Car question

[glatt]

Originally Posted by tw

I would not waste time cleaning things. Cleaning rarely fixes such obvious problems. And might even create more problems if disconnecting the coolant is necessary.

If I remember correctly, we went through a list of suspects from the manual and other suggestions. Eliminating many including fuel and O2 Sensor. Mileage and operation when not in idel mode eliminates most suspects (including motor mounts). Listen to the exhaust pipe both on idle cold and idle hot. Any missing that corresponds to the roughness?

When cold, the idle is probably faster. Might not feel rough. A problem not apparent in cold (faster) idle might still exist and be heard in the exhaust pipe. Missing in the exhaust pipe would eliminate motor mounts and other static reasons for roughness.

Does the roughness appear suddenly or gently become obvious as idle RPMs slowly decrease?

Does this fuel injection system do a fast idle by holding the throttle plate slightly open or by opening a fast idle air valve (not to be confused with the Idle Air Control valve)? IOW is there a relay or vacuum line that partially holds the throttle open for a fast idle?

As noted previously, an idle control system operates different from high speed operation. Parts that are common to both operations (ie spark system) can be eliminated since they work fine in high speed operation. Apparently performance and gas mileage say all high speed subsystems work OK.

Some previous questions I don't recall seeing answered. How does this car perform timing? For example, does the distributor have vacuum and centrifical advance and retard to mechanically adjust when spark plugs fire? Or is engine timing done electronically? Carchip may report timing as the engine gets warmer.

One part that operates only in idle mode (not in high speed mode) is the Idle control valve. A binding valve might explain rough idle. However, if difficult to access, then better is to eliminate other possible suspects first.

EGR valve can be in that suspect list. EGR valve is open during high speed operation. But must fully seal always during idle. That valve leaking exhuast gas during idle means it too would be a part that could only affect low RPM idle. And is often easily tested. Two reasons for that leaking would be vacuum on that EGR valve at idle (when no vacuum should be opening that valve) or the valve does not fully seat (close) at hot idle. The later can only be confirmed by temporary removal and inspection. Former becomes obvious by disconnecting an EGR controlling vacuum line and covering that hose with a finger. No vacuum should exist during idle.

I don't remember the long list of suspects. Don't remember any error codes. And don't remember why we spent so much time eliminating an O2 Sensor and gasoline from the suspect list. But gas mileage and high speed performance also eliminate both from the suspect list.

Thanks for the post! I'm not excited about removing and cleaning the IAC valve because I don't want to mess with the coolant unless I have to. I just read the section of the manual that talks about this, and the second step after relieving fuel pressure is to drain the coolant system. What a job!

A little more information:

1. I've only been using name brand fuel (Shell) for the last year.

2. Car has an O2 sensor next to the exhaust manifold by the engine, and one after the cat under the car. I've looked at the charts from the carchip pro, but don't really know how to interpret them. One potentially interesting thing that I noticed this weekend is that the outer protecting sleeve of the wires going the the first O2 sensor by the manifold is all chewed up and torn. There had been a cable tie holding that sleeve to a coolant hose away from the fan, and the cable tie was broken and slipped down the coolant tube. The O2 wire harness was hanging loosely between the fan and the exhaust manifold. But when I inspected the wires within that damaged sleeve, the insulation on the wires was undamaged, and the sensor sends readings to the carchip pro. So I assume it is fine, even though it looks like hell.

3. No error codes.

4. I think the timing is controlled by the computer. And I just confirmed. Manual says "All spark timing changes in the distributor are performed electronically by the Engine Control Module Powertrain Control Module." The OBD2 system will send data to the Carchip for "timing advance." But I don't know what those numbers mean.

5. Roughness is not present at cold start up, but I wouldn't say the engine is smooth either. The problem may exist when cold. Hard to tell. It gradually gets worse as the car warms up and the RPMs drop a little, but then suddenly gets a lot worse. I assume this abrupt change happens at the point when the car decides it's warm and changes from rich to lean mode.

6. There is a throttle opener that increases idle speed during start up conditions. A vacuum hose goes from the top of the throttle body to the throttle opener and appears to be what activates the throttle opener. I haven't studied this during a warm idle, so I don't know if it does anything after the engine has warmed up. The manual discusses ways to test this throttle opener, but it requires a real time scan tool to monitor RPM while you fiddle with the hoses at a specific RPM and then confirm that the engine drops to another RPM when you let go of the throttle with the hose plugged. I don't own a real time scan tool that will display RPM, so I can't test this component right now.

I am intrigued by the EGR. It looks easier to test than the IAC. I think I'll focus on that this weekend instead.

Also, listening at the exhaust pipe sounds easy. Could you describe in more detail what I should be listening for?

[tw]

Originally Posted by glatt

I am intrigued by the EGR. It looks easier to test than the IAC. I think I'll focus on that this weekend instead.
Also, listening at the exhaust pipe sounds easy. Could you describe in more detail what I should be listening for?

A good car will have a constant 'hum' (as it were). Misses will sound like a 'poofff'. Listen for this on any car. Best designed (high performance) engines have a smoothest and least noisy exhaust. (Listen to Harley, Camaro, Mustang and pickup exhausts to hear inferior engines.) But a cylinder miss (the pooofff) is quite obvious in an exhaust. Especially listening within feet of that exhaust pipe during idle.

Another sign of a defective engine is backfire as the car decelerates. Only scumbags think that crap sound is cool. An engine without defects outputs least and smoothest sound when a foot is removed from the accelerator.

EGR valve is typically so easy to access and test as to be a first thing to remove and inspect. (Some may require a new gasket to reinstall. Better cars typicaly have no gasket damage when an EGR valve is removed.)

EGR valve opens with vacuum that you can even create with your mouth. Flow from exhaust manifold to intake manifold can also be tested (for a good seal) with your mouth (and with something to keep your lips looking fashionable such as black lip balm).

If an EGR valve has a wire connection, then a computer is monitoring its operation. If no wire, then the computer might not know an EGR valve is failing. So disconnect a vacuum hose to that valve. Rev the engine. A vacuum will be felt only when a warm engine is at higher revs (above 1500). And never when the engine is at cold or idle. A leaky EGR valve can create rough idle.

Only EGR failure a computer cannot see is leakage from exhaust manifold to intake manifold. Your 'lip' test would expose a leak.

Disconnecting coolant hose is easy (once a few tricks are learned). Often best done by partially emptying the radiator into a pan via a cock located conveniently at the bottom of the radiator. Refilling is also easy on better designed cars. An air bleed port at the highest point of engine coolant is opened to let any trapped air bleed out when refilling. But disconnecting coolant hoses and removing the IAC valve is a pain that usually results in a little of your blood left in an engine compartment. (Or get construction gloves from a hardware store.) Requires care to not harm wires or control cables. And mark each vacuum hose before disconnecting.

BTW, one remote problem is an inspection mechanic who inadvertantly swapped two hoses. Better cars actually number each hose. Sometimes a vacuum hose diagram is on the bottom of the hood (bonnet).

EGR valve is typically 'inspected' with your mouth. IAC valve typically requires better equipment and knowledge. Just another reason why inspecting the EGR valve is better done first.

Noted previously - the idle system and normal operation operate completely differently. To idle, a car must change engine parameters to become very inefficient. And timing is different.

Car Chip can view engine ignition timing verses RPM and engine loading (if your computer reports loading to Car Chip ). During idle, timing is at zero degrees top dead center. Or a few degrees off center. As engine speed increases, timing (when the spark plug fires) advances. Generally, fuel is ignited before fully compressed. And continues to burn (pushing the piston) after the exhaust valve opens. When an engine becomes more loaded, then timing may retard closer back to zero. Causing more power but more unburned fuel.

To make an engine inefficient at idle (so that it can run at a pathetically slow 800 RPMs), timing is moved to zero. If timing does not move back to that point, then it might run rough at low RPMs. Shop manual may report what that timing number at idle should be.

Computer is apparently monitoring your timing. Therefore viewing these timing numbers would only be informative and interesting. The computer was not seeing timing 'out of spec'. So idle engine timing also should be OK. View those timing numbers for various RPMs to appreciate what is happening. And to confirm timing at idle is at the right spot. Obviously timing is OK at high speed (no knocking, no hesitation, good mileage). Timing was only relevant to your timing roughness when it should be at or near zero degrees.

BTW, TDC or Top Dead Center is when piston #1 is at its highest point. That is when a spark plug fires for idle. At higher speed, the spark plug fires much sooner (before cylinder gas is fully compressed). IOW timing advances ten or as much as 30 degrees Before Top Dead Center (BTDC). If not, then an engine can be less efficient and 'knock' due to unburned fuel. But again, your mileage number suggest timing during normal operation is correct.

Timing at idle sometimes changes with engine temperature. A cold idle might have a different timing from a hot engine at idle.

Engine timing was why so many spent maybe $100+ every three months for a tuneup (new points and condenser). EPA requirements (1975) made that tuneup unnecessary by replacing mechanical parts (1900 technology) with electrical parts (1962 techology). And later (1980s) with something also more reliable - a computer.

Four items to consider - EGR valve, IAC valve, engine timing, and swapped vacuum hoses. Also useful might be other items on their 'suspect' list. Since something unique to your car design (something else that works differently at idle than at normal speed) may be overlooked by me.

[glatt]

Glad I can test the EGR with just my mouth (I'll use some clean tubing so I don't get gunk in my mouth.) I was thinking I'd have to buy one of those hand operated vacuum pumps to test it. I like excuses to buy tools, but I've been spending too much lately.

There are so many vacuum hoses! I was looking for possible disconnected hoses and kept finding new ones to inspect. They are everywhere.

[tw]

Originally Posted by glatt

There are so many vacuum hoses! I was looking for possible disconnected hoses and kept finding new ones to inspect.

Better designed cars have a number on each hose. A vacuum hose diagram confirms each connection. I once found two hoses reversed. Car still started.

EGR valve will not open fully with 'mouth vacuum'. But just enough to confirm its works. More important is no leak (ie stray piece of carbon) from exhaust to intake manifold.

[glatt]

I listened at the exhaust, but didn't hear anything other than a steady hum. After a while some water started consistently sputtering out, and I noticed some pinhole leaks in the muffler where water was dripping out.

I turned my attention to the solenoid that opens and closes the EGR. I checked for the proper resistance between the prongs, and then check that there was no continuity with the outer frame.


I blew in the lower nozzle, and saw that the air came out the filter vent, and then I applied battery current to the prongs, heard the solenoid activate, and when I blew in the lower nozzle, the air came out the upper nozzle. According to the manual, this confirms the solenoid is OK

Then I went to the EGR transducer and checked to make sure the filter was clean. It was very clean. I blew in one nozzle, and covered the two opposite nozzles, and confirmed the air came out the sides.
According to the manual, this confirms the transducer is OK.

Finally I took off the EGR valve. This was a little trickier because it was underneath and there wasn't much room. But I got it off and saw that it wasn't terribly dirty.


A little dirty, but not bad. I tried sucking on the hose that leads to the diaphragm, and could easily open and close the valve by sucking on it.

The air intake was a little dirty too, but I couldn't get at it to clean it. It was hard enough to take this picture.

[glatt]

As long as I had the valve off, I might as well clean it. So I did. Sprayed lots of carb cleaner in the different holes and sucked on the hose that leads to the diaphragm so the cleaner would get in valve area as I sucked the valve open and closed.



Then I put everything back together and started the car. It still vibrates a little, but it's not as bad as it was before. I can't get it to make the really annoying vibrating and sound. I'm optimistic!

There were a couple other tests I could have done if I had a vacuum pump, a vacuum gauge, and a scanner tool that showed RPMs in real time.

Maybe this fixed it. I'll pay attention for a while and see if it's fixed. Hardly drive this car at all though, so I don't know when that will be.

[tw]

Originally Posted by glatt

There were a couple other tests I could have done if I had a vacuum pump, a vacuum gauge, and a scanner tool that showed RPMs in real time.

Did not realize you had a vacuum gauge. Smooth sound in an exhaust suggests the vacuum gauge should show a constant vacuum in the manifold at idle. However confirm this. Intake manifold vacuum at a constant above 17 inches should be observed. If the vacuum gauge needle vibrates, then the problematic cylinder is indicated when the gauge point dips to a slightly lower vacuum.

Assuming the vacuum gauge is contant at idle. Then this is puzzling. Vibration at idle implies a missing cylinder. And yet the exhaust pipe sound suggests no missing. Vacuum gauge will probably report the same 'no problem'. If so, move on to some other suspect. This engine appears to be operating smoothly on all cylinders during idle mode. And gas mileage suggests it is also working properly during normal operation.

Every part of a drive train and engine must be isolated from the car's body. Even a radiator does not make a direct connection; connects to the body via rubber isolators. Is it possible that something (ie exhaust pipe) is touching the body as it gets warm? For example, an exhaust pipe was hung slightly twisted? Or a heat shield place moves slightly with heat?

[glatt]

Well, I don't have a vacuum gauge. Maybe I should get one. Seems like a waste of money for a tool I'll only use once. But I wish I had one.

I listened at the exhaust, but don't trust my skill at identifying the sounds I hear. It sounded like an exhaust. Didn't seem abnormal at all.

I'm confident that I eliminated the EGR system as the cause of the problem, and even cleaned the EGR valve, but last night I drove the car to a party in DC (because it's small and easy to park) and it's as bad as it ever was. The problem is still there.

When I watch the engine, one of the radiator hoses is vibrating like crazy. The engine doesn't seem to vibrate much. At times the battery is vibrating a lot too, so you can't read the writing on it, but yesterday it wasn't so bad.

I guess it's back to the drawing board.

Under the section for vibration diagnosis, the shop manual talks a lot about road vibrations, but those don't apply because the problem is only noticed when it's stopped. The only part of the vibration diagnosis that applies is the engine speed sensitive section. They list a bunch of scary sounding possibilities:
-Belt or engine driven accessories like generator, A/C compressor, power steering
-drive belts and their tension
-bad bearings
-bent shafts
-faulty or loose flywheel

For rough idle, which seems to me like it might cause cabin vibrations and loudness, they list the following:
Sensor checks
Check O2 sensors
Check Throttle position Sensor
Check Engine Coolant Sensor
Check MAP sensor (but they don't say how)
Fuel System Checks
Check if condition occurs under Rich or Lean conditions
Check EVAP control system
Check fuel injectors for leakage
Check fuel pressure
Ignition System Checks
Check ignition voltage with spark tester
Check spark plugs for wet, cracks, wear, gap, burned, blistered, dirty
Check spark plug cables for resistence
Check ignition timing
Additional checks
Check for vacuum leaks (I did this)
Check ECM/PCM grounds for clean, tight, routing
Check ECM/PCM with scan tool to look for A/C signal
Check EGR (I did this in the post above)
Check battery cables and ground straps (battery cables look fine)
Check Crankcase Ventilation Valve
Engine Mechanical Check (It's like they are just listing engine parts now)
Motor mounts
valve timing
low compression
bent push rods
worn rocker arms
valve springs
camshaft lobes

[glatt]

I thought I would check the PCV valve since that was suggested. 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. It had been sucking air in like crazy.

The manual says that if you put your finger over the PCV valve inlet, it should snap back into place. I tried it and the valve did nothing. So I pulled the valve. It was perfectly clean, and I could shake it and hear the plug inside rattling back and forth. I guess the PCV valve is fine.

So then I started disconnecting the spark plugs one at a time and listening to the engine. Now I know what missing sounds like. It's very distinctive. The engine wasn't missing before. As I disconnected each spark plug in turn, the engine would sound worse each time. So I think it's not missing.

But now my check engine light was on! Disconnection a spark plug will do that. Fortunately, the CarChip Pro did exactly what it's supposed to do and I was able to turn that light back off.

So I haven't solved anything, but it's been fun playing with the pencils on the bench.

Maybe cars just sound like shit as they get older.

[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.