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Ask GM Tech (Cam) Your Own Question
GM Tech (Cam)
GM Tech (Cam), Chevy Technician
Category: Chevy
Satisfied Customers: 29457
Experience:  GM Grand Master Technician 2007. 14 years experience.
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I have a 1999 Chevrolet c1500 with the vortec 5.7 350. My

Customer Question

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: 1 year ago.
Category: Chevy
Expert:  Gary replied 1 year ago.

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

Expert:  Gary replied 1 year 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

Customer: replied 1 year 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?
Expert:  Gary replied 1 year 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

Customer: replied 1 year ago.
Also is there a way of diagnosing the spider without pulling the intake etc???
Expert:  Gary replied 1 year 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

Customer: replied 1 year ago.
All I can see where it says misfire monitor ok
Expert:  Gary replied 1 year 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

Customer: replied 1 year 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
Expert:  Gary replied 1 year ago.

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

Customer: replied 1 year ago.
would a bad injector only act up in that small Rpm range (1400-1800) 45-60 mph?
Expert:  Gary replied 1 year 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

Customer: replied 1 year 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?
Expert:  Gary replied 1 year 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

Customer: replied 1 year 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.
Expert:  Gary replied 1 year ago.

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

Customer: replied 1 year 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.
Expert:  Gary replied 1 year ago.

it works @ 50 % of the time

Customer: replied 1 year 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?
Expert:  GM Tech (Cam) replied 1 year ago.

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?

Customer: replied 1 year 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?
Expert:  GM Tech (Cam) replied 1 year 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?

Customer: replied 1 year ago.
the new distributor came with the cam sensor already installed.
Expert:  GM Tech (Cam) replied 1 year ago.

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

Customer: replied 1 year 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.
Expert:  GM Tech (Cam) replied 1 year 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.

Customer: replied 1 year 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
Expert:  GM Tech (Cam) replied 1 year ago.

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

Customer: replied 1 year 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?
Expert:  GM Tech (Cam) replied 1 year ago.

Let me think on this a bit and see.

Customer: replied 1 year 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.
Expert:  GM Tech (Cam) replied 1 year ago.

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

Customer: replied 1 year 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.
Expert:  GM Tech (Cam) replied 1 year ago.

I would please.