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I am having a strange misfire at part throttle. Between 30-35 position reading on my TPS from my R.E.M. It doesn't happen while idling or any position of the throttle above or below 30-35. I can see my knock sensor reading spike when it happens but there are not any codes or other anomalies in all the read outs that would point to a source of the problem.

 

I have swapped out my TPS with a spare I had and set it properly with no change. I have also changed my spark plugs distributor cap/rotor.

 

I am heading towards it being an ignition problem maybe the coil, ignition control module or distributor stator pickup.

 

There is an I.S. note about the distributor stator going out (I think it is 36e). Which is probably the next thing I will change out.

 

Any help would be greatly appreciated.

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8 hours ago, Ωhm said:

Sometimes a kick and a buck are caused when ECU drops out of Closed Loop (LP) for an instant period of time. See if that correlates with the REM.

The R.E.M. shows it stays in closed loop while it misfires. It is worse when the engine is cold than when hot.

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Thanks, on those fuel trim values. As you already mentioned, ST&LT read outs look good. Good call on possibly an ignition problem. If you haven't moved your distributor, it should be ok but still look at indexing (cruiser tip #13).

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On 2/25/2025 at 6:55 PM, Ωhm said:

If you haven't moved your distributor, it should be ok but still look at indexing (cruiser tip #13).

Good news I got my distributor indexed properly and the misfire issue has gone away.

 

Looking at the old cap the spark was arriving early catching just the edge of both the rotor and the cap. Surprised it ran as well as it did.

 

Bad news is my distributor gear is very worn and I will need to get it replaced soon.1783766214_IMG_20250226_1818576782.jpg.90a9f875755b934d672a6e25469be72c.jpg

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35 minutes ago, Ωhm said:

If your misfire just showed up one day, I’d wonder if that gear jumped a tooth.

It may have I don't remember it misfiring when I got the truck.

 

6 hours ago, watchamakalit said:

Might be worth sticking a camera down the hole to check the condition of the drive gear on the cam.

I have a borescope and will look at the cam gear when I get a new distributor.

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On a hunch I thought I would see if my extra 4.2 liter distributor had a better gear. It did and was a direct swap for the old worn out one.

2115571326_IMG_20250227_1850515452.jpg.83a6caec93905ccfa5023c1fd4247757.jpg

 

I looked at the gear teeth on the camshaft and they look like they are about as worn as the old distributor gear. Yet another project for down the road. When that happens I will probably take the opportunity to swap to a 7120 head with H.O. cam and intake manifold. 

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4 minutes ago, Ωhm said:

Check and make sure that the drilled hole for the spring pin alignment is in the same location on both gears.

I did check that both holes were in the same location both in relation the gear teeth and the bearing surface. I also used a added a lot of engine assembly lube to the "new" gear before reinstalling the distributor.

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