LS1 sudden onset won't run right

[someotherguy]

OK y'all, this is just a cut/paste of my post over on LS1truck but I figured this forum might get more traffic. Need to get this one figured out:

LS newb here so please be gentle

This is (likely) a '98 LS1 swapped into a '97 Suburban. I bought it already swapped, needs lots of wiring work, but ran and drove until now. I've put probably a hundred miles on it. Previously it ran OK; had a surge in the idle once it warmed up, but otherwise no obvious issues. Plenty of power including crazy passing speeds on highway.

Drove it a few days ago and all was normal as I described. Came home, it sat a few days, went out today to start it and it immediately backfired through the throttle body and stalled. Tried to start again and it's like it wasn't getting fuel. Threw gauge on it and key on/engine off is 60 PSI, watched while cranking and I can get it to run a moment while pressure is steady 60 PSI, stalls and drops off to 50 and verrrrry slowly starts dropping off afterwards.

Edited to add more info: trying to start (with everything connected as normal) gets an intial very high RPM then settles down to an extremely low erratically stumbling idle, with random popping in the intake, and may stall. If I feather the pedal I can get it to come up some but runs rough and it's got random popping in the intake. Engine not making any vacuum - brake pedal hard as a rock. I checked all vacuum lines I could find, no leaks, also disconnected brake booster and plugged its source - no change in how it runs.

If I unplug the MAF it will start but has an extreme but steady lope to the idle, down to almost a stall, then raises up high and down to almost stall again, over and over. If I feather the pedal it will come up and run reasonably smooth, no popping. Undriveable like this - it briefly lurches hard as if I was very quickly nailing the pedal. Doesn't even run well enough to reposition it in the driveway.

Ideas? Nothing done to the truck between running fine and today, it just sat in the driveway.

Thanks!
Richard

 

[polar]

Have you changed the fuel filter? I know it may sound dumb but if something is blocking the flow it will have a hard time idling

 

[someotherguy]

From running strong to this, and showing good pressure 60 PSI.. I kinda have my doubts that's it. I appreciate the idea and I'll probably end up changing it since I have no idea when it was done last, if that's all it is I'll kick myself in the ass, but I do really doubt it's gonna be that simple.

Richard

 

[polar]

Yea I always start with the easy stuff. I wonder if aj injection got something stuck in it?

 

[thz71]

Well i was going to say vaccum leak before I saw the edit. If there's a vaccum leak somwhere that should set a code for the MAF and make it run rich intake gasket?

 

[brycebba]

If it was already having issues is it possible that the last drive maybe fouled plugs or maybe a combination of things. Could backfiring through the throttle body be indicative of incorrect timing?

 

[someotherguy]

thz71 - I'm gonna have to step up and get an OBD2 scanner; haven't really needed one before. CEL isn't wired up or the bulb is out on purpose, either one, not that it does me much good because I can pretty much assume it has codes set even before this problem.

brycebba - prior to this it ran well with lots of power, just had a surging idle once it warmed up (assuming closed loop.) That's an issue it's had since I bought it a couple weeks ago. Anyway was running the same as it usually did, parked it and it sat a few days, then this new problem with it backfiring through the TB the first time I tried to start it, and now won't run right at all. It's an LS so I don't know how you would affect the timing other than custom tuning.

Richard

 

[hybrid1]

I would look into the crank position sensor. That is an exact list of symptoms I had on my old 5.0 vortec. Ran great, sat for a few days and would not start, just back fire. Ended up being the crank position sensor


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[hybrid1]

Also is this the red ls1 swapped burb that was on dfw craigslist not long ago?


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[someotherguy]

I'll check it out. I suspected one of those sensors, crank or cam, but don't know enough about LS's yet so I gotta ask.

And yeah, probably was the same truck; hard to imagine there would be more than one in the same general region. Truck was actually located in Houston though, across town from me.

Richard