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Ms. Wrench
Ms. Wrench, ASE Certified Technician
Category: Car
Satisfied Customers: 554
Experience:  ASE Certified Technician-10 yrs., Service Writer-Foreign, Domestic & Fleet, Alternative Fuel Expert
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94 Eldorado: MAP sensor..p071 and the resulting stall (p095)

Resolved Question:

Ongoing '94 Eldorado MAP sensor errors. Specifically p032, p071 and the resulting stall (p095). This has been ongoing for over a year. The following has been done: new PCM, new MAP sensor, new TPS sensor, replaced fuel pump, filter and FPR.
Submitted: 5 years ago.
Category: Car
Expert:  Ms. Wrench replied 5 years ago.
Hello,

This is Ms Wrench and I can help you with your 1994 Cadillac Eldorado Northstar.

You may have already verified this but just in case, check the vacuum going to the map sensor, the hose may be cracked or may be collapsing intermittently-to be sure replace the vacuum line there and anywhere else you see a potential problem.

If you have not already done so, a check for any other manifold vacuum leaks need to be performed. You can do this by carefully spraying carb cleaner in small bursts at different areas around the intake manifold to listen for a change in idle indicating a leak.

The harness connector should be carefully inspected at the map sensor, and the harness should be backtraced visually to find any problem areas.

Though you said the map sensor has been replaced, there is no gaurantee that the new one is good, I would suggest trying another new map sensor, especially if all other checks prove inconclusive.

Let me know about what I have mentioned, and if you could also record the MAP Pid values from the information center while the car is running that would be very helpful also.

Thanks for using JA

Ms Wrench
Customer: replied 5 years ago.
hi...already have inspected the elec. connections to the MAP, but was not aware of the vacuum line. I'll check into it and look for a manifold vac. leak after typing. Although I have no trouble entering the diagnostic mode on the dash for the pcm, I don't know how to access the MAP pid values. I'd be happy to do so if you could advise me how to do it.
Customer: replied 5 years ago.
re: vacuum connection to MAP sensor. One is not present, nor has there been. there is a nipple ontop of the sensor that appears to be able to accept a vacuum line (if one was present) connection, but I believe I was told by a local mech. that nipple is how it measure's the internal pressure.
Customer: replied 5 years ago.
oh...& no leak was detected when spraying carb cleaner around the intake maifold (while still bolted down, of course)
Expert:  Ms. Wrench replied 5 years ago.

 

Is that nipple exposed or inserted into the intake? Here is more testing info on the MAP, let me know about that vacuum nipple.

 

Ms Wrench

 

OPERATION

The Manifold Absolute Pressure (MAP) sensor measures the changes in the intake manifold pressure, which result from engine load and speed changes. The pressure measured by the MAP sensor is the difference betcovered in this guideen barometric pressure (outside air) and manifold pressure (vacuum). A closed throttle engine coastdown would produce a relatively low MAP value (approximately 20-35 kPa), while wide-open throttle would produce a high value (100 kPa). This high value is produced when the pressure inside the manifold is the same as outside the manifold, and 100% of outside air (or 100 kPa) is being measured. This MAP output is the opposite of what you would measure on a vacuum gauge. The use of this sensor also allows the PCM to adjust automatically for different altitude.

The PCM sends a 5 volt reference signal to the MAP sensor. As the MAP changes, the electrical resistance of the sensor also changes. By monitoring the sensor output voltage, the PCM can determine the manifold pressure. A higher pressure, locovered in this guider vacuum (high voltage) requires more fuel, while a locovered in this guider pressure, higher vacuum (low voltage) requires less fuel. The PCM uses the MAP sensor to control fuel delivery and ignition timing. A failure in the MAP sensor circuit should set a Code.

TESTING

The MAP sensor voltage reading is the opposite of a vacuum gauge reading. When manifold pressure is high, the MAP sensor value is high, and vacuum is low.

  1. Using the accompanying chart as a guide, determine the height above sea level and the comparative voltage range.
  2. Now, backprobe the MAP connector betcovered in this guideen terminals A and B.
  3. Key On, Engine Off, the voltage reading obtained should coincide with the chart; voltage and altitude should match.
  4. Now apply 34 kPa (10" Hg) vacuum to the MAP sensor, it should cause the voltage to be 1.5 to 2.1 volts less than the voltage at Step 1. Upon applying vacuum to the sensor, the change in voltage should be instantaneous. A slow voltage change indicates a faulty sensor.
  5. Check the voltage betcovered in this guideen terminal C and ground, you should read reference voltage (about 5 volts).
  6. If the reference voltage is right, and the sensor voltage is not within specs, replace the sensor.
  7. If the sensor and the circuits are functional, the PCM may be faulty.
  8. Be sure to test the vacuum hose for leaks or restrictions. This source must supply vacuum to the MAP sensor only.

Click image to see an enlarged viewMAP sensor specifications-note how altitude and pressure affect the voltage readingsREMOVAL & INSTALLATION

  1. Remove the intake manifold top cover.
  2. Disconnect the crankcase breather tube from the throttle body.
  3. Remove the MAP sensor from the intake manifold.
  4. Detach the MAP sensor electrical connector.
  5. Inspect the MAP sensor grommet for covered in this guidear or damage and replace as needed.

To install:

  1. Installation is the reversal of the removal procedure.
Customer: replied 5 years ago.
hmmm...the mounting position of the MAP sensor might just be the root of tyhis evil. It is the exact same as the illustration you sent, which shows the "nipple" facing downward, where I had mine installed facing up. I just spun it around, facing down and into the mounting harness and will see if that makes any difference. btw...how are your temps lately? Here in Havasu, we're still in the mid 100's with loads of humidity, but no monsoon activity! How nice we get the moisture without the show!
Expert:  Ms. Wrench replied 5 years ago.

OK so the nipple can mount into the manifold directly? One way or the other that nipple has to be connected to the manifold.

 

It is still about 105-110ish. Looks like this week is going to be humid-I'm not far from you, I'm in Vegas. The more troubles the rest of the country has I don't know why anyone would not want to live in the southwest. The heat more than make up for the lack of natural disasters-dont you agree?

 

Let me know what you find out with your map sensor. When you do get it mounted to vacuum, clear the codes and see what happens. I'm sure this is your problem.

 

Thanks for using JA

 

Ms Wrench

Customer: replied 5 years ago.

Yes...I agree...extreme heat beats hurricanes anyday!

 

There is nothing readily apparent, that connects directly onto the nipple. As I said, I spun it around (nipple facing downward, as in the illustration you sent), snapped it back into it's plastic harness, sealed up the manifold cover and headed out for a test ride, after clearing the history from the diag. display. After about 15 minutes of driving highway and local, there were no problems, but...this has happened before, only to have it re-appear a couple days down the road. I remain cautiosly optimistic.

 

One last thing re: the MAP sensor connector (nipple). You stated it has to be connected to the manifold somehow, yet, as I previously stated, there wasn't anything readily visible, for the nipple to connect into. If you think it would be helpful, I could remove the cover again and snap a couple shots of the connector, with the sensor snapped in and with it removed.

 

Let me know,

 

Thanks,

 

-Don

Expert:  Ms. Wrench replied 5 years ago.
Hello Don,

That would be perfect. I'll watch for the post.

Ms Wrench
Customer: replied 5 years ago.
graphicgraphic
Customer: replied 5 years ago.
graphic
Customer: replied 5 years ago.
pulled out of mount
Customer: replied 5 years ago.
for some reason, these snapshots aren't displaying full-sized. If this is the case on your end, I can put them up on a webpage for you.
Expert:  Ms. Wrench replied 5 years ago.

Hello,

 

The pictures displayed OK. When the map sensor is installed in its holder like that the nipple should be connected to the manifold with a grommett. What I dont understand is how if it has been unhooked all this time, that should have been a huge vacuum leak. Check it out and let me know what you see.

 

Ms Wrench

Customer: replied 5 years ago.

no grommet to be found. is the grommet supposed to be attached to something, like a vac. hose, like you previusly referenced? I also pulled up the fuel rail cover that surrounds the perimeter of the manifold and had a look underneath for anything that looked like it belonged attached to the nipple, but found nothing. Is there a way for you to confirm that something is supposed to be attached to the nipple?

 

-Don

Expert:  Ms. Wrench replied 5 years ago.

I just posted but it disappeared.

 

What I said was, there is definitely a port there, I can see that it is a direct port not a vacuum line connection. The port must be clogged up with carbon, that would explain why there is not manifold vacuum loss without the map installed.

 

Access your diagnostic mode and scroll through the ECM data until you see the MAP. Observe the MAP readings while driving and let me know what it says.

 

Ms Wrench

Customer: replied 5 years ago.
not sure how to access this info while driving, but I'll enter diagnostic mode in the PCM area and look for a MAP area as I go.
Customer: replied 5 years ago.

I just returned from another test drive and the damn thing stalled again! The old p032 and p095 are back and I just made an appointment with the local shop in town that last attempted to service this issue back in Feb, unsuccessfully. Are you aware of anyone here in Havasu that could make this problem go away?

 

-Don

Expert:  Ms. Wrench replied 5 years ago.

What shop did you have it at? I know exactly what is wrong with it now, we just have to find someone competent to listen to what it is and fix it. That might be easier said than done.

 

Ms Wrench

Customer: replied 5 years ago.
it's a local Shell station. The owner was said to be THE GUY for Northstar diagnostics, but I now have my doubts. I have already given this guy close to a grand to address this issue...in vain. He's the guy who installed the MAP sensor upside down in the first place! He couldn't believe the car ran at all with it being installed incorrectly, but it has managed to run for almost 6 months since I picked up the car from him. In fact, I've been to LA and back with no stalling issues. There are no Caddie dealers in Havasu. He said he was going to plug into it tomorrow afternoon and I got the impression he was going to do this gratis. What do you think the issue is?
Customer: replied 5 years ago.

Are you in today? Sure would like to click the accept button in exchange for resolution to this issue!

 

-Don

Expert:  Ms. Wrench replied 5 years ago.

Hello Don,

 

I beleive that your port for the map sensor in the intake is probably carboned up and plugged. I don't know of any other explanation as to how it could be running either-the vacuum port has got to be pluggd up, and the sensor has just been running in default mode.

 

Do you understand why the map has to be connected to manifold vacuum?. The way it has been running, the sensor has not actually been reading anything. The wires are just ground, 5v reference, and signal to the ECM, the signal however is from the pressure reading from the manifold.

 

Ask him about the port that it goes into-I would have my doubts too. So you went and talked to him and he said he was going to take care of it today or tomorrow? I would not pay him any more money-that is a pretty basic concept-that the map sensor has to be connected by vacuum to the manifold-hence the name manifold absolute pressure sensor! If the port was not totally plugged up the mistake couldnt even have been made because the vacuumm leak would be so bad it wouldnt run right anyway.

 

When you do get it resolved, make sure that the battery gets unhooked for at least 20 minutes to clear the capacitors out. The computer builds a kind of database while the engine is running and it is going to be completely out of whack becuase of running that way for so long. What has happened is the computer has substituted values or just recalibrated everything else around the fact that there is no manifold pressure data, so it needs to be completely cleared to start the fuel trim tables over again.

 

Let me know how it goes. I can't wait to hear what he has to say about the vacuum port! I should be in on and off all day so I'll watch for your post.

 

Thanks for using JA

 

Ms Wrench

 

Customer: replied 5 years ago.

I'm going to drop off the car this afternoon and hope for the best.

 

While the cover was still removed yesterday, I scrutinized what was underneath the sensor's mount. All I could see was the throttle body, off to the rioght, in the picture I sent you. I cleaned that with carb cleaner a few months ago, but...it didn't appear dirty at that time. Aside from that, there was no visual evidence of any "port" that you made reference to. Regardless, I'll bring him a copy of your comments, after editing out the payment references. ;)

 

More to come...

Expert:  Ms. Wrench replied 5 years ago.
Sounds good!
Ms. Wrench, ASE Certified Technician
Category: Car
Satisfied Customers: 554
Experience: ASE Certified Technician-10 yrs., Service Writer-Foreign, Domestic & Fleet, Alternative Fuel Expert
Ms. Wrench and 18 other Car Specialists are ready to help you
Customer: replied 5 years ago.
Still waiting...I won't get the car back until friday, but so far, he said that when he got the car to "act up", the reading from the MAP sensor was zero, which lead him to conclude the sensor needed to be replaced...again, only 6 months from the time he replaced it the last time and installed it upside down! Still waiting to see if he has the cajones to charge me for this.
Customer: replied 5 years ago.

hi...just got the car back and they guy said he replaced the MAP sensor and that he couldn't get the car to act-up. Because of your replies, you made me realize the MAP sensor was installed upside-down, which was a major discovery. Although your references to it being installed directly into a "port" remain ambiguous at best (could not find any sort of port that fits your description), I feel that you at least are entitled to the $9.00 for your efforts.

 

Have a nice extended weekend in the 100-teens! I know I will!

 

-Don

Expert:  Ms. Wrench replied 5 years ago.

Hello again,

 

OK there is nothing ambiguous about the fact that a MAP sensor has to be hooked to manifold vacuum to be functioning. I find it very amazing that a so-called professional technician is unaware of this fact. I think this is illustrated by the fact that he gets a zero reading and thinks the sensor must be replaced every time he checks the car.

 

For some reason, I cannot find a picture of the pre-1996 northstar map sensor anywhere, including in GM electronic service information. I did however, take the cover off a friends 1996 Northstar just to show you that I am not crazy. This MAP sensor is a little different style than yours but if you look at the picture you will see the port I was referring to that the sensor fits into.

 

I have figured out that on your car you are missing a vacuum line connection form the gold colored nipple close to the map sensor bracket to the sensor. The port is probably clogged up due to carbon so there is no leak.

 

I appreciate you accepting but I do not want to end this session with you thinking that there is not supposed to be vacuum to the MAP sensor. I dont care if you change it every week forever you will not get rid of that code until you hook it up.

 

graphic

 

 

So please take it somewhere and get someone to clean out that port and hook it to vacuum. I gaurantee this will solve you problem. And please let me know when you get it done.

 

Thanks again,

 

Ms Wrench

 

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