99 5.7 shuddering/low power, no codes

[JoshCampbell55]

I have a 99 5.7 that shudders on partial throttle/cruising and feels like its misfiring at idle. This truck has been sitting for 7 years and I'm trying to get it back on the road.
With my scan tool checking misfires per cylinder it seemed to mostly be 2 and 4 but most of the time it shows 0 misfires even when its shuddering so I might need to try another scanner to check that again. I only got a p0300 code once over the last few days so I cleared it and it never came back. Another thing I noticed is if the truck sits for an extended period of time it has a little bit of a harder time starting and then it runs poorly for a minute when it does start.

I replaced the crank sensor a while back because it didn't have a shim and it was hitting, installed new one with a shim. I made sure the distributor was installed correctly and got the CMP to 0.4 degrees with the scan tool at 1000 rpm and did the crank relearn.


I hooked up the fuel pressure gauge and I'm getting around 56 with the key on engine off then when I start it it drops to about 49-50 and doesn't go higher even when I rev it. It holds pressure even after turning the key off.

Are these acceptable fuel pressure numbers with the MFI injector system?

My thoughts are check the fuel pump wiring (maybe chewed a little bit? truck was sitting for a long time)
Or possibly a worn distributor gear?


Adjusted the valves
Compression test all around 150-160
o2 sensors show 0.1-0.9v
MFI Injectors
New Cap, Rotor, Wires, Plugs
new crank sensor
checked all grounds

thank you

 

[kolgeirr]

Any chance you have coolant missing? My Suburban had the same symptoms (plus coolant leaks, and was missing only on 5) when I got it which was resolved by replacing the lower intake manifold gaskets - they tend to crack with age and can result in coolant going places it shouldn't, and vacuum leaks.

 

[JoshCampbell55]

Any chance you have coolant missing? My Suburban had the same symptoms (plus coolant leaks, and was missing only on 5) when I got it which was resolved by replacing the lower intake manifold gaskets - they tend to crack with age and can result in coolant going places it shouldn't, and vacuum leaks.

Not that I have noticed but I really haven't driven it all that much I barely just got it going again a few days ago but I will definitely keep an eye on that, I do have a coolant pressure tester I can use.


Another thing I forgot to add is I have a new fuel filter on its way right now.

 

[kolgeirr]

A new fuel filter never hurts. Your pressure might be a bit low, I think these Vortecs call for 60-66 KOEO and tend to drop into the 50s while running. Generally a failed regulator causes the pressure to bleed off after shutdown in my experience, so at least your new injector units regulator seems OK. New filter should bring your pressure up a bit, so retest with the new filter on and see what you get.

Other than intake gasket, and assuming your new injectors aren't a factory defect part, I'd take a look at the distributor itself. If it's an original plastic-body unit they like to crack which can throw things off.

Also recommend taking a look at your idle air control valve. If that gets sticky or fails it'll starve the engine for air when the throttle plate is closed (like when coasting, or at full idle) and can cause rough running in those conditions.

 

[JoshCampbell55]

A new fuel filter never hurts. Your pressure might be a bit low, I think these Vortecs call for 60-66 KOEO and tend to drop into the 50s while running. Generally a failed regulator causes the pressure to bleed off after shutdown in my experience, so at least your new injector units regulator seems OK. New filter should bring your pressure up a bit, so retest with the new filter on and see what you get.

Other than intake gasket, and assuming your new injectors aren't a factory defect part, I'd take a look at the distributor itself. If it's an original plastic-body unit they like to crack which can throw things off.

Also recommend taking a look at your idle air control valve. If that gets sticky or fails it'll starve the engine for air when the throttle plate is closed (like when coasting, or at full idle) and can cause rough running in those conditions.

Thank you for the help yea the distributor is original. I’ll add these items to my list to check next weekend and report back

 

[Schurkey]

I have a 99 5.7 that shudders on partial throttle/cruising

Possible (not enormously likely) to be a torque converter clutch application problem. Clutch partially-engages, releases, engages, releases...feels like an engine shudder.

Rapid cycling of A/C compressor?

and feels like its misfiring at idle.

That would NOT be a torque-converter clutch issue.

Verify coil output voltage with a spark-tester calibrated for HEI.

I hooked up the fuel pressure gauge and I'm getting around 56 with the key on engine off then when I start it it drops to about 49-50 and doesn't go higher even when I rev it. It holds pressure even after turning the key off.

Don't trust the indicated pressure reading on the gauge until you've verified the accuracy of the gauge. Having the gauge "show" 56 psi could actually be 60-ish if the gauge is off even a little.

Adjusted the valves
Compression test all around 150-160

Probably indicates that the lifter's preload are within reason.

o2 sensors show 0.1-0.9v

How rapidly does it change voltage? Some scan tools show cross-counts, others display a graph of voltage over time. Fast switching is desirable.

Old O2 sensors tend to switch more slowly.

 

[JoshCampbell55]

I didn’t get too much time to mess with it today but I got a short log file with a borrowed HP tuner.
After letting it idle for a while my LTFT are bank 1 -5.5% and bank 2 -18% if that helps at all I don’t know too much about that stuff

 

[JoshCampbell55]

I uploaded a short video to YouTube if this helps at all

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[JoshCampbell55]

So I hooked up the smoke machine to the intake and found a small leak where the intake tube goes onto the throttle body. Do you guys think this leak could be doing that much? It’s kind of hard to see but I uploaded a video anyways

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[L31MaxExpress]

That would NOT be a torque-converter clutch issue.

Actually it very well could be, if the truck has a 4L80E. Some of the 4L80E calibrations were revised from GM to reduce idle line pressure. The high line pressure causes the TCC clutch to drag and the engine to feel like it is misfiring at idle.

Also fluid leaking into the TCC apply circuit could cause the converter clutch to randomly apply on any lockup converter transmission. Cross leaks from bad o'rings on the input shaft for example.