'88 5.0 - Probable MAP sensor related issue questions

[spectraman]

Hey all!

What does anyone know about the 5.0's MAP sensor (and/or connective vaccum line) being sensitive to heat?

My '88 all-original 5.0 LX just repeated an event I experienced about 2 years ago.

It seems like the MAP sensor is going crazy after a period of driving long enough to heat the engine compartment up.

Yesterday the temps were in mid 80's here in Michigan, and I was running my A/C. (These conditions were also true when the previous event occurred.)

NOTE: The sensor and vacuum line are both original (aka: 19 years old).

What happens is that the engine does the traditional bogging, backfiring, and ultimately refuses to run at all. After a period of cool down of a few hours, the car starts and runs just fine like nothing happened.

From what I've read, the map sensor reads low vacuum pressure as a sign the engine is running fast / loaded, and signals the computer to do things like enrichen the fuel mixture, adjust the timing, etc.

My question is: Would the collapsing of the vacuum line (thus less vacuum being read by the sensor) cause this type of problem?

Also: Has anyone had these kinds of symptoms (heat induced map sensor malfunction), and found that the MAP sensor is actually the culprit? And has anyone experienced this kind of sporadic issue due to an old/weak collapse-prone vacuum line?

I'd like to not spend $60-100 bucks for the sensor if simply changing a small portion of vacuum line can fix the problem!


Thanks in advance for any advice you guys can offer.

-SM

 

[jrichker]

Dump the codes and see what the computer says is wrong. A defective MAP sensor will set code 22

Code 22 MAP (vacuum) or BARO signal out of range. The MAP or BARO sensor is pretty much the same sensor for both Mass Air & Speed Density cars. The main difference is where it is connected. Mass Air cars vent it to the atmosphere, while Speed Density cars connect it to the intake manifold vacuum. Its purpose is to help set a baseline for the air/fuel mixture by sensing changes in barometric pressure. The MAP or BAP sensor puts out a 5 volt square wave that changes frequency with variations in atmospheric pressure. The base is 154 HZ at 29.92" of mercury - dry sunny day at sea level, about 68-72 degrees. You need an oscilloscope or frequency meter to measure it. There is no way you can use a common cheap voltmeter or DVM to accurately measure the output signal.

The MAP/BARO sensor is mounted on the firewall behind the upper manifold.

Baro or MAP test using frequency meter - run the test key on engine off. The noise from the ignition system will likely upset the frequency meter. I used a 10 x oscilloscope probe connected from the frequency meter to the MAP/BAP to reduce the jitter in the meter's readout.

If it is defective, your air/fuel ratio will be off and the car’s performance & emissions will suffer

Some basic checks you can make to be sure that the sensor is getting power & ground:
Note that all resistance tests must be done with power off. Measuring resistance with a circuit powered on will give false readings and possibly damage the meter.
Check the resistance between the black/white wire on the MAP/BARO sensor and then the black/white wire on the EGR and the same wire on the TPS. It should be less than 1 ohm. Next check the resistance between the black/white wire and the negative battery cable. It should be less than 1.5 ohm.

The following power on check requires you to turn the ignition switch to the Run position.
Use a DVM to check for 5 volts on the orange/white wire. If it is missing, look for +5 volts at the orange/white wire on the TPS or EGR sensors. Use the black/white wire for the ground for the DVM.

Codes may be present in the computer even if the Check Engine light isn’t on.

Here's the link to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great.
See http://www.troublecodes.net/Ford/

IF your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method. There is no functional check engine light on the 86-88's except possibly the Cali Mass Air cars.

Codes have different answers if the engine is running from the answers that it has when the engine isn't running. It helps a lot to know if you had the engine running when you ran the test.

Trouble codes are either 2 digit or 3 digit, there are no cars that use both 2 digit codes and 3 digit codes.

For those who are intimidated by all the wires & connections, see http://www.actron.com/product_detail.php?pid=16153 for what a typical hand scanner looks like. Normal retail price is less than $30 or so at AutoZone or Wal-Mart.

Or for a nicer scanner see http://www.midwayautosupply.com/p-7208-equus-digital-ford-code-reader-3145.aspx– It has a 3 digit LCD display so that you don’t have to count flashes or beeps.. Cost is $30.

 

[cenok is family]

jrichker, i was taking a look at my friend's 88 coupe last night just to check where a few of the vac lines and such go for when i put my motor back in today and i noticed the vac line coming off his MAP sensor was disconnected from the intake. i found an open hose fitting on the underside of his upper intake that i was pretty sure it was where it goes, just trying to verify. it runs to that black "tree" coming off the underside of the intake, right? thanks!

 

[spectraman]

Thanks JR for your detailed feedback.

Here is a follow up question:

I began looking into changing the vacuum line on my '88 5.0's MAP sensor, and I see where it quickly disappears under the plenum, rendering the attachment point on the manifold out of reach.

Do I have to unbolt the plenum from the intake to change this vacuum line?

Or do you professionals have any tricks to do this without disturbing the plenum?

Changing the MAP sensor looks like a piece of cake, compared to trying to get my hand or a tool under the backside of the plenum where the hose attaches.

Any tricks/tips you can suggest?

And again... has anyone out there experienced any kind of weirdness with their MAP sensor working properly when the engine gets hot?

-SM