Need Help Troubleshooting Code 31

June 27, 2005, 11:26:31 PM

Can anyone give me some guidance on how to troubleshoot a Code 31 on an 84 V8 T-bird with CFI?

The book that came with my code scanner says code 31 means EVP or EGR sensor below minimum voltage, but does not say how to troubleshoot.

If anyone could give me some troubleshooting advice, I'd really appreciate it.

Thanx!

Re: Need Help Troubleshooting Code 31

Reply #1 – June 28, 2005, 07:49:46 AM

EGR test and EVP sensor test
apply vac pres to egr vac line fitting.
if it holds the diaphram for a long time,, the diaphram is good
remove carbon buildup on egr base as well as egr base plate assy for 3.8l engines

EVP test (egr sensor)
wire id...
EEC PIN 26   "VREF" orange/white (parallels off to map)
EEC PIN 27   "EVP SIGNAL" brown/light green
EEC PIN 46   "SIG Return" black/white (parallels off to self test conn)

disconnect vac line
disconnect elec conn
hook ohm meter up to the VREF adn EVP Sig contacts
hook up vac guage or apply vac to egr
ohm meter should start out at about 5.5k ohms.
as vac is increased to max, resistance should bottom out to no less than 100ohms.

key on, engine off
cap off vac fitting on evp sensor
measure for 4 to 6 volts dc between VREF and SIGNAL RETURN (ground)

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in english,,,,,,,,,,, you probably dont have a good source of vac going to the egr sensor. Its source is going to trace out to the green vac line path.

on the 3.8L (if i recall correctly on the 87 and down cfi), the green vac line tees and one end will hook to a small round three way device the size of a silver dollar and about a half inch thick. Its listed as "something something CV" on your fan shroud emissions sticker. There are three connections on this purge valve thing a ma jig and if one of the elbow or fittings is cracked, you may loose vac to the egr thus reducing the vac on the egr, reducing the resistance or the applied voltage on the evp sensor.

in english again...........
do a visual inspection on the green vac line run. the source is behind the coolant fill tank, study and understand that the red vac line supplies the vac to a whole bank of vac actuated solenoids (tan,black,orange and red).
Unhook the green from behind the coolant fill tank (very very tricky but tricky and easy to break something here), unhook the other end of the green on the pass side engine bay and plug off the ends. put suction on the green line the old fashion way and see if they hold vac.

There is a vac distribution tube that is the main feeder along the solenoids behind the coolant fill tank in line with the red main feeder. the tube is sort of tan in color but inspect it for cracks also. basically you aught to do this to all the vac lines while your at it.

another option,, remove the egr , spray carb cleaner up in both holes while engine is cold, remove the large vac line on the rear of the cfi (large gold fitting and a part of the pcv path) and spray carb cleaner up in there also. Hook up cfi vac line,
start engine and romp the gas to blow out any carbon that decides to give up.

another option,, remove the cfi and remove carbon buildup near the PCV fitting and elsewhere if its there.

Re: Need Help Troubleshooting Code 31

Reply #2 – July 02, 2005, 06:15:40 PM

Thanks for the breakdown. I found the O-ring on the EGR sensor was leaking and not holding a vacuum. I replaced it and fixed the vacuum leak, then I pulled the throttle body and cleaned all the carbon out of the spacer, replaced the gaskets and the mass air flow sensor (which appeared to be the original).

The car is running smooth, idling smooth and accelerating without hesitation (except when the A/C is on - it surges at idle - gonna have to look into that one) - but other than that, it's running great. Thank you for your feedback - it was very helpful!

Re: Need Help Troubleshooting Code 31

Reply #3 – July 07, 2005, 09:52:49 PM

Great storybook ending...but how did you ever find a mass air sensor on that carburated car?

Re: Need Help Troubleshooting Code 31

Reply #4 – July 08, 2005, 04:38:08 PM

it's not carburated it's cfi, still no mass air though.

Re: Need Help Troubleshooting Code 31

Reply #5 – July 09, 2005, 08:59:23 PM

Great storybook ending...but how did you ever find a mass air sensor on that carburated car?

my mistake - thanks for pointing that out... it was the Air Charge Temperature sensor... and they all lived happily ever after.

Funky Cricket is right - its CFI.... and one very "moody" car.