MGB MGB Engine Problems Again ?

[Les]

Did anyone say MG's were reliable ? Just kidding. Here is what I've got on my 72 B:

- Runs Fine, good power.
- A rattling sound from the bottom end
- The rattling sound is contant no matter what you are doing (engine under load, idling, clutch in, clutch out, etc.)
- If you take each spark plug wire off one at a time while idling, the sound dosn't change, and the engine dosn't sputter/stall on any of the plugs
- If you take the oil filler cap off the engine chokes and stalls


Any thoughts would be great as I'm leaning toward the crank but then again I am not a great mechanic.

Thanks,
Les

 

[Les]

Further notes:

- I've checked the crank pulley and its fine
- I have a new alternator and water pump installed
- Oil pressure only slitghtly drops when idling
- Temp is normal

 

[Steve F]

Les,
You say the sound doesn't change at all under load or idle? Did this come on all at once, or was it gradual (i.e. did it rattle at idle, but get better under load?)

Other thing, simple as it may sound, is to check your exhaust pipe and make sure it's not hitting something.

 

[Les]

Steve,

I noticed it about 15 miles ago when I was driving on the highway. By the time I got home it was quite noticeable. I checked the exhaust and its in good shape (i.e. not touching anything), but I did ponder if it was a exhaust manifold leak, but I don't think the sound it right.

The more I think about the oil cap issue, I'm wondering if the oil pump is failing ?

Any thoughts ?

Thanks for the reply,
Les

 

[Steve F]

I'm going to bet it's not a mechanical issue, but a physical issue. In other words, something is just rattling under the car. Main bearings or rod bearings will gradually get worse over a period of time, and they will either lessen, or worsen under load (I can't remember which). Plus, you'll see a drop in oil pressure. If you really want to rule out bearing problems, run some STP engine treatment. The thicker oil will cause the rattling to quiet down.

I don't think it's oil pump either since you say oil pressure is fine. When you say "stall" you mean die completely? Many later MG's will run a little rougher when the oil cap is removed.

 

[Steve F]

Also to rule out an exhaust leak, either move your hand around the manifold (you may feel the pulse of hot air) or stick something over your exhaust tailpipe.

 

[tony barnhill]

Could be lots of things....is it internal to engine/tranny or external...is it front of car, middle or rear...does it occur only when car is moving or also when stopped...is it only i gear under load or in neutral coasting?

 

[Tom Bedenbaugh]

Something I do when I have a noise problem. Is get the car up to speed from 55-65 MPH. Clutch it and take it out of gear. Now shut the engine down and cost. If it's still there it's not the engine. If you still hear it move the steering wheel from left to right while coasting. If the noise changes you have a front wheel bearing going. If it goes away then you need to het the old long handle screwdriver out and start probing. I use a mechanic stethoschope but a screwdriver will work.

 

[Les]

I took the car to my buddy's auto shop and we found out the new water pump was the cause.

So that issue is solved, but I'd still like to hear any thoughts on why the engine would stall when the oil filler cap is removed ?

Thanks for all the replies.
Les

 

[aeronca65t]

If your car is unmodified, it has a PCV valve. The crankcase in your engine is sealed: the PCV valve allows any oil fumes to be drawn into the intake manifold and burned...this reduces pollution. If you unseal the crankcase by loosening the oil cap, free air will be drawn into the crankcase (since it's under a slight vacuum)...this will act like a vacuum leak in your intake manifold. Your engine is stalling because it is leaning out.

 

[Les]

So simple...Thanks