Replaced Engine revving up and down while idling

UDIT99

MEMBER
HONDA ACCORD

Hona Civic 1997 Ex 1, 24,000 miles

Hi
I just had my Honda Civic 97 Ex VTEC engine replaced with another Honda Civic 98 VTEC Engine. The salvage guy said according to some books he consulted, they're the same and they're interchangeable. So thie mechanic who was willing to do it for me replaced the engine and the car works now but there is one problem. When idling, the Engine starts revving up and down without anyone even touching the accelerator. He says his "computer" cant figure out whats wrong and he's having a difficult time figuring out this problem. He gave me two pointers:

1. He had to replace the O2 Sensor from the old engine to the newer engine since the newer engine sensor broke at the time of delivery. Possibly mismatching ohms he said

2. The wiring from the car computer to the engine wasnt exactly the same. There was one extra wire. Possibly on account of the newer engine being a 1998 than a 1997. Since the newer wiring didnt fit into the computer, he replaced the engine wiring with the older engine's wiring

Could wither of these have caused this problem?
Has anyone experienced this. Any suggestions I can pass on to the mechanic?

1 Reply

BRUCE HUNT

MECHANIC
3,753 POSTS

If it does it all the time have him check the intake manifold for a leak. A leak in the intake like say the gasket will cause more air to get in and confuse the computer. The result is the engine surging up and down.