Engine Missing While Cold

[Schurkey]

ANYWHERE you have moderate-to-high humidity and ~40 degree F. weather, you need the heated air intake.

 

[95 Tahoe]

I logged the engine from a cold start and thing I found a culprit or system The ECM show's the timing advance starting bouncing all over the place as can be seen in blue. Not sure where to head with this.

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[Road Trip]

I logged the engine from a cold start and thing I found a culprit or system The ECM show's the timing advance starting bouncing all over the place as can be seen in blue. Not sure where to head with this.

You must be registered for see images attach

Since this occurred at ~2:50 after starting, possibly this is when the system went into 'closed loop' operation?
And if I understand this correctly, the Knock Sensor signal is now enabled/added into the ECU timing calculations?

According to the blue legend, you went from a steady (open loop) baseline of 20°
of spark advance (from approx the 1:50 mark) to suddenly bouncing from 16-22°? I wish I
could see a graph from a 'known-good' engine identical to yours, for it looks like to
me that possibly your Knock Sensor is too sensitive? Or is this expected behavior?
(I wouldn't think so.)

My last question is, does the problem you are troubleshooting appear at the same
time as this 'noisy' spark advance graph starts to occur? Or is it bad the entire time,
and we're just looking at something interesting yet non-relevant? (I'm trying not to get
distracted from the root cause IF we don't have correlation between the timing of the graph
anomaly vs your rough idle issue?)

 

[Road Trip]

I do not have wide band 02 sensor. It is just part of the file in Tuner Pro I'm using. It was a profile that was already made that works with my ECM.

I am very close to seal level. My manual said compression should be 150 but I know that can change with elevation.

Today I ignored the proper procedure of setting timing. I set the timing while watching the vacuum gauge and rolled the distributor until it gave me the highest reading which was 18 -19 inhg. It seemed to run a lot better. Even tonight when I went out and cranked it after it cooled off it didn't stumble as bad. Tomorrow, I am going to verify TDC with a piston stop to be sure. I did not go back and verify with a timing light to see where it ended up. Maybe the mark on the harmonic balancer or timing cover is incorrect. Both were replaced during the rebuild.

Second thing I am chasing is the IAC. It will not reset to closed by jumping A&B on the OBD1 connector. However, it does work to adjust idle speed while the engine is running. I am suspecting the ECM could be failing.

Last thing I want to verify is if the fuel return line or the fuel pressure regulator is a problem. I rebuilt the regulator and throttle body when I rebuilt the engine.

The heated air intake is not connected. I live in Louisiana not sure if it's needed or not.

OK, I just reread the thread, and in this reply you set the timing for max manifold vacuum.

This is a valid troubleshooting step, but I'm curious as to how much spark advance you ended
up with? If it's a bunch, possibly the Knock Sensor circuit is relaying valid info to the ECU, and
it in turn is pulling a few degrees back out (yet hunting/oscillating) in order to keep the combustion
chamber peace despite the advanced base timing?

If you could, please document the timing that you ended up with. (Let's say it's +8 degrees from factory.)

Then, if you could temporarily reset the spark timing to the factory setting, and rerun the graph
that you have above? (ie: Cold start + 4-5 minutes?)

IF on the 2nd run the spark timing graph becomes much less 'noisy' then we have our answer. But if
the graph still has the same look, then we need to investigate the Knock Sensors for being
too sensitive/out of spec? (And if it was me, if the graph was still this noisy on stock timing I'd be tempted
to put the timing back to where the engine runs the best and continue to troubleshoot from there.)

The next step depends upon being able to figure out what the spark advance graph is trying to tell us.

 

[95 Tahoe]

OK, I just reread the thread, and in this reply you set the timing for max manifold vacuum.

This is a valid troubleshooting step, but I'm curious as to how much spark advance you ended
up with? If it's a bunch, possibly the Knock Sensor circuit is relaying valid info to the ECU, and
it in turn is pulling a few degrees back out (yet hunting/oscillating) in order to keep the combustion
chamber peace despite the advanced base timing?

If you could, please document the timing that you ended up with. (Let's say it's +8 degrees from factory.)

Then, if you could temporarily reset the spark timing to the factory setting, and rerun the graph
that you have above? (ie: Cold start + 4-5 minutes?)

IF on the 2nd run the spark timing graph becomes much less 'noisy' then we have our answer. But if
the graph still has the same look, then we need to investigate the Knock Sensors for being
too sensitive/out of spec? (And if it was me, if the graph was still this noisy on stock timing I'd be tempted
to put the timing back to where the engine runs the best and continue to troubleshoot from there.)

The next step depends upon being able to figure out what the spark advance graph is trying to tell us.

It starts out with a high idle sounding normal. When it starts to idle down it begins bogging and shaking. Which usually happens within the first minute. The engine was still in open loop when the timing became erratic. When I was troubleshooting adjusting timing by vacuum I ended up at 4 degrees advanced. During this log I was at 0 degrees BTDC. I am trying to figure out if the erratic spark is a cause or a symptom of something else. When I pull the spark plugs when the engine is acting like this they are all soaked in gas from a very rich condition. Once the engine is up to temp the 02 sensor leans the fuel trim out. It goes down near 100 BLM an 100 INT. I am starting to think the fuel pressure regulator may be allowing too much fuel. I rebuilt it with an AC Delco kit. I'm debating on replacing it with a new one or making it adjustable.

I show some knock counts at start up but do not show any retards.

 

[Road Trip]

It starts out with a high idle sounding normal. When it starts to idle down it begins bogging and shaking. Which usually happens within the first minute. The engine was still in open loop when the timing became erratic. When I was troubleshooting adjusting timing by vacuum I ended up at 4 degrees advanced. During this log I was at 0 degrees BTDC. I am trying to figure out if the erratic spark is a cause or a symptom of something else. When I pull the spark plugs when the engine is acting like this they are all soaked in gas from a very rich condition. Once the engine is up to temp the 02 sensor leans the fuel trim out. It goes down near 100 BLM an 100 INT. I am starting to think the fuel pressure regulator may be allowing too much fuel. I rebuilt it with an AC Delco kit. I'm debating on replacing it with a new one or making it adjustable.

I show some knock counts at start up but do not show any retards.

Thanks for the detailed update. Since you are watching the real time data I'm
thinking that having the ability to adjust the fuel pressure would give you
an additional degree of fine-tuning freedom? (The thought process is that
you could use the adjustable fuel pressure to help close the gap between the
stock cam the ECU is expecting vs. the RV/towing cam you upgraded to?)

I think I'm on the right track. I think it's the new map sensor is reading wrong or a possible wiring issue. While it's running rough and flooding I plugged a mightyvac into the map sensor and dropped it to 20inhg the idle immediately stabilized. The manifold pressure climbed from 16 to 18 inhg. I looked back at some of my old logs. The reading was 55 KPA which converts to about 13.5 inhg.

If you ended up only 4° advanced at peak vacuum tells me that the cam is
pretty close for what the ECU is expecting? (It's not like you have to crank in 12+
degrees at idle in order to compensate for a performance cam with a lot of overlap.)
This makes sense given the RV/towing cam you chose.)

You know, since we have a nonstock cam in place, what if you were to put the timing
back to where it was for peak manifold vacuum, and then repeat the use of the Mityvac on the map sensor,
I wonder how well it would then start up and go through the warm up process?

IF this clears up the too-rich open-loop fuel delivery, does this mean that the MAP sensor
is out of tolerance? Or does it mean that the MAP sensor is OK and you need a custom tune
like @Komet mentioned in reply #4?

Or, in the interest of science do you upgrade to the adjustable fuel pressure regulator,
keep the timing at the +4° setting, and see if you can fine tune this combo to really run sharp?
(Thinking of the closed loop trims that you mentioned.)

****

And of course it would be nice to have an explanation for the noisy-looking KS signal? It was
working textbook until suddently around the 2:40 mark? But at the same time, if your roughness was present
when the KS signal was smooth & sane, then obviously these are 2 different issues, and this can
be investigated down the road...

For what it's worth, we have a couple of TBI devos in the forum, here's hoping someone like
@PlayingWithTBI can find this thread and possibly share some insight with what you are
trying to accomplish.

One last thought. The adjustable fuel pressure regulator could be used to fine tune either the existing stock
tune or the aftermarket tune. So you wouldn't be throwing any money away by getting the adjustable FPR
and then ending up with a custom tune that better matches your build.j

Keep us in the loop. Feels like there's a lot of good relevant info to be shared here.

Cheers --

 

[95 Tahoe]

I dropped in a junkyard ECM and the erratic timing stopped. However, I went to do the IAC reset procedure again and it would not respond to jumping A&B. The truck did idle a little rough until the IAC picked up the RPM to 900 until the truck was warm.

 

[Road Trip]

Glad to hear that the replacement ECM cured the erratic timing. Getting
the spark timing variable sorted out will make troubleshooting the rest
of the system that much easier. Assuming you don't get a code down
the road for an inop KS subsystem, this is progress!

At this point I have to step aside & let the TBI devos jump in and help
with that IAC reset procedure.

Best of luck --