Poor MPG, one error code: E30

DEVILLE MAN 93

MEMBER
1993 CADILLAC DEVILLE
4.9L
V8
4WD
AUTOMATIC
150,000 MILES

Hello,

My car has been acting up as of late. The car is using too much gas and the only error code that is receiving is an E30, which pertains to the Idle Speed Control Motor rpm out of range. I plan on replacing the motor but wonder if there could be any other problem.

Note: I have changed the Air Filter and I have seen no vacuum leaks within the vehicle.

Thanks!

1 Reply

STEVE W.

MECHANIC
12,950 POSTS

The idle speed control shouldn't effect mileage, it can cause the wrong idle speed or an unstable idle though. Do you see any black or dark smoke from the exhaust? Any loss of power or is it just the poor mileage? I would probably do a leak down test on the injectors to see if one or more is sticking or leaking and also check that the fuel pressure regulator on the fuel rail isn't leaking into the vacuum lines. To do that simply pull the vacuum hose on the regulator off while the engine is running, if you see or smell gas in the hose or dripping out of the regulator, it is faulty. That would let the car pull fuel through the intake that wouldn't be monitored.
Also being a 93 I would check the fuel lines under the car for any leaks or issues.

This is a video on one way to test the injectors for leakage:
https://www.youtube.com/watch?v=K7STocr9FBI

Image (Click to make bigger)