CarTalk.com Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Map sensor

what are the sign of a bad map sensor.

Stumbling/misfire under acceleration.

Tester

Here’s the deal.

Your engine is a pump. Every time the intake valve opens and your piston goes down it tries to pull air through the intake manifold into the cylinder. Since the throttle plate (and a few other items) are in the way of the airstream, the pressure in your intake manifold becomes lower than the air pressure in the intake airstrea, it becomes a vacuum. Your manifold absolute pressure (MAP) sensor reads this pressure.

When you stomp the pedal, a few things immediately happen. One, the throttle position sensor tells the computer that the throttle has been stomped. Two, the throttle plate is now out of the airstream’s way and the absolute pressure in the manifold immediately rises toward the air pressure in the engine’s instream of air. The MAP sensor also sends a signal accordingly. The engine’s computer uses these signals to adjust the injector pulsewidths to meet the new engine demands.

Conversely, when you now let your foot off the accelerator, the throttle plate again restricts the airflow. The engine is now at the same RPMs (manual tranny) and trying to pull in the same amount of air but cannot, so it creates a high vacuum (low pressure). The MAP again senses this and the computer again adjusts the injector pulsewidth.

So, a bad MAP sensor can cause stumbling in acceleration but also an inappropriate level of fuel being fed in deceleration.

The problem is there are a whole plethora of other things that can cause these same problems. Without knowing a lot more about your symptoms, you make/model/year/engine/mileage, and what’s been looked at so far all we can do is explain a map sensor. That may or may not help you fix your car.

We’d be happy to help you troubleshoot your car, but we need a lot more information.

This is why I love this board. I learn new things all the time from people like Tester and mountainbike. Where else can you get this much expert information for free?

Yes, I know, an engine is a pump, but mountainbike’s explanation of how the pump works is simple and to the point. Fantastic!

I only hope the OP takes the time to read mountainbike’s response.

[b]I Had A MAP Sensor Go Belly Up, One Time … [/B]

… When I bought a new one at the dealer, I had to perform a rather intricate relocation of the sensor according to a Technical Service Bulletin (20 degree cold garage operation, not much fun). I had to move it from inside the car (on the ECM), drilling a 1/2" hole through the firewall, to mount it under the hood on the strut tower. The factory had them mounted too low and moisture would work its way into the system and gravity would take it from there. It took several years for that to happen. The new location, slightly higher than the intake manifold, and the new sensor, did the trick.

Oh, what were the signs it went belly up? Decelerating from highway speed (foot off the gas pedal), the engine would just die rather than idle. Also, the car set a code for MAP sensor/sensor system.

And, how to test it with a digital voltmeter: http://www.autozone.com/az/cds/en_us/0900823d/80/04/ae/32/0900823d8004ae32/repairInfoPages.htm

And, how to watch a YouTube video clip of it: http://www.youtube.com/watch?v=ijK2LsN8ipw

I find myself extremely flattered and humbled, McP. Sincere thanks.

Unfortunately, the OP hasn’t rersponded. Bedrock? Are you there?

I belive the MAP sensors is used in a differant way on cars that have both a MAP sensor and A Mass Air Flow sensor. I think in this situation the MAP sensor monitors EGR function. Mabe someone can confirm this.

All modern cars monitor both the pressure in the intake manifold and the amount of air flowing into the engine (via a mass airflow sensor). In truth, those are only two of the signals that are used by the computer to determine the proper signals to send out to the injectors, the coils, and other control functions.

These signals and the others are also monitored by the computer and of they’re out of their normal range when considered with all the other signals the computer stores a “fault code” and trips a CEL light.

Other signals are the crankshaft speed, the crank (or camshaft) position, the engine temperature, the position of the throttle, the vehicle speed, the signal(s) from the upstream oxygen sensor(s) (the downstream sensor(s) is(are) used to monitor the performance of the cat converter), and probably a few that I’m forgetting.

Think of it as a formula with a string of variables, except that each variable has to be within a specific range when considered in relation to the specific numbers in all the other variables at any given moment.

See last paragraph of this wiki. http://en.wikipedia.org/wiki/MAP_sensor . This is what I was refering to as the primary purpose of a MAP sensor on cars using a MAF sensor as the primary metering of air.

Cool link. It goes well beyond the depth that I was trying to stay at.

My sense is that the OP was looking to tie a specific sensor (MAP) to some specific problem that he’s having. My goals were twofold, to point out that there are a number of signals used by the ECU to meter fuel and get him to tell us more. Stochiometric measurements being important, while the formulas may vary and the means of taking the readings may vary I’m unaware of any vehicle that doesn’t include in it’s calculations both the mass of air incoming and the pressure in the manifold.

The OP wanted a simple answer such as the engine stumbles therefore changing the MAP sensor will fix it. I was trying to emphasize that it isn’t that simple.

Excellent link, though. I thank you for it. And I did expand my own knowledge by reading it.

Unfortunately, the OP has “left the building” and will probably spend the next three months changing parts until he finally hits on the right one.