454 TBI surging/loping
 

I made a thread a few months back about my TBI surging, I thought I had fixed it however its back and appears worse.

so backstory- truck was getting horrible MPG. unknown history so I decided to rebuild throttle body. in process found fuel lines rusted so replaced those.

when I installed rebuilt throttle body and did IAC relearn, I was having issues. turned out to be a torn o ring but I had already bought a new one so swapped it.

everything seemed ok, but every once in awhile truck would lope- like it had a cam in it. drives great, only does this at idle or creeping in low (truck is NV4500).

so here is everything that has been done to the truck:

previous mods:
new ditsrubutor and coil
adjustable FPR (set to about 12 PSI)
TB spacer and better air cleaner

in past 3 months
rebuilt TB with new injectors
new fuel lines
new IAC
new Throttle Position Sensor
new MAP Sensor
new O2 sensor (heated upgrade)
new coolant sensor

I dug out my old Chinese tech 2 and figured out how to read the data on the truck. I attempted to set the IAC count down, and it would be good, but when I start truck up it goes right back up. so for example I try to set to 15. its good, restart and its up to 30.

I tried doing the IAC reset jumping terminals- when I disconnect IAC the truck lopes VERY bad and barely runs- it was not doing that 3 months ago. I also noticed if Im sitting in the garage and I raise the rpm slowly, it will climb to about 2400 or so, and then randomly drop to 17-1800, and then rise again, and just do that over and over.

ive checked timing, its good steady not jumping. I checked for vacuum leaks with propane, nothing. I checked EGR and PCV, both seem good to me.

at this point im lost. I dont know where to turn next. every time I think it's one thing and I change it, it runs good for a few miles, then random lope again.

when this first started back in march- it was NOT loping when I tried to do IACA relearn, so to me it seems to be getting worse.

I have a couple screen shots I took of my tech2 screen today when I had IAC set good and it was idling nice. then I shut off restart and it lopes. maybe someone can tell me if anything looks off?

Re: 454 TBI surging/loping
 

Pics from tech 2

Re: 454 TBI surging/loping
 

turn

Re: 454 TBI surging/loping
 

What year truck? Some of the 454 TBI units need a lot more fuel pressure than 12psi

Re: 454 TBI surging/loping
 

its a 92 so that fuel pressure should be good. i think its the 94-95 trucks thats need the like 30 PSI.


so updates if anyone is reading

sunday i pulled the cap off my newish to me dizzy (probably 10k miles) and the pickup coil looked crappy to me. so yesterday i dropped a new distributor in.

truck is running like crap.

troubleshooting some last night- vacuum appears a little low- showing about 16 on gauge, i found posts saying factory would be around 20. truck smells rich

here is the really odd part- when i pulled vacuum line for MAP off TB- it had traces of gas in it?

i tried to fool the MAP sensor, i put vacuum pump on it, pumped it up to 18. truck idles smooth and doesnt smell rich anymore. i verified wiring at MAP is correct.

im going to double check timing tonight. thinking of trying to advance it some to see if it brings vacuum up. the fuel in the vacuum fitting has me puzzled? could that be a result of the loping?

Re: 454 TBI surging/loping
 

When you set the timing, be sure unplug the timing wire otherwise it won't be set correctly. If you do advance it, you can go up to about 3-4 degrees. Some of the TBI motors like it better than 0 degrees