Home Forums Hunting & Shooting Hunter's Campfire Low RPM miss, 93 GMC Sierra

Cap and rotor

Such an engine that runs strong at highway speed is not likely to have a primary ignition problem - secondary, maybe. The suggested plug check is good - maybe use insulated pliers to grip.

I would suppose that the in-tank electric fuel pump is not sensitive to rpm rate - so not likely an issue involving fuel delivery to TBI - but there may be an injector problem at body. Maybe run some Seafoam or injector cleaner through those.

Don't know your vacuum setup, but a vac leak can cause such a problem on carb engines. So can high fuel pressure - maybe an injector is letting in too much fuel at idle. Is the engine loading up so that you are getting dark smoke/unburned fuel out the exhaust at idle? That = too much fuel for available air.

I know almost nothing about sensor operations, and the guys here talking sensor checks may be onto the problem.

Pop the hood in total darkness, see if you have a "light show " going on, with it running of course.

Long shot, but check the thermostat sensor plug at the thermostat and make sure it's not toasted.

If it is, the truck can think it's cold and it'll run richer by adding fuel (causing bogging down and surging at low RPM's) but the problem won't manifest itself at high RPMs.

IAC
idle air control valve.

I had a 91 1500 that had the low speed/idle miss issue....it eventually started acting up at highway speed and it turned out to be the in tank pump...easy to test the pressure.

That Gen of Chevy's had terrible fuel pumps. My buddy was like a NASCAR pit crew changing them he had done it enough times between 2 pickups of that Gen

I had absolute fits with my 87 throttle body injected 350. I transferred the old distributor to a new engine. It ran good for a year or so. Then it began to have a rough idle and sometimes stalled yet it ran great at higher RPMs. My tech tried switching ignition modules and it wouldn't run with the new module! He suspected a worn out distributor causing poor triggering of the pickup coil at idle. Higher RPMs would allow the distributor shaft to have less runout so it properly triggered the pickup coil. I put in a remanufactured distributor with all new parts and it solved the issue. 6 months later the crappy distributor cap and rotor had to be replaced with a good one as it began to cause issues too. (if I EVER get another distributor you can bet I'll be putting blue streak parts in it before it goes in!!!)

First off I'd change the cap and rotor. While the cap is off inspect at pickup coil. You never mentioned mileage on this engine. If it has high mileage you might put in a new distributor and be done with it.

Let us know what ends up fixing your problem.

I love these threads. Replace all of this stuff and then you take it to the shop. First thing you say "It can't be much as I already replaced everything".
Chuckle, chuckle

Get the thing Scan Tooled First, instead of guessing...

I think Autozone will do that for free... don't know how thorough they do..

but it would be a start... instead of replacing a lot of things in an attempt
to guess, or get lucky on the solution.

sure would be nice if my buddy had a competent shop in Delta to take it to.... that would make our searching life on his problems a LOT easier.

Couldn't get to it yesterday....hoping for this evening.

It has 199,6XX miles
2 Fuel pump replacements over the 15+ years that I've owned it[the last included a new fuel tank about a year ago].

Thanks for all of the replies

Please enlighten us on that easy test procedure. I've been looking for a way to do it.


Jerry

They used to make an adapter that would replace the fuel filter in the line.

I believe it had a standard Schrader type valve and thread.

The pumps can be a bit of a mystery. For example, a Vortec engine can be hard starting if the pressure is a pound or two under spec....but run like a raped ape once you get it going.



Unfortunately, the OBD system on the old throttle body set ups are a bit lacking. Fortunately, they are pretty simple.

My 92 did the same thing.

WHy?

Long read but one of the better descriptions of how it all ties in for a TBI system.

http://www.autozone.com/repairguide...-Fuel-Injection-TBI/_/P-0996b43f803938c8

The purpose of the Idle Air Control (IAC) system is to control engine idle speed while preventing stalls due to changes in engine load. The IAC assembly, mounted on the throttle body, controls bypass air around the throttle plate. By extending or retracting a conical valve, a controlled amount of air can move around the throttle plate. If rpm is too low, more air is diverted around the throttle plate to increase rpm.

During idle, the proper position of the IAC valve is calculated by the ECM/PCM based on battery voltage, coolant temperature, engine load, and engine rpm. If the rpm drops below a specified rate, the throttle plate is closed. The ECM/PCM will then calculate a new valve position.

Thanks, Swifty52 - informative and helpful.

I thought GM TBI on pickups was 88-92 but the TBI is dirt simple, I have installed two systems on two different Jeeps, one 258 and one 360 modifying factory harnesses, running it on the stock 4.3 and 5.7 chip and having a custom tune after datalogging and providing the data to Bill.

The GM TBI is pretty darn good at running and continuing to run even as the engine wears out. Running the scan is too easy to not stop by any parts house and having it checked.

In my opinion, the first place to start after eliminating fault codes is GROUNDS!

GM grounds through multiple points and ties ground wires together in the harness so there is redundancy but your truck is old and has a lot of miles. Locating and cleaning the connections on the block may solve the problem and costs nothing before you start throwing parts at it.


And there are a crap ton of wires and grounds in an LS harness too, and I am NOT and electrician <grin>




Is it a miss or running rough?

Tom mentioned the IAC, since the throttle body blades are closed the IAC allows air to bypass and the computer controls idle.

If the issue occurs at idle only it may be the IAC has gone wacky.

It's also possible one or both injectors are leaking, loading up at low rpm but smoothing out as rpms increase.

Vacuum leaks are also an issue that confuses the ECM.


.

Plugs seem to have fixed it.
Took it for a ten mile spin after changing yesterday.
No miss/jerk/whiplash at any speed/RPM, it's just completely gone.
Thanks for all of the info.