TBI questions

[EMSjunkie]

A freinds son has a 94 chevy pickup with a 350 with tbi. He rebuilt it, added a lunati voodoo cam, and a gear drive timing set. He set the timing at 4 degres advanced, not sure why, but anyway... it is running very rich, to the point of stalling. I know enough about tbi to know I don,t know anything about them.  I am not sure of the exact cam specs,  but it is not radical by any means. Around 270 duration and 460 lift.
Would the cam profile make the computer think it is lean?
Where else should I look
Would the 4 degrees advance confuse the compuer?
Any help would be greatly appreciated!

[chimp koose]

I know even less about the EFI than you but I wonder if the system has a detonation sensor. If it does I wonder if the gear drive is giving the detonation sensor false detonation signals and it backs off the timing /richens the fuel to cure the perceived detonation.

[EMSjunkie]

That's a good point, never thought about that. I did find a broken temp sensor by the thermostat housing, replaced it, and it is running again. Not sure if it is still running rich.
Thanks for the info Chimp, I do appreciate it!

[wayne petty]

gm TBI systems require several things...


decent manifold vacuum.. at least 12 inches at idle in park..

thats really not decent.. but 15 inches is better..  big cams freak out the ECM programming.

the timing spec should be 0 on that motor.. with the tan and black stripe wire unplugged..  its usually hidden above the passengers toes.. but it could be elsewhere on a 94..  its location is on the emission label..

to deadstick time the engine.. bring the crank damper around in the normal direction of rotation... stopping at ZERO TDC... or the spec on the emission label on the  core support...

pull the distributer cap.. see if the teeth of the pickup coil and the teeth of the reluctor are aligned perfectly... this allows you to dead stick time the engine..  and get it within a few degrees..  once the engine is above 400 RPM.. the electronic spark timing is in command..  this dead stick allows you to KNOW exactly where the base timing is.. before starting..

the rotor could be pointing at #1 or #6 terminal in the cap since the crank turns twice to once of the distributer..


you might also want to pull the rotor and look at the reluctor for cracks in the magnets that are visible around the shaft.. these cracks cause ghost waveforms and drive the electronic spark timing crazy..

the electronic spark timing counts down from a sine wave signal..  cracks cause random waves in the signals.. so the spark timing controller starts counting down from the random waves..  and oops.. its wrong.. making it misfire.



~~~~~~~~~~~~~~~~

the engine coolant temp sensor.. 2 wires   this is an SU102 from AZ or wells... under 20 bucks.. replace the connector also..  this sets the fuel mixture according the engine coolant temp.. when the sensor goes bad.. the ECM might think the engine is 250 degrees or more..  if the pigtail goes bad.. the ecm might think its -20 F..



192/195 thermostat unless you have a prom chip with that setting changed..
~~~~~~~~~~

make sure the engine to computer harness grounds are INTACT and clean...   the ECM is ONLY GROUNDED to the ENGINE.. if these grounds are left off or just stripped wire wrapped around a stud.. its NOT going to work well.. recall.. electrons flow from negative to positive..  no ground.. no electrons to muscle the devices..

could be at the back of the heads... could be on the intake behind the thermostat housing.. could be in both places..


~~~~~~~~~~~~~~

are the oxygen sensors hooked up properly..  

the system will NOT self learn without them..

he will also need to check for exhaust leaks ... before the oxygen sensor..  air injection system issues... if the check valves are rusted out.. if the diverter valve is leaking UNCOMMANDED air into the upstream exhaust before the oxygen sensor it will really screw with the ECM ... as the ECM looks at oxygen content in the exhaust stream.. if there is a leak..  it will have additional oxygen in the exhaust stream and cause the ECM to go crazy..

this test is easily done... unhook the air injection hoses at the inlet of the check valves.. plug the check valve openings with a short section of heater hose and a spark plug or some other good stopper..  if the engine runs better.. then you can plug one of the disconnected hoses...

get to the other hose..   with your thumb or finger over it.. with the engine running at idle.. there should NOT be any airflow...   if you goosed the gas hard... it might have some air flow.. but thats risky.. it will have air flow commanded for 10 seconds or so on any start..

with a digital volt meter.. NOT AN ANALOG (needle type)  clip into the purple wire from the sensor..   with the engine running at idle and also at 2500 steady rpms..   you should have swinging voltages from 0.1 Volts  to 0.9 volts try it in both AC and DC..


~~~~~~~~~~~~~~~~

make sure the MAP sensor has a good source of manifold vacuum...

some TBIs have vacuum  multiple taps .. some manifold.. some ported.. hook it to a ported vacuum where there is NO vacuum at idle.. you will have a heck of a time getting the engine to run...


~~~~~~~~~~~~~~~~~~~

engine off..  grab the throttle lever.. open it part way..  see if there is ANY shaft play.. from front to back... this is a problem area on most GM TBI units and a LOT of carbs..

when there is worn throttle shaft bores..  the throttle can close to slightly different angles each time... this confuses the ECM as it uses the MINIMUM TPS voltage to go into IDLE FUEL TRIM..   when the TPS voltage is varying when closed.. it could cause a stall.. or a rich mixture as if the voltage is slightly higher than the lowest.. it will think that you have your foot on the gas... like you are drag racing and have both staging lights on.. it keeps the injector pulse width wider than it needs to be ..

1.3 or 1.4MS at idle is good..


please print all this out..  go over it one section at a time..

or do it the easy way.. find a scan tool that displays OBD1 live data stream

sit there at idle.. with the scan tool hooked up.. and go thru the live data stream values.. write each down..

then do it again at a steady 2500 RPM...

write each one down...

then post them...

i will tell you what's wrong...  or where to look.

most scan tools can also take a snap shot of the data stream..  if yours will.. it will be a time saver..  as you can take your time to go thru the list..

i have been learning these systems since electronic fuel controls came out in the 1980 models..  actually some 79s had them...   sevilles and eldorados also had electronic fuel injection in the 76 or 77 model year.. i know those well also. but there is no scan tool for them..

[wayne petty]

scan tools that will work on OBD1 GM cars and trucks...
if they have the proper cartridge and cables..

94 and 95 are OBD1.5 usually and some autoxray models not shown here may not do more than get codes..

most consumer scan tools on the market today that list OBD1.. don't read live data.. only codes..
even the newest trick of the week snap on scan tools only read codes.. thats a 10 grand oops..







there are a bunch of versions of the monitor 4000E... monitor extended.. extended monitor.. monitor enhansed..
the MONITOR 2000 will NOT work on models newer than 89.. so they are great cheep used ones if you have an 82 to 89 GM car or truck..


this is a OTC genesys... there are various versions of this used and NEW available.. actually generations of it..  most will work with a 94..  depending on the software and cables installed..


the 4 above can be had on craigslist sometimes.. search for scantools... scan tool...   actron....   OTC...  snap on...   MT...
look in autoparts and in tools..

below is an OTC 3499N KIT... around 500 bucks.. and will do OBD1 and OBD2 ...

[EMSjunkie]

thanks wayne i appreciate the info!
That gives me quite a few things to check out.

[papastoyss]

If you're lucky your friend swapped out the thermostat for a 180 or so unit instead of the 195 part as Wayne specified. The ecm sees a cooler engine temp than it is programed to expect & thinks the engine is still in warmup mode & richens up the fuel mixture like a carb engine still on the choke. Also the cam change could be screwing up the vaccum signal to the MAP sensor.Check all the basics  Wayne suggested first ,esp the ground connections since the engine has been R&Red. GOOD LUCK!

[UGLY OLDS]

I agree 110% with Papastoys ... I repaired a motorhome last week  with a 454 TBI .. Running rich ..Junk mileage - Idle would not slow down at stoplights ..Driver was putting into neutral ..    
  Thermostat was stuck part way open ..engine would only reach 184*..
  Was NOT setting codes ... Installed new 195* t/stat ...  resolved ALL the problems ...Owner was dumbfounded .. He was chasing the problem since last year .... Follow the steps that Wayne said .....
Oh .. Try to get the EXACT specs on the cam .. None of the factory programing likes cam changes .....

Bob .....  

[EMSjunkie]

Thanks for the info guys, I really do appreciate it!