QuestionHello Barrie,
I have a 1979 MG Midget 1500 that has been rebuilt aproximately 25,000 miles ago. The car has been running very good for me since the rebuild up until a few days ago. Having a faulty temperature gauge in the car, the engine had became hot on me I believe. This happening from being stuck in traffic for a long period of time on a 90 degree day. The car started knocking & pinging badly as well as backfiring through the carburetor. I shut in off immediately and opened the bonnet to inspect the cause. I didn't noticed anything unusual except for a small amount of antifreeze that had come out the overflow on the expansion tank and had sprayed underneath the bonnet. After releasing pressure on the radiator cap you could hear the radiator gurgling as if it were hot. This had only lasted a short period of time. I then checked the engines oil level and it was 1 1/2 quarts low but otherwise looked in fine condition with nothing unusual on the dip stick after adding oil and running the car a few seconds to circulate the oil. No water was visible on the dip stick mixed with the oil. No white smoke or any smoke was visible coming out the exhaust as well. I thought the car had just plainly "JUMPED TIMING" some how due to it getting hot. Well today I inspected the engine a bit further and in doing so removed the valve cover to see if I might happened to notice anything broken. Upon removing the cover I immediately noticed that the 4th. pushrod back coming back from the front of the engine to the rear of the engine had come out from underneath the "ROCKER ARM". It doesn't appear to be bent thank goodness, but I don't know what could have caused this to happen. The adjustment nut and screw for that particular valve is still tight and didn't come loose. I'm thinking that when the engine became hot, the metal components expanded causing in to slip out from underneath the "ROCKER ARM". Do you think this is what happened to cause this? Also could you please tell me which valve this is (EXHAUST or INTAKE) that this pushrod came out from underneath the "ROCKER ARM"? Again, it's the 4th one back from the front of the engine going towards to rear of the engine. I hoping that I was very lucky that I was going a slow speed when this occurred and that no damage was suffered to my engine. Whew! I thought for sure it was my timing chain all along until seeing that the pushrod had became dis logged from underneath the "ROCKER ARM". This being the case... that would cause the backfiring through the carburetor along with the chattering and clunking noises I was experiencing correct? I'll be patiently awaiting your reply. Thank you. Your reply will be deeply appreciated!
Sincerely,
Jim Yeager
AnswerHi Jim
I think the combination of hot weather, slow traffic and low oil level has caused you car to overheat badly.
Your engine should never need 3 pints of oil, because more than half of the heat generated by the engine is normally dissipated through the oil, not via the radiator.
Also, you should never remove the radiator cap from an overheated engine while it is still hot. The water is pressurized and could be at 110 degrees celsius. Releasing the pressure allows it to boil instantly, and you could have badly scalded yourself. I hope you are using antifreeze and not just water. It contains inhibitors, so it is needed all the year around, not just in cold weather. I would fit a new temperature sender so that I could keep an eye on the temperature in future.
The valves are inlet, exhaust, exhaust, inlet, inlet, exhaust, exhaust, inlet.
I think that one of the exhaust valves got so hot that it stuck in its guide, fully open.
As the camshaft continued to rotate, the pushrod was released and became dislodged.
You just need to re-locate it.
It is possible that the valves have become gummed up due to lack of oil. This may be a one-off, but if this happens again, you will have to take the head off and get it decoked and checked out by a mechanic.
