TBI Idle/highway trouble

[snowball]

Hi all,

I'm having a bit of trouble with my 90 burb, R1500, 350 TBI 2wd.

This has been an ongoing issue, for the past few weeks. Starting to get annoying.

Truck hunts at idle. When I get to a stop, initially fine, but as I wait for a stop light, it gets progressively worse. RPM's drop and rise and drooop, and rise and drooooooop... it's even stalled a few times.

At highway, sometimes, it feels like for a second, the injectors stop firing. gives you that thump and your head bops forward.

And sometimes, during load I get a backfire. not sure what end.

and sometimes, I'll keep my foot on the gas pedal steady, and it will downshift, and bog, and upshift, and downshift, and bog and upshift.

and sometimes it runs like there is nothing wrong. strong, full power, full acceleration. no issues.

Yesterday it was "cold" for texas. got into the mid 40's. Started the truck, and stalled right away. repeat. stall. put my foot on the gas steady. start, release stall. start again with pedal, romp on the gas. runs rough and stalls at 30 feet away as I back out of the driveway. repeat, and got her going.

I checked the CTS when I got home that night. 242 ohms and dropping. 30-40min later it was ~450ohms. so it looks like its working right.

TPS was replaced with an ACDelco. Ran for 3 months and one of the reading was stuck at 450 Ohms. Replaced it with one from oreily. looks good. both sides no dead spots.

Fuel pressure is 13PSI, 2 year old carter pump and new filter.

No codes stored.

Any ideas would be greatly appreciated.

[D13]

Distributor worn, affecting pick up coil performance, or pick up coil and/or module going bad. Gets confused as to RPM and/or timing. Confused enough it will shut off fuel. Bad plug wires backfeeding the module caused my miss then no start issues on my 1990.

Massive vacuum leak, intake gaskets or TBI shaft although that usually is less sensitive to high throttle.

IAC stuck but again less sensitive to high throttle.

There ought to be TBI diagnostics floating around here somewhere...

[snowball]

Thanks D13,

So a little more history, new GM crate motor was put in last year. New plugs and wires were installed at that point. Not sure if they reused the distributor, but that was replaced a few years ago. I will pull the spark control and check the ohms on the pickup coil.

Vacuum read 20" at idle. steady until it starts loosing RPM.

I'll pull and clean the IAC.

[Mattchu60]

I would check the cap/rotor. I had similar issues a few years ago when my rotor cap button was worn off. I did not think it was the problem as I had just replaced it a year prior, but it was the problem.

[Kudzupatch]

One thing I never or very rarely see mentioned is a dirty injectors. I had a VAGUELY similar issue and about pulled my hair out. Then one day I noticed that the spray/cone of fuel coming from the injectors didn't match. One was obviously stronger than the other. It is hard to see so use a strong light. Hooked up the computer and took it for a drive and I could see the O2 sensor levels drop[ing way lean under a load. New injectors solved my issue.

May not be your issue but I it is easy to check.

[snowball]

OK. not the ICM. tested it 5 times and it passed

I did get more bucking this week and got a code 45. running rich.

Any more thoughts? ordered another cap and rotor.