Car randomly dies and doesn't want to start back up, MAP sensor?


#1

Hello
I have a 2000 pontiac grand prix (3.8) and when i was driving my car randomly shut off. When i tried to start it again, the car would start, rev a 1000 RPMs then dies a second later. If i pump the gas i can keep it running at around 2000 RPMs until i let my foot off the gas, then it dies After repeated attempts the tow truck was in need. the following day i ran some tests. when starting the car the fuel pressure would stay between 40-50 psi and doesnt drop below that. (excludes fuel pump, filter, etc…) After running tests the car suddenly started up and continued to run with the rpm bouncing (+/- 100 rpm) i let the car run for 10 minutes and it began running perfectly. This has happened repeatedly in the past 3 days. I have noticed that when letting the car sit while runnig, it will usually die within 2 minutes, however if I am driving. It will sometimes be fine for hours. When attempting to start it with the help of hitting the gas, the car will sometime sputter at 300 RPMs when hitting the gas will do nothing. in some situations i will hit the gas and the rpms will decrease and rise randomly. (letting my foot off the gas, rpms will hesitate before reducing). When the car dies, i am at times able to get it running after spending 30 min fiddling with it. But half the time it wont start. I have a -P0107 and P0108, codes for high input and output of the MAP sensor. When i unplug the sensor it seems to have minimal effect to the running condition of the car. When i unplug it at times the car will keep a steady rpm while hitting the gas and the car acts no different when i unplug the sensor when the car is running well. I ordered a new MAP sensor which should arrive today so i can see if that is the problem. If you can offer any suggestions, it will be greatly appreciated, thanks!


#2

Given those codes and the fact that problems there can cause things like stalling and hard starting, then the place to start is by checking the wiring for the MAP sensor and the MAP sensor itself.

If you have access to a scantool that does live data then you can look at the MAP sensor signal to get some idea of what is going on. But sooner or later you’ll probably also need a way to test it.

The MAP sensor gets a 5V reference signal from the PCM over one wire, and on another wire sends back a signal to the PCM that will range from about 1V (high vacuum - e.g. throttle plate closed) to 5V (low vacuum - e.g. WOT). So the MAP should have 3 wires - one 5V ref, one signal return and one ground. You can do some checking if have a digital multimeter. Check that the ground is grounded and doesn’t have continuity with either of the other two wires. With the key on check that one wire is getting a constant 5V. It shouldn’t really fluctuate. If you can keep the car running then you can check for the signal voltage - e.g. at idle with no revving, the signal should be down around 1V. If you open the throttle the signal should climb. It would be great to have a vacuum gauge on the car because it really does go with vacuum rather than with how much you rev it). You can test it even if the car won’t run - pull the MAP off but then plug it back in. Put the key in the Run position and use some other way to apply vacuum to the port on the MAP. Low or no vacuum means the signal wire should do about 5V. As you apply more vac, it should drop down to as low as 1V. If you have a way to measure it (like a MityVac) a healthy engine pulls about 17-22inHg at idle and you can use that as “high vacuum”

If you don’t have a steady 5V ref signal then your problem lies in the wiring between MAP and PCM. If the signal from the MAP seems to be fine, then you need to check the signal return wire back to the PCM. If there is any indication of a ground problem (e.g. no continuity with ground, continuity with one of the other wires), then you have to trace that out.


#3

Good explanation above. I’ll add that w/my Corolla at least, if the MAP sensor fails, the ECM will substitute a fixed value for the MAP sensor input, so that the engine will continue to function, just won’t perform as well as with the MAP sensor working.


#4

True, should not ignore code for map. But, if car stalls again, try leaving maf unplugged.


#5

Replaced the MAP sensor and the car seems to be working fine. When removing the old sensor i accidentally broke the top part off (oops!) and it seems to have a decent amount of oil on it, along with the nipple of the sensor. Is this normal to have oil in the sensor? (Old car with 225000 on it so leaks are likely.) And thanks for everyones help, its reassuring having others opinions!