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5.3 Engine Stumble / Misfire On Hard Acceleration


splat10

Question

99 silverado with 5.3 z71, 148k miles, and brand new tranny (condition existed both before and after tranny rebuild)

 

on acceleration in 1st or second gear (don't usually get 3rd gear to 3000 rpm) when i get to around 3000 rpm the engine stumbles real bad. It jerks and loses all power, resulting in a downshift. There are no signs of a misfire at any other condition and there are NO codes on my Acton scanner (don't know about the tech II, can it read codes mine can't???)

 

When it is stumbling there is a clicking noise coming from the engine compartment that almost sounds like it rev limiting (might be a fuel pressure reg???)

 

Reading some other posts here's what i've done to try and fix.

1- unplugged my electrical EGR. No improvement

2) replaced the fuel filter - seems a little better but not cured

3) New fuel pump several months ago (don't know if the problem started then or not. can't remember)

4) Plugs and wires are new about 3 years 50k miles ago. Haven't replaced them yet. The coils packs are original.

 

I'm leaning to fuel issues but don't know if i should start with the plugs and wires but that's $ i don't have right now with the tranny rebuild if I'm just throwing parts at it.

 

Should i just bring it to the dealer to scan it? Can they scan more than my cheapo scann can on the engine?

 

Thanks

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replaced the Fuel pump with a same part airtex pump from advance (wanted to try free first) and it drives worse than before. Have an AC delco on order and will report back. I'm 95% that the cheapo fuel pump was the root of all my problems.

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new Fuel pressure reg actually made it worse (probably because it's not gummed up or the spring is new??? i have no idea). Fuel pressures dropped to low 40's at idle and run at about 50 under acceleration. Look like the crapo fuel pump was never any good. Hopefully i can get it replaced and a new crapo will give more pressure. If not hopefully they'll give me a bosch at a reduced price or something.

 

Threw a P0336 Crank pos code this morning. I think that's from the engine missing due to low fuel. Does that make sense to anyone?

 

What are the proper pressure specs for the 99 Vortec 5.3?

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put in an AC Delco fuel pump (not from the dealer) and the pressure is still the same and the truck runs the same. I'm running out of ideas at this point. New filter, new regulator, new pump and no improvements. It's like it's leaking pressure from somewhere but i don't get any of the symptoms of leaking injectors. It's starts easy in all conditions, runs great unless you give it the gas under load. Fuel milage is still good (actually better because i can't whomp on it). I don't want to spend a lot on dealer repair or diagnostics since the truck is all rusted but i have a new tranny and tires on it so i'd like another year or so out of it.

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Back TTT for some new diagnostics.

 

Finally broke down and bought a fuel pressure gauge. Results are enlightening.

1) Key on engine off fuel pressure is 52 PSI

2) Engine on fuel pressure is 48 PSI

3) Revving engine fuel pressure drops to low 40's

4) Pull vacuum line from regulator and pressure jumps to rock solid 58 psi continuous.

 

Other notes:

1) Replaced fuel pump about 10 months ago with a cheap parts store pump but it seems to be working fine with 58 PSI? or is that weak? I can pull it and return it to the store and get a different crap one for free and try that.

2) CAN I DAMAGE ANYTHING BY UNPLUGGING THE REGULATOR FROM THE VACUUM LINE?? to do a drive test to see if the hiccuping goes away? It will just run rich right?

3) Will replace regulator and report back.

 

What are the correct pressure ranges for this engine?

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well it's not really WOT. I rarely if ever go WOT. Usually just enough throttle to prevent it from upshifting at the 2200-2500 shift point will cause the condition. If i downshift to pass someone or just get off the line a little harder it will do it. Hauling my snowmobile or ATV in the bed will exagerate it (or bring the rpm range downward a little to where it starts) obviously since it's more load.

 

Once in a while i can run it right up to 4000 clean but rarely and usually when it's dead cold (i think). all this testing is making my fuel milage suffer though.

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cleaned MAF and replaced plugs tonight. Got to 3500 RPM before problems started. First pull was trouble free. Had seen improvement since i seafoamed this weekend.

 

This is the clue that i think is important but don't know what it means.

 

When it's stumbling, i am hearing a distinct clicking noise in time with the stumbles. Almost like the fuel pressure reg is popping open when it whouldn't be. I don't see how it could be though when the truck should be pulling low pressure under WOT? What else could cause a mechanical clicking noise related to engine firing? Could be a relay too i guess??? Would a rev limiter do that somehow???

 

Thanks

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Is this why the trans was rebuilt?

 

Almost 1/2 of the slipping trans complaints end up being a fuel delivery issue. I could make a fortune off of rebuilding transmissions when a fuel filter was the real issue if I was crooked.

 

Can the T-2 see things a cheap scanner cannot? Yes, when I get a trans complaint first thing I do is check codes and the second thing I do is watch fuel trims on a quick road test. If the trims are way positive I look for a low fuel volume issue, if they are around 0 I start digging into what the trans is doing.

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Try cleaning the EGR first & see if anything changes. At least that won't cost anything.

Been there done that. Filled it up with Throttle body cleaner and did the best i could. I've gooten random EGR codes for a long time. Always just cleared them and they didn't come back for many months. The pintle is free and moves. Unless it's leaking but that would cause rough at low idle which is not a problem for me. I may try and block it off with a piece of aluminum just to elimante any EGR to test though. The plugs did look a little lean when i changed them though.

 

 

Maybe the crank position sensor weirds out at around those rpms, so the engine looses track of what it's doing...

 

That's what i'm guessing. It's not really RPM dependent as much as it is loading. If i have a big load (like my snowmobile or ATV in the bed) and go up a hill it can be as low as 2000 rpm. Flat land and empty truck and it will be over 3000 rpm.

 

I'm going to check the wiring, but i think that the reason the crank sensor code it there is because it's sensing the misfire and hesitation in the engine speed and not throwing a misfire code for what ever reason. the crank code can't throw if theres a misfire code (i think i read that, or maybe it's the other way around).

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no. The tranny has nothing to do with it. The only reason i mentioned that it was brand new so that i wouldn't get a bunch of tranny slipping responses. The tranny was rebuilt because it reached it's end of life as is normal for these things at 10 years 145k miles.

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Condition is getting worse ( or maybe i'm just not babying it anymore that i have a new Tranny). Anything over about 2250 - 2500 RPM and it's bucking like crazy. Can not get over 3000 rpm without it downshifting due to lack of power. Not intermittent, not variable at all. Never misses or stumbles under normal driving conditions. Hot or cold doesn't seem to matter.

 

Couple more questions.

1) Would a clogged Cat cause this type of stumble? (only under hard accel)

2) Seems fuel related to me since if it were spark related it would do it under all conditions??? (i know i need to rule out fuel pressure and get my hands on a guage)

3) still no codes

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Yeah i went through a problem this summer of no start with a bunch of relay noise. Found lots of corrosion on pos terminal was the problem. I fixed that and it's been solid since cause i've been checking on it.

 

I'm about to give in and bring it to a dealer to watch the outputs. Could still be O2 or something like that but that noise is what has me stumped. All the other misfires i've dealt with in the past there wasn't this clicking that i have

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