GM TPI stalling diagnosis

BombshellCW · Feb 10, 2019

With luck ,this is the right spot for this thread. About 6 months ago I installed AGM TPI conversion on to the 305 in my buddies 85 Monte Carlo SS. The whole conversion was previously installed on a 1970 Camaro and is therefore secondhand to us. All of the components match up with about a 1991 Camaro 5.0 Vin F lb9 engine. When we first installed it back in August it ran really really rich and a local tuner created a new prom and tune that was more suited to the application. At the time we were having trouble with the idle and a surging issue. The idol turned out to be a cracked idle air control valve and the surging turned out to be a mismatched timing Tab and balancer. Now I finally got the base timing set at 6 degrees with the black and tan wire disconnected. I've set my base idle and my TPS at .54 volts. The trouble I'm having now is after about 10 minutes of run time the engine will randomly stall. I have narrowed it down to an ignition issue as when the engine stalls my noid light continues to flash until the engine stops spinning but my timing light stops flashing immediately. Upon restart the engine will run for 1 to 2 minutes before stalling again. The pickup coil in the distributor has been replaced and the resistance false within spec. The ignition module has been replaced as has the ignition coil. The ignition coil is mounted directly to the intake manifold with a steel bracket. When the engine stalls I still have 12 volts at the ignition coil. I was in the process of checking the four circuits between the ECM and the ignition module when I developed a fuel leak and had to stop for the night. I'm using Factory Chevrolet service manual for the 91 Camaro for reference Speaking of fue, my pressure seems to hang right around 40 PSI. I am using the small cap distributor they came with the conversion, I have the adapter between the ignition module and the ECM harness that goes from a large 4 pin to a small 4 pin. I have noticed an interesting quirk in that if I disconnect the coolant temp sensor, the engine will run longer before Stalling. it is also worth mentioning that I have picked up a second ECM with the appropriate part number ending in 7730 and currently have it installed with no change to my stalling issue. it seems as though I'm having a problem with the bypass ignition system. Am I headed in the right direction? Any advice you folks have would be greatly appreciated!

Maniacmechanic1 · Feb 10, 2019

If its stock layout GM Tuned Port injection it will be Pass Key 1 Vats security system.
You have to match the Passkey external module resistance value it was programmed with.
Or custom tune out Passkey 1.

There is an oil pressure bypass switch on all Tuned port engines that all fail after time or 50,000 miles.
It turns on electric fuel pump after cranking the engine over & it fires up.
Its located on the rear of the block right above the intake block china wall.
On a strange Christmas tree.
Its suspect bad too.

Grumpy · Feb 10, 2019

reading through the start sequence may help isolate the problems source
Id sure try to isolate the issue to a problem with sensors, ignition or fuel delivery.
owning a factory shop manual for your year corvette and a multi meter is a big help as is a code reader.

c4 & c5 corvette trouble codes

if your corvette won,t start or run, don,t panic its usually easy to locate the problems source, buy a CHEVY SHOP MANUAL FOR YOUR YEAR CORVETTE! RULE#1 never assume a damn thing ISOLATE ,TEST AND VERIFY...

garage.grumpysperformance.com

curing that problem with how your c4 corvette runs badly or won't start

A recurring theme has been replayed over and over, READ THE LINKED INFO CAREFULLY THERES NOT A DAMN THING YOU CAN'T FIND AND FIX WITH A FEW TOOLS, AND SOME FAMILIARITY WITH THE CORVETTE< A SHOP MANUAL IS A HUGE HELP Id strongly suggest you systematically verify the cause rather than making...

garage.grumpysperformance.com

Brian may be onto something, the oil pressure bypass switch in the TPI is a very common source,
if oil pressure falls below about 6 lbs the ignition cuts out by design
for similar issues ID suggest replacing it as they are cheap and easy to replace.
the switch may not be the only potential issue but its a known potential problem.
are you pulling any trouble codes?
whats the fuel pressure ?
is it consistent?
it should be in the 37 psi-39 psi range!
when you turn the key the fuel pump comes on, but it will cut off if oil pressure drops below about 6 psi
Id connect a multi meter to the coil voltage and see if its cutting out causing the ignition to cut off.
You need to think carefully, don,t assume anything.
TEST AND VERIFY EVERY COMPONENT AND SENSOR FUNCTION!
Id suggest pulling trouble codes and verifying the basics of engine function,
are you actually getting spark at ALL the plugs?
are you getting injector pulse from the injector wire harness, do you have fuel pressure?
have you been checking the too verify your actually getting fuel out of the injectors,into the cylinders?
are all the fuses in the panel good?
have you verified that your alternator produces 13.5 plus volts,
that your getting spark at all the spark plugs ,have you verified the spark plugs are actually arcing and not fuel fouled?
your getting compression in all cylinders, all the rockers move, as the engine rotates?
ALL the sensors are fully functional,
your getting at least 10 psi of oil pressure as the engine spins while its trying to start,
you need to see the fuel pressure is consistently at 38-42 psi.
you need to verify the fuel filter and fuel pressure regulator and return line are functioning as designed.
verify all the electrical grounds are good,
and you use a noid tester to verify your getting the injector pulse.
what most people fail to do is test and verify, you can,t assume anything.
youll want too verify, youve got no plenum or runner vacuum leaks, and the catalytic converters are NOT restricting exhaust flow, theres no excessive exhaust system back pressure.
just because the injectors have pressure in the fuel rail, and get a pulse does NOT insure fuel flow thru them,
and having fuel rail pressure does not insure ITS GOOD fuel,
it can be tainted with water or rust particles in the fuel,
preventing fuel atomizing in the cylinders.
clogged catalytic converters can easily cause the engine to fail to flow air,
checking the plenum vacuum and exhaust back pressure and reading the exhaust fuel/air ratio,
use of an infrared temp gun, to verify cylinder exhaust temps are reasonably equal and a multi meter to set the TPS sensor and verify the ignition wire resistance will help.
simple stuff like verifying the firing order that you just know is correct (which may not be , so VERIFY IT) get over looked
and testing the coil and oxygen sensors and temp sensors sure helps.
After you verify theres consistent strong electrical spark at the spark plugs when your trying to start the engine and fuel pressure is consistent at the fuel rails,and your getting at least 7-8 psi of oil pressure,
IF you suspect a fuel delivery related issue, a good long spray of starter fluid into the throttle body and trying to start the engine with that extra fuel source may prove informative, if it trys to start with the extra fuel in the plenum but failed to start without it, its a good indicator the injectors are plugged or defective

have you removed and cleaned the IAC?
IAC and TPS Adjustment
Idle Air Control
Tools needed:

- Torx bit # T-20
- Paper Clip
- Small Punch

Take the paper clip and open it up and form it into a big "U" shape. Insert the clip ends into the ALDL in the ‘A’ and ‘B’ pins.

Turn on the ignition, but don’t start the engine. Wait 30 seconds. Now, go remove the connector from the IAC.

Start engine. You are now going to adjust "minimum air". There is a Torx screw on the side of the throttle body. This is what needs to be turned to adjust minimum air, or more commonly known as "idle speed". It comes from the factory with a protective metal cap over it. If the cap is still there, use a small punch to knock it out. Set the idle speed to 450 rpm, rotating the Torx screw clockwise to raise rpm, and counter-clockwise to lower rpm. Once the idle rpm is set, turn off the engine.

Re-connect the connector onto the IAC. Start engine. Idle speed is now once again governed by the ECM, but your idle should be smooth and steady, approximately 600 rpm in Drive (for unmodified cars).

If you set an SES light by having the IAC disconnected, then after shutting down the engine disconnect the negative battery terminal. Wait 5 minutes. This will clear the ECM of all trouble codes. Re-connect the battery and drive the car for 20 minutes to allow the ECM to relearn your driving style.

Throttle Position Switch (TPS)
Tools needed:

- Digital Volt-Ohm-Meter (VOM)
- Jumper Wires (make your own)
- Auto Xray Scanner (if available) will eliminate the need for VOM and jumper wires.

Turn on ignition, but don’t start the engine.

With a scanner: plug in the scanner and read the TPS voltage. It should be 0.54Volts +/- 0.075Volts

With VOM and jumper wires: disconnect the connector from the TPS. Using your jumper wires, make a connection allowing some room for the VOM terminals to contact the jumper leads and read the TPS voltage.

If out of spec, loosen the two screws holding the TPS to the throttle body, and slightly rotate the TPS up or down, reading the voltage until it comes into specification. Tighten screws. Using the throttle lever, rotate the throttle to WOT (wide open throttle). The TPS voltage should be over 4.0 volts. Close the throttle again, and then slowly open it to WOT, observing the voltage reading. It should increase progressively and in a linear fashion. If it sticks or jumps or falls off at all while doing this check, that could mean a bad TPS switch and could be a cause of stumbling and driveability problems.

After setting the correct voltage, turn off ignition switch. Remove jumpers/scanner and reconnect the TPS connector as required.

have you measured the resistance ohms on every injector?

BombshellCW · Feb 11, 2019

The VATS had been turned off by the tuner. I think the oil pressure switch has been removed from the swap as well. I don't recall hooking one up and the only extra wire I have is for the torque converter lockup. I will check tomorrow.

BombshellCW · Feb 11, 2019

I have checked all of the injectors, and they all have similar resistance which Falls within spec according to my book. This engine ran fine with a carburetor before the EFI swap so camshaft timing and valve adjustment are not suspect. I have 40 PSI according to the gauge that is mounted on the injector rail. I have set the base idle, and the TPS to .54 volts. There are no catalytic converters. Washington state only requires Vehicles 25 years or newer to Pass emissions. Coolant temp sensor, intake air temp sensor, idle air control valve, ignition coil, ignition module, pickup coil, engine control module, are all new and measure perfect spec. I'm not having a problem with fuel delivery so much as I lose my spark. It's like I just reach up and turn the key off. My noid light continues to flash after the engine has stalled though my timing light stops flashing immediately. I will go through your suggestions tomorrow when I get back to the shop.

Maniacmechanic1 · Feb 11, 2019

Check the Ignition Module or Replace with another known Good IGN module & Test again.

BombshellCW · Feb 11, 2019

Ignition module is new and good. Previously replaced with a different brand. That new module threw a code 42 immediately. No code, new module.

BombshellCW · Feb 11, 2019

Interesting new development, my factory manual says I should have 1.3 volts with the engine running on circuit 423 which is my EST. This is measured at pin bc8 it is a white wire. I have .19 volts! Occasionally my dvom will spike to 4 to 18 volts Unsure how that's possible as my alternator is sitting at 13.5. Thing is, I'm not sure what this means or where to go from here ...

Edit: circuit 430 and 453 also have erratic readings. Faulty connection?

BombshellCW · Feb 11, 2019

think I found it...

Grumpy · Feb 11, 2019

reading links and sub links can help
replacement connectors and pigtails are available

http://garage.grumpysperformance.com/index.php?threads/lots-of-wiring-info-diagrams.317/#post-83877

http://garage.grumpysperformance.co...-auto-elecrtrical-connectors.3105/#post-68805

http://garage.grumpysperformance.co...cting-a-distributor-for-your-application.855/

http://garage.grumpysperformance.co...-idles-and-sometimes-stalls.10688/#post-46397

http://garage.grumpysperformance.co...lay-switch-locations-and-info.728/#post-17654

GM TPI stalling diagnosis

BombshellCW

Member

Maniacmechanic1

solid fixture here in the forum

Grumpy

The Grumpy Grease Monkey mechanical engineer.

c4 & c5 corvette trouble codes

curing that problem with how your c4 corvette runs badly or won't start

BombshellCW

Member

BombshellCW

Member

Maniacmechanic1

solid fixture here in the forum

BombshellCW

Member

BombshellCW

Member

BombshellCW

Member

Grumpy

The Grumpy Grease Monkey mechanical engineer.