Vortec 350 Misfire But Engine Light

flagvan2000 · April 7, 2011

i have a 2000 chevy k2500 with a vortec 350. It has a jasper motor with 10000 miles on it. i tuned it up 3 months ago. cap, rotor, plugs, wires, new fuel pump, and fuel filter. It has a misfire at idle and a very random misfire while driveing. mostly at steady speeds. feels like surges. the misfire just started 2 days ago. I am not getting any engine light to help me out. the truck has 190,000 miles on it. i feel like the power of the truck is bad. my 1998 chevy 1/2 ton had more pulling power than this 3/4ton. i change the map sensor last night. still have problem. and i have noticed with that newer motor i am burning about a quart of oil a month or about 1500 miles. i did get a engine light at lunch that says po108 (map circuit high input). but i changed that map sensor last night.

tim73 · April 10, 2011

Have you checked your intake for oil? Also, have you checked the actual fuel pressure at the rail? ALso made sure the vacuum line going to your fuel pressure regulator isnt cracked, also does your map sensor have a vacuum line? If so check it. Aslo make sure the reference voltage going to the map sensor is good(4.5-5VDC)

Stevens03 · April 11, 2011

Who installed the motor? How do you know your power is down?

flagvan2000 · April 11, 2011

I installed the motor. I no the power is down because i had the same motor in my 1/2 ton Chevy and it only had 3.42 gears and this truck has 3.73 there should be more get up and go that my 1/2 ton. my 1/2 ton would pull my car hauler a lot better than this 3/4 ton. the fuel pressure at the rail is 55 at idle. goes to 60 when i push on the gas. there isn't a vacuum line at the map sensor.

I took the connector off the map sensor and and tightened up the pins. and changed the distributor cap and rotor. and now i don't have a miss at driving speed it is a lot better. now it just lopes at idle and misses while accelerating. and i can't get an engine light to come on. not even a misfire code.

tim73 · April 11, 2011

I installed the motor. I no the power is down because i had the same motor in my 1/2 ton Chevy and it only had 3.42 gears and this truck has 3.73 there should be more get up and go that my 1/2 ton. my 1/2 ton would pull my car hauler a lot better than this 3/4 ton. the fuel pressure at the rail is 55 at idle. goes to 60 when i push on the gas. there isn't a vacuum line at the map sensor.

I took the connector off the map sensor and and tightened up the pins. and changed the distributor cap and rotor. and now i don't have a miss at driving speed it is a lot better. now it just lopes at idle and misses while accelerating. and i can't get an engine light to come on. not even a misfire code.

Have you pulled codes or just looking for the CEL? Some times there will be a code without the light. ALso, any corrosion on the pins you can see? Perhaps put a lil dab of noalox on the pins. cant hurt. Finally...and this is a long shot...but is there any way one of the spark plug wires is defective? You can check them all by checking resistance...they should all be the same.

flagvan2000 · April 12, 2011

i checked the resistance on the plugs they were all the same. the old shop i used to work for said i have a dead cylinder. i got new plugs, wires dist cap and rotor. does anyone no what kind of problems they have had with the spider injection on the vortecs?

txab · April 12, 2011

Fuel specs show:

Key on engine off 60-66 psi

Engine on, idle. 60. Accelerate should hit the 66 mark.

You're on the low end according to specs

ChevyTech77 · April 12, 2011

Sounds like a low/inconsistant vacuum concern due to internal engine issues or a fuel miss.

The fuel injector tubes (poppet valves) were known for clogging if not maintained regularly. A good pressurized fuel system clean procedure will clear them out. If no change, they may be leaking and will have to be replaced.

On the vacuum issue though, I have seen a TON of failed Jasper engines over the years. Put a vacuum gauge on it and note its reading and response first. Compression and cylinder leak down tests would be my next, after the vacuum test.

flagvan2000 · April 12, 2011

On the fuel pressure answer. I have had 4 different 350 vortec's and have had to change fuel pumps in 3 of them i have never had a new fuel pump hit over 62psi. my book says the same thing but i don't agree with the specs. and i have only use delco fuel pumps which are gm's pump.

I have the old motor and spider injection. so i will swap the injection out to see if there is a change.

thanks

rdnkcpa · April 12, 2011

I'd lean towards compression check. The two Jasper engine I was exposed to, my neighbor's Corvette, the engine was never right from day one. The other Jasper engine was in a Taurus, 3.8L I think, it was a hunk of junk. I've not heard a lot of good things about Jasper.