Car question

[Clodfobble]

There was a review on the Amazon page by a dad who had purchased it to monitor his son's driving habits... and when his son got a ticket for going 90 in a 55, the son swore he wasn't going that fast, and the dad's all, "Yeah yeah, sure son. Let's check the record, shall we?" And it turned out, no, the CarChipPro had recorded a speed of only 62 at that time. Still speeding, of course, but obviously not 90. They presented the data in court and got the ticket thrown out.

[infinite monkey]

Originally Posted by Clodfobble

There was a review on the Amazon page by a dad who had purchased it to monitor his son's driving habits... and when his son got a ticket for going 90 in a 55, the son swore he wasn't going that fast, and the dad's all, "Yeah yeah, sure son. Let's check the record, shall we?" And it turned out, no, the CarChipPro had recorded a speed of only 62 at that time. Still speeding, of course, but obviously not 90. They presented the data in court and got the ticket thrown out.

Nice! I like!

[glatt]

A year and 150 posts ago, I complained in this thread about a rough idle my car has. If you are a glutton for punishment, you can go back and re-read it all, starting at post 55. The problem is still there.

The executive summary is that after driving the car for a while and the engine gets warm, I stop at a light and put the car in neutral or step on the clutch, and it begins to idle. And vibrate. A lot. It's annoying. It's loud. It didn't use to be like that. It used to be smooth and quiet. Now it's rough and loud and vibrating. However, the fuel economy is as good as ever. I got 39.5 mpg on the highway on a trip back from Maine over the summer. Very pleased with that. Normally it's city driving and around 30.

We recently bought a second car, which means I can take a day here and there to work on this one, and the other one is available for use. No pressure to get it fixed in a half hour window before the next soccer game. So I am returning my attention to this problem.

So far, I've tested the engine mounts by putting on the parking brake and trying to drive forward and reverse while watching the engine. The engine barely moves. I think the mounts are good. I ran a few bottles of fuel injection cleaner through the gas tank according to instructions. No change. I paid the corner mechanic to clean the throttle body when they were doing an oil change. No change. But I recently took the air filter off to check the throttle body, and noted that it was clean, so they did the work. I tried to clean the mass airflow sensor, but discovered that this car (1996 Geo Prizm) does not have one. It has a pressure sensor instead and I understand you don't clean those.

Here's the throttle body.

Looks clean to me. The little square-ish hole at the bottom of the throttle leads down to the Idle Air Control valve. It looks like it might be a little dirty down there, so I may try cleaning that next. It's kind of complicated though. I'd need to remove a few hoses, including coolant lines, some data cables, and a throttle cable before I can get the throttle body off, and then take the IAC off of the throttle body and clean it out. Maybe this weekend. Seems like if I'm having idle trouble, the problem might be the idle control valve.

How do you take off a couple of coolant hoses without getting coolant everywhere or bubbles in the coolant system? Do I need to flush or bleed the coolant afterwards? Sounds like a lot to go through to clean something in the hopes it will fix the problem.

[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.

[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.