Mystifying misfires on a 99 Tahoe

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I have a 99 Tahoe with random misfires on 2 and 3 that I have been trying to diagnose for months. I have replaced the cap, rotor, wires and plugs with ACDelco parts. I swapped out the coil and ignition control module with a spare I had and still have the misfires.

I put a PCM from Black Bear Performance in it and when I was taking it a mechanic to get the crank relearn done to get rid of the P1336 code, that is when the jerking on acceleration started. I had changed the crank sensor about 6-8 months earlier and this behavior was similar to what I was experiencing then, so I changed it out before having the crank relearn done. Unfortunately, this didn't solve anything.

I thought the new PCM was causing this, so I swapped it back to the original, but again nothing changed.

I unplugged the MAF sensor and nothing changed. I took it to a mechanic and he diagnosed it as being the distributor. So I replaced it with an ACDelco unit ,but it didn't solve anything.

I also checked fuel pressure:
KOEO: 54psi
Idle: 49-50psi
Operating range: 48-58psi (driving home)
Bleed down results:
56psi-0min
51psi-18min
38psi-36min
34psi-45min
33psi-49min
31psi-54min
30psi-58min
I used an endoscope to see if the injector nozzles on 2 and 3 were leaking. The image was a little grainy, but I didn't see anything. I am leaning toward the spider, because I don't know what else it could be. It seems to me that if it was the fuel pump, my problems would be more evenly distributed over all cylinders than just 2 and 3.

Well I replaced the spider with an ACDelco 217-3029. Fuel trims got way better so initially I thought it was finally fixed. NOPE! Believe it or not, now the misfiring cylinders are 1 and 2. I am beginning to think that my problem is a bad ground.
 

NastyNate98

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I have seen bad/corroded grounds do very strange things to the diesel trucks I work on. Definitely find your engine to chassis, chassis to body, and body to engine grounds, if you have all 3, and clean them up with a wire brush. I've gone down the rabbit hole just like you in the past, only to find out it could have been fixed in 20 min by checking and cleaning ground cables! Good luck
 
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I appreciate your response. I just wish I had a decent schematic to go by. I'll look for these 3 grounds you are talking about and replace any I can't find. Thanks again.

Jeff
 

east302

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Are you losing any coolant? A pressure test may be in order.

When were the lower intake gaskets last replaced?




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east302

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I appreciate your response. I just wish I had a decent schematic to go by. I'll look for these 3 grounds you are talking about and replace any I can't find. Thanks again.

Jeff

There is the ground screw by the battery to fender (one ground wire from the headlight harness and one from the battery cable sharing the same bolt, one by the harmonic balancer, one by the thermostat and one on the back of the passenger cylinder head, roughly where the transmission dipstick tube runs by the firewall.


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Are you losing any coolant? A pressure test may be in order.

When were the lower intake gaskets last replaced?




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No, I am not losing any coolant. I put an updated steel insert intake gasket approximately 15000 miles ago
 
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There is the ground screw by the battery to fender (one ground wire from the headlight harness and one from the battery cable sharing the same bolt, one by the harmonic balancer, one by the thermostat and one on the back of the passenger cylinder head, roughly where the transmission dipstick tube runs by the firewall.


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Thanks
 
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There is the ground screw by the battery to fender (one ground wire from the headlight harness and one from the battery cable sharing the same bolt, one by the harmonic balancer, one by the thermostat and one on the back of the passenger cylinder head, roughly where the transmission dipstick tube runs by the firewall.


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I have all of these and measured 0.4 ohms or less from the negative battery post to the engine, frame rail and body panels.
 

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Was the CMP/CKP offset (CMP Retard) set when the distributor was replaced?


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Was the CMP/CKP offset (CMP Retard) set when the distributor was replaced?


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I have Dashcommand and was able to set it to within +/-0.5 degrees.
 

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