Any ideas??

[Kitch]

My TBI Vortec headed 94 G20 conversion van doesn't seem to be running right at the moment, the engine just feels a bit flat.

It was like that when I parked it up a few months ago, I thought I might try and see what going with it over the holidays.
I decided to focus on one aspect and go from there, so I started simple with looking at leaning out the idle AFR's slightly.
I run a constant open loop idle and have a wideband O2 gauge that I can see while I drive and it logs to TunerPro.

I took the van for a drive to get it up to temp and then looked the the idle AFR logs when I got back home.
@600 RPM in drive it pulls about 45Kpa, with 27° of timing at 81°C and an AFR of around 12.3-4, IAC at 3

The next day with tracing turned on in TunerPro I then went and lowered the values and then smoothed the "Idle VE vs MAP vs RPM" table and loaded it into the Autoprom.
I logged a run to the local testing station, the engine had just reached temp by the time I got there and the intake manifold probably hadn't heat soaked yet.
From that log I was getting:
@600 RPM in drive it was pulling about 45.5Kpa with 27° of timing at 80°C and an AFR around 13.5-6, IAC at 9

The increase in AFR seemed reasonable given how I'd lowered the idle VE values, so far so good.

On the trip back home and hour later I noticed the in gear idle AFR's seemed a bit rich again, when I got back home and looked at the log I saw:
@600 RPM in drive it was making about 45.5Kpa with 27° of timing at 83°C and an AFR around 12.5-6, IAC at 7

So it seemed to be back to how it was before I lowered the idle VE values.

This may be a symptom of something else going on but at this stage it's the only thing I've noticed that doesn't seem right based on the one change I've made.

Looking back at some much older logs (with a different tune) it looked like the engine previously idled @600 with around 43-44Kpa.

I considered my wideband gauge might be playing up so I cross referenced it with the heated narrowband I run, when the wideband was showing mid 12's the narrowband was showing around 970mV and when the wideband was showing mid 13's the narrowband was showing 940mV. Based on the trend from the narrowband I assumed that the idle AFR must definitely have changed.

I'll do some more testing and see if the richer reading at idle is constant.

I could see how an increase in fuel pressure could do that and I'll check that out, but it seems like it would generally be an unlikely thing to randomly happen (I'm running a later model Vortec fuel pump @20.5 PSI).

Any other ideas what might cause this?
I could be too close to the problem and missing the bigger picture so to speak.

[Fast355]

My TBI Vortec headed 94 G20 conversion van doesn't seem to be running right at the moment, the engine just feels a bit flat.

It was like that when I parked it up a few months ago, I thought I might try and see what going with it over the holidays.
I decided to focus on one aspect and go from there, so I started simple with looking at leaning out the idle AFR's slightly.
I run a constant open loop idle and have a wideband O2 gauge that I can see while I drive and it logs to TunerPro.

I took the van for a drive to get it up to temp and then looked the the idle AFR logs when I got back home.
@600 RPM in drive it pulls about 45Kpa, with 27° of timing at 81°C and an AFR of around 12.3-4, IAC at 3

The next day with tracing turned on in TunerPro I then went and lowered the values and then smoothed the "Idle VE vs MAP vs RPM" table and loaded it into the Autoprom.
I logged a run to the local testing station, the engine had just reached temp by the time I got there and the intake manifold probably hadn't heat soaked yet.
From that log I was getting:
@600 RPM in drive it was pulling about 45.5Kpa with 27° of timing at 80°C and an AFR around 13.5-6, IAC at 9

The increase in AFR seemed reasonable given how I'd lowered the idle VE values, so far so good.

On the trip back home and hour later I noticed the in gear idle AFR's seemed a bit rich again, when I got back home and looked at the log I saw:
@600 RPM in drive it was making about 45.5Kpa with 27° of timing at 83°C and an AFR around 12.5-6, IAC at 7

So it seemed to be back to how it was before I lowered the idle VE values.

This may be a symptom of something else going on but at this stage it's the only thing I've noticed that doesn't seem right based on the one change I've made.

Looking back at some much older logs (with a different tune) it looked like the engine previously idled @600 with around 43-44Kpa.

I considered my wideband gauge might be playing up so I cross referenced it with the heated narrowband I run, when the wideband was showing mid 12's the narrowband was showing around 970mV and when the wideband was showing mid 13's the narrowband was showing 940mV. Based on the trend from the narrowband I assumed that the idle AFR must definitely have changed.

I'll do some more testing and see if the richer reading at idle is constant.

I could see how an increase in fuel pressure could do that and I'll check that out, but it seems like it would generally be an unlikely thing to randomly happen (I'm running a later model Vortec fuel pump @20.5 PSI).

Any other ideas what might cause this?
I could be too close to the problem and missing the bigger picture so to speak.

Distributor slipping, retarding the timing? Would reduce the idle timing which would pull less vacuum, causing more fuel delivery.

Also worth mentioning is intake manifold temp and the air cleaner preheat. If the engine is not fully warmed up it will usually read leaner than a fully heat soaked engine. The BLMs will always drop some on a HOT engine as the induction system heat soaks.

[Kitch]

Hi Fast,

Good point.
I was thinking of rechecking the base timing again, the engine certainly feels flat almost like it's lost timing....It would be great if it was that simple, sounds like tomorrows first job.

[Kitch]

Base timing is still dead on 0° but from a symptom stand point it sure sounded like a good thing to check.

The engine seems to visibly shake more when idling than I remember, I dropped off the spark plug lead to one cylinder at a time but I couldn't notice much of a difference between all 8 cylinders. It only dropped 1/2" of vacuum and didn't noticeably shake much more whenever I dropped a cylinder off.

One probably unrelated thing I noticed was when I raised the no load RPM up above the idle threshold it would go into closedloop and the AFR's would cycle between 12 and 17 and the engine would hunt up and down.