Good Idle untill... you touch the throttle?!?

[Hame]

Hi all! I have a client/work collogue with an LS swapped 62 Chev. I tidied up the VE table on an SD tune from an off the shelf package he'd been given, ran great, no issues. He drove it for about 100 miles and then during the trip al of a sudden it was misfiring and wasn't running so good.

Here's a log of me starting the car up. Perfect smooth idle 650-700ish, after you give it a rev on decel it starts to misfire and is way too rich. Half way through the log you'll see I manually adjust the IAC opening to bring it back but the idle desired airflow and dynamic airflow refuse to line up, even though they were beforehand. I've tried another MAP sensor and IAC valve with no change. I've unplugged the O2's as they were trimming and making it worse.

I'd just appreciate any input as I don't really want to make any changes to the tune seeing that it was running fine for a long time, I just can't figure out what has failed that could cause it run fine at start then misfire after throttle input!

[gametech]

The tune did not cause a misfire to appear after 100 miles. Start with the basics, like looking for a burned plug wire, and check the plugs. Your log with no o2 sensors is sort of useless, because we can't even see which side of the engine is misfiring. Also, the VE table could still use quite a bit more work. That is obvious just from looking at it, but has nothing to do with the current issue.

[blindsquirrel]

Fuel pressure...?

[Alvin]

Be mindful of a sticking TPS. Yours is sticking slightly. idle learning and trimming is paused on GEN3 anytime the vehicle moves or the throttle is opened.

Be sure your IAC is actually working. Drive it around in the scanner VCM controls and look for a response.

There is nothing to check your fueling in the log. Most idle issues are actually fueling issues.

[Hame]

Thanks for the input guys.
Sorry, I had disconnected my wideband when I did this log It was in the .8 lambda range when misfiring, but strangely wouldn't recover even when revving, it would continue to run rich and misfire, which like you say it can't be the tune, I'll confirm fuel pressure and do some more checks. Thanks!

[edcmat-l1]

The tune did not cause a misfire to appear after 100 miles. Start with the basics, like looking for a burned plug wire, and check the plugs. Your log with no o2 sensors is sort of useless, because we can't even see which side of the engine is misfiring. Also, the VE table could still use quite a bit more work. That is obvious just from looking at it, but has nothing to do with the current issue.

Driving it around on a goobered up toon, in open loop, with the O2s disconnected, can damn sure make it start missing after 100 miles. So, in a sense, the toon could cause the misfire to appear after 100 miles. Once it's coked up the plugs enough.

[gametech]

Driving it around on a goobered up toon, in open loop, with the O2s disconnected, can damn sure make it start missing after 100 miles. So, in a sense, the toon could cause the misfire to appear after 100 miles. Once it's coked up the plugs enough.

When he said he disconnected the o2s after the problem started, my mind equated that to having been in closed loop with o2 feedback before the problem, since he said the o2s were trimming it worse. Of course the exact history on these problems is not always accurately stated, so who knows?

[Hame]

When he said he disconnected the o2s after the problem started, my mind equated that to having been in closed loop with o2 feedback before the problem, since he said the o2s were trimming it worse. Of course the exact history on these problems is not always accurately stated, so who knows?

Yea the O2s were trimming -20% and then some because of the misfire so I wanted to remove that factor when isolating the cause. I tuned it open loop with my wideband feed into VCM suite. Then when I reconnected the narrow band sensors afterward trims were only one or two percent..

After the issue arose, I had dual widebands in and it was pretty even across both banks .8-.75 lambda. I'll spend some more time with it soon and get some better logs if I cant find anything. Thanks guys

[edcmat-l1]

Yea the O2s were trimming -20% and then some because of the misfire

That's not how it works. Misfires show up as lean spikes (bursts of raw O2) which will actually drive the trims rich. Fuel trims don't go negative 20% because of a misfire. The only thing that causes -20% fuel trims is it's off by that much at that point in the tune.

[Hame]

That's not how it works. Misfires show up as lean spikes (bursts of raw O2) which will actually drive the trims rich. Fuel trims don't go negative 20% because of a misfire. The only thing that causes -20% fuel trims is it's off by that much at that point in the tune.

Apologies, you're right, it was positive trim not negative. Like I say, I'll get better logs next time sorry!

[edcmat-l1]

Need to put a new set of plugs in it, hook the O2s back up, put it back in closed loop, then data log it.

[gametech]

Just seeing the spark plugs should tell a story. 99% of the time I hear that something ran good for 100 miles, then started missing, it ends up being 1 or more burned plug wires.

[Hame]

Sorry for the delay, thought I'd better follow up on this. Turn's out the 'new' motor was a dud. Had a broken valve spring and oil pressure issues...

[bk2life]

Turn's out the 'new' motor was a dud. Had a broken valve spring and oil pressure issues...

everyone is quick to blame the tune....