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Can a MAP Sensor cause idle way to high?


happydog500
11-21-2007, 02:01 AM
I got done with a U & LIM on a 1997. The car runs way to high idle. I can go 50mph without touching the gas peddle.

I got a Check Engine Light, which the code was the MAP Sensor. It figures this would be the one, since I broke it when I took it off (came apart in half).

Can a bad MAP Sensor cause it to go real high idle as soon as I start it up?

I would think the computer would stop it from going up so high.

Thanks for anyone who can explane this.

Chris.

HotZ28
11-21-2007, 09:53 AM
When you have a PCM that uses MAP to calculate air mass, it is using the speed-density method. This type of system is used in lieu of the Mass Air Flow (MAF). In addition to air mass, the PCM depends on engine speed (RPM) & air temperature to complete the speed-density calculations. As with most other sensors, the PCM sends a 5-volt reference signal to the MAP sensor. When pressure changes in the intake manifold, the electrical resistance of the MAP sensor also changes. By monitoring the sensor output voltage, the PCM can determine the manifold absolute pressure. The higher the MAP voltage output, the lower the engine vacuum and that calculates into a demand for more fuel. The lower the MAP voltage output, the higher the engine vacuum, which translates into a command for less fuel.

Most of the time, problems with MAP readings are not the fault of the sensor itself (unless broken in half as yours was). Usually the problems are associated with loose or broken vacuum hose or the rubber seal that seals the MAP sensor to the intake manifold. If this seal/hose is loose, leaking, or plugged, the sensor cannot produce an accurate signal.

In addition, if there is any other vacuum leak such as; intake gasket leak, EGR valve that is stuck open, or a leak around the PCV hose or seals, the MAP sensor's readings will be skewed and engine operating parameters will be affected. The engine may run with a bad MAP sensor, but it will hesitate under load, emissions will be out of range and have an overall lack of performance.

happydog500
11-22-2007, 07:54 PM
The engine may run with a bad MAP sensor, but it will hesitate under load, emissions will be out of range and have an overall lack of performance.So this means my engine running at a very high idle is not from the bad MAP Sensor?

I have two Vacuum hoses that come over and T off, plugging into the Throttle Body (one plugs in above the other so there is a upper and lower on the TB).

They continue over to the left and both plug in. Any way to find out which one goes in the upper and lower for sure? Does the one that goes upper go to the left or right where they plug in down line?

I called up my friend to see if I could come over and look at his Lesabre to compare, but he sold his.

Thank you for any help,
Chris.

HotZ28
11-22-2007, 10:43 PM
Usually, a MAP sensor will not cause a "high idle" unless it is leaking vacuum. Have you checked the idle air control solenoid or the TPS voltage? Also, make sure the TB is completly closed @ idle.

IIRC, the TB vacuum connection is like a T fitting, with one port going to the fuel pressure regulator and the other going to the evaporative purge solenoid. Some may have a third vacuum line going to the transmission. It really makes no difference, which one is which, because they all come from the same single vacuum port on the throttle-body. For more details, Click Here (http://www.autozone.com/az/cds/en_us/0900823d/80/15/e5/16/0900823d8015e516/repairInfoPages.htm)and select the correct Federal Emission code for your vehicle. The code should be located on the emissions label under the hood.

happydog500
11-23-2007, 02:48 AM
What I don't understand is how can changing a gasket make a sensor quit working? If the idle air control solenoid or the TPS is bad, wouldn't I get a code with that?

I changed the TPS about 11 months a go. If a TPS goes out, it should go into a "Default" mode, so even if it is bad again, shouldn't cause 45mph at idle.

One thing I need to say. During the almost week of driving the car, twice it has ran at the right idle. Both times I went from my house, runing high, then stopped (at a fast food restaurant). When I went home, it ran at regular idle on the way home.

The only way I can check the TPS Voltage is to have it plugged in. If I have it plugged in, I can't get check the voltage.

I want to thank you very much for replying. It's getting cold here and I hate having a car I can't drive good.
Chris.

happydog500
11-23-2007, 03:50 AM
After reading some about the IAC, It's almost 1:00am and need to get to bed. To make a long story short, on the IAC it says;

"If installing a new IAC valve, measure the distance between the tip of the IAC valve pintle and the mounting flange. If the distance is greater than 1.102 in. (28mm), use finger pressure to slowly retract the pintle. The force required to retract the pintle of a new valve will not cause damage to the valve. If reinstalling the original IAC valve, do not attempt to adjust the pintle in this manner.."

How come I can't attempt to adjust the pintle of a original, but I can on a new one?
Chris.

HotZ28
11-23-2007, 09:53 AM
What I don't understand is how can changing a gasket make a sensor quit working? If the idle air control solenoid or the TPS is bad, wouldn't I get a code with that?
I am not suggesting that a sensor has gone bad, only that you need to check them for proper operation. An improper reference voltage or ground to the sensors through the wiring or connectors can cause erratic readings. Do you have a scanner that will read live data, if not you need to get one in this case. High idle can be a number of things, including the PCM. Without knowing what the sensor readings are, you can only speculate as to what the problem may be.
The only way I can check the TPS Voltage is to have it plugged in. If I have it plugged in, I can't get check the voltage.

BTW, you can use a paper clip to probe the blue wire pin on the TPS terminal for checking voltage. Even if the voltage checks within range (.45-.52v @ idle) on the blue wire, you still need to look at the voltage reading on a scanner. They may not be the same!

happydog500
11-23-2007, 01:22 PM
I'm wondering if this could be the problem. We (my friend) took off the IAC, to show me how it worked. He had it in his had and told me to turn the key on. The point moved way out. He then pushed it back in by hand.

I see now where it says if installing a new valve, you can manually compress as required. If reinstalling the original IAC valve, do not attempt to adjust the pintle in this manner.

We (he) did adjust the original in this manner. That's why I was wondering last night why you can't adjust the original?


Thank you,
Chris.

jerryls
11-26-2007, 03:13 PM
I recently had the same problem on my 94. If you search for my post, you'll see I had the same situation- car driving down a level street at 45-50 mph without my foot on the gas. It turned out to be an open TPS. I replaced it and it fixed the problem. It's just a potentiometer, about 5K ohms across it. The pointer varies the resistance from about 400 ohms to 4500 ohms depending on the throttle position. There are 3 wires going to it. Gnd,+5V and the output varies between the two depending on the position. You can check it with the car not running.

happydog500
11-26-2007, 05:06 PM
My TPS was changed about 10 months a go. Plus, I think when the TPS is out it goes into default mode and doesn't idle high?

happydog500
11-27-2007, 06:59 PM
I read where A bad IAC will cause the Check engine Light to [I]stay on all the time.[I] Plus, to reset the IAC, you turn on the ignition, then off. Then start the car.

Check Engine Light stays on all the time.

When I first start the car, it idles high. Driving around town, at every chance I shut the car off, turn the ignition switch on, off, then start the car. After a few times the idle goes down. This makes me think the IAC is bad, since two things point to it.

When I test it, A to B, C to D is within the 40-80 ohms (around 56 if I remember correctly).

I hate to buy a new TPS if I got a new one less then a year a go, but that's one of the codes I get.
Any input?

Thanks very much.
Chris.

happydog500
12-04-2007, 05:32 PM
OK, finally got my MAP Sensor in the mail. Put it on (cleared codes) and it fixed the high idle. Car ran like a rocket, everything ran fine. Later on that day, I got a "Check Engine" Light.

Car still ran good. I checked and got two, P0171 Codes. (I got this code, changed the 02 Sensor before I did the Manifold).

One more problem I've had (no code when this started happening) for quite a while is, when I go out to start the car (first time) I turn the key to start, the engine turns over, na, na, na, na, na, na, na, na, without anything. When I turn off the key, then try it the second time, varoom, Starts right up. This does it on the second time, each time. When I go somewhere, when I come back out, it starts right up the first time.

Should I start with a fuel filter replacement? What else could I look into?

Thank you,
Chris.

jerryls
12-04-2007, 05:45 PM
Glad to hear the MAP sensor fixed some of the problems. The alternating starting problem sounds strange. Maybe related to the ECM. How about if you just turn the key on then off, then start it, or only one na rather then a bunch. I could live with that. Sort of a added anti theft feature!

happydog500
12-04-2007, 06:27 PM
No, from what I remember if I turn the key and then try, it doesn't start. I have to turn the key to start.
I like mine to start right up like a good car.

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