15 in. Vacuum, need some advice


Founding Member
Jan 5, 2000
Las Vegas, NV
My 89 gt has been running a little off lately, just didnt sound right. It still has plenty of power, but the idle sounds choppy, like its missing. I did the cap, wire, rotor, plug routine. I even have a new coil on it because the post on the stock one broke off. I have been unable to pull codes because my computer wiring was messed up. Anyway, to make a long story short after I fixed the wiring I pulled codes 22, 31, 67, 85. 22 is baro sensor out of range. I dont know how to check a bp sensor so I decided to do a vacuum test. I had a steady 16 when I started it, then it slowly pulsated its way back down and held at a steady 15. I checked all of the vacuum lines that I can see and dont see any thing that looks suspicious. Anyone have any suggestions? Can I spray some tb cleaner around and see when the engine revs? or will that mess things up. Thanks for the help/
  • Sponsors (?)

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 and 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.

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

CODE: 31 (KOEO) - EVP circuit below minimum voltage. Vref missing or broken wire or bad contact in circuit. Check for 5 volts on the orange/white wire. With the sensor removed from the EGR and still connected, press the plunger and watch the voltage change on the brown/lt green wire. Check for the same results on pin 27 on the computer. Fix this problem before attempting to fix any other codes, as the EGR may be opening under computer control and leaning out the air/fuel mixture. Quick test of this leaning out the air/fuel mixture theory: remove the vacuum line from the EGR or install EGR blocking plate. Drive car and see what happens.

Code 85 - CANP solenoid (ALL 1989) - Solenoids - the Carbon Canister solenoid is inoperative. Check vacuum lines for leaks and cracks. Check electrical wiring for loose connections, damaged wiring and insulation. Check solenoid valve operation by applying 12 volts to the solenoid and blowing through it.

Code 67 - clutch not depressed (5 speed) or car not in neutral or park (auto). Possible neutral safety switch or wiring problem. You can generally ignore this code, since it has no effect on engine performance.
Thanks for the help! I do have a frequency meter, so I will test the baro later. I must fix teh egr first? Would this cause the engine to pull such low vacuum? I was more concerned about the vacuum then the codes. I do not have an egr blocking plate, so I dont know how to make sure it is working. Am I correct in thinking that the problem would most *likely* be carbon buildup holding it open? Should I rip it apart and clean it out? otherwise I dont see how it would be causing a problem at idle. I thought the computer only opened up the egr under cruising conditions. Forgive me if I am wrong, I am just trying to sort this out in my head. It is raining outside, and I dont have a garage so I have a little while to think about this.

The neutral safety switch I bypassed because it was bad, so that would explain 67. Thanks for the help!!
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.

Carbon under the EGR pintle valve will hold it open and cause a code 34, but not a code 31. Code 31 says the 5 volts from the computer is missing or the computer isn't seeing a signal from the EGR sensor. Since you didn't get any TPS codes which use the same 5 volts from the computer, code 31 points to bad wiring or bad connections.

Disconnect the vacuum from the EGR valve and see if the symptoms change.

Here's an EGR test proceedure I copied from cjones:

to check the EGR valve:
bring the engine to normal temp.
connect a vacuum pump to the EGR Valve
apply 5in vacuum to the valve.
if engine stumbled or died then EGR Valve and passage(there is a passageway through the heads and intake) are good.
if engine did NOT stumble or die then either the EGR Valve is bad and/or the passage is blocked.
if engine stumbled, connect vacuum gauge to the hose coming off of the EGR Valve
snap throttle to 2500 rpms (remember snap the throttle don't hold it there).
did the vacuum gauge show about 5in vacuum?

if not, check for manifold vacuum at the EGR vacuum valve.
if you have manifold vacuum then connect vacuum gauge to the EGR valve side of the vacuum valve and snap throttle to 2500rpms.
should read about 5in vacuum
Ok, I havent fixed the bap yet, but the egr connector had some corrosion, I cleaned it up and made sure this time it had a "tight" fit. The only codes now are 22 and 67. The problem now is I cant run the engine running test, when I start the engine I hear the car almost die for the low idle, but then it just kicks on the light, doesnt do any more testing. I tried turning the power steering, and flooring it, but it doesnt continue... What should i check, I have the service manual, but cant find in it where it has eec trouble shooting. Thanks for the help