For those of you who may have missed the thread regarding my idle surge, here’s a brief summary:
- One hot Saturday in June, I was driving happily along the freeway. Upon exiting the freeway and reaching the stoplight at the end of the ramp, I pressed the clutch pedal and much to my surprise, the car stalled. It restarted right away, but then stalled three more times before reaching my destination.
- Within a day, the stalling problem had been replaced by an idle surge that seemed to only occur when the car was hot. At no time did the computer ever report an error code.
- On Monday, I asked for advice from the list and received various suggestions, most of which related to the throttle body. At this point, however, I was reluctant to perform any service on my own when I might be able to get the dealer to do it under warranty.
- On Tuesday, I took the car to the dealer and asked them to figure out what was wrong. They proceeded to misdiagnose the car as in need of a tune-up including: plug wires, fuel filter, and some simple cleaning for *only* $500. Needless to say, I retrieved the car without having any work done and spent the next 3 weeks living with a very annoying idle surge and trying everything I could think of to identify the problem on my own.
That’s as much as I’ve posted so far.
Q: So what’s new?
A: Over time, the problem evolved from: "Only when hot" to "Intermittent" to "All the time". As any troubleshooter can tell you, many problems are impossible to identify until they reach stage three.
Q: So what was the problem anyway?
A: One of the coils in my Idle Speed Control (ISC) had become intermittent and eventually opened up. Since the ISC is a stepper motor, a bad coil means that the motor can’t do much more than twitch. As a result, the ECU did not have positive control over the idle speed. Occasionally, the stepper did make progress in one direction or the other, but not on a regular or controlled basis. This explains why my idle ranged from stall to surge. From one stoplight to the next, the idle might change, but while stopped at a light, it would be consistant.
Q: So what is an ISC and what is it good for?
A: The ECU attempts to control the idle speed by opening or closing the ISC. The valve on the ISC allows extra air to bypass the throttle plate. Since a stepper motor does not provide any feedback on its actual position, the computer simply counts steps and assumes that the valve lands in the correct place.
During normal operations, the ECU uses the ISC for some very interesting things besides simply adjusting the idle speed. The ISC is completely shut when you kill the engine to make sure it stops quickly. The ISC is opened completely at startup to provide plenty of air. It is also opened during normal driving to prevent the car from bucking when you take your foot off the throttle to shift. If you run the air conditioner or turn the steering wheel, the ECU compensates immediately by opening the ISC a little to keep the revs up. This also happens during cruising to mask the effect of the increased load on the speed of the car.
Q: So what, exactly, is idle surge?
A: Idle surge is the condition that occurs when something in the ECU idle speed feedback loop fails to operate as expected. Since the ECU is not quite as smart as you are and has a very microscopic view of its world, its behavior may seem very strange to you, but seem perfectly normal to the computer. The result is an idle speed that oscillates from ~1200-1500 RPM about 2 times a second and no error codes are ever displayed.
My idle surge was a result of the following situation:
- The idle switch is depressed (foot off the gas)
- Too much air is getting through the throttle body (ISC stuck open)
- O2 sensor registers lean mix (too much air)
- ECU adds fuel to achieve correct mix
- RPMs increase due to increased fuel/air supply
- At 1500 RPM the ECU goes into decel mode and cuts fuel (*)
- With only air coming into the engine, the RPMs drop rapidly
- At 1200 RPM the ECU turns the fuel back on
- Goto 2.
*) In our cars, the ECU cuts the fuel supply completely when the car is decelerating. It determines that you are trying to decelerate because the idle switch is depressed and the RPMs are above 1200. This is done to prevent an over lean condition and to assist in engine braking.
Q: Are there any other annoying side effects of this problem?
A: The only other major side effect is that with the throttle plate all the way closed, the engine still wants to run at 2000 RPM. The result of this is that it is very difficult to drive around parking lots. Parking lot speeds in 1st gear put you just under 2000 RPM. At this point you have the throttle barely cracked open. It’s about all you can do to keep the throttle almost closed, but not depress the idle switch. If the switch gets hit, the fuel gets cut (see (*) above). You can’t let the car just idle, because the surge is severly aggravated if the car is in gear. The gentle 2 cycles per second surge will change to a 5-6 cycles per second violent bucking of the whole car. Rather unpleasant.
Q: That’s nice to know, but what can I do about the idle surge on my car?
A: Glad you asked. In the time it took for me to identify my problem, I was forced to examine the fuel control and air supply loop of the engine very thoroughly. As a result I have developed the following diagnosis guide. Since I have obviously not had the opportunity to test all of these conditions, there is a fair amount of speculation involved and I probably missed some important stuff, but it should at least get you on the right track.
It helps to have a shop manual available while following this guide. Turn to chapter 14, Fuel System, find the pages for your engine, and use them to identify the parts I refer to.
Throughout this entire procedure, make sure that the A/C and all other accessories are turned off. Extra engine loads may confuse the diagnosis.
- My car has an idle surge
- While the car is surging, smell the exhaust (ie. hold your hand in front of the exhaust pipe for a few seconds, smell your hand) It should smell like partially burnt fuel (rich).
- Pull the connector off the idle switch. The idle should stabilize somewhere above 1500 RPM. Smell the exhaust again, it should smell like a proper mix, ie. no odor. If you have access to the O2 sensor test connector under the dash, you can confirm a proper mix by checking it with a DVM. The O2 sensor normally outputs a value between 0 and 1 volt. (0=lean, 1=rich)
NOTE: Do not trust the O2 sensor output unless the engine RPM is steady. The lag time in the sensor makes it useless for correctly reading a surging engine.
- If the exhaust smells fine and the O2 sensor is sending reasonable values, go to step 2.
- If the exhaust still smells rich or lean (the vapors sting your eyes), think about getting an emissions inspection and suspect that you may have a bad oxygen sensor. If you’d like, you may follow the instructions for inspecting the O2 sensor as found in the shop manual.
- My exhaust smells fine now, but why are the RPMs so high?
- With the idle switch disconnected, the ECU thinks you are pressing the accelerator and will allow the engine to run at any speed. It simply adjusts the fuel supply to match the incoming air.
- Shut off the car and let it cool enough so you can work on it.
- Remove the metal snorkel from the front of the throttle body. This usually requires that you remove the air intake hose and the iron brace first. When removing the snorkel, be very careful to not damage the gasket.
- If you have a non-turbo engine, you may not have a snorkel on your throttle body and need only loosen the hose clamp on the air intake hose to get at the throttle plate.
- Inspect the barrel of the throttle body. Check the throttle plate for free movement and make sure it closes completely. If there is excessive carbon buildup behind the throttle plate, get some carb cleaner and follow the instructions in the shop manual for cleaning the TB. The manual shows that it is not necessary to remove the TB for cleaning, but you can do a much more thorough job if it is not on the car.
- If you decide to remove the TB, you must disconnect as many as 7 hoses, including the two water supply lines, the throttle cable, and two electrical connections. There is a disassembly list in the manual. I found it easier to disconnect the water lines at their far ends as the hose clamps on the TB itself were obscured. If you do not drain the coolant system before removing the water lines, expect to lose over a half gallon onto the ground. Make notes in your shop manual about what hoses connect where. Most are obvious, but you should record the colors of the vacuum lines for reference at reassembly.
- Give particular care to the gasket between the TB and the intake manifold. It is marked as non-reusable, but can be reused if it is not damaged. My gasket was stuck to the TB with sealant, so I just left it there during the cleaning process rather than risk tearing it.
- Be aware that it is normal for a small gap (~.010) to exist at the edge of the throttle plate when it is fully closed. DO NOT adjust the idle switch to change this gap. If for some reason you suspect that the idle switch is improperly positioned, follow the instructions in the shop manual to set it. It is highly unlikely that this will be necessary.
- After cleaning, reassemble everything, refill the coolant if necessary.
- NOTE: When reattaching the TB, carefully observe the torque specs. You are driving those bolts into an aluminum manifold that will strip very easily.
- Start the engine. If the surge persists, go to step 3.
- My TB shines like new, but I still have a surge
- You still have too much air getting past the TB.
- If you have a turbo, a vacuum leak probably isn’t causing your surge, but it won’t hurt to check.
- If you have a hand vacuum pump, use it to check all the vacuum lines on the TB and intake manifold. Check the shop manual for which lines are supposed to hold vacuum and which aren’t. Don’t be fooled by the EGR which is supposed to bleed vacuum on the yellow side.
- If you don’t have a hand vacuum pump, you can still do a simple test for leaks. Start the car. Disconnect the hoses one by one and cover the nipple with your finger. If this seriously impacts or even fixes the surge. Inspect that hose for leaks.
- Don’t forget to check the brake booster hose on the back side of the manifold and the fuel pressure regulator hose on the front side buried under the injector electrical harness.
- If all the vacuum lines check out ok, go to step 4.
- Ok, no vacuum leaks, but it’s still surging
- You still have too much air getting past the TB, but you have narrowed the number of possibilities down to 4.
- There are only 5 normal ways for air to get past the TB: The throttle plate, the base idle speed set screw (BISSS), the ISC, the fast idle air valve (FIAV), and the EGR valve. You already cleaned the throttle plate and made sure it closes properly, that eliminates 1.
- With the engine running and hot (temp>=180), pop the rubber plug off the front of the TB to expose the base idle speed set screw (BISSS). Disconnect the idle switch and the ISC connector so the engine runs at a steady speed and the ECU can’t fiddle with the ISC without your knowledge. Count the turns of your screwdriver as you screw the BISSS closed (clockwise). The RPMs should drop. Have a friend watch the tachometer to tell you when it reaches 750. If you run out of screw before the engine drops to 750, back the screw out to where you found it. Replace the rubber plug.
- If closing the BISSS did not drop the engine to 750 RPM, you have eliminated another air path as the culprit, leaving only 3 more.
- NOTE: If you did get the idle to drop to 750 using the BISSS, don’t get too excited yet, the real problem may still be hiding and is merely masked by closing the BISSS. Reconnect everything, go for a drive and see if the problem reoccurs. If not, consider yourself lucky.
- Stop the engine and wait for it to cool a bit.
- If you removed the TB to clean it, you may have noticed that the FIAV is completely sealed. As a result it is nearly impossible to determine its health or lack thereof, so for now, we’ll assume that it’s working. That leaves only 2 air paths as a potential problem.
- Follow the instructions in the shop manual for checking out the ISC. The easiest thing to start with is checking the resistance of the four coils using a digital multimeter. All four should be approx 30 ohms when cool and no more than 40 ohms if the car is hot. It is possible to perform this check without removing the ISC from the TB. If all four coils are within spec, go to step 5.
- If you found one or more of the ISC coils with a resistance much higher than 30 ohms (100 ohms or higher, mine was around 2K ohms) or if a coil is open, then you’ve found a bad ISC. The ISC is covered under the 5/50 emissions warranty, but the hard part is convincing the dealer to do the work for free. After being told by the dealer’s mechanic that, "I’ve been working on cars for 18 years and no customer is going to tell me how to do my job.", it took a call to Chrysler Regional and a long talk with the service manager before I was allowed to come down to the dealership with my ohm meter and take 60 seconds to demonstrate the problem. It is unfortunate that it took all this to get them to agree to do the work.
- If you are actually in this situation, the dealer may refuse to follow your advice simply because the ECU is not reporting an error for the ISC. As Todd can confirm, the ECU will never report an error with the ISC. A stepper motor is inherently open loop and can not be self diagnosed.
- A new ISC lists for $247. Try going for warranty service first.
- Ok, the ISC coils look good, what’s left?
- If your symptoms sound a lot like mine, especially the part about picking a slightly different idle speed every time, your ISC might still be at fault.
- Remove the ISC from the TB. It is not necessary to remove the TB from the car to do this, but it might help to get a few other things out of the way. BE CAREFUL to not lose the o-ring seal.
- Plug the ISC back into the electrical connector and have a friend turn the car on and off. Don’t turn the starter, just flip slowly back and forth from ACC to ON. The ISC plunger should move smoothly in and out as power is cycled. Lightly rest a finger on the tip of the plunger to check for weak spots in the step cycle.
- If the ISC seems to behaving normally, go to step 6.
- If the ISC doesn’t move in and out smoothly, but just vibrates in place without making any real progress, then something is wrong. Check the coils again, just to be sure. If they are ok, then find a way to check the voltage coming out of the harness connector. I suspect the best way to do this would be to construct a 4 LED matrix as a DVM wouldn’t be fast enough to follow the steps. The easiest way to do this would be to solder a 1K resistor to each of 4 LEDs and stick the LED/resistor pairs into the harness connector. What you should expect to see during stepping is power across the pins in an order similar to this:
1-2 & 4-5 2-3 & 4-5 2-3 & 5-6 1-2 & 5-6 back to 1-2 & 4-5, etc...
- Judging from the shop manual, I would expect to see 6 volts applied to each coil. If one set of pins never seems to get power, flex the harness while watching the LEDs to check for a broken wire. (BTW, make sure to get the polarity of the LEDs right to avoid false alarms…)
- Now I’m sure the ISC is good, aren’t we running out of options?
- Yep. About the only things left are the FIAV and the EGR.
- The EGR is located on the intake runners immediately below the TB. It is difficult to remove with the TB installed, but not impossible. For easier access, remove the air intake hose and battery during this procedure.
- Remove the EGR and clean the carbon deposits from the valve and seat with carb cleaner.
- Follow the procedure in the shop manual for inspecting the EGR. Applying vacuum to the green hose side of the diaphragm should move the valve plunger, opening the valve. If your EGR assembly has attachments for two green hoses, make sure to block one off. Releasing the vacuum should allow the valve to close completely. With the valve closed (I know this is gross, but what the heck, you could be paying somebody $55/hr to do this…) blow on the valve port furthest from the diaphragm. No air should leak through the valve.
- If the EGR checks out, reassemble it. Go to step 7.
- A new EGR valve lists for only $120, what a bargain, but should also be covered under the 5/50 emissions warranty.
- As an alternative, you can always go with Frank’s EGR block off, but only if you’re not in California. The California cars will detect your deception and the ECU will flag an EGR failure.
- Ok, only one thing left, the FIAV
- I never did figure out a good way to diagnose that thing. Even the shop manual doesn’t offer any help. It simply states (on page 14-49 for those reading along at home) that if you’ve tried everything else, suspect the FIAV, replace the TB. Considering that the FIAV may be replaced separately from the TB by just ordering the TB lower assembly (the good news), but that the TB lower assembly lists for $833 (the bad news), and it’s useless to suggest warranty diagnoses to the dealer unless you can back them up 100%, you’re probably better off trying alternatives.
- Block the FIAV permanently. I’m not sure if this will buy you anything. Those who experience colder weather may not be able to start their cars without a working FIAV.
- The FIAV shares air intakes and exhausts with the ISC. Blocking just the FIAV requires some creative metalworking (or maybe just some hi-temp RTV for a non-removable block off). Be especially careful when separating the upper and lower TB halves. Coolant flows in the chambers between the halves. A damaged seal here could allow coolant to enter your intake and seriously damage or destroy your engine.
- Ok, so now what?
- In discussing this article with a friend, he suggested that an idle surge might also result if an injector got stuck open. The catch is, this situation would affect driving performance as well as idle. This can be easily checked by inspecting the spark plugs. One of the plugs will show up as fuel fouled or at least very black from a rich mix, while the others would show signs of running very lean (white). If a bottle of injector cleaner doesn’t solve the problem, show the dealer the condition of your plugs and get him to replace the injector under the emissions warranty.
- If you’ve made it this far and you’ve still got an idle surge. I’m not sure I can help you. Bring it up for further discussion on the list and maybe somebody will come up with a bright idea that solves your problem. Other than that, you can always throw money at it until the dealer actually finds something or you can just learn to live with it.
Good luck and have fun,