Tbi 350 has no power on the road

[Mitchell93]

1991 K3500 with a Tbi 350 and 4l80e
Idles fine, will revs up no problem, in park... when I take it out on the road, it won't go over 30 mph unless I go downhill, it just falls on it face, can hold it wot and it just don't Rev past round 2000, and just doggy, no power. It has transmission codes 85 and 87, but I fixed the broken wire for the input speed sensor. So as of now there is no codes. Previous owner put a ton of shiny new parts on it trying to fix it, to no luck, has a new fuel tank, fuel pump, fuel filter, distributor (whole thing, so module, pickup, cap and rotor are new) also has new ignition coil, acdelco plugs, plug wires, egr valve, iac valve, throttle position sensor, coolant temp sensor on intake, has a new timming chain, I tore it apart today to find a new double roller that is clocked correctly. I found a bad pigtail for the right knock sensor that I replaced, I also replaced the entire throttle body with a known good one, no difference was made there, so that kinda rules out the injectors and pressure regulator... set base timming to 0°
The transmission will shift though all gears, going downhill I had it in overdrive and doing 55 60... also on my hoist I can run up the rpm and tire speed with no problems, only when I hit the road...
Anyone hear have similar issues that would care to share their knowledge and experience, I would appreciate it

 

[Erik the Awful]

Look to see if both of your injectors are firing. If you've replaced the TBI with a known good module, one of your injector connectors might be crunchy. My son's truck wasn't firing one of the injectors. We identified that it was the connector, so I cut it off and installed a new lead - still no fire. I had to cut the wiring back about 4" to get to good wire that spliced well.

 

[GMTMark]

Check your exhaust. The catalytic converter could be stopped up or the muffler could be restricted.

 

[Road Trip]

Check your exhaust. The catalytic converter could be stopped up or the muffler could be restricted.

Greetings @Mitchell93 and Welcome to the GMT400 forum!

Given the repair history, the current performance symptoms, plus the lack of codes, I think
that GMTMark's idea is a very good one. Even better, if you temporarily disconnect your
exhaust system from the manifolds (and securely wire them up to the frame with baling
wire or the equivalent) it won't cost you anything (well, maybe new donut gaskets and new hardware)
you will know almost immediately whether or not you are barking up the wrong troubleshooting tree.

NOTE: Assuming your computer is working correctly, no doubt that O2-related error codes
are going to set, but they can be cleared once the exhaust system is reconnected. (either
your existing if the engine behavior doesn't change, or the new system if you determine
that the hidden 'potato in the tailpipe' exhaust performance is the root cause.

Looking forward to reading your report of what you found and how you fixed it. Sounds
like the PO gave up just a little too soon.

Happy Hunting. And don't forget the earplugs. :0)

 

[GMTMark]

Very good and inexpensive way to troubleshoot.

 

[Jroctizzle]

Lazy fuel pump? Not enough pressure under load to maintain the increase demand?

 

[JDGMC]

1991 K3500 with a Tbi 350 and 4l80e
Idles fine, will revs up no problem, in park... when I take it out on the road, it won't go over 30 mph unless I go downhill, it just falls on it face, can hold it wot and it just don't Rev past round 2000, and just doggy, no power. It has transmission codes 85 and 87, but I fixed the broken wire for the input speed sensor. So as of now there is no codes. Previous owner put a ton of shiny new parts on it trying to fix it, to no luck, has a new fuel tank, fuel pump, fuel filter, distributor (whole thing, so module, pickup, cap and rotor are new) also has new ignition coil, acdelco plugs, plug wires, egr valve, iac valve, throttle position sensor, coolant temp sensor on intake, has a new timming chain, I tore it apart today to find a new double roller that is clocked correctly. I found a bad pigtail for the right knock sensor that I replaced, I also replaced the entire throttle body with a known good one, no difference was made there, so that kinda rules out the injectors and pressure regulator... set base timming to 0°
The transmission will shift though all gears, going downhill I had it in overdrive and doing 55 60... also on my hoist I can run up the rpm and tire speed with no problems, only when I hit the road...
Anyone hear have similar issues that would care to share their knowledge and experience, I would appreciate it

What do the spark plugs look like- before and after? What is your fuel pressure? How does the injector spray pattern look at high rpm? What is your intake vacuum? How does the trans fluid look? If you just got the vehicle, check for the correct ECM, EPROM, or incorrect tuning.

 

[Schurkey]

Rather than disconnecting the exhaust system, merely remove one or more O2 sensors AHEAD OF THE CATALYTIC CONVERTER.

 

[Mitchell93]

Thank you all for your quick response, I'll go down the list...
So both of the injectors are differently working, I can see a clean healthy looking cone shaped spray from both, I have not held it wot wile watching it, it does Rev up great in park, I'd have no way to watch it wile I drive it...
I did forget to include that it has a shiny new muffler with the catalytic converter cut out, and replaced with a chunk of pipe, no tail pipe, has a turn down right off the muffler...
Also has a new o2 sensor, sorry I completely forgot to mention exhaust, the air cleaner is new as well...
I have not been able to check fuel pressure because I did not have the correct adapter, I have it ordered and will definitely be checking that, but the pump and filter are both new, and I know new parts can be bad... previous owner put all these parts in it, so who knows how cheep the parts are..
I have not checked vaccum, I will though, and the new spark plugs look kinda black, I can get a picture of them. I work 3 hours from home so I'm only home on the weekends so I can check fuel pressure and vacuum, I'll have my wife snap a picture of the plugs. Atf looks and smalls clean, nice bright red. I do not know how to know if the pcm or prom is correct, how would I verify this?

 

[tayto]

Lazy fuel pump? Not enough pressure under load to maintain the increase demand?

YES!