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My 1991 Celica GT (5sfe engine, manual transmission) cranks but won’t start. I’m getting a code 31. My mean reason in writing is to learn what can cause this code: could it be from insufficient vacuum, or can code 31 only result from an electrical problem (open or short)? I’m at a loss as to what to check next.

Full story: car recently started with some difficulty and surged instead of driving smoothly. Once warm the engine turned off at idle. I was able to get it home (didn’t get more than 1-2 miles away) by keeping the revs high when stopped. I don’t recall getting a check engine light. The first thing I changed was the spark plug wires, to no effect. A bit of research made me suspect too much exhaust was being recirculated into the intake. I removed the EGR valve (moved throttle body out of the way and detached the exhaust tube at the exhaust manifold). The EGR diaphragm was able to hold vacuum, and I cleaned the nipple to the EGR modulator, which was mostly or possibly completely clogged. Though I did not detach the exhaust hose at the bottom of the EGR valve, carb cleaner flowed past the pintle, making me think that it was not able to close. With some spraying/soaking, I was able to fix it so that carb cleaner only flowed past the pintle with vacuum applied. I replaced the vacuum hoses between throttle body and EGR modulator and also replaced the EGR modulator itself with an aftermarket unit after breaking the nipple of the original trying to remove a vacuum hose. I also replaced the big vacuum hose between the valve cover and the intake.

After reassembling everything, the car cranked but wouldn’t start and triggered a code 31. The vacuum sensor seems to be ok: applying voltage (~4.0V, from 3 rechargeable AAA batteries in series) and vacuum resulted in approximately the expected voltage drops. Turning the ignition to ON gave about 5V on the plug side. I did not do any testing at the ECU side. The EC2 pin had continuity to ground. The PIM pin to ground resistance was open, and the VCC to ground resistance was about 0.6 k Ohms. When cranking I measured about 20 inches of vacuum between the intake manifold and the vacuum sensor.

https://www.toyotanation.com/threads/car-sputters-and-stalls.361498/#post-3364040 is a report of fixing code 31 by replacing spark plugs, wire, rotor, and cap. This gave me pause because I started troubleshooting by replacing the spark plug wires. My distributor is leaking oil, and I temporarily disconnected the harness to the side of the distributor in order to improve access to the tube from exhaust to EGR valve, which I reconnected after reinstalling the EGR valve.

So back to my original question: should I be troubleshooting an open or short in the vacuum sensor circuit, a vacuum leak, or something having to do with the distributor/spark plugs/wires? Or should I be troubleshooting all of the above: assuming the code is an electrical problem (e.g., short or open), would that prevent the car from starting, or would that code just put it in some kind of limp/degraded mode but otherwise allow the car to start and run?

Thank you.
 

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Hang in there. I have a complete set of manuals. It will take some time to look it up. You are not forgotten.
The good side is you are at least getting a code. My '91 Gt 5s stalls without a code....still trying to work that one out.
 

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I have a very difficult time trusting the internet guru "experts" that's why I have a full set of shop manuals plus a Haynes and Chilton book(s).

Here's the official Toyota shop manual explanation.
Code 31 is thrown by a signal from the vacuum sensor.
Diagnosis is an open or short detected continuously in the vacuum sensor circuit.
The area of trouble is an open or short in the vacuum sensor itself or....The ECU. I seriously doubt the ecu
Might be able to google how to test the sensor.
If you don't have a vacuum leak anywhere, with all your efforts did you accidently bump or detach a vac line?
At least it might be narrowed down for you.
Two weeks ago I replaced what was called the ported vacuum switch on the cooling manifold. The plastic connector was broken off. It's a temperature activated switch, one hose leads to the vacuum sensor the other hose leads to the charcoal canister.
Just a thought.
Good luck. Keep posting.
 

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Try taking off the Air Cleaner and spraying some starting fluid into the Intake Port. If the the Engine starts, try it with gasoline (be SAFE). If it still acts as you indicate, then at least you have eliminated the Fuel Pump as the cause. Note that the Fuel Pump is under the back seat. If you need to replace it, have an experienced Toyota Mechanic do it as it requires Ventilation, Battery Disconnection, and experience to extract and reinsert the Pump. Also, check the wiring on the Fuel Pump for corrosion and fraying.
 

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Hang in there. I have a complete set of manuals. It will take some time to look it up. You are not forgotten.
The good side is you are at least getting a code. My '91 Gt 5s stalls without a code....still trying to work that one out.
My 91 did this and it turns out it was the wiring from the igniter. What i did to test this was i pluged in the igniter and with a small allen key i pushed the pins on the plug the started no issues. I then wen on to replacing the plug on the ignited and havent had issues since
31563
 

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There's another situation. Always good to know these things and what to look for.
These babies as wonderful as they are do begin to show their age.
 

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Discussion Starter #8
Fixed!

Thanks everyone for your responses. I ended up taking it to a mechanic, whose troubleshooting indicated that the vacuum sensor was indeed bad. Luckily I was able to find a vacuum sensor in a local junkyard and am on the road again. The mechanic's explanation was that my use of an external 4V source to test the sensor was the problem, as the procedure calls for 5V. He also found that the sensor voltage (PIM to E2) with no vacuum applied was only 1.6V on the bad sensor, much lower than expected (this typical voltage is not in the online BGB manual, which only reports the expected voltage drop as a function of vacuum). Thank you Larry!
 

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Good news. Great that the problem is solved. Now if anyone can help me find what's making my '91 stall after 5 mins of idle. No codes. Now thinking failing battery or maybe the vacuum sensor also.
Going to a bone yard today to see what I can find.
 
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