1998 c1500 5.7 very bad misfire

[Ousten Pruitt]

Ok yall need some help. 98 c1500 5.7 131k miles very very bad misfire and almost no vacum at idle to the brake booster. I've pulled the intake and cam and lifters are all good no wiped lobe changed the updated mpi out with a new mpi again newer plugs less then 15k on them new cap rotor and plug wires still have a bad misfire, was moving from Jonesboro to bentonville arkansas and midway in Conway I saturated running real bad it's always had a misfire under load on cyl#4 haven't run a compression test but there's only a gallop on 1 cylinder and has always been like that it flooding out 2 or 3 cylinders and I'm stuck idk where to go have the distributor set correctly using a scan tool what should I look at I'm gonna pop the y pipe off layer and see if I have a clogged cat but idk any inside would be helpful thanks. I don't have any egr codes if someone could give me the values for sensors I should be looking out for to see if mabe a o2 is going bad toss me the values and I'll see what myne are at thanks

 

 

[richard wysong]

02 won't cause a misfire, what are you getting for codes?

Edited June 29 by richard wysong

[asilverblazer]

On 6/28/2024 at 12:59 PM, Ousten Pruitt said:

98 c1500 5.7 131k miles

Very bad misfire and almost no vacuum at idle to the brake booster.

I've pulled the intake, cam and lifters; all are good and no wiped lobes.

Changed the mpi out with a new mpi.

New plugs less then 15k on them, new cap, rotor and plug wires.

Moving from Jonesboro to Bentonville Arkansas, midway through Conway it started running real bad, it's always had a misfire under load on cyl#4.

Haven't run a compression test but there's only a miss on 1 cylinder and has always been like that. It seems like it is flooding out 2 or 3 cylinders and I'm stuck.

IDK where to go have the distributor set correctly using a scan tool. What should I look at?

I'm gonna pop the y pipe off later and see if I have a clogged cat but idk any insight would be helpful. I don't have any egr codes. If someone could give me the values for sensors I should be looking at to see if maybe an o2 sensor is going bad to compare what mine are.

Slow down and start with the basics. There is nothing in the exhaust that is going to cause a miss fire.

 

Try a power balance test, unhooking one spark plug wire at a time to try to isolate the miss firing cylinder. Use a scan tool and view cylinder miss fire counts to find a cylinder missing more than the others.

 

If the above doesn't narrow it down. Check fuel pressure and the air intake track. A true random miss fire would be caused by something that affects all the cylinders. This rules out plugs, wires and injectors. 

 

On a 98 model there is no adjustment to the distributor. 

 

If you have had the intake off, I'd do a careful search for a vacuum leak at the manifold. Also, retrace your work to find an obvious fault like forgetting to reconnect something.

 

Confirm that the only codes are P0300.