what can cause a converter to periodically unlock while cruising

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BOTTLEDZ28

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Wonder if doing a crank relearn would help you out any?

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Not sure how that works considering I have a 0411 PCM in it. I dont remember doing a relearn when I swapped it in. The truck fired up on the first shot.
 

BOTTLEDZ28

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Update, the misfires are getting worse. Ironically, they got worse after I changed the plugs and wires. I had the parts in the shop so I figured why not change them. I used AcDelco parts.

I picked up the dreaded P0300 code 3 days after doing the work. It was a pending code and was stored.

At first, it was just cylinder 4 that was giving me a handful of misfire counts. Then, cylinder 7 picked up a handful. Both were at idle. At 2K RPM cylinder 4 starts misfiring a lot. To the tune of 50-75 over a a 30-45 second period. Long term fuel trims are now between 10 and 15% on both banks. The P0300 was triggered when cylinder 6 misfired 144 times. Total misfire counts around the time the light was flashing was 196.

Not sure if I mentioned it before but a new AcDelco cap was installed maybe a month ago. I had it in the shop.

Tonight, I will be attempting to do a crank relearn on the PCM and see what happens. If I remember correctly, I was unable to do it the last time I tried. Something about the PCM not allowing the Scanner to do. I am running a 0411 PCM.

I am also going to try and smoke the engine this weekend to check for vacuum leaks. After 170K miles and a lot of messing around under the hood, there is a good possibility that maybe there is a rubber hose that is dry rotted or cracked and allowing unmetered air to enter the engine.

If anyone else has any other educated suggestions then please post them up. I will get to the bottom of this.

PS. I will also be doing a compression test at some point
 

BOTTLEDZ28

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Crank relearn did not fix the misfire. Injector balance test did not fix the misfire issue.
 

east302

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You mentioned that it got worse after replacing the plug wires. Is it the original distributor...are the plastic tabs that the cap screws into cracked, causing the cap to be loose?

GM has a couple of bulletins that may be worth looking into:

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Otherwise, here is the diagnostic for P0300 on a 1998, maybe it will help.

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Fuel system diagnostic referenced in the above tables:

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BOTTLEDZ28

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Yes, original distributor. I pulled it a couple years ago to do the lower intake gaskets an inspected in. Nothing abnormal was found so I reinstalled it. I am going to look at it again tomorrow and see if the screens are blocked up on it. Not sure how critical those are but this type of cap doesnt really allow a lot of air flow and if the screens are clogged, it ma be holding in moisture.

All the parts that I have ever used on this truck have always been AcDelco. The spider was a Dephi unit.

Not sure if I mentioned this already but this problem has been going on for a few years. It did it with the original ECM as well so I can rule out any issues with the Gen3 PCM and wiring. One of my cats is rattling now. Its a 2 year old Magnaflow dual cat ypipe setup. Not sure if the cat just failed on it own or if it failed due to the misfires occurring. Either way, I will have to replace the ypipe assembly at some point this year.
 

east302

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Did the cam/crankshaft sensor correlation (CMP Offset or Retard) get reset when the distributor was pulled? I don’t think it, by itself, would cause a misfire, but the VCM does use it to identify which cylinder is misbehaving.

I’d wager that the converter issue is due to your misfire or running rich. Does swapping the oxygen sensors to the other bank change the misfire location?


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BOTTLEDZ28

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I used a Snap On scanner to reset the CMP back to 0 degrees.
I havent bothered with O2s because I am getting misfires on both banks. Worse one is 4 though and its really bad on 4 at 2K RPM. Cylinders 2 and 6 get a handful and so does 7 which is on the other bank. I think 7 started after the plugs and wires but I cant confirm since I do not remember if I was logging data and watching misfires prior to the parts change. I have already went back in and double checked the plug and wire too.

Im not ruling out a bad spider, even though its only a couple years old. I just dont want to guess at it and spend another $300 to swap it out and find out it wasnt the problem. The engines runs fine. It has power. It doesnt shake either.
 

Eveready

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I have a 98 Tahoe with a 4L60E with 177K on it. Every once in a while, the converter will just unlock while cruising. It will go back into lock up eventually but its been happening everyday now. It used to only do it when it was raining out but not every time it rained. I thought I fixed it with a plug and wire change but it came back. Its not the brake switch.

I thought it wasn't the brake switch on my old Dodge but it turned out that the mount was just loose enough that it would rattle the switch when moving. Sitting still for diagnostics it would not move at all so no indication of a problem.

Probably OK but that one bit me on my van.
 

BOTTLEDZ28

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Current fuel pressure with engine running.

With the Ignition On/ Engine Off and fuel pump priming, pressure reads 59-60psi

Once pump stops priming after 2 seconds, pressure drops to 52psi .
With engine running at 2k rpm, pressure is around 54psi.
I’ll be doing a fuel filter tomorrow and see if it helps.
 

Christian Steffen

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Current fuel pressure with engine running.

With the Ignition On/ Engine Off and fuel pump priming, pressure reads 59-60psi


Once pump stops priming after 2 seconds, pressure drops to 52psi .
With engine running at 2k rpm, pressure is around 54psi.
I’ll be doing a fuel filter tomorrow and see if it helps.

I doubt that is your issue, as long as it primes good key on engine off, and doesn't leak down at a stupid fast rate after shutdown, its probably fine.

Doesn't hurt to replace the filter though, they're cheap.
 
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