I went to look at a S70 today for a friend. The car was a mess and the motor is knocking. The lady was told that there was a knocking rod. It sounded like it was up top to me, like a valve stuck open. At any rate, there is no CEL or codes, however the car will not idle on it's own. You have to use the gas pedal to start it and keep it running.

The lady said she purchased the car and took it to have the oil changed 2 weeks later. Immediately after the oil change it started knocking. Will a stuck valve cause the car to stall or do you guys think it's a piston problem? I alway though that the motor would lock up if you busted a piston

 

Rod know usually is more like a metal to metal thunk (to me anyway).

If its up top, and the motors a mess, could the oil have been changed as a result of concern to some issue that was going on. Like the red light came on and time to change the oil?

If the pre-change oil was really old it could have theen thing and varnishy. Sometimes thick oil can hide a lot of sins. When it was changed, this could have made the noise more pronounced.

Lifters? If they were gummed up they can make noise. Usually more a clacking but I have heard load lifters when they get plugged up.

Stuck valve or broken spring?

If it won't idle but it runs, that could a valve open or maybe when the oil was changed, something was broken or knowed loose? A significant vacuum leak woudl cause that... same with the stuck valve.

I would find it hard to imagine a broken piston. If it broke I would think any run time would cause it to seize the engine and drop metal pieces. If the piston is indeed broken in some way I would think you could pull the dipstick and hear/feel the compression coming from there?


How many miles on it? The engine is full of oil? How does it run if you use the gas pedal?

 

The car has 212,000 on it. Yes the motor oil is full and clean. Matter of fact, they put another quart in it after being advised to do so by a friend, so it has 1 quart to much. It seems to idle pretty smooth with gas pressure. It did NOT toss a code or anything. Just noisy as heck with the knocking.

 

Did you try using a long screwdriver or something matallic that you can put to your ear and listen at verious points on the engine.

I know valve noise tends to have a higher pitch to it. The connecting rods tend to make a lower pitch tone - much more a true knock.

A piston hitting the valve is loud too but I would think you'd hear that pretty loud and clear.

I can't speak to the noise but I wonder if you may have a vacuum leak somewhere which would definitely cause an idle problem.

Maybe an ignition problem? Cross fire from one of the coils or a plug? I know I bought a set of Bosch plugs gthat were supposed to be for the S70 but they were quite a bit longer than the plug I took out.

 

Well, due to the fact that the car was running well until the oil was changed makes me want to rule out things like bad plugs. I'm kinda leaning more towards a valve stuck open or maybe even a broken valve.

I could not get close to it while keeping it running. I was somewhat alone with no help.