Questions and Answers

Dodge Shadow Low But Steady Idle Q. Hi Vincent, I really like your site. It's a great help to us that have to do their own repairs. I have a problem that is leading me around in circles. I hope you can steer me in the right direction. Here's the basic info. 1991 Dodge Shadow 2.2 litre Automatic transmission 160,000 kilometers Fuel Injected

The car has what I would call a very "delicate" idle. When it's cruising down the road there's no problem. Lots of pep. When I have to come to a stop though the engine seems to idle too slowly. In fact it will idle itself to sleep if I don't give it some gas or put it in neutral. It's not really a rough idle just very slow.

I replaced the PCV Valve yesterday and in the process broke the little Tee Hose connector underneath. I knew there was going to be no vacuum in the PCV line but I had to get to the auto parts store to get a new connector. While driving I noticed the idle speed was increased, which I guess it should be, with no problems at stops.

When I got home and put everything back together, making sure there were no cracks in any of the hoses or connectors, it ran slightly better than before, but the idle speed is still too low, just barely keeping the engine running while sitting at a stop light.

After doing some reading up on how PCV systems work I understand now that the vacuum should be highest during idle and lessen during acceleration. I was wondering if there was anything that could cause the vacuum to be too high, suck too much I guess, and cause the condition my car is going through at the moment.

I also have a problem with it using up gas way too quickly with obvious poor gas milage. Could these two problems be related?

Here is a list of everything I have replaced in the last six months.

Radiator (leaky)	Head Gasket (leaky)
Spark Plugs	Spark Plug Wires
Distributor	Rotor
PCV Valve	Tee Hose Connector
Air Filter	Any vacuum hoses that looked suspicious

Thanks very much for your time.
Dave

A. Engine vacuum is created by the pistons and cylinders. The better they are, the higher the vacuum. Normal vacuum is between 16" and 21". The engine will not make any more than that since air is coming in through the air induction system.

If the air was cut off, assuming for a moment the engine will keep running, the most it can develop is 29" of vacuum, a figure dictated by the laws of physics.

The idle speed is not adjustable and is controlled by the PCM. What you can do is check the PCM for codes to see if any are stored. If there are, correct them before doing anything else. They will, most likely, be related to the problem you're having.

The fact that the idle came up and smoothed out with the vacuum line off would make me think the fuel mixture is a little rich. This can be caused by a bad O2 Sensor(HO2S), bad Throttle Position Sensor(TPS), a bad Coolant Temperature Sensor(CTS) or a bad Manifold Absolute Pressure Sensor (MAP).

I would first try a cleaning of the throttle chamber and Automatic Idle Speed (AIS) Motor. About 80% of the time this alone will fix the problem.