QuestionQUESTION: My 1998 Neon, 5 speed, sohc with 240k miles has a serious driveability problem that I have been throwing parts at. I have relaced the fuel pump module, ecm, back o2 sensor, crank position sensor, ign. coil, plugs and wires.
The car starts when cold and runs fine until it warms up. Then it starts to miss, sputter, and when I come to a stop it dies. Sometimes it will restart, most of the time not. Just a 43 engine code which is pretty generic in it's description. I pull the fuel pump fuse to reset the computer, wait untill the engine cools, then I can replay the whole scenario over. i also have replaced the injectors and went over all the sensor connectors and the wiring harness. I bought this car new.
ANSWER: Hi Robert,
The temperature dependence of the problem would lead me to look at the coolant temp sensor and the MAP/intake air sensor units. The coolant temp sensor is on the rear of the engine and has a 3-wire connector. Remove the connector and check its resistance when cold, then notice as it warm up whether the resistance drops(measure between the pin A and B, which are connected to the black/light blue ground wire, and the tan/dark blue signal wire, essentially across the pins that are adjacent to the release tab). It usually will read about 10,000 ohm and then will drop gradually to around 700-1,000 ohms when the engine is fully warmed. The IA sensor is mounted on the intake manifold and has 4 wires because it also serves the manifold absolute pressure. The signal wire of the sensor is "B" and is black/red. I would suggest that you pierce the insulation of probe the plug socket with a pointed probe and measure the voltage on that wire compared to engine ground as the engine warms up. You should see a voltage change.
Either of those sensors not responding properly would cause a mixture mismatch that would then cause misfire. Otherwise, there are a great many engine mechanical issues that would need to each be checked as to possible cause, but the temp dependence observation should be the first priority.
Roand
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QUESTION: Ok Roland ,
I metered the temp sensor and it works fine. I metered the map sensor and it reads around 2.5 VDC with the engine fully warmed up. (is this normal? )
I filled the gas tank from 1/4 to full and took it out for a ride again. Ran great all day.
This morning, with the tank still reading full, I took the car to work. It started it's familiar routine of missing, popping, and hesitating while cruising in top gear. I would downshift to raise the revs, and it would clear up and run fine. I got on the freeway and began cruising at 60MPH. There, it starts to stutter and miss again. If I leave it in 5th and just floor the petal, it just moans and pops-no acceleration at all.
I would down shift to raise the revs where again it clears up and runs fine. Shifting into 5th and running at 70MPH, it runs much better than at 60 mph. BUT, when the grade changes from flat/downhill to an uphill grade, the stuttering and missing comes back !
As long as I don't go any slower than 70mph, It clears up once I am flat/downhill again.
ANSWER: Hi Robert,
I am unclear about whether this problem is related to rpm/engine load/manifold pressure (which is a combination of rpm and throttle opening)/or fuel demands. All I can suggest to do (without having a diagnositc readout box on board and watching the sensors' values while you are driving) is: to measure the resistance readings across the throttle position sensor range (measure between the center and either side pin with an ohmmeter while you move the throttle lever over its complete range of movement) and,
to monitor the MAP voltage with a probe on the wires as you have done, but to do it with long enough wires as to be able to observe it while you are driving so you can see the response of the voltage under normal/problemmatic behavior.
It appears to me that you are not getting a proper mixture and those two sensors are the ones I would believe are acting up. The MAP voltage you measured (I assume at idle rpm) was OK. But how it is tracking at road speeds, road elevations, rpm, is of interest as well. It could be a faulty sensor that is causing this.
Roland
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QUESTION: Roland,
Can I assume that the TPS is good if I measure it with the car not running ? I assume that it has to be disconnected anyway to test it.
I only measured the 2.5VDC at the MAP sensor with the car fully warmed. What SHOULD that sensor read if I start measuring it's voltage when cold and going through warm up?
AnswerYes, Robert, I meant that the TPS should be tested when the engine isn't running and unplugged.
On the MAP, I assume we are measuring the intake air sensor signal, not the MAP (this is a dual function unit). I can't be sure but I believe the intake air sensor is simply a thermoresistor, like the coolant temp sensor, so I would not be measuring the voltage across it but rather the resistance as a function of the temperature of the intake manifold in the vicinity of the sensor. I don't have a tech spec for the Neon, but earlier model engines had resistance profiles for the throttle body and intake air sensors simial to that of the coolant sensor. So measure the resitance between pin A and B when cold and after driven on the highway long enough to heat the manifold. It probably will be similar to what I first answered for the coolant temp sensor if my surmise is correct and it is functioning properly.
Roland
