Engine-Carb Swap: 4L80E Driving me Loco - Upshift Blues

[Medic0893]

I have a 1992 GMC C2500 with 5.7L Auto Trans 4L80e. I write this because the issues I encountered are common and the suggested resolutions are... precarious:

1. History: The motor (with TBI) went at 295K. No complaints. I got from a kid who beat the tar out of it, so I wasn't expecting much. When I got it he just got a rebuilt trans installed and I expected to have to replace the motor so I acquired a gently used crate motor which wound up spending 2 years on the engine stand, waiting patiently.
2. Engine/Carb Swap: I chose to replace the intake with a Holley 600 (electric choke, vacuum secondary) and manifold. Shorty Headers and true dual exhaust. Electric fans. HEI ignition (White and Purple/white wires must get the tach signal). I kept the original fuel pump and installed a regulator. Acculink TPS. Manual gauges (my kingdom for an affordable, functional water temp gauge).
3. Temperature Gremlins: Everything went smoothly. Started right up and ran good. Then it kept overheating. Parts cannon, aim fire... turns out the water pump fins disintegrated while it waited patiently. Notable at this point is concerning the two original temps sensors (manifold & Left head): The head sensor operates the original temp gauge, that got the manual gauge lead. The manifold sensor provides a signal to the trans for proper shifting. I say this because...
4. Trans Challenges: With the temp under control i was able to broaden my perspective and realized the trans wasn't shifting properly. A few experiments with the TSP sensor angle and it seemed good (all 4 gears)... but (nothing before the word "but" means anything) As I started driving it farther from home I noticed on the highway it would shift down on acceleration/hills then it wouldn't shift back into OD. If I shut it off and started it back up it would function normally until the next hill... I set out in search of enlightenment on this issue. I found a plethora of contributions (varying in efficacy). Make no mistake this was months of aggravation and frustration. This is where I learned the original manifold temp sensor has to be connected. It turns out the forward trans input sensor was bad (I swear I never touched it!!). I can't swear either way if it shifted correctly with the original motor, but I never detected this before the swap. Note: I'm not a big believer in coincidence, I'm just prayerfully hoping that is was indeed serendipity. Now it shifts no problems, up or down and back again.

I'm not suggesting this is the solution for every situation, but it has seemed for me.

At this point all is right with the world, but...

 

[HHoldred88]

I have a 1992 GMC C2500 with 5.7L Auto Trans 4L80e. I write this because the issues I encountered are common and the suggested resolutions are... precarious:

1. History: The motor (with TBI) went at 295K. No complaints. I got from a kid who beat the tar out of it, so I wasn't expecting much. When I got it he just got a rebuilt trans installed and I expected to have to replace the motor so I acquired a gently used crate motor which wound up spending 2 years on the engine stand, waiting patiently.
2. Engine/Carb Swap: I chose to replace the intake with a Holley 600 (electric choke, vacuum secondary) and manifold. Shorty Headers and true dual exhaust. Electric fans. HEI ignition (White and Purple/white wires must get the tach signal). I kept the original fuel pump and installed a regulator. Acculink TPS. Manual gauges (my kingdom for an affordable, functional water temp gauge).
3. Temperature Gremlins: Everything went smoothly. Started right up and ran good. Then it kept overheating. Parts cannon, aim fire... turns out the water pump fins disintegrated while it waited patiently. Notable at this point is concerning the two original temps sensors (manifold & Left head): The head sensor operates the original temp gauge, that got the manual gauge lead. The manifold sensor provides a signal to the trans for proper shifting. I say this because...
4. Trans Challenges: With the temp under control i was able to broaden my perspective and realized the trans wasn't shifting properly. A few experiments with the TSP sensor angle and it seemed good (all 4 gears)... but (nothing before the word "but" means anything) As I started driving it farther from home I noticed on the highway it would shift down on acceleration/hills then it wouldn't shift back into OD. If I shut it off and started it back up it would function normally until the next hill... I set out in search of enlightenment on this issue. I found a plethora of contributions (varying in efficacy). Make no mistake this was months of aggravation and frustration. This is where I learned the original manifold temp sensor has to be connected. It turns out the forward trans input sensor was bad (I swear I never touched it!!). I can't swear either way if it shifted correctly with the original motor, but I never detected this before the swap. Note: I'm not a big believer in coincidence, I'm just prayerfully hoping that is was indeed serendipity. Now it shifts no problems, up or down and back again.

I'm not suggesting this is the solution for every situation, but it has seemed for me.

At this point all is right with the world, but...

Are you happy with the carb/intake swap? I've thought on doing something similar. I used to have a neighbor said his truck ran a lot better. But he doesn't live around here anymore to follow up with. Also said fuel mileage went to 20mpg.

 

[Medic0893]

Absolutely happy. Fuel mileage went up and throttle response is awesome. I did a lot more than just the carb. Headers, distributor, electric fans. I will tell you to be careful if you have an electric trans. Getting that right has been a real challenge.

 

[NickTransmissions]

Absolutely happy. Fuel mileage went up and throttle response is awesome. I did a lot more than just the carb. Headers, distributor, electric fans. I will tell you to be careful if you have an electric trans. Getting that right has been a real challenge.

Do you know if the wiring harness and solenoids in the trans were replaced when it was recently rebuilt?

Does your set up use a manual lever position sensor (MLPS)? If so, was that replaced?

Sounds like the input speed sensor was not so thinking the other stuff wasnt either (all that stuff must always be replaced on those units).

If not, Id plan on replacing them all soon.

 

[Schurkey]

Perhaps a moderator will move this thread to the LSX + Carb Swaps sub-forum.

 

[MyChevyisaBeast]

Hi this my first post. I Love my 99 k2500 flatbed He is my Beast. XFiance did the spider to carb swap n caught on fire top distributor some wires. I just put holley 600 , intake , n hei distributor Wiring harness still nightmares Broke my heart again when I learned that my 4l80e not shift if I did buy everything for it to shift it make it a manual.
So mister Can u please school me on how you did it?
Oh thank everyone I've learned so much on here.

 

[Medic0893]

Do you know if the wiring harness and solenoids in the trans were replaced when it was recently rebuilt?

Does your set up use a manual lever position sensor (MLPS)? If so, was that replaced?

Sounds like the input speed sensor was not so thinking the other stuff wasnt either (all that stuff must always be replaced on those units).

If not, Id plan on replacing them all soon.

Hey Nick, sorry it took so long to reply: I have no information on the trans rebuild, only that it was done.

I’m not familiar with the MLPS, Any insight would be greatly appreciated.

Can you give me a list of things that should be replaced?

I’m still dealing with the downshifting issue. Around town it shifts through all the gears and locks up as it should. When I get on a highway and keep my speed above 55: It downshifts on an upgrade as it should, but will not return to 4th or OD. I have to shut the key off for about 10 sec and restart to get it back in 4th/OD. It will cruise fine until the next incline and the process repeats. I initially had a TPS from HGM (support sucked b/c I didn’t buy their TCU for $1000). I recently switched to the Holley TPS, but it still does the same thing. Sure seems to me to be a throttle position issue, but WTF do I know at this point.

 

[Medic0893]

Hi this my first post. I Love my 99 k2500 flatbed He is my Beast. XFiance did the spider to carb swap n caught on fire top distributor some wires. I just put holley 600 , intake , n hei distributor Wiring harness still nightmares Broke my heart again when I learned that my 4l80e not shift if I did buy everything for it to shift it make it a manual.
So mister Can u please school me on how you did it?
Oh thank everyone I've learned so much on here.

welcome… there’s a lot of great info here and everyone is genuinely helpful. When I swapped the TBI for a carb I replaced the entire motor (original crapped out at 295K when I tried to tow something heavy, no complaints). While I was doing that I also did headers, electric cooling fans, true dual exhaust, HEI. I also had the opportunity to re-wire everything. Couple things to remember:
1. Keep the original temperature sensor and wiring that mounts on the intake manifold. The one on the driver side head is for the cluster gauge, I installed manual water and oil gauges so I eliminated that one.
2. There is a Purple/White wire coming from the original coil that must be hooked to the tach signal along with the white wire from the original distributor.
3. You have to get an aftermarket TPS kit. I suggest Holley’s if you’re using a Holley carb. The HGM (Acculink) only works with their $1000 TCU (computer).

 

[NickTransmissions]

Hey Nick, sorry it took so long to reply: I have no information on the trans rebuild, only that it was done.

I’m not familiar with the MLPS, Any insight would be greatly appreciated.

Can you give me a list of things that should be replaced?

I’m still dealing with the downshifting issue. Around town it shifts through all the gears and locks up as it should. When I get on a highway and keep my speed above 55: It downshifts on an upgrade as it should, but will not return to 4th or OD. I have to shut the key off for about 10 sec and restart to get it back in 4th/OD. It will cruise fine until the next incline and the process repeats. I initially had a TPS from HGM (support sucked b/c I didn’t buy their TCU for $1000). I recently switched to the Holley TPS, but it still does the same thing. Sure seems to me to be a throttle position issue, but WTF do I know at this point.

No problem. Electricals include the harness, shift solenoids, electronic pressure control solenoid and pressure switch manifold assembly.

MLPS = manual lever position sensor...Here's some more detailed info:

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Your downshifting issue is likely TPS, especially if it's aftermarket as they can be tricky to get dialed in. It can also potentially be your 3-2 control solenoid but I'd def rule out the TPS first before swapping in another solenoid...But if you do have to drop the pan, I'd replace everything electrical/electronic down there.

 

[yevgenievich]

Two Speed sensors on transmission and tps are two common reasons for defaulting to third when at speed. Worth checking data from the ecu on what it thinks tps data is at that speed and then reference to the data table for that computer on what it has set. On early computers with 4l80e after about 80mph it is set to allow lock up in 4th gear even at 100% throttle