Mysterious lean run engine destruction

Mrlees · September 24, 2016

Hi all. Purchased a 98 Sierra 1500 with the 5.7 with 100k miles to tow my race car last year please bear with me this is a long intensive troubleshooting process with little luck.

Soon after buying notice major power drop at WOT when I exceeded 3500 rpm.

I also noticed coolant consumption, took it to a friends shop and sure enough, the intake manifold gaskets were cracked and leaking coolant into the intakes.

Repaired that, but the low power at high rpm problem continued. Started logging data during driving and noticed severe lean issue (~.030 v on both O2 banks) when loaded at high rpm. MAF, MAP numbers were good,assumed low fuel pressure, went ahead and replace pump because on testing it was about 55 psi, not super low but out of spec so I did it.

No joy, replaced MAF and MAP and CATs (one after the other as each failed to resolve the issue) no solution yet.

Took it to a shop and had tech observe data during lean conditions while I drove pulling a trailer (loading it helps bring on the symptoms) held it lean to long, broke an exhaust valve Arghhhhh! Otherwise during driving conditions live data showed no faults or unusual data. Btw, cats had gotten white hot during last lean run, confirming lean readings were accurate

Buy new long block, installed with all top end components of old engine, of course the problem follows the electronics. WTF is next? I'm tired of throwing money at it and I'm out of money.

Summary:

Lean over 3500 RPM specially on WOT

New longblock

New fuel pump

New fuel filter

New MAF, Map, coil, O2 sensors, rotor, wires,

Confirmed 65psi fuel at WOT when lean

WOT MAF ~120

WOT MAP 80 not sure what units the system is logging

Also, runs great cruising, good O2 numbers just high RPM is when it happens. Love the truck but can't use it if I can't tow with it.

And no fault codes with the new engine (I could hold it lean long enough to get low O2 voltage codes on all sensors, but found out the hard way that is a horrible idea)

I really need help here, can't figure it out and 2 separate shops say everything looks good, no idea.

Any ideas? Thanks!

Lee, newly broke guy

barry G · September 25, 2016

there is too many things to make it judge lean.. I'd get another ecm.

left bank, right bank, final trim, there is even a cat code checker with o2.

throttle position, map, maf, coolant sensors..

on and on.

for all those things to say "go lean"..that is ridiculous.

maybe a limiter is failing or something, again I look at ecm.

another thing is spider injection or the hot tamale california version?

I would not trust the cali one.

EGR was a weirdo from 88-95..but 98. those usually do good.

Mrlees · September 26, 2016

Thanks Barry, (nice truck, it's a mirror image of mine)

I ordered a reflashed used ecm right after I made the original post, it's the only thing left aside from the injector.

That's the kicker, all the electronics should be saying richen up. MAF indicates high airflow, map says low vacuum, tps says WOT, O2 are screaming lean. Should be richening up.

I hope we are right about the ECM, only thing left is the injector, but generally those either work or have lots of misfires even at idle (or so I hear). Not the Cali version btw.

Thanks a lot for the response, I considered a stuck egr too, can't monitor if with my scanner, I'd guess it'd always be lean though regardless, maybe more so at light throttle. Maybe I should disconnect it and see if the problem persists.

Mrlees · October 1, 2016

Well, installed a new ecm, no change. No idea where to go from here. Here's some freeze frame data. The killer for me is that as lean as the data shows fuel trim is barely trying to richen up, I just don't get it.

Stft bank 1 0

Ltft bank 1 6.25

Stft bank 2 0

Ltft bank 2. 7.03125

MAP in hg. 22.14758

Rpm 3863

MAF lb hr 15.95273

Throttle position 94.5098

O2 Bank 1 volts 0.065

O2 Bank 2 volts 0.04

Calculated Load 65.4902

Coolant temp F 199.4

Ignition timing advance 20.5

Intake air temp 122

Voltage 13.5

Tried running with egr disconnected, no change. If the spider injector was bad wouldn't the fuel trim still be trying to richen up? Like greater than 10%. Also does the 98 have the old style poppet injector or 8 seperate ones.

Thanks again for any help