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GM Tech (Cam)
GM Tech (Cam), Chevy Technician
Category: Chevy
Satisfied Customers: 27664
Experience:  GM Grand Master Technician 2007. 11 years experience.
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Chevrolet K1500 4x4: I have a 96 k1500 4x4 with the 5.7L Vortec

Customer Question

I have a 96 k1500 4x4 with the 5.7L Vortec motor.
Symptoms
At freeway speeds I was recieving a p0300 code which would go away when driving in town. I could feel the slight misfires while in cruise but it ran strong under load and acceleration.
Most recently the fuel was low and I stopped to get gas after the fill up I got back on the freeway. The truck got up to freeway speeds and then began to misfire very badly to the point I had to pull off the road once at idle the truck promptly died. Now its parked at my house and it starts just fine goes into high idle then levels out for a few seconds when the rpms drop as does the fuel pressure to 45ish (checked with gauge) the engine begins to sputter and then the fuel pressure goes back up to roughly 60 and it recovers. It goes through this cycle 3-4 times each time it has a harder time recovering until it dies out. If I give it gas the misfires start up again and the motor shakes hard enough to rattle the exhaust.

I hooked up a fuel pressure gauge and with key on/engine off I get a fuel pressure of roughly 60psi. When it runs that drops down to roughly 54psi until it starts to do its dying thing. When I rev the engine it seems to hold around 59psi.
What I have tried so far: Cap and rotor, plugs, wires, EGR valve, fuel filter, CPFI ot MPFI conversion, I checked the distibutor gear it looks good but I havent checked it since the new symptoms started, coil pack, checked what I could on the vacum lines and intake manifold for leaks but I dont have the propper tools for that.

Any thoughts? I am not quite sure where to go from here.
Submitted: 2 years ago.
Category: Chevy
Expert:  Gary replied 2 years ago.
so when the fuel pressure drops down to 54 is when it starts to die out ? when the fuel pressure is up to 59 - 60 it will run fine ?
Customer: replied 2 years ago.

basicaly think of it as a tapering sin wave.


Turn the key, pressure comes down engine starts and does the initial high idle pressure stays pretty constant. idle comes down to normal and engine runs briefly just like it should then rpms drop into the 200s or less and fuel pressure drops down into the high 40s during maybe before the drop. Engine revs on its own to above idle speeds and then comes back down fuel pressure goes back up at the same time to roughly 60psi then it goes through the same cycle each time the recovery time gets worse until the motor dies out. This lasts maybe 30 seconds from start to die. It reminds me of a cold started weed eater when you open the choke too early...

Expert:  Gary replied 2 years ago.
ok im so curious why that pressure drops down like that.its either not making the pressure due to a bad pump or the power or ground to the fuel pump is causing this. what you can do is connect a multi meter to the Grey and Black wire on DC volts and watch the voltage right at the fuel pump when this acts up see if what the voltage reads when the PSI is dropping
Customer: replied 2 years ago.
I can give that a try. I did buy a new pump but havent installed it yet. I can try that this evening. Any other suggestions or procedures I might go through so I can provide you with the best response?
Expert:  Gary replied 2 years ago.
if you connect the meter red lead to the grey and black lead to the black wire set it to DC volts when its loosing fuel pressure if the voltage drops below @ 10 then jumps back up 12 volts or more then the pressure jumps back up we have a power or ground problem to that pump causing this problem.if the voltage stays constant when the PSI drops and then rises then more than likey the fuel pump is bad
Customer: replied 2 years ago.
I will try it tonight then and get back to you.
Expert:  Gary replied 2 years ago.
no problem let me know
Customer: replied 2 years ago.
So the voltage read normal at the fuel pump... I also tested the coil just to be sure and it tested out just fine. I pulled the plugs to look for fowling but they are relatively new and didnt have any concernable carbon build up. I also checked the smell of the oil for gas and it smelled normal. The distibutor doesnt have any left to right play but I can rotate it back and forth a very small amount. Cap and rotor look normal no marring or scraping... Pressurized the fuel system and key on engine off held at 60lbs for 20 mins then I turned off the key. Checked all the grounds and the straps look good. Any other thoughts? There arent a lot of vacum lines to check and the fan is too strong for me to check the intake manifold with my smoke machine. MAF looked shiny and clean but I dont know how to test that.
Expert:  Gary replied 2 years ago.
ok get it to act up and see what the voltage of the fuel pump is when the PSI is dropping down
Customer: replied 2 years ago.
I did it was acting up it doesnt seem to be the fuel pump.
Expert:  Gary replied 2 years ago.
so when it was acting up the fuel pressure stayed at 60 PSI when it was acting up ? you told be before the PSI droped down then it died
Customer: replied 2 years ago.
No what I mean to say is that the fuel pump reads within normal voltages while the truck is acting up. That was what I understood you wanted me to test last night. The pressure still begins to drop out as the engine is dying. I did forget to mention that as soon as the engine dies you can hear the pump pressurize the system and the pressure returns back to roughly 60psi after it dies.
Expert:  Gary replied 2 years ago.
yes i did i was concerned the voltage was low when it was acting up so the voltage is good but that PSI still drops down and it acts up correct?
Customer: replied 2 years ago.
Yes same symptoms.
Expert:  Gary replied 2 years ago.
ok if that voltage stays constant when the PSI drops there has to be a bad fuel pump.it cant be the regulator because you replaced it with the conversion of the injection system
Customer: replied 2 years ago.
Well I have a new pump to put in it so I can give that a try and the bed is off still so I will get that done tonight and see if that fixes her up.
Expert:  Gary replied 2 years ago.
ok let me know
Customer: replied 2 years ago.
Replaced fuel pump and no change. I did however look closer at my distibutor rotor and found that some how a plastic fin had been sheered off and was lodged by the sensor I removed that and went and bought a new cap and rotor just to be safe. Its starts and runs now without dying during the idle but it is still missing and giving me a p0300 random misfire on the MIL. So running but not running if you know what I mean. Any thoughts?
Expert:  Gary replied 2 years ago.
for the 300 code we need to see which cylinders are currently skipping with a scan tool viewing the misfire data list which cylinders show a current misfire?
Customer: replied 2 years ago.
Suggestion on a scantool? Mine is OBDII to USB type that connects to my laptop but the softwarde does not show me a misfire data list.
Expert:  Gary replied 2 years ago.
so your getting a 300 code how does it drive and run ?can you feel a misfire or a skip ?
Customer: replied 2 years ago.

Yes it goes into drive and it will drive but you can feel the misfires like random little losses of power but only starting at about 45mph and the occur frequently. It starts fine and has a little shake while idling accelerates without any missing that I can feel. It just seems to do it when the load levels out (cruising at 45) or at idle.

Expert:  Gary replied 2 years ago.
boy without guessing which cylinder or cylinders are skipping it can be a wire,plug,injector or possible compression problem.but if you feel it we need to know which cylinders are in fact misfiring
Customer: replied 2 years ago.
Ok well I can go rent a scantool did you have a suggestion on which one would show the data you need?
Expert:  Gary replied 2 years ago.
they varry.just ask them if it will show a data list of the current and history misfires
Customer: replied 2 years ago.

Ok will do on my lunch

Expert:  Gary replied 2 years ago.
let me know which cylinders are skipping
Customer: replied 2 years ago.

Ok so I have the report I put about 300 miles on it p0300 code still on.


 


Cylinder 1 18K misfires


Cylinder 2 6500


Cylinder 3 23k


Cylinder 4 32k


Cylinder 5 28k


Cylinder 6 10k


Cylinder 7 20k


Cylinder 8 27k


 


I have more data from the program I use if you need it but for the life of me I am lost here. Pulled the distributor this weekend there is some wear on the distributor gear but not more than I would expect with 120k miles on it.


 


So when it misses the worst and I can watch the misfires climb is on the flat at anything above 65mph when holding a constant speed and when going down hill in cruise or just keeping a constant speed. It does not miss at all when idling (so says the computer) or when pulling a load or going up hill or during acceleration.

Expert:  Gary replied 2 years ago.

lets see if we can get another opinion from another expert i am missing something here.hang on lets see if another expert can take a look and give you his opinion.hang on another expert will reply back to you

 

Customer: replied 2 years ago.
ok
Customer: replied 2 years ago.
Question says closed and I am still waiting on a reply?
Expert:  Gary replied 2 years ago.
I am still awaiting another expert to take the question this is a very tough one once an expert reads all we did and has any idea's they will reply to you
Customer: replied 1 year ago.
I never heard anything back... Still having the same issue I thought we were waiting on another opinion
Expert:  GM Tech (Cam) replied 1 year ago.
Hello, I'm Cam.
Have you checked yet for the intake gaskets leaking?

And you did replace the pump, verifying that the pressure never drops below 55 psi when running?
Customer: replied 1 year ago.
Checked the intake gaskets as best I could I used the soapy water method with pressurized air and the pump does hold pressure above 55 psi
Expert:  GM Tech (Cam) replied 1 year ago.
Okay, this test. Remove the PCV and fresh air hose from the valve covers. Plug the port in the valve covers as well as the hose for the PCV. Take the oil cap off, and with the engine running spray small amounts of either into the crank case Oil fill and see if there is any reaction from the engine. Let me know what happens.
Expert:  GM Tech (Cam) replied 1 year ago.
What did you find out?

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