Misfire on idle after fixing code 42

[IsaakMooar]

I’ve got a weird one.
I recently bought my 1994 k1500 with the 350 from a guy in my area. He had owned it for a short time and the owner before him put an aftermarket tbi setup with a spacer and different ram style intake in it. To my knowledge that is the extent of the aftermarket parts atleast on the engine side of things.
It ran fine but had a dtc light so I checked it and it read code 42 and that was the only one. After going through the diagnostic trouble tree I found that the connector used to make an open on wire 424 (ic bypass) to set base timing was unplugged. So I plugged it back in and started it and the light went away. I then soon realized it had developed a misfire at idle. The idle seems to be at the same rpm although maybe slightly lower around 700 or so (I have a cheapo tac from autozone so who really knows) and the misfire is pretty steady but the idle doesn’t seem rough other than the one misfire that happens every 1-3 seconds. If i wack the throttle it doesn’t miss or hesitate. And while accelerating or cruising it doesn’t seem to miss at all. Only when on idle. Could it be a bad pcm or icm. It ran fine on base timing. It was slow but I figured it’s a tired ol 350. It also isn’t throwing any other codes. I’ve also only driven it about 5 miles since fixing the original code so it may throw one later on. Please help I’m very confused.

 

[Erik the Awful]

Is the miss on a specific cylinder, or does it roam around?

Check the cap and rotor for condition. Look carefully for cracks or arcing. Check the plug wires. Check for vacuum leaks, especially if the misfire appears after the truck warms up. Check your O2 sensor swings to make sure it's not getting worn out and lazy.

 

[Schurkey]

Doesn't miss with the timing connector open, misses when it's connected and the electronic timing advance is working?

Verify initial ignition timing, assure that the torsional damper hasn't shifted, and the condition of the distributor.

TBI distributor magnet is a common issue.

 

[IsaakMooar]

Doesn't miss with the timing connector open, misses when it's connected and the electronic timing advance is working?

Verify initial ignition timing, assure that the torsional damper hasn't shifted, and the condition of the distributor.

TBI distributor magnet is a common issue.

Yes with it just running off base timing it doesn’t miss. But with the electronic advance connected it misses but only on idle. Does the electronic timing control anything other than just ignition timing? Because if not wouldn't it stand to reason that there is a fault in that system if it ran fine off base timing with no electronic advance. Or maybe the previous owner adjusted the base timing to run fine at a bad afr or with vacuum leaks. Would it make sense to check timing at idle with and without the electronic advance hooked up and see what the difference is. If so what should it be at idle.

 

[IsaakMooar]

Is the miss on a specific cylinder, or does it roam around?

Check the cap and rotor for condition. Look carefully for cracks or arcing. Check the plug wires. Check for vacuum leaks, especially if the misfire appears after the truck warms up. Check your O2 sensor swings to make sure it's not getting worn out and lazy.

How do I check these things. How do I identity which cylinder is missing and o2 sensor swings

 

[Erik the Awful]

If it's a rhythmic miss it's a single cylinder. Elsewhere Schurkey has posted how to insert nails into the spark plug boots (with the engine off!) so you can ground them one at a time while it's running to see which one doesn't make a difference. With a rhythmic miss and your other symptoms, it would likely be a secondary ignition issue, a problem between the distributor cap and the spark plug.

If the miss is arhythmic, it's a primary ignition issue, a mechanical issue like a burned valve or vacuum leak, or a fuel issue. Since you state the engine runs fine on base timing, that makes the mechanical and fuel issues much less likely.

You can check your O2 sensor condition by watching the O2 sensor voltage while its running. Rev it up to 2000 rpm and you should see the voltage "swing" aka "crosscounts", between lower and upper voltage values at least seven times in ten seconds.

 

[IsaakMooar]

If it's a rhythmic miss it's a single cylinder. Elsewhere Schurkey has posted how to insert nails into the spark plug boots (with the engine off!) so you can ground them one at a time while it's running to see which one doesn't make a difference. With a rhythmic miss and your other symptoms, it would likely be a secondary ignition issue, a problem between the distributor cap and the spark plug.

If the miss is arhythmic, it's a primary ignition issue, a mechanical issue like a burned valve or vacuum leak, or a fuel issue. Since you state the engine runs fine on base timing, that makes the mechanical and fuel issues much less likely.

You can check your O2 sensor condition by watching the O2 sensor voltage while its running. Rev it up to 2000 rpm and you should see the voltage "swing" aka "crosscounts", between lower and upper voltage values at least seven times in ten seconds.

What all controls timing on these trucks. I’d assume the knock sensor does is there a map sensor and if so does it have an effect on timing. I’m wondering if there could be a simple vacuum leak or something that wouldn’t cause too much of a problem when the electronic timing isn’t working which could make sense if a map sensor has an effect on timing.
Also would it make sense to check timing with and without the electronic timing working to see what difference there is at idle.

 

[Schurkey]

What all controls timing on these trucks.

The computer, the EST module which may be separate, or integrated into the computer, and a handful of sensors; all working off of a programmed spark-advance table.

would it make sense to check timing with and without the electronic timing working to see what difference there is at idle.

Yes. The sooner the better. "0" with the timing connector disconnected, and maybe 11--12--13 degrees advanced with it plugged-in would be fairly common. Some guys advance the base timing from "0" to 2--4 degrees which may or may not make the engine run better.

Wild Guess with no evidence: Someone has jacked-up the initial timing for reasons of their own. When the computer advances the timing, you have too much advance at idle, and bad things happen.

Be sure to verify the timing mark on the damper in relation to "0" or TDC on the timing pointer, when the crank is turned to real, true TDC of #1. Ideally, you'd use a piston stop or cylinder leakdown tester to locate genuine TDC, and at that point the damper mark would align with the "0" on the pointer. If the damper is defective, or incorrect for the application, the "indicated" base timing might be correct, while the actual timing is far off.

 

[IsaakMooar]

Just checked vacuum and it’s extremely erratic at around 20inhg Almost looks like the needle is buzzing between 18 and 21

Is this a valve not fully closing? Or a vacuum leak.
And could this cause the mis if it was running fun on base.
I did the test with the truck not missing

 

[Schurkey]

Could easily be a valve-sealing problem.

Cranking compression test, perhaps a cylinder leakdown test. If you've got the leakdown tester connected, it'll be easy to find TDC #1 to verify the timing pointer/damper alignment.