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• Why does my engine stall at idle?
• How does an idle air control valve work?
• Why does my engine idle high?
• Why does my engine idle low?
• How to check for an engine vacuum leak?
• How does a MAF mass air flow sensor work?
• How to test an oxygen sensor
• Why does my engine hesitate?
• Howling noise from car
• Why is my car vibrating?
• My car is making a whistling sound
• How to test a fuel injector

MORE: Engine idle questions

Answer: We think you have done a really good job of diagnosing your problem. It does sound like a faulty fuel injector.

Question: 2001 Isuzu Rodeo mileage: 45,000. My fuel injection (ITEC) has a hard starting problem each morning. It idles very roughly. The check engine lamp comes on periodically. Checked and noticed the problem is from the oxygen sensor (sensor not yet replaced). I think I'm not getting enough power from this engine. I'm way over here in Cameroon, West Africa, a little too far from good test equipment.

Answer: The first thing to do is to replace the oxygen sensor. A faulty oxygen sensor would adversely affect engine performance.

Question: 2002 Mazda 3 mileage: 54,000. My Mazda has the following problem: when cold, no problems are apparent. Once at normal operating temperature, it idles erratically and stumbles during normal acceleration - the stumbling is similar to water in the gas or misfiring. It operates well at a constant speed (say highway speed). The "check engine light" has come on. Reviewing the Haynes repair manual, it appears that the problem might be with part of the emissions control systems (say oxygen sensor). Haynes has a description of how to get the codes from the OBD by using a jumper wire (between terminals STI and GND) and watching the light to get the number of flashes (i.e. the engine's Morse code). It appears from the diagram of the OBD connector that my car has the OBD-1 system (a group of 25 or so terminals in a compartment under the hood (not an under dash model that seems to be the OBD-II. The directions that they give do not work. Furthermore, in another part of the book they present another example of this same OBD-I connector and give the same terminal (STI) a different name (TEN) that matches the terminal name and position as shown in my car.

I am guessing that they have mixed up the diagrams and terminal names. Can you give any information on how to get the codes from the OBD system as a help? I have been told by an auto service provider that the problem with my car is that the oil seals around the spark plugs (o-rings) are shot on the front 3 cylinders, and that the head cover gasket needs to be replaced, the plugs need to be replaced, and that the special spark plug wires need to be replaced. However, they could not get the codes. I cannot trust them to make the right call if they can't even access the codes (they say they don't have the computer hook-up) and we do not live near a dealer. Your assistance would be greatly appreciated as a second opinion - I am not thrilled about forking over a lot of money to people who can't even get the right codes.

Answer: You car requires an OBDII scanner. This should tell you if there are any fault codes in the computer. In addition, check the air intake boot for cracks between the airflow box and the intake manifold.

Question: 2003 Dodge 1500 Truck, miles: 68,000: Seem to have a problem with the engine it idles up and down and I was wondering how do you check the fuel pressure. I have replaced everything that would cause this and everything is new, but the thing is that the computer does not display any codes. How does the fuel pump completely quit or does it fade to quit? The engine just runs rough, mostly it idles high at cold temperature and idles up and down when at operating temperature. I also checked all the vacuum hoses too, and also tried some injector cleaner which didn't work. I am so stumped if you have any suggestions that would be great. Here is the list of things that I have changed, Fuel filter, Oxygen sensor, alternator, battery, new belts, Idle Air control, (I cleaned out the grease and dirt with STP throttle body/carburetor cleaner before installation of new IAC throttle position sensor, air filter, new spark plugs and wires.

Answer: Fuel pressure can be checked with a gauge installed in the auxiliary port in the fuel injection system. If your car is not equipped with that port, use a tee in the fuel pressure line. Today's truck depend on it's fuel pump more then ever, If a fuel pump is weak or faulty it can cause many different problems. Also the fuel filter for this truck is in the fuel tank mounted to the fuel pump.

Question: 2002 Mitsubishi Montero mileage: 10,921. My truck was working fine but due for a major tune up. I took it into SpeeDee tune and they performed the work. When I picked up my truck it drove fine for about 20 minutes. Then when I came to a stop I noticed my truck's idle was very high and fluctuating - from 2500 to 3500 RPMs. The idle should be between 500 and 1500 RPMs ideal is at 750 according to my manual. I took it back to them and they adjusted the throttle. I drove off again and in 20 minutes my idle was at 4500 RPMs. I found a mechanic at 6:30 PM on Saturday, Memorial day weekend, I slipped him $60.00 - he said the distributor was anywhere from 45 to 90 degrees out of position and for the adjustments they made to my throttle they should have been fired. I brought my truck back in today and again they did the timing and adjusted the throttle. They hooked it up to the diagnostic and nothing came up as a problem. Lo and behold, 30 minutes of driving it out of the shop my truck's idle was back to fluctuating between 2500 and 3500 RPMs. Any ides what they could have done during the major tune up to cause this? As the problem was not there before the major tune up? Vacuum hose? Throttle sensor? It has to be something that builds up pressure since it only starts acting up after 20 to 30 minutes of driving. I am baffled I really hope you can help.

Answer:   If a strong fuel injection cleaner was used on the fuel system it may have burned out the Oxygen sensor. Oxygen sensors fail and if it was close to failing at the time a fuel injection cleaner was used, the cleaner would have been enough to push it over the hill. Replacing the Oxygen sensor in this case should remedy your problem.

Question: 2002 S10 Blazer mileage: 85,000. I am having a rough idle, rich mixture, smell from the exhaust and black carbon coming from the exhaust as well, and I have to crank it for a while and hold the gas pedal to the floor for it to start when it is cold. When it is just shut off for a minute or so and I restart it, it will start immediately. I have put in a new fuel filter, new spark plugs and wires, distributor cap and rotor, O2 sensor, and EGR valve. I am not getting any codes CONSISTENTLY. A couple of times I got codes, one was MAP sensor/EGR valve. The other was EGR valve / O2 sensor. But the codes weren't back to back and they went away shortly after they came on.

Answer:   Check to see if the fuel pressure regulator has a ruptured diaphragm allowing raw gas into the intake manifold. This is a common problem.

Question: 2001 GMC 1500 4WD mileage: 98,000. Idles good for about 5 minutes. Starts idling rough once engine warms. I have so far replaced the following items and performed following repairs: 1. New cap/rotor, plugs, wires, coil and distributor. 2. New EGR valve. 3. New Coolant temp sensor. 4. New fuel filter. 5. Cleaned all electrical grounds and battery posts. Idles like it's missing. Doesn't stall but is rough. The IAC valve checks good and pintle is clean. Timed engine and noticed power improvement as well as mpg. Baffled as to why it idles rough only when warm?

Answer:   This is a difficult problem to detect because the computer can not sense a malfunction, the vehicles computer system thinks everything is ok. In reality what is happening is the electrical component inside the MAF sensor becomes contaminated by air impurities. These containments imbed themselves to the electrical component that senses the air flow causing the sensor to be slightly off, but not enough to trigger the MIL (malfunction indicator lamp). Replace the MAF sensor with a new. NOTE: (Mass Air Flow MAF sensor or Air Intake Sensor AIS, depending on manufacturer. It does the same thing only with different names).

Question: 1999 Dodge Viper mileage: 13,000. The battery recently went dead, after I had left the rear hatch open for a few days. After charging the battery, the car will not idle as before. It is difficult to keep it running. The idle was always too low. It idles at about 500 rpm. Is there an idle adjustment?

Answer: There is no idle speed adjustment on your car. If you drive the car, the computer will re-learn the correct idle speed.

Question: 2003 Honda Accord mileage: 70,000. Two problems, maybe related, maybe not. When cold starting the car, engine idles at 1500 rpm (normal). After a couple of minutes, the idle fluctuates between 1000 and 1500 rpm and continues; "jumping" until the engine is fully warm. It then idles at 800 rpm. The "jumping" will sometimes recur in cold weather when the defroster is on. Second, periodically, whether the engine is cold or warm, it will crank but not start. Seems to not be getting fuel. Hold the pedal to the floor and crank, it will eventually start and once started, runs fine. No computer codes have been set. I removed the throttle body and cleaned it, the throttle position sensor tested OK. I removed and cleaned the idle speed motor (it was clogged with carbon) and actuated it with a 9-volt battery. Worked fine. The coolant sensor was recently replaced and the EGR valve moves freely although it is original. The oxygen sensor is original and $120 to replace. Could the manifold air pressure sensor or the intake air temperature sensor have a bearing on these problems? Any other items come to mind that I should check?

Answer: We would replace the PCV valve and also the oxygen sensor.

Question: 2004 Mercedes Benz CL500 mileage: 33,000. The car idles fairly smooth, but when put in gear the idle becomes quite rough. I have performed maintenance of all filters, plugs, and oil change. I have not changed the plug wires (they appear to be original) the plugs are not discolored and seem to be firing correctly when checked with a timing light. The wires are expensive so would prefer not to change them if not needed. The car runs well at speed but am afraid the roughness is being masked by the increased Rpm's. This problem is not always present, but I have been unable to determine a pattern. We have been using 93-octane gas as recommended. We recently purchased this car so do not know the history of this problem.

Answer:   This is a difficult problem to detect because the computer can not sense a malfunction, the vehicles computer system thinks everything is ok. In reality what is happening is the electrical component inside the MAF sensor becomes contaminated by air impurities. These containments imbed themselves to the electrical component that senses the air flow causing the sensor to be slightly off, but not enough to trigger the MIL (malfunction indicator lamp). Replace the MAF sensor with a new. NOTE: (Mass Air Flow MAF sensor or Air Intake Sensor AIS, depending on manufacturer. It does the same thing only with different names).

Question: 2001 Chevy 1500 mileage: 126,000. Rough idle when starting cold or hot and when put into gear it stalls out. When it's idling, rough it also smells like it's running very rich. If you let it run for several minutes the idle smoothes out then you can put in gear and it doesn't stall. It runs great after you get it going. I have had it scoped, probed and fondled by several different mechanics and they can't seem to find anything wrong. I had a tune-up with new wires and the whole 9 yards done and it ran perfect for about 2 months but then went back to it's old ways. Seems to be a little worse in the summer months.

Answer: Check the fuel pressure regulator. On this truck it is an easy task, 15 mins.

MORE: Engine idle questions

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