Car Details: 1978 with Goodwrench 350. Converted to TBI from a 94 GM truck. All else stock, except headers and TKO600 trans. Catalytic converter we removed long ago by PO.

Engine has a slight stumble under a light load. I took it out today to run some tests on a road with a long, slight incline. I wanted to test the vacuum system as it just felt like it was running lean, and filling the tank up today lead to HORRIBLE gas mileage calculation - approx 7mpg. Most of my driving is short bursts and low speeds and I typically average 10-12 mpg. Headlights function normally.

On my test runs, I kept the engine at 30mph and right around 1800 rpm, in third gear. On my first test, I called both the vacuum lines that T off from the manifold. That test seemed to run the best with the least amount of stumbling. Still there, but not as pronounced. When I connected either one of the lines, stumble came back like before.

Capping both lines here gave best results, but did not eliminate the st7mble under light load. Check valve is fed by fitting on intake manifold and goes to reservoir (bottom) and headlight switch (right).


At home, hooked my Mityvac to all the vacuum lines I could think of. Results:

Brake Booster: Held vacuum
EGR: Held vacuum
HVAC: Held vacuum, although might have a very small leak. Possibly just where I connected, but not certain.
Light switch: Held vacuum and dump valve functions properly
Hose to Reservoir: Arm wore out before I could get a reading on the Mityvac, but heard sound of air rushing in when I disconnected the tool.

I built a smoke generator today, again thinking I had a vacuum leak. Hooking up to each of the hoses did not show any obvious leaks.

Car idles fine, and runs great under throttle, just this light load, partial throttle issue. ECM not throwing any codes.

I would greatly appreciate any feedback on where I should be looking next.

 

Well, as you know, it could be a lot of things. Hard to fix it if were not there.
however, I had a low RPM miss awhile ago that I chased for quite some time. In the end I found a bad plug wire.
I would suggest you start with the basics .plugs, wires, cap and rotor. Check, set timing.
then go from there.

 

Just a couple quick things to check, absolutely free.

Remove the distributor cap and maybe 3-4 plug wires on one side. Flip the cap over and with a DRY shop towel (no cleansers) wipe the entire inside of the cap. Note any moisture or "carbon tracking" on the shop towel. Rub the rotor center & tip contacts, put it back together.

Then, trace cyl numbers 5 & 7 plug wires from cap to plug. Make sure those two are well separated in the looms to prevent cross-fire.

 

Thanks guys! I will definitely check cap and rotor and the plug wires. I unplugged the EGR solenoid (my fuel system is essentially 1994 GM truck) and that seemed to fix the issue. Going for a long cruise today so I will get some solid wheel time to check it out. I recall that I had to reuse the distributor cap when I did my conversion because the one that was shipped had a broken carbon button and I was too impatient to wait for a replacement. Definitely worth checking!

 

I just did a whole refurb on a 1994 K1500 350 TBI.....ran like ****, bad gas mileage (about 9 or so), would not rev...etc.
I replaced two sensors (CTS, MAP), injectors/regulator diaphragm, leaky header gaskets (throwing off O2), and the timing chain (TBI is triggered by the distributor....loose chain can cause all sorts of running issues).
It went to 15-16 MPG, runs strong and revs out nice........5 speed stick, pulls past 5000rpm.
The CTS is critical for the computer to go into closed loop.......MAP and O2 control the injector "cone" and adjust it accordingly.....if you are getting 7 MPG....do what I mention here and your problems should go away.
Are you running a stock ECM?

Jebby

 

The EGR could also not be closing all the way......on the pickup it will throw a code but a lot of people deleted them in the 90's anyway.

Jebby

 

I'm running the stock ECM for the 94 setup (16197427). Been using tunerpro rt for the past few years on it. Good idea to recheck the data feeds from the CTS and MAP. I know the CTS is reading accurately as I was watching coolant temp yesterday when datalogging. Will dig into my log for the MAP values. Took the Vette on a long cruise yesterday and it ran great. On mostly highway driving, I get up to 15mpg, which I can accept given the 3.73 read end gearing.

Thanks for the ideas!