98' Chevy

[eyehunter78]

My Dad has a 98' Chevy 350 vortex. We where gonna pull the wheel house the other day but it had a bad miss when we would start to speed up. He had just replaces plugs and wires. WE where woundering if it could be the cap? And a guy had told him something under the intake. Just woundering if anyone has heard of this intake problem. (has new intake gaskets) Thanks in advance!!

[Jeremy airjer W]

If it's misfiring the check engine light should be on, that's a good place to start. It will at least identify the cylinder or cylinders that are not working properly.

The CPI unit is what I think they are referring to under the upper intake plenum. Back in the day these failed quite frequently.

How long ago was the tuneup done?

What brand plug was used?

How long ago was the lower intake gasket done?

[Macgyver55]

Does it run ok when not under a load? Was the work performed to correct this problem or did the misfire start after the work? If the plugs and wires were just done is there any chance there could be a couple plug wires crossed?

#5 and #7 are easy to mix up since they are next to each other on both the cylinders and the distributor cap.

If those are correct for sure, yes, the distributor cap/rotor can be a source for a misfire. Take off the cap and check the center terminal carefully. (it should protrude slightly) Look for any cracks, carbon tracks or moisture. Replace it and the rotor if you see anything, or if it has high mileage on it.

[eyehunter78]

Replaced plugs and wires to fix the problem back in October.

I believe bosch plugs, but not sure.

Truck only has 99k miles and intake done around 75k to 80k.

[eyehunter78]

Truck does seem to run just fine with no load. No miss fire. Yes, check engine light was one and flashing. Took to local shop and no codes showed up!

Gonna try a GM cap on Monday! Hope this solves it. Sure don't want to take intake off again! Thanks guys for the help!!

[Jeremy airjer W]

I believe bosch plugs, but not sure.

You might want to check those. If they are bosh plus 2, or plus 4 pull them all out and discard. Replace with O.E. ac/delco plugs (available at your local NAPA).

[BoxMN]

Replace with O.E. ac/delco plugs (available at your local NAPA).

I have found this to be important on my 98 350. Also plug wires, spend the money and make sure they are OE as/delco. It costs more, quite a bit, but worth it.

[eyehunter78]

I was wrong, they are AC/ Delco . GM parts. Hoping to get a GM cap by Wednesday. Thanks guys for the help!!

[eyehunter78]

Well finally got the new GM cap Friday and installed. Gonna give it a try pulling fish house today. Hopefully this will solve the problem!

[eyehunter78]

Well the new GM cap did not do the trick! Do you think it could be the injectors?? The code comes up as a miss fire. Thanks for any and all the help guys!

[53orbigger]

I'm not as knowledgeable as the others here, but I had a random misfire on my 2001 6L and it turned out to be an O2 sensor. Just a thought.

[Jeremy airjer W]

P0300 - Random or multiple cylinder misfire?

If so a good scan tool will be your best friend. There will be misfire history for all the cylinders that will help you narrow down the cylinder(s) or possibly a bank of cylinders. A plugged cat on either side will cause one bank to misfire while the other bank is fine. Also fuel trims and O2 data can also help narrow down other problems (there should be too rich, too lean, etc codes if there was an issue with fuel/air).