problem with my truck loosing power

**joegreen** · 07-02-2014

i couldnt find anything on this but here is how it goes. i was driving to school today and my truck instantly cut power while i was driving. when this happened the truck slowed down and would pop through the intake if i tried to accelerate. i did manage to floor it and it would only accelerate to 3000 rpm or less max but it would hold a steady rpm and speed when floored maybe 40 mph. now the kicker, if i put it into neutral and turned the truck off and back on it would be fine again for a few minutes until it instantly lost power again. it feels like someone just flicks a switch. is this some kind of limp mode? i think my 1227747 ecm has limp mode. i dont want to be stuck somewhere and have barely any power. what could this be.

**Maxzillian** · 07-02-2014

The intake backfiring and loss of power sounds like the engine is running lean (either due to the PCM or a lack of fuel pressure). My truck has had a similar behavior when the O2 sensors were reporting that the engine was running richer than it really was and causing the PCM to cut back fuel by 25%. Cycling the ignition would reset the closed loop timer and allow the engine to run normal for about 20-60 seconds.

In my case I found that my exhaust wasn't grounding very well and could actually measure up to a volt between the exhaust pipes and the chassis ground (no idea how it got that high). I could never measure any current so I figured it was either some thermoelectric effect or something else. Datalogging showed that while this was happening, the O2 sensors were registering around 0.4-0.6 volts minimum. Ultimately adding some simple ground straps to the exhaust fixed the problem.

**joegreen** · 07-02-2014

o2 sensor is a new 4 wire delco afs75. exhaust ground is not an issue. just checked fuel pressure a week ago at 13lbs steady. truck stayed running fine longer than 1 minute.

**joegreen** · 07-03-2014

One problem i have is i do not have the check engine light hooked up. I cannot figure out why it does not work. so what i have is a 12 volt light bulb that goes into a small connector with a positive and negative wire. when i test it on the battery it lights up fine. i have tested everything i can and i do not understand why the light bulb will not light up when connected to a power source and the ecm ground wire for the light. so to start off i set my volt meter to volts and tested between a hot wire and the cel ground on the ecm. when i turn the key to the on position the meter reads 11.9v. next i did an ohm test between the body of the vehicle and the cel ground. again with the key on the on position it showed a complete circuit. when i hook up the light bulb it does not even glow dimly. i would like to see what codes i have.

**joegreen** · 07-04-2014

i do not think this is the case but could i have the wrong bulb. i used a bulb and socket that came with a gauge set i have. i only used 2 out of the 3 gauges so i had one extra light. i tried looking on forums and parts suppliers for the proper bulb for the cel but i could not find any info on what bulb to use. i am interested in what others use. what voltage and wattage. if i take my dvom i can get the ecm to flash out codes by jumping the a+b but i cannot tell what the codes are because the dvom does not change voltage display fast enough.

**joegreen** · 07-05-2014

Anybody. I have been searching the webs full of inter for a few days but cannot find anything. Everyone elses cel magically works. I know its probably something simple.

**EagleMark** · 07-05-2014

Got to be your light bulb, if they are to big they don't work, or LED no worky, Got power going to the light bulb and the other side of bulb to ECM for ground? Really simple! Is your power always on? Some key on looses power while in crank?

billygraves · 07-05-2014

Originally Posted by joegreen

i couldnt find anything on this but here is how it goes. i was driving to school today and my truck instantly cut power while i was driving. when this happened the truck slowed down and would pop through the intake if i tried to accelerate. i did manage to floor it and it would only accelerate to 3000 rpm or less max but it would hold a steady rpm and speed when floored maybe 40 mph. now the kicker, if i put it into neutral and turned the truck off and back on it would be fine again for a few minutes until it instantly lost power again. it feels like someone just flicks a switch. is this some kind of limp mode? i think my 1227747 ecm has limp mode. i dont want to be stuck somewhere and have barely any power. what could this be.

With no CEL and no check for codes, I STILL would check the fuel pressure even though a week ago it was good. I suggest you examine the data for codes and unusual data when the fault occurs. Un plug the O2 to put it in Open loop you could try. I think you should look at your GROUNDS. When the fault occurs try TAPPING on the ECM and if the engine changes tone or picks up or even stalls, I suspect the ECM has loose solder joints. Time for a reman. then. Most of the time when the solder joints fail, the CEL will flash or be on for some length of time.

**joegreen** · 07-05-2014

What bulb do you guys use voltage and wattage wise. I will go to the store and see what i can find for a good bulb. Thanks.

billygraves · 07-05-2014

Normally I see a 194 bulb in the socket of them. It's a common one. You could set up an external bulb. One wire to switched 12 volts and the other to the ECM CEL terminal. Try tapping on the ECM when it occurs.
Your Distributor. Is it the small dia HEI? If so look at the shaft magnetic for cracks and oil around the base. If so you need a new shaft. If you pull the dist insure the magnet is strong as you slowly turn the shaft. Look at the pick up coil terminal. If corrosion or whitish growth, I suggest to replace it and relube the module. Clean out the inside of the cap. I would replace the cap/rotor. Insure the Ign coil is not to close to the small dia dist. I think GM issued a bulletin on flipping the coil around to reduce EMI. Later GM Modules had been "Hardened" against EMI.
Insure both wires from your TBI Injectors are soldered inside the harness where they change back to the normal wire. Check the ECM terminal ends for size with the butt of a drill bit. I don't remember the size but I use a mike to size them before I use a carb guage to check for oversized terminal on the ECM harnees female connectors.