S-10 Forum banner
1 - 12 of 12 Posts

·
Registered
Joined
·
6 Posts
Discussion Starter · #1 · (Edited)
As the title states I've had a very rough idle on my truck for the last month that I cannot solve. Also has anoticeable power loss. Truck has 80,000 miles

Throwing 1 code: P0300 random misfire

All plugs are getting spark to the plugs.

Compression is 180 PSI across all cylinders
Vacuum pressure test reveals 18 PSI at idle
(I'm told this is normal)

As far as i can tell there are no leaks of any kid.

Replaced:
Dist cap and rotor
Plugs
Wires
spider injector
Crank sensor
Cam sensor
Idle air valve
EGR valve
Map sensor
Coil

Possibilities:
MAF sensor
O2 sensors (i was told these dont cause a misfire though)
 

·
Wanna Play a Game?
Joined
·
823 Posts
I'm going to put money on a worn out distributor gear. Pull the distributor out and take a look at the gear on the bottom of the shaft. They wear out and cause these kinds of symptoms when cross fire occurs under the cap.
 

·
Registered
Joined
·
6,908 Posts
I'd bet on distributor gear ^^, or MAF or low fuel pressure.

Clear the codes, unplug the MAF and drive it. The check engine light will come on for the MAF but you should have no misfire codes.

Do you have a scanner that can read live data? You can monitor O2 voltage to determine if the fuel delivery is sufficient.

I doubt an O2 sensor is your problem.
 

·
Wanna Play a Game?
Joined
·
823 Posts
A bad O2 sensor can cause momentary misfire to occur but the PCM will pick up the data difference and let you know if one is going out. I've notice this happen to my truck but that only happened as the PCM was going from open loop to closed loop.
 

·
Registered
Joined
·
142 Posts
spray carb cleaner around the LIM gaskets where the intake ports are ( middle sections of both sides of the lower intake manifold where the gaskets set) if you notice a change in the idle.....then theres ur vaccum leak. But could very well be one of the mentioned above I would check all of them.
 

·
Registered
Joined
·
6 Posts
Discussion Starter · #6 ·
Awesome ideas guys I'll be checking on those items in the next few days and I'll report back. Also did a fuel Pressure check and it read about 15 psi steady with the key on and about 45 psi steady with engine on. Another thing I'm going to check is to make sure i seated the injectors properly, they seem like they can pop out easily and leak fuel into the intake.

Last thing... i asked a mechanic friend today about this and he suggested swapping out the coolant temperature sensor. I thought there's no way that has anything to do with a misfire/rough idle. But after researching it they actually do control the fuel injectors to a degree. (Much to my surprise)
 

·
Registered
Joined
·
6 Posts
Discussion Starter · #7 ·
Also, I do have a scanner that reads live engine data. The truck wont even start if I disconnect the MAF sensor. I sprayed that seafoam carb cleaner into the vacuum without any change also.
 

·
Registered
Joined
·
2,608 Posts
Awesome ideas guys I'll be checking on those items in the next few days and I'll report back. Also did a fuel Pressure check and it read about 15 psi steady with the key on and about 45 psi steady with engine on. Another thing I'm going to check is to make sure i seated the injectors properly, they seem like they can pop out easily and leak fuel into the intake.

Last thing... i asked a mechanic friend today about this and he suggested swapping out the coolant temperature sensor. I thought there's no way that has anything to do with a misfire/rough idle. But after researching it they actually do control the fuel injectors to a degree. (Much to my surprise)
Fuel pressure and leak down must be checked when the fuel pump is running and the engine is off. GM does not publish any "engine running" specs for this engine. When the ignition is initially turned to the RUN position, the fuel pump should run for 2 seconds and shut off. While the pump is running, pressure must be 60psi to 66psi. Pressure must remain above 55psi for at least 10 minutes after the pump shuts off. If it fails one or both parts of the test, there is a problem with the fuel delivery system. Further diagnosis is necessary to determine what the problem is.

If fuel pressure is below 60psi, there will be driveability issues. If fuel pressure is between 55psi & 60psi it may or may not start without having to manually add fuel. It will however, run. From 55psi down to 43psi, good luck When pressure drops below 43psi, the poppet valves at the intake ports will not open and squirt fuel. The engine will not start or run.
 

·
Registered
Joined
·
6 Posts
Discussion Starter · #10 ·
Tried the Engine Coolant Temp Sensor and no change to the idle or performance. The fuel pressure is steady at 55 PSI up at the rail test port. I'm thinking maybe an injector slipped out during installation. I'll have to take it part and have a look.
 

·
Registered
Joined
·
7 Posts
98 vortec v-6 two wheel drive...runs good wout fuel cap... put on dealer cap and engine coughs out and stalls, starts but runs crappy...at this point no check engine light, but it has come on so i know it works...engine stalls out i take off fuel cap lots of blow off pressure comes out of cap...also increased pressure in tank pushes up fuel gauge and a;lso leaks fuel out of purge valave located above rear axle until i remove cap and stops leaking... can somebody please help!!!
 
1 - 12 of 12 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top