2000 OBS K3500 454 miss firing

[liv2shoot]

2000 chevy 454 OBS K3500 Crew cab dually truck

Started out the thermostat needed replacing because it only got up to 110 degrees. Which caused us to change the original heater hoses dated 1999. Then decided to change the spark plugs (badly worn) then also plug wires. Broke distributor cap and screw mounting bosses on the distributor. Replaced complete Distributor with new complete Delco distributor. Had to take the intake plentom off to get the distributor out. Replaced plentom gasket.

Test ran the engine. Ran great.

Took the truck to Madison (300 mile round trip) to pick up some machines (using a snap-on scanner) coming back the engine started to constantly miss fire on #1 cylinder idling The other 7 did not miss fire. So we changed the spark plug on No.1 cylinder no change. We ran the truck home watching the miss fire on the scanner. When running down the road cursing at 55 mph cylinder 2 would continue to miss fire and cyl 1 will not miss fire. When you step on the gas peddle they would switch Cyl 1 miss fires and Cyl 2 would not miss fire

Any Ideas where to start to look. Thank you for any responce.

 

[wtd]

Have you looked at your fuel trims after the truck warms up? Since cyl 1 & 2 are the first two cylinders on each bank, maybe you didn't get the intake plenum gasket on the front seated correctly. The fuel injectors in these trucks also can become problematic as they age.

Is the cap and rotor AC-Delco or an aftermarket brand? What about the spark plugs? These trucks can be picky about what ignition components are used. I own a 98 K3500 with the 7.4L but have not experienced what you are with the misfires.

 

[wwillson]

the engine started to constantly miss fire on #1 cylinder idling The other 7 did not miss fire. So we changed the spark plug on No.1 cylinder no change.

Was the plug from cylinder 1 fouled?

 

[liv2shoot]

Plug no.1 had 200 miles on it and was not fouled

The GM distributor is all plastic the ears that hold the cap on broke. So we replaced the complete distributor with genuine GM. set the cam timing at +2 We used new aftermarket plug wires. And new AC delco double platinum plugs. We used the propane test on the plenum gasket then tried starter fluid the same way. no change in RPM. So we figure the gasket must be okay. Cannot find any vacuum leaks

We also checked for plug wires arcing

So what we are doing right now is New GM brand cap, rotor , plug wires, coil, ignition module, all GM brand

What should the fuel trim be on the scanner? and what to look for with the fuel trim?

Do you think if you change one injector all should be changed with 105,000 miles
What injector would you recommend?

What confuses us is it ran great for 200+ miles and then all of a sudden once it started to miss fire cyl. 1 and 2 would alternate miss firing depending if you were on the pedal or just cruising.

Once we figure it out I will post the cure.

Thank you guys for the response

 

[burbguy82]

Since you removed you distributor.....check the cam correlation. It should be less than 2 degrees, or 1.5 from memory at idle. If not you can move the distributor to get close. Also, make sure the hold down clamp is tight.

 

[burbguy82]

What should the fuel trim be on the scanner? and what to look for with the fuel trim?

less than 10 + or -

Do you think if you change one injector all should be changed with 105,000 miles
What injector would you recomend?

not one, all.........bosch style is what I used....if you do this replace the FPR aswell and the little vacuum tube. Be careful with the plastic fuel rail.

What confuses us is it ran great for 200+ miles and then all of a sudden once it started to miss fire cyl. 1 and 2 would alternate miss fireing depending if you were on the pedel or just crusing.

again check cam correlation on scanner if possible. and check mounting bolt

 

[clinebarger]

You have 26 year old Multec/Rochester injectors....They can cause a lot of issues & not set DTC's, I've seen shorted injector circuits not set codes on these, Vortec small blocks & Gen II LT engines.

Get some Bosch injectors of the same flow rate & impedance, Trav @Hurst fuel injector service.

L29's have a 4X crank sensor tone wheel....That's pretty low resolution for a accurate misfire monitor & should be taken with a grain of salt.

 

[wwillson]

Get some Bosch injectors of the same flow rate & impedance, Trav @Hurst fuel injector service.

You can contact @Trav here in the forums. He is very knowledgeable on fuel injectors and runs a business cleaning and overhauling them.

 

[Hirev]

How good are your plug wires?

 

[Number_35]

Besides all the good suggestions above, I would hook up a vacuum gauge to test vacuum at idle. That will confirm you don't have a vacuum leak somewhere.

As well, paging @clinebarger who is an authority on GM V8 engines.

 

[Rob_Roy]

The fuel injectors on the Vortec 5.7L and 7.4L engines are known for issues. If you don’t find any ignition issues, I would go after the injectors too much.

 

[wwillson]

As well, paging @clinebarger who is an authority on GM V8 engines.

He already gave his suggestion:
https://bobistheoilguy.com/forums/threads/2000-obs-k3500-454-miss-firing.407802/post-7619192

 

[Lawnman23]

Since you removed you distributor.....check the cam correlation. It should be less than 2 degrees, or 1.5 from memory at idle. If not you can move the distributor to get close. Also, make sure the hold down clamp is tight.

Cam Retard value +/- 2* from 0 and blipping throttle north of 2k RPM’s, adjusted by rotating the distributor. At least for the small blocks. I have a few newer scanners that don’t show this value, only one that I bought back in the day for this exact purpose does.