L31 Vortec questions

[travis]

Last attempt at fixing this !@&)(! thing before I yank the motor...

1999 Suburban 3/4 ton, L31 350 Vortec/4L80 trans. I’ve been fighting through a multitude of issues and thrown way too much time and money at this thing. Still having an issue where it stutters and stumbles at anything except WOT. Idles smooth as silk all day long, good oil pressure, never overheats, throws no codes, etc. Maintains 58 psi fuel pressure at all times, but is still borderline undrivable.

I have replaced so much stuff...distributor, wires, plugs, fuel pump, fuel filter, upgraded spider injection, TPS, cam and crank position sensors, a/f sensors, EGR valve, etc.

The thing acts like it has way too much timing in it. It cranks much slower than it should, even after replacing the starter and cables. And the bucking and stumbling when trying to drive it normally acts just like any carbed engine with the timing way too far advanced. But again...no trouble codes. I can twist the distributor a bit and get it outside it’s operating window, and it throws a corresponding code (I don’t remember exactly what that code was...it’s been a while).

The history of this engine is unknown. The truck has something like 280k miles on it, but someone has been in it before I got it. It cranks 180psi on all cylinders except 1 that is at 165.

What does the crank position sensor get it’s signal from? The sensor bolts to the bottom side of the timing cover. Is there some kind of reluctor wheel on the front of the crank behind the timing cover? If the timing chain was heavily worn and sloppy, could that cause the computer to crank the timing way up to compensate, even though the scan tool shows everything is within range?

[turbo camino]

Does the scan tool live data agree with what it "feels like", or are you just guessing?

[rebelrouser]

Not really enough info, but. I will assume it has no codes active or in memory. Remember if you clear the codes they will not reset until you have driven it until all the monitors are set. Some scan tools will let you see the monitors and if they have been run. First thing I do is look at the short and long term fuel trims, if they are more than 10% rich or lean I look for fuel system issues and vacuum leaks. Next trick you may try is to do a cold soak, let sit over night, hook up the scan tool and do not start, just scroll and note all the temperature sensors, they should all read the same, if one is more than 5 degrees off change it out. Then always check the fuel pressure under a load driving and compare to specs, weak fuel pumps cause all kinds of issues, and a weak pump does not set any codes, maybe a lean O2 code sometimes. Another thing is to check the mass air flow sensor, take it out and do a WOT 1-2 upshift, record the grams per second at the top of the gear at max RPM. use this website https://atgtraining.com/atg-volumetric- ... alculator/ it will let you calculate about what the reading should be, if it is off more than 10% you need a new mass airflow sensor, or you have a clogged exhaust system some place. A simple vacuum test will verify the clogged exhaust, a normal engine should read 18 to 22 inches at idle, when you crack the throttle the vacuum should drop and then slowly rise back to 18 to 22 inches as you hold it at a steady rpm, a clogged exhaust will not it will stay low. Also when you hook up a vacuum gauge needle should be steady, if it bounces you got valvetrain trouble. Just a few ideas for you.

[travis]

Does the scan tool live data agree with what it "feels like", or are you just guessing?

Everything on the scan tool live data looks perfectly normal. So yes, I’m grasping at straws at this point

[travis]

Not really enough info, but. I will assume it has no codes active or in memory. Remember if you clear the codes they will not reset until you have driven it until all the monitors are set. Some scan tools will let you see the monitors and if they have been run. First thing I do is look at the short and long term fuel trims, if they are more than 10% rich or lean I look for fuel system issues and vacuum leaks. Next trick you may try is to do a cold soak, let sit over night, hook up the scan tool and do not start, just scroll and note all the temperature sensors, they should all read the same, if one is more than 5 degrees off change it out. Then always check the fuel pressure under a load driving and compare to specs, weak fuel pumps cause all kinds of issues, and a weak pump does not set any codes, maybe a lean O2 code sometimes. Another thing is to check the mass air flow sensor, take it out and do a WOT 1-2 upshift, record the grams per second at the top of the gear at max RPM. use this website https://atgtraining.com/atg-volumetric- ... alculator/ it will let you calculate about what the reading should be, if it is off more than 10% you need a new mass airflow sensor, or you have a clogged exhaust system some place. A simple vacuum test will verify the clogged exhaust, a normal engine should read 18 to 22 inches at idle, when you crack the throttle the vacuum should drop and then slowly rise back to 18 to 22 inches as you hold it at a steady rpm, a clogged exhaust will not it will stay low. Also when you hook up a vacuum gauge needle should be steady, if it bounces you got valvetrain trouble. Just a few ideas for you.

The fuel trims are the only thing that looks suspicious. Exhaust back pressure is about 1/2”@5000 rpms before or after the cats. Fuel pressure is a rock solid 58 psi under any load after replacing the fuel pump some time back.

I’ll pull the scan tool data again maybe tomorrow

[prairiehotrodder]

when did it start acting up ? Was it right after someone worked on it ? Put a wrench on the crank bolt and turn it back and forth to see how much slop is in the timing chain. i kinda doubt that is the problem however but i've been wrong before. A guy recently brought me a motor and that wouldn't rev up. I played with everything trying to figure it out. It also had real high cranking pressure like yours. Turns out someone had done the timing chain and they lined up the dot on the cam gear with the keyway on the crank gear.

EDIT: someone has been it it before i got it. I see that now.

Brian

[rfoll]

Those engines have problems with the distributor caps. My neighbor changes his twice a year.

[dannobee]

The crank sensor gets its signal from a 4 window reluctor ring sandwiched between the balancer and the crank gear, keyed to the crankshaft. The code you likely got when fiddling with the distributor was probably P1345, cam/crank correlation problem. Turning the distributor only changes the cam sensor retard offset.

If you have access to a Tech2 or better, see what the cam offset shows. And command the EGR full open and see if it stalls the engine. We had lots of problems with EGR passages plugging up in the intake.

[rebelyell]

as said, simple crank position sensor reluctor sandwiched between damper snout & crank gear.
cam position sensor is in distributor; plug's on side of dist.
yes, L31, L30 & V6 L35 dist caps can be troublesome. I chased and chased and used a $5K snapon analyzer to no avail. Cap looked good BUT it wasn't. Changed cap. Immediately stumbling & hard-starting over & done with.

[travis]

Any particular brand or PN of distributor cap y’all would recommend? I’ve replaced it multiple times over the years and a couple times recently just troubleshooting this issue.

This thing ran great for several years after I got it, even used it as my daily driver for quite a while. One day driving home from work it started popping and backfiring badly...it hasn’t been right since. A bunch of things have made it better, but it isn’t 100% yet.

One thing I have noticed is that the exhaust note is a lot softer than it used to be. The truck is bone stock other than a custom spintech muffler I had made for it...it had a “muffler delete” pipe on it when I got it. The exhaust note at idle with the spintech muffler was borderline loud...now it is noticeably quieter. The other weird thing it does is that the rpms flair up for a second or 2 when you put it in gear even without touching the gas. That started too after the popping out the exhaust issue happened. The popping issue was resolved when I replaced both the cam and crank position sensors. The crank sensor had some pretty ugly grooves worn into it

[turbo camino]

The cap issues those engines are notorious for also stress out all the other secondary ignition parts as well. Many times I have seen them arc through the side of the coil.

[Dirtybob]

I've had our 2000 GMC Savana 3500 5.7L act like you describe a few times. Distributor cap is the most likely culprit (usually the truck will be hard to start if this is the case). Another time the MAF was bad (no codes and I couldn't 'see' it in the live data), if the truck runs better with it unplugged you will know. More recently the gear on the distributor was worn paper thin...
HTH

[dannobee]

The quiet exhaust is sometimes the tell of a plugged cat, but you checked backpressure and it's ok? From my experience, driving around with a bad distributor cap usually kills one of the converters and gives you the the P0420 or P0430 code. And how are you checking backpressure? I used to drill a 1/16" hole in the pipes before the cats and had an old mechanical pressure/vacuum gauge that I adapted with an old straight spark plug boot. Have someone topside work the throttle while you check pressure at the drilled hole. But if the cat's bad, cracking the throttle would usually reveal the nasty hissing sound out of the drilled hole without even having to check the pressure.

[rebelyell]

JMO best cap or rotor is gen GM / AC Delco.
There should be an OE small air vent in base of OE distributor. You make Certain vent hole's clean & clear.
Was that a "muffler delete" or a cat "test tube" pipe ?
Do you have a working cat or not ?
Do you still have both O2 sensors; one upstream of cat (or test tube) and one downstream ?

[travis]

Exhaust back pressure test was done with a proper back pressure test setup. Pressure gauge connected to a long hose that screws into the O2 sensor bungs. It seems to be a very free flowing system for this power level.

When we bought this thing several years ago, the stock muffler (2 inlets, single outlet) had been replaced with a dual in/single out y-pipe. It was loud and crackly...always sounded like you had your foot buried in it even when driving normally. The stock replacement muffler and the only aftermarket muffler (flow master) with the right configuration and pipe sizes haven’t been available for years, so I had spintech custom make me one. I’m happy with it...it sounds good and no interior resonance and not too loud. The factory cats are still in place...whether they function correctly or not I don’t know, but they aren’t clogged. The stock exhaust on this thing is short 2” head pipes, ballooned up to 2 3/4” duals back through 3” in/out cats, then to the dual 3” in/single 3” out spintech muffler, and a single 3” tailpipe. It’s all stainless factory big block stuff mated to a 350, done this way from Chevrolet only on the 3/4 ton suburbans.