Surge at Idle, TBI L03

Okay this is kinda nutty. As you all know I bought Chris' OCC a few months ago. Been driving it issue free for months now. Recently though, when I start it and it's cold, it surgres at idle. Not enough to stall but noticible.

Fast forward to yesterday and I started Ruby up. Guess what, same issue but worse! It surges but will stall unless you run the idle up for about a minute. then once the engine warms up it smooths out.

Any ideas? Vacuum line maybe? Ruby has been run about every 10 days, but not on the road in awhile (over 2 years) but is about to go back on the road.

What year vehicle Sproket? TBI or LT1


You are in open loop for the first 90-120 seconds. Vacuum lines are a strong possibility, as is the IAC step motor being dirty internally.

The title says L03 TBI, and, it's an OCC.

ones a 91 the other a 92, both stock L03s.
the 92 is the one that stalls.

A shot in the dark here, but take the distributor cap and rotor off and take a look at the trigger piece as shown in the picture below.

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If there are any cracks like the ones seen here you should replace the distributor shaft and pickup coil. When the magnet cracks like shown it creates a weak magnet. Look closely near the rivets, oil tends to creep up the shaft and soaks the magnet causing it to swell and eventually crack.

While in there remove the ignition module and put some fresh dielectric grease underneath it for good measure.

This particular image shows a V6 distributor shaft, as noted by the 6 star points. This particular piece is defective and should not be reused.

Came across a slightly better snapshot of what I am talking about with caption.

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Good info. Do they crack from age or oil soak or both?

Hmm, interesting. I had a 79 Cutlass with a 260 and the wire that attached to the pickup coil broke but the insulation was still there. when the vacuum advanced the timing at 70 MPH, the engine would die, you'd coast until 60 and then it'd restart. Took me a lil while to diagnose.

I'm curious though as on the 91 my DD, it's fine as soon as I drive it. It was hot out and this afternoon, it didn't do it. (external temp gauge said 98 when I started the car).

I'm still betting on open loop issues.

You get an injection of recirculated exhaust until warmup is done as well as part of the AIR emissions events. Check for holes in the A.I.R. tubes as well.

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Good info. Do they crack from age or oil soak or both?

I would have to say it a combination of the two.

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...I had a 79 Cutlass with a 260 and the wire that attached to the pickup coil broke but the insulation was still there. when the vacuum advanced the timing at 70 MPH, the engine would die, you'd coast until 60 and then it'd restart.

The '79 is a different setup, as you mentioned it had a vacuum advance, and the pole piece would move causing the wire to flex, intermittently opening and causing the stall. The TBI engines are not like this.

I have replaced many distributor shafts for many different driveability problems. It was always one of the places I looked when I came across some weird issues. It's worth a look.

lakeffect wrote:

I'm still betting on open loop issues.

You get an injection of recirculated exhaust until warmup is done as well as part of the AIR emissions events. Check for holes in the A.I.R. tubes as well.

I would have to disagree with the exhaust gas recirculation theory, the EGR valve does not open until the engine has warmed up to operating temperature and with the correct operating parameters. (If it did recirculate/open; engine stalling, rough idle and rich tailpipe emissions (black smoke) would occur.) This may mean that you have to drive the vehicle down the road, some EGR systems will not function until it sees vehicle speed.

The S.A.I.R. on the other hand, if equipped, operates for several moments, on a cold start up or whenever the proper conditions are met, to get the catalytic converter up to operational temperature faster so that it can start correcting tailpipe emissions. Holes in the A.I.R. pipes would likely show a lean condition when looking at a scan data and likely not effect engine start up, due to open loop. Block learn and Integrator numbers would show high, as "extra" air is getting into the exhaust stream. Oxygen sensors would then pick this up (once in closed loop) as a lean exhaust and report to the ECM, ECM would then make the correction by adding more fuel, increasing pulse width, to compensate for this.

There could be many things that may cause this, but keep in mind the surge is being caused by an input to the ECM, or ignition module, the ECM could be telling the IAC to adjust the idle speed. Timing will also effect engine speed, hot or cold. This is why I am suggesting to take a look at the distributor shaft trigger magnet, an incorrect timing signal will change timing.

On my DD it's so subtle I'm not worried about it, on the 92 it'll be an issue when the car hits the street in a few weeks. I'll dive into it in the coming weeks and keep y'all updated.

First order of bizniz is getting that Cougar back to the house.

Going to be a few days to get that done.