P1351 one time before MPFI conversion, worse after (MISFIRE CYL 4)

[bigfootchiro]

Need some help here. I've had an intermittent slow start issue for quite some time now. Would only do it on occasion after truck was warmed up. Did a leak-down test on Spider, and found a very slow leak. Decided to do the MPFI upgrade. On Friday, I got a P1351 with a no start after truck was slightly warm, and the Brake Module code, but eventually got it to start and truck ran fine. Yesterday I did the MPFI conversion and replaced the plenum gasket. Looked like someone had been in there before me, as the plenum gasket looked fairly new, but was chewed up toward the rear next to the distributor. After driving it, I noticed a good misfire that would cause the truck to hiccup around 40-45 with a significant drop in RPMs, especially with lower RPMs and under some load (after TC lockup, for example). I started going through the P1351 checklist, but couldn't get past Step 2, which says "Go to Diagnostic Aids". I checked the VCM, and the terminals look good. I'm thinking a short circuit or bad ground, but not sure where to look! ICM is new (P1351 developed after I installed the new one, swapped out for old today and no change), coil is new, plug wires new, past owner said distributor is new, fuel pump new (good fuel pressure).

Any help would be greatly appreciated! Do not want to take it to a shop and have them rack up the bill, but may have to if I can't figure it out. Thanks in advance!

 

[bigfootchiro]

Also, seems to accelerate fine under WOT and/or heavy throttle, but under light acceleration from a stop, you can feel slight misfires. I should also mention that I'm not getting the P1351 code right now, as it starts just fine, but still getting the misfire. At times, if I blip the throttle after putting in drive or reverse, it will hesitate/studder, then return to normal. Not sure if related to P1351.

 

[east302]

The "Diagnostic Aid" portion of the P1351 test reads as follows:
-------------------------
Diagnostic Aids--

Intermittent test - If the connections and the harness check OK, monitor a digital voltmeter connected between VCM terminal C3 pin 9 and C4 pin 18 while moving related connectors and wiring harness.

If the failure is induced, the voltage reading will change. This may help to isolate the location of the malfunction.

An intermittent may be caused by any of the following conditions:

*A poor connection.
*Rubbed through wire insulation.
*A broken wire inside the insulation.

Thoroughly check any circuitry that is suspected of causing the intermittent complaint.
------------------------





Hope this helps.


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[bigfootchiro]

Thanks east302. I went through that, but not sure if I did it properly as I did not see any voltage change. Could be that I switched back to the original ICM, if that were throwing the P1351. I still have a major hesitation/misfire though. Drove it down the freeway and under light to moderate throttle while TC is locked up, will cause it to buck.

 

[east302]

Do you have a scanner that can read misfire counts?


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[bigfootchiro]

Do you have a scanner that can read misfire counts?


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I have a WiFi OBD reader hooked up to my DashCommand app. I know it monitors misfires, but not sure about counts.

 

[bigfootchiro]

Cylinder 4 misfires 609 times versus 0-6 times on all other cylinders :O

 

[east302]

How do the plugs look? Swap #4 with another cylinder and see if the misfire follows.

Is the distributor cap new? Many have better experience with Delco versions.

In DashCommand, see if the camshaft position sensor offset (CMP Retard) reads zero degrees (plus or minus two) at 1000-1100 rpm. Rotate the distributor to compensate.

How does the distributor gear look? Are the tabs for the cap hold down screws cracked or broken?

Last, a compression test wouldn't hurt, or pop off the valve cover and see if you spot anything on #4.

Did the original spark control module get transferred to the new VCM? Assuming the VCM was programmed, was the CKP sensor relearn done as well? Pretty sure that is required.


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[bigfootchiro]

How do the plugs look? Swap #4 with another cylinder and see if the misfire follows.

Is the distributor cap new? Many have better experience with Delco versions.

In DashCommand, see if the camshaft position sensor offset (CMP Retard) reads zero degrees (plus or minus two) at 1000-1100 rpm. Rotate the distributor to compensate.

How does the distributor gear look? Are the tabs for the cap hold down screws cracked or broken?

Last, a compression test wouldn't hurt, or pop off the valve cover and see if you spot anything on #4.

Did the original spark control module get transferred to the new VCM? Assuming the VCM was programmed, was the CKP sensor relearn done as well? Pretty sure that is required.


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Thanks for the quick responses!


Truck is still hot, so will wait for it to cool down before checking plug.

Distributor is supposedly new, replaced about a year ago.

Checked CMP retard, and it reads between .8-1.0 at 1150 RPM.

 

[bigfootchiro]

This is the spark plug from Cylinder 4. Burned?