Misfire

[98k2500]

What are the logical steps to diagnose a misfire?

Truck starts very easy. Starts right up.

I replaced the spider fuel injectors with the mpfi replacement hoping the misfire would go away. But no luck.

I have a p1351 code. But reading about this it sounds like most folks trucks won't start at all when they have this code. Could the p1351 code, icm voltage cause a misfire?

I seem to be getting spark to the plugs just fine. Any ideas?

 

[Schurkey]

What vehicle?

What are the logical steps to diagnose a misfire?

If OBDII, connect scan tool. Find out which cylinder(s) are misfiring. If you get a P0300 code,verify that the scan tool is capable of displaying individual-cylinder misfires. Most aren't.

If OBDI, perform cylinder-balance test to find misfire.

When you locate misfiring cylinder(s), verify ignition including spark-plug condition.

Then verify cranking compression pressure.

Then verify no vacuum leaks into that runner/cylinder.

Then verify fuel delivery.

I have a p1351 code. But reading about this it sounds like most folks trucks won't start at all when they have this code. Could the p1351 code, icm voltage cause a misfire?

https://www.gmt400.com/threads/p1351-code.50753/

I seem to be getting spark to the plugs just fine. Any ideas?

Spark to the plugs does not mean you have spark across the plug gap. Or that there's a burnable air/fuel mixture compressed in the cylinder.

 

[98k2500]

What vehicle?

If OBDII, connect scan tool. Find out which cylinder(s) are misfiring. If you get a P0300 code,verify that the scan tool is capable of displaying individual-cylinder misfires. Most aren't.

If OBDI, perform cylinder-balance test to find misfire.

When you locate misfiring cylinder(s), verify ignition including spark-plug condition.

Then verify cranking compression pressure.

Then verify no vacuum leaks into that runner/cylinder.

Then verify fuel delivery.

It's a 98 2500.

I'll check these things tomorrow. Thanks for the reply.

 

[east302]

You can download copies of the 1998 service manual at this link below. It will have the GM diagnostic (troubleshooting) tables for that code. An alternative is a subscription to alldatadiy.com which has the same information and is a bit easier to navigate.

Paper copies of the four volume GM set can be had on eBay as well.


https://www.gmt400.com/threads/88-95-service-manuals.43575/


Sent from my iPhone using Tapatalk

 

[Pinger]

What are the logical steps to diagnose a misfire?

Identify which driving circumstance provokes it.
Misfires due to loss of high voltage ignition show up under load. A misfire in that situation can still be fuel related but fuel related tends to show in a variety of driving situations. That's my start point.

 

[98k2500]

Thanks for the info. It has a misfire while idling. I just bought this truck to fix it up.

I accidentally hit clear codes and the truck is not drivable. I drove it down my street but when I hit the gas it goes nowhere. Very bogged down. Could be clogged cats but one thing at a time I guess.

Since I cleared the codes how long will it take for any codes to show back up if I can't really drive it anywhere to complete a drive cycle?

 

[Schurkey]

Stop looking for "codes", start looking at cylinder misfire counts.

Or at least look for cylinder misfire codes: P0301--P0308. If it misfires at idle, those codes should show up fairly quickly.

What is the fuel pressure? How old is the fuel filter?

 

[98k2500]

I checked the fuel pressure today. At idle it is at 52. If I bump the throttle it goes to 62. I read somewhere that at idle it should be around 60 to 66? Does that sound right. I'm pretty new to all this so I really appreciate the help.

 

[98k2500]

Also when turning key on ,engine off it went up to almost 63