Check engine light? Idle problem - TBI 350 (1 Viewer)

[Senor Aguas]

Hello. My check engine light comes on at least half the time I drive my 40. I've hooked up a code scanner and have gotten no trouble codes. I do get code 12 which means everything is supposedly working as it should.

If I pull over and turn off the engine and restart, the light goes away. I think I might notice a little less power after the light comes on, but it might be my imagination, too.

Also, (this may be unrelated, or it may be part of the same problem), my idle oscillates up and down a couple hundred rpm, sometimes. From reading various places on the 'net, it sounds similar to the problem that some have if no speed sensor is installed. So, I got a speed sensor, and it didn't change anything.

Here's what I know about the engine and TBI.

Engine is a '77 or '78 350 from a van, I believe. Rebuilt about 5k miles ago. It got an "RV cam" (whatever that is--unknown specs) and was built for a carb setup. The next owner put the TBI on it and had the idle problem since the beginning.

The TBI is from a 1990 Caprice Cop car. It's got a newly rebuilt TBI assembly on it. FWIW, it looks like a good installation on the TBI (wiring, etc).

Are these 2 symptoms likely the same issue?

Would a cam made for TBI likely fix my problem? Which would you recommend--stock TBI or performance TBI grind?

A custom TBI chip (programmed with unknown cam specs)?

Could this be a sensor, or a wiring issue?

Where should I start in diagnosing this?

Many thanks, in advance.

David.

 

[Brian in Oregon]

Since the problems are intermittent and there's nothing you can positively nail down, I'd check into the possibility of a poor electrical ground for the electronic control module first, followed by checking the power input.

 

[Senor Aguas]

Thanks, I'll check out the grounds. Just remembering, I'm not sure how the ECM is grounded. The box is actually just velcro'd in place in the glove box, but I suspect that maybe there's a ground wire coming from the box.

 

[theo]

There should be a grounding lug on the box.

Poor idle can be caused by about a hundred different things. For surging idle on a TBI I would start with a thorough check for vacuum leaks, especially at the throttle shaft and at the base of the throttle body. The gasket is notorious for failing. Then I would test and adjust the TPS (Throttle Posisiton Sensor), then clean the IAC (Idle Air Control valve).

An RV cam is a mild grind that lowers the rpm range of the power band - race cams raise it - so you have better torque right from idle through ~3000 rpm. RV cams don't cause poor idle.

You don't need a scan tool to check for codes:
Your high performance 'airflow' parts source for GM / Chevy Tbi / Vortec / Marine + Racing

HTH (Hope This Helps)