High mileage 5.7 Vortec, Heavy sputter under load - no codes!!

[SinnSix]

Hi fam, long time Chevy owner, first time posting.

I have a new to me 1996 5.7 Vortec 2 door 4x4 Tahoe. 300k on the ODO, swapped in motor has 180k. 40k on trans.
Newer fuel pump, new fuel filter.

Cap, wires and plugs unknown (they look heavily worn.)

I’m in California and subject to smog. Which once this issue is resolved I will be heading straight to a smog station.

I have read through all the posts I could find about sputtering and misses, but none seem to act the way mine does, with the vortec injection.


Truck starts with one or two extra uggah-duggas and runs wonderfully. Treated it with a few cans of seafoam in the tank and 1/2 in the intake.

Truck runs 80mph on the freeway with no issues if I ease into it.

The issue is If I get more than 1/2 throttle under load at any speed, she bucks and sputters, it gets worse the harder I accelerate. Seems to have a miss but I can’t tell if it’s fuel (injection spider, fuel pump) or spark causing it. I am leaning towards fuel. One of these next weekends I’m going to borrow a fuel pressure gauge from auto parts place to make sure I have 55-60 psi fuel pressure.

I’m on a tight budget so I kind of have to pick something to replace it will have to be the injection spider or the spark kit. The kicker about this whole thing is that it hasn’t thrown a single code. The longer I run it after the seafoam treatment, the better it seems to run, but still sputters under heavy throttle. Has anyone else had an issue like this?



I read somewhere on another forum a post about the Crankshaft position sensor possibly needing shims due to crankwalk under load on high mileage motors. Please let me know if anyone has experience with this.


Any input would be greatly appreciated.



Thanks

Sinn

 

[kolgeirr]

I think you've got the right ideas on tracking it down. Injector spider, plugs/wires/cap/rotor/coil, distributor, CPS and vacuum leaks are the most common causes of a miss on a 5.7 in my experience.

If cap, plugs and wires are heavily worn, you'll want to address that whether they're the problem or not, really.

Even when it's not setting a code you can use a scan tool to check the logged misfire numbers by cylinder. If it's all cylinders I'd say upstream fuel issue. If only one you can at least narrow it down a bit more to that cylinder's injector/plug/wire and distributor bits.

 

[Schurkey]

Newer fuel pump, new fuel filter.

A fuel-pressure test is recommended. Glad you're working on that.

Cap, wires and plugs unknown (they look heavily worn.)

Spark plugs heavily worn? Get new ones in there.

Take the cap and plug wires off, verify the plug wires with an ohmmeter for resistance, and visually inspect the insulation for cuts and burns. Inspect the cap (common problem on Vortecs, I hear.) I don't hear of many problems with rotors, but might as well replace the rotor if you're replacing the cap.

Test spark with a spark-tester calibrated for HEI. If it reliably fires the spark tester at the spark plug end of every plug wire, the ignition system should be OK, and you'd kinda expect that with new cap 'n' rotor, new or inspected plug wires. If it will not reliably fire the spark-tester, work backwards to the distributor cap terminals (failed plug wires) then to the coil wire (failed cap/rotor) then to the coil itself (failed coil wire.) If you don't have reliable spark at the ignition coil, you may have a failed coil, or you may have issues with the crank sensor, ECM, ignition module, or the wire harness connecting them.

Use this, or a similar spark tester having an actual spark gap suitable for HEI, NOT A STUPID LIGHT-BULB that glows when there's spark.
www.amazon.com/dp/B003WZXAWK/?coliid=I3S98D7T1J0RLJ&colid=2VLYZKC3HBBDO&psc=1&ref_=list_c_wl_lv_ov_lig_dp_it

xc_hide_links_from_guests_guests_error_hide_media

I’m in California and subject to smog. Which once this issue is resolved I will be heading straight to a smog station.

Once you have everything sorted-out, you'll probably need to drive around for awhile so the computer can get adjusted to running right again.

Truck starts with one or two extra uggah-duggas and runs wonderfully.

Does the fuel pump prime when the key is turned from "Off" to "Run"? If not, there's trouble in the fuel pump relay, the ECM, or the wire harness that connects them.

Truck runs 80mph on the freeway with no issues if I ease into it.

The issue is If I get more than 1/2 throttle under load at any speed, she bucks and sputters, it gets worse the harder I accelerate. Seems to have a miss but I can’t tell if it’s fuel (injection spider, fuel pump) or spark causing it. I am leaning towards fuel. One of these next weekends I’m going to borrow a fuel pressure gauge from auto parts place to make sure I have 55-60 psi fuel pressure.

Yes, it could be fuel, or it could be spark. You already know that the ignition system needs attention, and the ignition system parts are somewhat easier to replace than tracking down fuel pump wiring, or replacing the spider/regulator.

Yes, verify fuel pressure.

I’m on a tight budget so I kind of have to pick something to replace it will have to be the injection spider or the spark kit.

Don't "guess", diagnose to find the real issue. But as said--you already know the ignition system needs attention.

I read somewhere on another forum a post about the Crankshaft position sensor possibly needing shims due to crankwalk under load on high mileage motors.

Maybe. Use a scan tool, verify that the computer has an uninterupted RPM signal from that sensor when the engine is running.

Doesn't really seem likely that that sensor would only cause problems on acceleration, though.

Connect a scan tool, look for failing sensors, and individual cylinder misfire counts to localize the problem. O2 sensors are a "wear item", they get lazy with age. Their response time increases until they "work" but don't provide accurate data.

 

[SinnSix]

UPDATE: Thank you for the quick responses. I am a weekend warrior so most of my wrenching is done on the weekends.

Bought new plugs and installed them, I will work on testing the plug wires and inspect the cap and rotor when I have a bit more time this weekend. Plugs were worn, but nothing crazy. #8 had was in the worst shape. This fixed the miss, but there is still performance issues while under load, also the truck is getting progressively harder to start, which is also pointing me in the direction of fuel.

Trying to get a Fuel pressure tester this weekend to test the fuel pump. I'm pretty sure this is the issue, but until I can put hands on one I have done the following:

Tested the following with my DVM and they were all in spec.
AIT sensor
MAF sensor
TPS sensor

A fuel-pressure test is recommended. Glad you're working on that.


Spark plugs heavily worn? Get new ones in there.

Done! It took care of the miss, see attached photos, but it still can't perform under load.

Take the cap and plug wires off, verify the plug wires with an ohmmeter for resistance, and visually inspect the insulation for cuts and burns. Inspect the cap (common problem on Vortecs, I hear.) I don't hear of many problems with rotors, but might as well replace the rotor if you're replacing the cap.

Test spark with a spark-tester calibrated for HEI. If it reliably fires the spark tester at the spark plug end of every plug wire, the ignition system should be OK, and you'd kinda expect that with new cap 'n' rotor, new or inspected plug wires. If it will not reliably fire the spark-tester, work backwards to the distributor cap terminals (failed plug wires) then to the coil wire (failed cap/rotor) then to the coil itself (failed coil wire.) If you don't have reliable spark at the ignition coil, you may have a failed coil, or you may have issues with the crank sensor, ECM, ignition module, or the wire harness connecting them.

I will do this next.

Use this, or a similar spark tester having an actual spark gap suitable for HEI, NOT A STUPID LIGHT-BULB that glows when there's spark.
www.amazon.com/dp/B003WZXAWK/?coliid=I3S98D7T1J0RLJ&colid=2VLYZKC3HBBDO&psc=1&ref_=list_c_wl_lv_ov_lig_dp_it

xc_hide_links_from_guests_guests_error_hide_media

Once you have everything sorted-out, you'll probably need to drive around for awhile so the computer can get adjusted to running right again.


Does the fuel pump prime when the key is turned from "Off" to "Run"? If not, there's trouble in the fuel pump relay, the ECM, or the wire harness that connects them.

Yes, sounds the same every time. If it hasn't been ran in several hours/days, it primes a scosch longer, but under repeated cycles it sounds even and consistent. (if I'm honest tho, it doesn't sound like the last 3 obs's I've owned

Yes, it could be fuel, or it could be spark. You already know that the ignition system needs attention, and the ignition system parts are somewhat easier to replace than tracking down fuel pump wiring, or replacing the spider/regulator.

Yes, verify fuel pressure.


Don't "guess", diagnose to find the real issue. But as said--you already know the ignition system needs attention.


Maybe. Use a scan tool, verify that the computer has an uninterupted RPM signal from that sensor when the engine is running.

Doesn't really seem likely that that sensor would only cause problems on acceleration, though.

Connect a scan tool, look for failing sensors, and individual cylinder misfire counts to localize the problem. O2 sensors are a "wear item", they get lazy with age. Their response time increases until they "work" but don't provide accurate data.

Yesterday I spoke to a body shop I have a good relationship with, he is going to let me bring the Tahoe by and let me use his Snap-On scanner to look at diagnostics while its running.

 

[SinnSix]

SOLVED:
Finally had a chance to get ahold of a fuel pressure tester. It was only charging up to 49 psi. It was enough to get her to run, but no power. Replaced the fuel pump and she runs great! Thank you all for your help and support!