 TROUBLESHOOTING DRIVABILITY PROBLEMS
by Mark Davidson
To control vehicle emissions and improve performance and fuel economy, automobile manufacturers began installing “on-board” computer systems on vehicles in the late 1970’s. This provides a monitoring system that continuously checks the readings from various sensors and turns on the “Check Engine” or “Service Engine soon” light when a sensor is not providing the correct information to the computer. Whenever the “Check Engine” or “Service Engine Soon” light appears, there is a service code, often called the trouble code, stored in the computer’s memory circuit that can be extracted and used to help troubleshoot the problem. Today’s computerized vehicle systems can actually help you diagnose and troubleshoot themselves if you have the right information and a few basic hand tools.
If you are experiencing a drivability problem, your first step is to extract any “trouble codes” from the vehicle’s computer. The following are the most commonly experienced drivability problems and a short description of what components may be affected. In each case, these suggestions are assuming the engine is mechanically sound (camshaft, pistons, timing chain or belt, valves, etc). The engine must also be in good tune (spark plugs, wires, distributor cap and rotor, air and fuel filters been recently replaced, etc). The ignition timing and base idle speed must also be adjusted properly.
Complaint #1 – I HAVE AN INTERMITTENT “CHECK ENGINE” LIGHT FLASHING
An intermittent “Check Engine” light is an indication that a service code has been stored in the vehicle’s computer system. First, extract the service codes prior to making auto repairs. A common sensor that creates an intermittent “Check Engine” light is the Oxygen Sensor.
Complaint #2 – MY CAR STARTS HARD
Hard starting can be caused by a number of different items. First, inspect all vacuum lines on the engine and replace them if any cracks or brittleness is found. If your vehicle is carbureted, the choke system must be functioning properly before any sensor testing is performed. If it is operating correctly, the Coolant Temperature Sensor could be affecting how well the vehicle starts. On fuel injected vehicles, the sensors that can commonly be attributed to this problem include the Coolant Temperature sensor, Manifold Absolute Pressure (MAP) Sensor, or the Mass Air Flow Sensor.
Complaint #3 – MY FUEL INJECTED CAR IDLES ROUGH AND STALLS AT STOP SIGNS
Stalling at stop signs while the vehicle is in gear on carbureted vehicles could be caused by a failing Idle Speed Control Motor. On fuel injected vehicles, you should look for failure in the Manifold Absolute Pressure Sensor, Mass Air Flow sensor, or restricted fuel injectors. If you suspect restricted fuel injectors, we suggest using an in-tank fuel injector cleaner prior to automotive troubleshooting procedures.
Complaint #4 – MY CAR IDLES ROUGH
Rough idle is most commonly caused by a tune-up problem or vacuum leak due to deteriorated vacuum lines. Once these parts have been replaced or ruled out, the sensors that could be at fault include: Coolant Temperature Sensor, Manifold Absolute Pressure Sensor, Air Charge Temperature Sensor, or Idle Speed Control Motor. Check your auto repair manual for the testing of each of the sensors.
Complaint #5 – THE ENGINE SEEMS TO IDLE ERRATICALLY, WAY UP AND THEN WAY DOWN AGAIN
The items that may cause an erratic idle include the Throttle Position Sensor and the Idle Speed Control Motor.
Complaint #6 – THE ENGINE HESITATES ON ACCELERATION, BUT ONLY WHILE THE ENGINE IS WARMING UP
If you own a carbureted vehicle, first make sure the choke system is functioning properly. A hesitation on acceleration during the engine warm-up period may indicate a failure in one of the following areas: Coolant Temperature Sensor or Air Charge Temperature Sensor.
Complaint #7 – THE ENGINE HESITATES ON ACCELERATION AFTER IT IS FULLY WARMED UP
A hesitation or stumble during acceleration is most likely caused by the Throttle Position Sensor (whether the engine is carbureted or fuel injected).
Complaint #8 – I AM EXPERIENCING POOR FUEL ECONOMY, AND OCCASIONALLY SEE BLACK SMOKE OUT THE TAILPIPE
This situation is typically caused by an overly rich air/fuel mixture. This is commonly caused by a defective Oxygen Sensor, Coolant Temperature Sensor, Manifold Absolute Pressure Sensor, or a bad Mass Air Flow Sensor.
Complaint #9 – I HAVE POOR FUEL ECONOMY AND PERFORMANCE, BUT THERE AREN’T ANY SERVICE CODES IN THE COMPUTER
Sluggish performance is often attributed to plugged or restricted injector nozzles, in which case you can use an in-tank fuel injector cleaner to help restore performance.
Complaint #10 – THE ENGINE SEEMS TO “PING” OR “KNOCK” AFTER IT IS WARM
The first item to check is the base ignition timing. If that is correct, the electronic component to test, if your vehicle has one, is the Knock Sensor. An inoperable knock sensor will not send a signal to the computer to retard timing therefore pinging may result.
Complaint #11 – MY CAR SURGES AT HIGHWAY SPEEDS
Surging at highway speeds is commonly caused by a lean air/fuel mixture. This may be due to a failing Coolant Temperature Sensor, a defective Manifold Absolute Pressure Sensor, or a Mass Air Flow Sensor problem.
Complaint #12 – MY ENGINE HAS A CARBURETOR AND IT DIESELS (“RUNS ON”) AFTER I TURN OFF THE KEY
Engine “run on” is commonly attributed to a high idle speed setting caused by a defective Idle Speed Control Motor, an incorrect adjustment, or excessively advanced ignition timing.
Complaint #13 – I AM EXPERIENCING A “ROTTEN EGG” SMELL OUT OF THE TAILPIPE
Rotten egg smell out of the tailpipe is caused by an overly rich air/fuel mixture or by running leaded fuels in a vehicle equipped for unleaded. Overly rich fuel mixtures can be caused by a leaking fuel injector, a bad Oxygen Sensor, or defective Coolant Temperature Sensor.
Complaint #14 – MY CAR “BUCKS” AND OCCASIONALLY BACKFIRES
Other than a mechanical or ignition type of problem, bucking or backfiring can be caused by a defective Manifold Absolute Pressure Sensor, a failing Mass Air Flow Sensor, or a bad Throttle Position Sensor.
As you can see, many of the sensors of a computerized vehicle system can create the same types of drivability problmes. But with a few inexpensive hand tools and the right troubleshooting information you can diagnose and repair the electronic computerized systems on today’s vehicles. Remember the above are only to be used as a guideline for testing. Many times, other components or sensors can cause the same symptoms. Consult the manufacturer’s auto repair manual covering your specific vehicle before proceeding with auto repairs.
(Mark
gave up on sports when the Browns left Cleveland and now spends his
Sundays working under a shade tree in the back yard tuning his son’s
soap box derby car.) |
Thanks a lot, i am experiencing some of those problems
Fuel pump failed on my 2006 Lexus GS 300 with 33,000 miles on it while on the highway. No warning as the failure takes place and no power, steering or brakes. My family in the car. Is this reasonable? Is there a car that when a fuel pump fails I can still have some steering and brakes or at least gives a warning. Very concerned.
Chris
98 Jeep Grand Cherokee 4.0 2wd. Starts may cut right back off or will run all day. Stumbles and dies backfire some times. Blew the muffler off but kept running. It will start back up after a few mins. I checked and there was no fire put on a pick up coil tried another coil that was running on another engine, still does it It acts like it is loosing electic power but the dash lights don’t go off. Some one else has put a pcm and fuel pump in. Checked the grounds around the engin comparment also. Any Ideas?
http://www.batauto.com/Forums/index.php?topic=12646.msg81465#new
This late comment is for future readers,with a similar problem,although it usually stops the engine suddenly. Since you eliminated the possibility of a failing ignition coil,and you know you are losing spark, and it restarts after awhile,usually 5-20 minutes I suspect the ignition timing sensor,(on later models called, crankshaft position sensor) is bad.This sensor starting being used after ignition points were discontinued.First in the distributor in their place,then down by the crankshaft pulley, then inserted into the bell housing near the flywheel.It consists of a tiny pickup coil,wound with hair thin wire.When the coil gets overheated (such as leaving A/C on while idling for long periods on a hot day)the wire will expand and break. The engine will then stop.Minutes later after it cools down enough,the broken ends will retouch and it will work again until it gets warm enough to expand and split apart again.When it gets to the mechanic is usually has cooled down and is now working preventing the mechanic from finding the problem.If you test the sensor continuity immediately after it fails (unplug it and use a VOM meter),it will show an open circuit.Minutes later after cooling enough it will show a normal closed circuit.It is easy to replace using 1 or 2 screws and the ones not in the distributor usually don’t need adjustment.They generally cost between $15-$50.
i am using a 2zz engine on high way speed at 120kph it will start to be on high refs, like it is on gear 3(auto) high fuel consumption what could be the cause?
I’m still new around here but it seems nobody is talking about how not to lose your car keys except for The Car Key Shop at 5614 Warrington Avenue, Philadelphia, PA 19143, USA. I found a lot of their material deals with the dropped wallet, broken cel phones and replacement car keys. When I called them at (215) 407-5942 I think it as the same guy who answered the phone who is in the videos. How come nobody else talks about this ? The video stories are hilarious at youtube but most of them are on the web site. Am I the only one who noticed this ?
HI Marc. I have a alfa romeo spider 1987. I bought it this year it ran flawless all summer. The first time it died on me was in late september. I went to gas up the car and it would not start. I finally goth it to start,. I drove the car back home and it staled after i shifted on first. I had to tow the car to a garage . The mecanic was relutant to work on it. He called me two days later and told me it was just the wire going to the coil, he told me to change the wiring.I drove the car about two days later and the car would buck when I would go into first and second gear. I did a complete tune up including spark plugs, roter,distributer cap , coil, gas filter. I’m still waiting for the air filter and new wiring from my order, I tried the car and it was having problems starting. I recharged the battery it helped the car started . But the car still bucks on 1st and 2nd gear, I going to check all the fuel system and hope ‘it is not the transmission . could you give me some info what it might be. thank you.
I had a complete tune up(on my 1999 chevy blazer) but my car still hesitates why??
Check your TPS (throttle position sensor), ECT sensor, and engine mapping sensors , map/maf
But before alll of that , check for codes, vaccum leaks, visual inspection
Check the timing belt and timing settings.
I have a 2005 Ford Expedition 5.4L 2WD. I had my CEL light flash on/off, then stay on. It had a P0303 code. I had my plug and coil changed. It was jerky when idled and around the 40-60 mph range. It is doing the same thing again, the day it was changed.
my nissan pathfinder 2002 v6 3300l , the fourth gear is not changing, i have checked by obd2 and the reults was just one error which knock nsenor fult, do a bad knock sensor produce the same symptoms??
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thanks alot
No, generally not. You could have a shifting solenoid out.
The knock sensor is just there to detect an overly lean mixture and to adjust things accordingly , such as engine timing , and air fuel ratio.’
My car just started bucking what causes this
I have a 98 gmc Sonoma with a 2.2 I have pulled the fuel pump really and I have fuel dumping into the cylinder so I pulled the injectors out and just got the battery hooked up and all four injector a are straying fuel out of them this is with the key off and the fuel pump relay out so does anyone have any idea of what causes this problem can call me or text me at 336 233 1462
Sorry i mean 336 233 1463
Hello: I have a ford pu 4.6 2003 shortbed that has very low mileage,like in the 30’s but on a 6 mile trip it started acting up , the tack bouncing and the engine acting as if it had run out of fuel,a friend scanned the obd link and there was no codes, it starts very good no excessive cranking, and runs perfect when cold but when it warms up it starts acting up, can be visibly be seen shaking and not running right,you can feel the fender and know its not running right,it runs from 800-1000rpm on tach but the tack is not visibly showing the rpm changes now, do you have any clue what is going wrong this is a really clean truck like new, thanks carlos
I had camshaft sensor put in car still bucks not as bad as before but it still bucks when it reaches 45mph and 55mph I have a 2007 ford Taurus any suggestions in what might be going on?
Hi i have a toyota yaris,after about 30min it starts to lose power and jerks until it stops.Idling gets rough just prior to stopping.
2O min later it drives fine again.Mechanic checked computer all good when we brought it in.we checked air flow meter.
Any ideas,thanks steve
brake lights stay om after engine is shut off