Shaky Idle

[Schurkey]

The bank one LT fuel trim seems a bit high. Ideally it should be less than 10%...but 2-3% is better...

...Your static fuel pressure does seem low, but that would lead to a lean condition, and you have slightly rich condition...

I'm not used to a scan tool that reports fuel trims in percentage. I'm used to fuel trims being reported as higher or lower than the mid-point of 128. A report of less than 128 is a lean-command, the computer is removing fuel from what it would normally be programmed to squirt. A report of more than 128 is a rich-command, the computer is adding more fuel than it's regular programming would provide.

So when I see + and - percentages, I'm thinking that (+) 12.5 and (+) 7.8 percent is a rich command, because the mixture is too lean and the computer is trying to correct that leanness. That would agree completely with low fuel pressure.

 

[stutaeng]

I'm not used to a scan tool that reports fuel trims in percentage. I'm used to fuel trims being reported as higher or lower than the mid-point of 128. A report of less than 128 is a lean-command, the computer is removing fuel from what it would normally be programmed to squirt. A report of more than 128 is a rich-command, the computer is adding more fuel than it's regular programming would provide.

So when I see + and - percentages, I'm thinking that (+) 12.5 and (+) 7.8 percent is a rich command, because the mixture is too lean and the computer is trying to correct that leanness. That would agree completely with low fuel pressure.

Oops! My mistake...those are LEAN numbers.

Rich conditions would be shown as negative values...ECU is cutting back fuel. Not the case here. Sorry.

The RPM increase monitoring fuel trims still is useful to see. The system would run much leaner with increase in rpm if a fuel delivery issue, probably would also cause driveability issues? On the other hand, a vacuum leak would cause a change in fuel trims vs RPM.

Also, fuel delivery usually tends to cause fuel trims to be almost exact bank 1 vs bank 2.

 

[OB1214]

The bank one LT fuel trim seems a bit high. Ideally it should be less than 10%...but 2-3% is better.

Question: what happens to the fuel trims if you increase the RPM? Say at 1k, 2k, 3k RPM? Stay the same? Decrease?

Your static fuel pressure does seem low, but that would lead to a lean condition, and you have slightly rich condition...

Need more testing.

The EVAP purge solenoid could be stuck open, causing higher fuel trims. Find a way to temporarily plug it and see if there are any changes.

I do think there may be something wrong with my EVAP solenoid, as I can hear it clack away. This is the second new one I have put in, granted it a NAPA branded one. I will check later tonight what my fuel trim is at higher engine speeds while I am driving and report back to you. I will come up with something to delete the EVAP solenoid tempeoratirly to see what the fuel trims are.

 

[OB1214]

I'm not used to a scan tool that reports fuel trims in percentage. I'm used to fuel trims being reported as higher or lower than the mid-point of 128. A report of less than 128 is a lean-command, the computer is removing fuel from what it would normally be programmed to squirt. A report of more than 128 is a rich-command, the computer is adding more fuel than it's regular programming would provide.

So when I see + and - percentages, I'm thinking that (+) 12.5 and (+) 7.8 percent is a rich command, because the mixture is too lean and the computer is trying to correct that leanness. That would agree completely with low fuel pressure.

Yes the truck is running lean and the computer is telling the injectors to run,12.5% and 7.8% MORE fuel to get to 14.7:1 Air/Fuel.
I did my lower and upper intake gaskets with sealer and new Throttle body gasket. hooked up a vacuum gauge and it was showing 20inHg at an idle.

 

[Jimmy D]

I'm not experienced reading scan data but doesn't that -24* timing look suspicious?

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[stutaeng]

I don't think so...but you MAP sensor reading does look kinda weird. I think it should be around 29 in Hg., unless your elevation is really high?

 

[OB1214]

I don't think so...but you MAP sensor reading does look kinda weird. I think it should be around 29 in Hg., unless your elevation is really high?

my elevation is about 600 feet according to google. 29inHg is no vacuum, which would be key on the engine not running, or engine running at WOT, I think. correct me if I'm wrong

 

[stutaeng]

my elevation is about 600 feet according to google. 29inHg is no vacuum, which would be key on the engine not running, or engine running at WOT, I think. correct me if I'm wrong

Oh, yes. That's correct. I always confuse those...

 

[Schurkey]

I'm not experienced reading scan data but doesn't that -24* timing look suspicious?

Minus 24 degrees almost certainly means BTDC. In other words, 24 degrees advanced. At idle rpm, probably correct.

you MAP sensor reading does look kinda weird. I think it should be around 29 in Hg., unless your elevation is really high?

29inHg is no vacuum, which would be key on the engine not running, or engine running at WOT.

VERY common for folks to incorrectly think of the MAP sensor as a vacuum sensor, not a pressure sensor.

Us old coots, especially. Years of reading a vacuum gauge makes us think in terms of vacuum levels, not pressure levels. So MAP readings are backwards to us. 10 inches of MAP = 20 inches of vacuum, more-or-less. 29.x inches of atmospheric pressure, minus 10 inches of MAP, equals 19.x inches of vacuum.

 

[Billythekid]

I work at chevy as a lube tech and we just had a garage kept 98 454 come in that almost never ran and it ran really shaky with no light. I told my advisors and they ignored me and it rolled out. Came back a few days later with a misfire code. I dont remember what the fix was but i talked to the tech who i know pretty well and said it was fuel related if i remember correctly, maybe a clogged injector or something, but ill look into what he said tommorow to see if it happened to be the same thing seeing as both trucks mainly sat.