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I have a 1999 Chevrolet c1500 with the vortec 5.7 350. My

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problem is it's missing at...
I have a 1999 Chevrolet c1500 with the vortec 5.7 350. My problem is it's missing at speeds above 45 mph with steady or light throttle. Let off the gas it quits or give it more than a 1/4 gas it quits. No check engine lights and no codes when I plug in my scan tool. I have changed plugs,wires,cap,rotor,fuel filter,and the distributor. This is getting expensive and without a code I'm just throwing parts at it. Any advice?
Submitted: 2 years ago.Category: Chevy
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Answered in 45 minutes by:
8/16/2015
Chevy Mechanic: Gary, Chevy Technician replied 2 years ago
Gary
Gary, Chevy Technician
Category: Chevy
Satisfied Customers: 17,282
Experience: 15 years GM master tech
Verified

I am sorry for the long wait

at this point with NO codes at all and to NOT throw parts at this truck is a scan tool needs to be connected to this truck and monitoring current misfires on the live data list when it is missing and see if it registers a specific one or two cylinders misfiring

its a little odd on a 1999 truck to have a misfire and it not trigger at least a PO300 code

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Chevy Mechanic: Gary, Chevy Technician replied 2 years ago

we have seen issues with the fuel injector spider cause this type of issue to sticking valves in valve guides case this type of issue

but at this point we need to see if it is a specific cylinder or not misfiring when it is acting up

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Customer reply replied 2 years ago
I hooked my scan tool up and was watching the live data but not sure what I should be looking at? This is the 5.7 not the 5.3. This engine does not have individual coil packs. Can the computer still tell which cylinder is missing?
Chevy Mechanic: Gary, Chevy Technician replied 2 years ago

it will still show misfires if your scan tool will show live data of the current misfires in the misfire data list or one of the data lists

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Customer reply replied 2 years ago
Also is there a way of diagnosing the spider without pulling the intake etc???
Chevy Mechanic: Gary, Chevy Technician replied 2 years ago

you usually find out which cylinder is misfiring then connect a fuel pressure gauge and perform a fuel injector pressure drop while firing each injector with a scan tool seeing if all 6 injectors drop the same amount of fuel pressure

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Customer reply replied 2 years ago
All I can see where it says misfire monitor ok
Chevy Mechanic: Gary, Chevy Technician replied 2 years ago

see if someone has a scan tool that can see any current or history misfires in a misfire data list or any of the live data lists

if you can not get to see a misfire and it has a new cap and rotor and it is not setting a P0300 code try an injector pressure cleaning if it still acts up try a new GM crank sensor if by chance it still acts up then it is either the spider injection or possibly a valve binding in a valve guide which is very time consuming to find which valves are binding

again i am surprised its not triggering a P0300 code these systems will usually pick up a misfire and set a code very quick when misfiring

the problem you have is not being able to test when it is misfiring as the truck needs to be driven down the road and it becomes almost next to impossible to test it when it is acting up

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Customer reply replied 2 years ago
Also all the spark plugs looked the same. I thought maybe if there was injector problem one would look different unless all injectors were bad
Chevy Mechanic: Gary, Chevy Technician replied 2 years ago

it is just so hard to tell without knowing what cylinder is possibly misfiring

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Customer reply replied 2 years ago
would a bad injector only act up in that small Rpm range (1400-1800) 45-60 mph?
Chevy Mechanic: Gary, Chevy Technician replied 2 years ago

yes if the poppet was restricted or spraying the fuel in a NON mist straight direction due to it being dirty and when it can not mist the fuel raw fuel can not burn as good as atomized misted fuel being sprayed in a nice misting cone pattern out of a fuel injector

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Customer reply replied 2 years ago
most likely something simple like a vacuum leak wouldn't cause this? I've also read about transmissions and converters causing it to fell like its bucking at that speed. Being I can get it to do it in 3rd gear as well as overdrive does that rule out the transmission?
Chevy Mechanic: Gary, Chevy Technician replied 2 years ago

yes if it does it in 3rd the transmission is not in lock up so it is not the transmission

a vacuum leak would set codes and case a rough or bad idle and not usually this kind of issue

going over some of my notes lets try 2 simple things

remove the MAF and clean it out good with carb cleaner or MAF spray cleaner if it still acts up unplug the electrical connector to the EGR valve and drive it and see if it still does this

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Customer reply replied 2 years ago
I forgot to mention I already tried unplugging the electrical plug on top of the egr. No change. i will try cleaning the mass air.
Chevy Mechanic: Gary, Chevy Technician replied 2 years ago

ok clean the MAF if it still acts up then have the injectors pressure cleaned and see if that helps

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Customer reply replied 2 years ago
Ok thanks.Does the pressure cleaning work more than not? I don't won't to waste money that could go towards a new spider.
Chevy Mechanic: Gary, Chevy Technician replied 2 years ago

it works @ 50 % of the time

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Customer reply replied 2 years ago
Ok I cleaned the mass air and no change. I replaced upper and lower intake gaskets and spider injection still no change. What's next?
Chevy Mechanic: GM Tech (Cam), Chevy Technician replied 2 years ago
GM Tech (Cam)
GM Tech (Cam), Chevy Technician
Category: Chevy
Satisfied Customers: 29,591
Experience: GM Grand Master Technician 2007. 14 years experience.
Verified

hi there, New expert here. I am Cam. Have you tried disconnecting the map sensor to see if that makes a change in how it runs? How about the crank sensor wiring, have you inspected that for signs of chafing and such along the side of the block?

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Customer reply replied 2 years ago
thanks for your reply. No I have not tried unplugging the map sensor. Only thing I unplugged was the egr and I still missed. So I just unplug the map sensor and drive it? I have not done anything with the crank sensor yet. The wires go instantly into a loom with other wires. Should I peel back the loom and inspect? Do you think changing the crank sensor would help or would a bad crank sensor always throw a light?
Chevy Mechanic: GM Tech (Cam), Chevy Technician replied 2 years ago

OK unplug map and drive it.

Yes if the MAP does not change it, then peel back wiring loom and inspect. Also you replaced the distributor completely? including the cam sensor inside of it?

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Customer reply replied 2 years ago
the new distributor came with the cam sensor already installed.
Chevy Mechanic: GM Tech (Cam), Chevy Technician replied 2 years ago

OK, lets see what the map unplugged does and check that wiring.

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Customer reply replied 2 years ago
Ran the same with the map unplugged. Don't see any obvious damage to the wires tracing them up to where the loom connects with all the other wires.
Chevy Mechanic: GM Tech (Cam), Chevy Technician replied 2 years ago

Shoot. So then we need to see more live data, what that MAP reading, MAF reading is, Engine coolant temp reading. Intake air temp reading, knock sensor reading to see if any are erratic or out of spec.

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Customer reply replied 2 years ago
The readings I have are with the truck running between 1500-1800 rpm (50-60mph). That is where the miss/surge happens.
Maf averaging 5.8 fluctuates a little (may have been feathering the pedal)
Map 17.7-18.9
Chevy Mechanic: GM Tech (Cam), Chevy Technician replied 2 years ago

If the Mass Air Flow sensor (MAF) was unplugged, did it happen still?

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Customer reply replied 2 years ago
Still did the same thing with maf unplugged. I went and bought a new tps sensor,crank position sensor and both O2 sensors and it didn't help either. I don't know what is left?
Chevy Mechanic: GM Tech (Cam), Chevy Technician replied 2 years ago

Let me think on this a bit and see.

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Customer reply replied 2 years ago
Thank you. I don't know what to do. I bought this truck thinking a tune up would fix it and 1400 worth of parts and repairs thrown at it and nothing's worked. I know it's tough to diagnose through the Internet but if you can some how figure it out I'll be more than glad to throw a healthy tip. I hooked back up vacuum gauge earlier and getting a steady reading of 19. I'm guessing that's within spec.
Chevy Mechanic: GM Tech (Cam), Chevy Technician replied 2 years ago

Ok thanks .COuple questions. First what was the fuel pressure. Second, does the vacuum stay that high when driving and it acts up?

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Customer reply replied 2 years ago
Fuel pressure was 60 with key on engine off. When I started the engine it was 51 and stayed there. I haven't checked vacuum while driving. The hose on the gauge is too short to reach the cab. I'm sure I can rig something up if you want me too.
Chevy Mechanic: GM Tech (Cam), Chevy Technician replied 2 years ago

I would please.

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