Nissan - Sentra :: 2003 - Began To Shake At 40 MPH

Three mechanics have given us three different opinions about what is wrong with our 2003 Nissan Sentra. I'd like to hear from the Car Talk community.The problem started when our car began to shake when we drove above 40 miles per hour. We got the radiator replaced. However, the car still would not always start. We took it to the Nissan dealership. The mechanic there said it needed a new fuel pump.

We took it to mechanic #2 for a lower-cost fuel pump replacement. However, the car has started every time for them. So they cannot be confident that there is something wrong with the fuel pump. The fuel pressure does drop off after turning the key off. This mechanic said our main problem is that there is antifreeze in cylinders No. 2 and 3 of the engine (likely from when our radiator broke). There is either a crack in the cylinders or the head gasket has failed. They are estimating $1,200 to $1,600 to fix this. They said if we don't take care of this, the antifreeze will continue to circulate and damage the engine further.

The Nissan dealership says if there were antifreeze in the engine, the check engine light would have come on. Also, they say they would have noticed if there were antifreeze in the cylinders. Their opinion is the fuel pump is only thing that needs to be fixed.

We then called mechanic #3 to get another opinion. This mechanic specializes in Hondas/Toyotas/Nissans. He hasn't looked at the car yet, but he thinks that the motor could be damaged since the car was shaking when the radiator broke. He says he thinks it is unlikely that the fuel pump needs replacing, and he thinks the check engine light would not detect the antifreeze in the engine.

Fuel pump, antifreeze in cylinders, or motor? Or all three? What do you think?The car has about 85,000 miles on it.