hydraulic tappets...

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I think you've already hit on the answer with the hydraulic lifters (tappets). If the oil pressure gets low enough in the engine, the lifters will bottom out and will not allow the engine to suck in the proper amount of air/fuel, nor open enough to allow enough exhaust out. This is a very bad situation.

Don't think the girls boyfriend got away scott free, though ... more than likely if they were down enough oil to cause this to happen, they've also introduced a significant amount of wear into the engine at the bearings and other wear points. Who knows how much added wear there ended up being, but you can be assured there was some.

One of the bigger things to figure out here is why was the engine so low on oil? It may have a leak or this may be the first time the oil has been checked in a very long time. You may want to enquire about his maintenance habits with the vehicle and get on him a bit if he's slacking on this ... you don't need your daughter getting stranded because her knucklehead boyfriend cannot even keep up on the very basic maintenance. Maybe this is a chance for you to get to know him a little better, eh? (Sorry, off on a tangent ... this isn't Parenting.SE, lol!)

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Any engine that's 3 quarts low on oil will knock and run poorly. It simply does not have enough oil in it to maintain oil pressure. This is extremely bad for the engine and will cause premature wear.

All engines consume some amount of oil over time. Engines in good/new condition tend to consume a very small amount which is not really noticeable within the oil change interval, but older cars can use more to the point where you need to add oil between changes.

Monitor the oil consumption - it's convenient to check the level when filling up the gas tank. Add oil when it's low. If the engine only needs a quart or two between changes, that's not very unusual for a tired engine and is generally considered acceptable on older vehicles. If it's significantly more than that, you should check to make sure it's not leaking out. If it is, you'll need new seals. If it isn't leaking, that means it's burning it, and you'll probably have to rebuild or replace the engine.

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