Car question

[footfootfoot]

Originally Posted by glatt

I've got a car question.

Rough idle.
The car is old. After it warms up from driving around, when I stop at a light and push in the clutch, or shift out of gear, it goes into an idle where it's pretty rough. The CDs in the door pocket start to rattle against each other. The hard roof liner starts to vibrate against the roof. If I apply a little pressure to the accelerator and ever so slightly rev the engine, the vibrations and roughness go away. It does not have a rough idle when cold. (I understand the computer sends different signals to the engine when it is still cold than when it has warmed up and the fuel mixture is different then.) It is not rough when driving. This has been going on for a couple months.

How to improve? We had new spark plugs and wires (and presumably distributor cap?) replaced about 30K miles ago as part of a tune up done then, so they should still be good.

It has 115K miles now. We did that first (only) tune up at about 85K.

I could just go replacing stuff, but would like to know where a good place to start is. I was thinking fuel injectors. We've never done anything with those. Are clogged fuel injectors likely to cause symptoms only during idle?

Oh, this is a 1996 Geo Prizm LSI with the 1.6 liter engine and manual transmission. 115K miles. Starts fine. Drives fine. But idle is rough. Never stalls.

Glatt, good news! That is a super easy fix.

Look to the right of the steering wheel for one of these. Twist the knob on the Left CLOCKWISE until the sound of the rough idling disappears. If you turn it all the way clockwise and you still hear the rough idle you may need to get a new one.

[Beest]

Originally Posted by footfootfoot

Glatt, good news! That is a super easy fix.

Look to the right of the steering wheel for one of these. Twist the knob on the Left CLOCKWISE until the sound of the rough idling disappears. If you turn it all the way clockwise and you still hear the rough idle you may need to get a new one.

When we were on our last day of driving to Yellowstone, we were about 20 miles out in farmland when a loud hiss arose form the front of the car, Oh Crap!, is it overheating? gauge says not, just the AC wheezing ? then it dies away, weird we drive on.
About ten minutes later it starts again getting louder and softer, sounds right at the front, like it's coming out of the heating vents. After moving my hands around covering things up to see what changes the sound I notice the radio is on, but there are no stations out there it's just the static coming and going

[glatt]

It is fuel injected.

So in addition to hooking it up to a code reader, the shop manual says to:
- check oxygen sensor
- check throttle position sensor
- check engine coolant temperature sensor
- check MAP sensor
- check if problem occurs when engine is running Rich or Lean (I think it's lean when it's hot, which it is)
- check evaporative emission (EVAP) control system
- check fuel injectors
- check fuel pressure
- check ignition voltage output
- check spark plugs
- check spark plug cables
- check ignition timing
- check for vacuum leaks
- check the ECM/PCM grounds
- check ECM/PCM for A/C signal
- check EGR
- check battery cables
- check A/C refrigerant pressure
- check PCV valve
- check for broken engine mounts
- check valve timing
- check low compression
- check bent push rods
- check worn rocker arms
- check valve springs
- check camshaft lobes

Holy crap. I can rule out about half this stuff, because it doesn't make the metal clanking sound of worn valves or bent push rods or anything like that. And it has nothing to do with the A/C being turned on. And we just replaced the PCV valve. The spark plugs and distributor cap are not as new as I thought they were (they are about 30K miles old,) but the cables look good and clean on the outside.

[tw]

Originally Posted by glatt

It is fuel injected.

So in addition to hooking it up to a code reader, the shop manual says to: ...

Many of those items need not be checked either because they should never fail in the life of the car, or would be obvious from sound and observation.

Never need replacement include spark plugs and wires, rotor and cap, and PCV valve.

Would be obvious for other reasons include: fuel pressure, fuel injectors, battery cables, broken engine mount, and valve timing and those other related entries including camshaft lobes.

These entries should result in a check engine code: oxygen sensor, throttle position, coolant temperature sensor, MAP, most injector problems and low fuel pump pressure, EVAP problem (that will even detect a missing gas cap), ignition timing, and EGR.

I have no idea why refrigerant pressure is on the list unless they are discussing a mechanic who over pressurized the system.

That should seriously shorten a list of top suspects. Having said that, I don't know how 'good' your car's diagnostic software is. But being 1996 or later means a diagnosis is much easier due to that computer diagnostic port.