1996 Camry extreme thud noise on cold start

Maintenance/Repairs
#1

I have a 1996 Camry XLE 4 cyl with about 65000 miles on the clock. When starting first thing in the a.m. (or after standing for hours on a colder day) there is a thud-bang-like noise coming from under the floor boards. Almost like a hole in the exhaust system or at the manifold. When starting with a warm engine the noise is there but barely detetectable. The muffler shop where we have had our mufflers lovingly replaced for nearly 15 plus years says the exhaust system is okay. When stopped at a light with the car in drive and my foot on the brake there is a lot of vibration. No vibration in neutral. I had two mechanics look at the car after leaving it overnight. Noone else experienced the problem. I just had the front and side motor mounts replaced because I was told they were broken. The problem is still there - the noise and the vibration. Some mornings I expect the engine or transmission to wind up on the pavement!

Any ideas?

#2

I’ve read this post several times, and could it be that what you’re hearing is backfire when the engine is first started? If so, you might want to look at the injectors. Not just the four for the cylinders, but also the cold start injector.

If any injector leaks any fuel, it can load one or more cylinders with raw fuel. Then when the engine is started after it has sat, this fuel is ignited in a cylinder that shouldn’t ignite. And this causes a backfire.

Tester

#3

Two guesses: The engine isn’t always firing on all cylinders–the unburnt fuel mixture is igniting in the exhaust system and causing a ‘backfire’ This would also translate to rough running–but it would run rough all the time, not just at idle. The other guess is that you have a bad motor or transmission mount–when you start the car, the torque is causing the engine or transmission to flop around and bang into a frame member or other component. This could also cause a rough feeling at idle when the car is in gear, as the faulty mount isn’t isolating the engine vibration. Of my two guesses, I’d have the engine/transmission mounts checked first.