Tunerpro log - rough cold start 1995 7.4L

[termite]

I could use some assistance with deciphering the attached from my 1995 K2500 Suburban with a 7.4L. It has had an intermittent rough cold start and I attempted to catch a datalog of it today but admittedly have little knowledge of what I am looking at. Can any of you offer some guidance on making sense of the log file.

Delete the .txt from the file name to get the correct .xdl extension.

Edit: Acts like a few dead cylinders on cold starts but inconsistent on doing it. One day, starts smoothly, next day, rough again.

 

[PlayingWithTBI]

Can any of you offer some guidance on making sense of the log file.

What service number do you have? Did you capture the "intermittent cold start" in this log? What does it do or not do?

 

[termite]

What service number do you have? Did you capture the "intermittent cold start" in this log? What does it do or not do?

'Doh, that should have read "intermittent rough cold start". When it acts up, its a solid miss with no abnormal lifter noise. Sounds like a few dead holes though. Clears up after a minute or two of idling.

That datalog had the rough start and about 20 minutes of idle before I left the house. I'll have to figure out the service number again. I suppose the definition file for tunerpro would work if I can't get the service number easily?

 

[termite]

service number

See photo.

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[PlayingWithTBI]

I'll have to figure out the service number again. I suppose the definition file for tunerpro would work if I can't get the service number easily?

Yeah, I thought it was probably a 7427 but wanted to confirm. I looked at it with $OE and $OD. I could see where it's idling rough but, I didn't see anything obvious other than your O2 sensor is reading low but, that doesn't do anything on startup until it goes into Closed loop. It may be an indication of low fuel pressure. I'll try to look at it again today. Anyone else is welcome to add their $0.02 if they like.

When was the last time you checked fuel pressure and did a regular tuneup like, plugs, cap, rotor, wires (maybe), etc?

 

[termite]

When was the last time you checked fuel pressure and did a regular tuneup like, plugs, cap, rotor, wires (maybe), etc?

Tuneup wasn't many miles ago for plugs, new dizzy (cap, rotor included) wires new when I did intake routed per FSM.

Haven't checked fuel pressure. Need to get an adapter for the tbi connection.

Its got a one wire O2 sensor that only goes closed loop under throttle. Idle is always open.

New acdelco egr valve showed up yesterday that will maybe remedy the intermittent code for it.

 

[PlayingWithTBI]

Haven't checked fuel pressure. Need to get an adapter for the tbi connection.

Keep in mind, if you don't already know, '94 and '95 7.4L TBIs run ~28 PSI (26-32) fuel pressure.

 

[PlayingWithTBI]

Here's a log from 0:00 minutes/seconds through 3:20.

OK, this is counter intuitive, the O2 sensor (teal line) is reading lean when cold (red line) and getting richer as it warms up. The Timing (dark blue) and MAP (purple) are jumping all over the place. Which is unknown but maybe causing the rough idle (light blue).

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Even at 186.6°, timing is still jumping some.

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Here's the dashboard showing sensors and feedback. They seem to be working OK.

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I think we need to figure out why it's running so lean when cold. Whether it's fuel pressure and/or a bad sensor? Then look at timing jumping around causing the rough idle.

 

[Road Trip]

Then look at timing jumping around causing the rough idle.

Really interesting stuff! Q: Is the 291 Knock Counts during a warm-up session like this
something that is expected/always seen, or is it a hint that the KS is failing via the
hypersensitive mode?

What if we were to temporarily disconnect the Knock Sensor and attempt another
cold start in as similar conditions as possible? Assuming the Knock Count goes to zero,
and also the timing stabilizes, then I think this would be a valuable troubleshooting clue?
(And we could get this info for zero additional cost?)

So let's say that during this proposed test the Knock Count stays at zero, the timing
stabilizes, and even the cold idle smooths out? We then substitute a 'known-good'
Knock Sensor, clear the CEL, but during the next cold start the Knock Count returns,
along with jumpy timing? Is it time to start searching for excessive engine noise during
the initial firing? In English, a little bit of piston slap when stone cold, which disappears
after a short period of time? Or there's something in the serpentine belt drive area that's
coupling a thump, bearing rumble, or other faux 'knock' sound that's faking out the ECU?

****

Apologies in advance if this issue eventually gets traced back to something non-exotic like
a marginal ground path. But when I see raw data like this it's hard to resist trying to make
sense of it all. :0)

Good stuff -- can't wait to see how this unfolds. (!)

 

[termite]

I think we need to figure out why it's running so lean when cold. Whether it's fuel pressure and/or a bad sensor?

I'm adding 3-wire 02 sensor to my short list. Pretty sure the existing is old enough to drink from its appearance.

@Road Trip Im going to verify my exhaust (OE still) isn't knocking on anything as well. At one point it was, but had been corrected. Will have to double check it.