MG Car Repair: Pushrod coming out from underneath rocker arm, 2 quarts, radiator cap

Question
Hello Howard,
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

Answer
Hi Jim,

You need to do several tests right away. First with the engine cold remove the radiator cap and look at the coolant level and have someone start the engine and let it idle while you watch the coolant level. The coolant level must not rise. Then get the radiator dye from any auto parts store that tests for CO in the coolant and run the test on the instructions. This is a test for a leaking head gasket.

Run the engine and keep feeling the upper hose on the radiator and keep watching the temp gauge to get an idea when the thermostat opens

Then remove the valve cover and set the crank so it is at TDC of #1 & #4 piston and then rotate the crank 90 degrees and note that some valves are depressed and some are not. On the valves that are not depressed, put both thumbs on the valve collar and depress the valve spring and note how hard that is to do. (it should be difficult) check all that are not depressed. Then rotate the engine about 180 degrees and note that the valves that were depressed are now not depressed so you can check them too. After checking all the valves you should find that all are very hard to depress with your thumbs. This is a rough test for a weak spring due to over heating. The very first thing that over heating does is to weaken valve springs and if the over heating continues it weakens the tension of the piston rings and will start burning oil and loosing compression.

A push rod will jump out of a rocker arm when the valve springs become weak due to over heating. Also over heating can cause a valve to stick in a guide especially if the clearance it too close. An engine that is over heating can burn the oil out of a valve stem and guide thus causing the valve to stick and the push rod will jump out. Very high RPM can do it too even when an engine is not overheating. Triumph does not use valve stem seals and that can be a factor at times. (The 1500 MG midget uses a Triumph Spitfire 1500 engine)

Howard