Rsulliva

I have a 1994 350 TBI that has a significant lope to the point of stalling, especially when in gear. When in park it will lope between 800 to 1000 rpms and when in gear sitting still will lope between 200 and 700 rpms and every so often, stall out. You can feel an intermittant stumble when actually driving although tach doesn't show it then. The stall will also occur sometimes when coming to a stop. The motor has plenty of power. Changed almost all sensors/components - see below for list of replaced items. I can temporarily fix the issue if I unplug the Coolant Temp Sensor the truck will stumble really hard several times and if it doesn't stall, it will smooth out completely 800 RPM for about 1 minute then starts to stumble again. Putting the vehicle back into park, plugging the CTS back in then putting the vehicle back into gear for about a minute then unplugging the CTS again will once again will repeat the temporary fix most of the time. Plugging the CTS back in once the lope has started has no effect. I have checked the voltage going to the CTS and it remains a steady 5V. I have tried 3 CTS's with the same results. Also about one in every 20 trips the vehicle will show no signs of any lope or stumble or very little. Could this be a PROM issue?

Replacements already performed:
ECM(computer) - had to use old PROM as new ECM doesn't come with it.
Coolant Temp Sensor (x3)
EGR and solenoid
IAC (x2)
Vacuum lines everywhere
MAP sensor
Fuel Pump
Fuel Filter
Distributor (whole thing)
Plugs/Wires
Coil
Injectors
Fuel Pressure Regulator
Intake gaskets
Removed and cleaned all grounds
Alternator
TBI Gasket
Thermostat 195

Suggestions from any GM Genius'?

tylercamaro

iTrader: (1)

im having the same issue as my 93 im pulling code 43 though which is either the esc and as yuo checked the computer is good or replaced so hopefully good. try your knock sensor and check the signal to pin b15 on the red connector to the computer.

Rsulliva

On my 94 I don't have either a Knock Sensor on the block (looked everywhere then was told mine didn't have one) or ESC unless its a chip somewhere in the Computer. Before replacing the distributor, I did change the ICM in the distributor which had no effect whatsoever.

Schurkey

Stop replacing parts, perform actual diagnosis using a scan tool to look for commanded idle speed, coolant temperature, short- and long-term fuel trim, O2 crosscounts, manifold vacuum, or anything else that looks odd. A fuel pressure test is strongly recommended, because replacing the fuel pump and filter does nothing useful if the problem is a faulty wire harness which drops the voltage supplied to the fuel pump motor.

If you have a knock sensor--and I'd have bet money that you do--it'll be screwed into the block ahead of the starter motor. The hole used is what would have been used for the right-side coolant drain plug on the block, so when you remove the knock sensor, the coolant drains out unless there's so much corrosion built-up in the water jacket that the hole is plugged with rust particles.

tylercamaro

iTrader: (1)

every tbi truck has a knock sensor, that could be your issue right there, occasionally the wire leading to it gets burnt off by exhaust and what not as well.

Rsulliva

Guys, I will check again for the knock sensor where the coolant drain port is near the starter. I just remember seeing a plug there, nothing else, but I could be wrong. Will also test fuel pressure for any drop outs. May have to get a ALDL cable and a laptop for real time sensor readings. Any suggestions on a USB interface cable source or best free software I should use?

tylercamaro

iTrader: (1)

tuner pro, and aldl cable.com also check out gmt400 and gearhead efi for tuning and diagnostics theres mulitiple threads on both sites for tbi trucks.

Rsulliva

Tried two more things. You guys were correct in the fact that it does indeed have a knock sensor. Replaced it - did nothing to help. Was suggested that I replace the O2 sensor - also did nothing to help. Idle was smooth as can be, but in gear still large lope (200-700 rpm's). So we are in no better shape than before.

However, today in addition to the loping, pulled up to a stop light and went to take off and almost had no response until pedal was pushed way down, then took off like a bandit. Did this a lot today and at almost any slower speed. It would be great for about 2 blocks then act stupid again. Replacing TPS didn't help - ohm'd out TPS's and all pin outs were smooth. Kind of leading me back to the computer/PROM again. Was diging around under dash and noticed another box behind middle lower part of dash that looks like another computer (silver metal box like PCM/ECM, maybe slightly smaller) turn up on its side, any ideas on what this might be? Looks factory installed. The truck does have an aftermarket radio that the previous owner installed.

tylercamaro

iTrader: (1)

pics. factory amps were silver

Rsulliva

Guys,

The vehicle finally threw a code. It has been both acting like the post above and acting perfect on trips. The code finally thrown was a 16. For my particular vin it according to http://www.troublecodes.net/GM/94-95_57J/ means "DIS fault line voltage was low for 2 seconds when EST was disabled." Any ideas on how to trouble shoot this one?

Thanks,

rsulliva

morgsie

iTrader: (1)

Get a scantool or data logger. What is your charging voltage @ idle? It MUST be at or above 13.5V.

Rsulliva

Working on getting an ALDL Cable. Found an identical PROM and installed - No change. Rerouted plug wires away from everything electrical just in case and retimed to 0 degress when comp is unplugged (runs about 30 degrees advanced when warm and comp is under control) - No change. Also tried 2 and 4 degrees advanced - no change. Have noticed that when it is running bad, it will pull a dollar into the exhaust every so often, and then if shut truck off and immediately restart it sometimes will run great and doesn't pull dollar at all. Almost like a complete misfire then firing perfectly . Timing light also seems to show a lack of fire as the light will flash about ten times then seemingly skip and flash intermittently. It does this on all plug wires cylinders. When running bad the throttle sometimes acts like it doesn't have any response. Any suggestions?

RFmaster

When you replaced distributor did you get a new ICM or did you re-used the original unit from old dizzy??? Which brand of ignition control module do you have?

Get a scan tool or ALDL cable with WinALDL to see what ECM sees.

Rsulliva

In an effort to fix the problem, I initially replaced the ICM and the pickup coil on the orig dist before replacing the whole distributor. The new distributor came as a complete unit with ICM and pickup, cap, rotor. The new dist made no difference in the way it performs.

RFmaster

Hmmm ok

You've mentioned spark drop-out

1) Check Supply voltage (+12Volts) pink wire to ignition coil plug. Must be rock steady - no drop-outs!
2) 2-wire harness between dizzy ICM and coil. This harness may have been damaged or have intermittent pin contact.

//RF

morgsie

iTrader: (1)

have you measured voltage at the battery with engine running yet? under spec voltage will cause ecu to do weird things. odd for an ecu to fail out right as well. usually from jump starting backwards or introducing voltage where it's not suppose to go.

tylercamaro

iTrader: (1)

is tbi really that picky/ ive had alternators die and not do a thing to the engines functioning, but ive been tracking an issue where nothing seems to be at fault itl idle great but under load it wont run wortht a damn, could it really be the alternator?

morgsie

iTrader: (1)

Any EFI will not run right if the alternator is not out putting the correct voltage. Read this thread here: https://www.thirdgen.org/forums/diy-...breaks-up.html The car would cut out or miss on WOT, usually above 4000 RPM. He tried everything from tune up, tearing down the top end, switching the tune from MAF to SD, etc. Close to 2 years later it ended up being a dirty alternator, which he later noted was evident because of interference heard when his radio was on. I am not saying it is your problem, but it is a simple thing to check and is one thing I always check.

tylercamaro

iTrader: (1)

mine was the timing the esc wire wasnt disconnected when the shop said they retimed it.

morgsie

iTrader: (1)

Your SES would have been on if that was the case. A datalogger or scan tool would have also showed this.

Schurkey

If the ESC wire wasn't RE-connected, the SES light would be on.

If they didn't DIS-connect the wire, I'm not convinced the SES would light. Engine would probably run poorly, though, as I expect the timing would be significantly retarded.

tylercamaro

iTrader: (1)

timing was significantly retarded and they yanked the wire so even when i installed it it shorted out because they broke the connection

morgsie

iTrader: (1)

As soon as you pull the ESC bypass wire the SES light goes on and will not go off until the connector is plug back in and the vehicle is cycled off/on. If you're talking about the ESC module being unplugged I don't think the car would run at all.

Schurkey

Re-read what he posted:
they timed the engine without DIS-connecting the ESC wire.

In other words, they erased a bunch of advance without upsetting the computer or turning on the SES light.