Car question

[tw]

Originally Posted by glatt

I pulled off the vacuum hose to the PCV valve, and the engine almost stalled and then started revving and then cycling between to two. When I put my finger over the end of the vacuum hose the engine settled down into its normal rough idle.

Understand what you did. Fuel injection monitors intake manifold vacuum. If the vacuum decreases, then the injector system assumes you have opened the throttle plate (pressed on the accelerator). So it pumps more fuel into the engine to accelerate. But then it sees the throttle not open (throttle sensor) and faster engine RPMs. So it cuts back on fuel (there is an RPM number above which injectors cut off all fuel when the throttle is fully closed). Confused because the amount of fuel to maintain intake vacuum is not desireable if you have taken your foot off the accelerator. So it stops injecting fuel. But then intake vacuum falls too much. So it again thinks you have pressed the accelerator.

Exhuast - is the sound constant? Then no "poofff" exists. Its simple. Either the sound is constant - good engine. Or is has a change (or short loss of sound) - cylinder miss. If in doubt, go around listening to other cars. Then notice, for example, how much smoother Acura, Lexus, Audis, etc are because they have higher performance engines.

Mouth the word "poofff" using lungs and not using any vocal cords. That is exactly what you are listening for. A not properly working car should make that sound.

Sensors will be seen by, for example, Car Chip as the O2 Sensor was also reviewed. MAP changes with throttle changes. The shop manual may provide some graphs that relate voltage to air pressure. Which could probably be confirmed by viewing those voltages via CarChip.

Don't know how you checked vacuum lines for leakage since that usually requires a vacuum gauge. Some lines can be tested by simple squeezing the line to learn what happens. Then comparing that line to its function defined in the shop manual.

A small vacuum leak could be apparent by temporarily plugging the offending line and its manifold connection. You saw what happened with a major leak. What would a minor leak do? The engine would constantly be searching to inject enough fuel to maintain vacuum but then lower the resulting high idle.

I do not know what they recommend for a throttle sensor. In at least one case, that throttle sensor was only factory adjustable. Any inadvertant change meant buying a new throttle sensor assembly. Don't know if that applies to your design. A precaution about doing any throttle sensor adjustments without consulting the shop manual.

EVAP system would create problems similar to a vacuum system leak. And located by similar procedures.

Low fuel pressure would result in poor acceleration. You would need a pressure gauge to measure a maybe 45 or 60 pounds pressure on gasoline. I built one once by using a water pressure gauge from Lowes and some fitting from a company selling pneumatic hose fittings. Then discovered low fuel pressure on a pump the mechanic said was good.

He could not bother to read spec numbers. He saw pressure. Then assumed everything was OK. And yes, I was pissed I had to do his job. Low pressure did not affect idle. But did restrict acceleration and eventually (but rarely) triggered the check engine light if I did full petal accleration with multiple, consecutive engine restarts. I had to create the same problem with every restart by doing a full pedal acceleration - that I never do.

Why did I find it? I would be damned if a check engine light was reported by a mechanic as an 'unknown problem'. He failed to do what all solutions require - numbers.

Low fuel pressure would be apparent when accelerating in normal mode. Pressure would be higher (and sufficient) in idle due to less fuel consumption.

Any ignition system failures would be indicated by a missing cylinder (as reported by the vacuum gauge or "poofff"). Eventually by poorer mileage. And by poor operation (ie inconsistent acceleration) during normal operation mode.

Valve timing, bent valves, low compression, worn cam lobes, valve springs, etc would result in poor normal mode operation or in noisy 'ticking' in the overhead cam. I am surprised they also did not list valve lash which is adjustable on overhead cams for better engines. I cannot think of any condition here that would cause rough idle, not create the 'missing cylinder' symptoms, but permit good normal mode operation.

Those visible vibrating parts may simply be frequency reasonant to the vibration. May stop vibrating as engine speed (and therefore vibration frequency) increases slightly.

However, is some other part (ie heat shield on the exhaust pipe) vibrating against the body only when it also resonants at that frequency? Might be heard inside but not outside the car.

[glatt]

Originally Posted by tw

Don't know how you checked vacuum lines for leakage since that usually requires a vacuum gauge. Some lines can be tested by simple squeezing the line to learn what happens. Then comparing that line to its function defined in the shop manual.

I saw in some youtube video that you can spray carb cleaner onto the exterior of the vacuum hoses. If there is a leak in the hoses or at the connections, the carb cleaner gets sucked into the vacuum hose and the engine either stalls or revs. This guy also said you can use an unlit propane torch to do the same thing. I was out of cleaner, so I used a propane torch. But it was a little bulky and hard to get into some locations. So I should try it again now that I have a pretty full can of carb cleaner.

[xoxoxoBruce]

Make sure it's not idling rough because the rpm is dropping too low when it's warmed up.

[glatt]

It's dropping down to about 700. I can't find the number anywhere for what the idle should be for a 1996 Geo Prizm Lsi, but I think 700 is in the right ballpark. It's certainly getting rougher as it drops. Maybe it should be 800? You'd think the shop manual would give me that number, but I haven't come across it in any of my reading. Maybe there is a chart of specs in the front or back or something. I should look harder to make sure that number isn't hiding somewhere.

[xoxoxoBruce]

That's not uncommon these days since the mechanic isn't required to set the idle speed. It's all the computer saying stand back, I'm in charge here. But that makes it hard to find out if the computer is fucking up because of bad feedback.

[tw]

Originally Posted by xoxoxoBruce

But that makes it hard to find out if the computer is fucking up because of bad feedback.

Back feedback means a computer also fires spark plugs at the wrong time.

[xoxoxoBruce]

No it doesn't. The engine management system gets input from several sources, and uses it to decide what directions to send to several components. Not all input is used to calculate all directions, only what's pertinent.