95 TBI Hot Miss troubleshooting

[The Iron possum]

I haven’t posted in a while but now need some help troubleshooting. Truck runs fine for about 15 minutes but then starts missing, especially on acceleration or under load. It ran fine for the last 6 months after engine swap. Many of the parts that were transferred over during the swap had been replaced and Run in for 3-4 months before the swap on the old engine. I think it is an electrical issue since it seems to be heat related.
My first thought was the new tune, but I put the original tune back in and problem is still there. I can data log, but haven’t seen any issues in the data except a drop in RPMs when the miss occurs.
Truck is 95 K1500, 4L60E
383 stroker installed Nov 2018.
New injectors
New plugs and wires
New fuel pump with adjustable regulator set at 15lbs
New ignition coil (run in)
New distributor (run in)
New Map sensor (run in)
New EGR (run in) (dialed out in tune)
New coolant temp sensor on intake (run in)
New ground strap from frame to firewall and firewall to engine block

Since the problem started I have-
Ohmed out plug wires - all good (even changed sides 1-2, 3-4, etc)
Checked voltage on TPS - smooth through range
Changed plugs (old ones still looked good)
Changed out ignition control module on distributor
Changed out ignition coil

Sometimes I get a backfire, sometimes it feels like the spark is grounded and I just loose power for an instant. Almost like a surge during acceleration. Fuel pressure gauge is where I can monitor it and it stays solid at 15 psi.

Other than the grounds on the intake and frame/engine, where are some other grounds I could check?

Thanks for any input!

 

[The Iron possum]

Has anyone ever had an ignition switch start to go bad and cause a miss? From the Haynes book, power to the coil comes directly from the ignition switch. If the switch contacts weren’t making up all of the way, could that cause an issue? I’m running out of things to test or replace.

 

[Schurkey]

Truck runs fine for about 15 minutes but then starts missing, especially on acceleration or under load.

I think it is an electrical issue since it seems to be heat related... ...drop in RPMs when the miss occurs.

New distributor (run in)
Changed out ignition control module on distributor
Changed out ignition coil

Is this a Chinese replacement distributor, or a "real" Delco distributor? Did you ever inspect the pickup coil?
Have you inspected the power supply to the distributor/ignition coil? (see below)

Has anyone ever had an ignition switch start to go bad and cause a miss? From the Haynes book, power to the coil comes directly from the ignition switch. If the switch contacts weren’t making up all of the way, could that cause an issue?

Not a bad idea. Not the first thing I'd be looking at, though. The distributor and ignition coil need to be powered--a fault anywhere from the battery to the ignition switch, or from the switch through the harness to the distributor and coil could cause ignition misfire.

 

[The Iron possum]

Is this a Chinese replacement distributor, or a "real" Delco distributor? Did you ever inspect the pickup coil?
Have you inspected the power supply to the distributor/ignition coil? (see below)


Not a bad idea. Not the first thing I'd be looking at, though. The distributor and ignition coil need to be powered--a fault anywhere from the battery to the ignition switch, or from the switch through the harness to the distributor and coil could cause ignition misfire.

Distributor is not a Delco. It’s the autozone Duralast. The pickup coil I removed didn’t look burned, but I replaced it anyway with no change. Even with a new pickup and ICM, could there be enough slop in a 6 month old Distributor to cause a miss during acceleration?

With truck idling (cold), I had 13.6V from ignition switch to the ignition coil. I have a spare map sensor to put in tomorrow to see if that makes a difference.

Thanks for the input.

 

[The Iron possum]

Replaced distributor just to rule it out. Also put in my spare MAP sensor. No change in performance. Still drops out like a miss or stumble during acceleration. Will replace plug wires tomorrow (always wanted a spare set) even though these have ohmed out good. Running out of things to test/replace. May do a compression test to rule out cracked head or head gasket. No water in oil and no smoke, so not expecting to find anything. Only 500 miles on engine.

 

[PlayingWithTBI]

Can you get a data log to see what the timing and AFR are doing?

 

[The Iron possum]

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Here are 2 screenshots of rough acceleration. This is just normal coming up to speed with 2 shifts. Top image is RPM (green), spark advance (blue) and MAP (red). Bottom pic is RPM, Advance, and O2 (red). I don’t have a wideband O2, so no data there. I have a full set of tuner pro Rt data logs posted at gearhead efi in .XDL format under same name. Not sure I could attach that file type here. I’m starting to think timing signal is getting “disturbed” and causing dip in rpm. MAP is just following rpm.Those dips in rpm feel like surge/miss in the drivers seat. Maybe bad wire or ecm going bad? I can take screenshots of any other parameters if anyone is interested.
Thanks for the interest! I can email the log if you would like to look at it.

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[The Iron possum]

I’m cautiously optimistic that the miss is fixed. Bought a cheap set of plug wires just to rule out wires as suspect. Truck runs great with no miss or hesitation. Plug wires were Taylor pro 8mm and only 4 months old when issue started. Headers must have gotten them. Strange because they ohmed out good and there was no change when I swapped wire for wire across the engine.

Oh well, as least I’ll have a spare set of troubleshooting plug wires after I get some new ones and high temp sleeves.
Thanks for the suggestions and help!

 

[PlayingWithTBI]

Strange because they ohmed out good and there was no change when I swapped wire for wire across the engine.

When you "Ohm" out a wire with a DVM you won't see a ground if the wire isn't actually touching the ground. That's why you need a Megohm Meter, it shoots up to 5K Volts which will find the potential short. We used to do that on our big electric motors (50HP +) periodically to predict when they will go out and schedule when to change them without experiencing unexpected downtime.

Good thing you got it. On my 350 I had to put in heat boots on #s 5 - 8, my headers melted 2 of the boots within 2 hours of running. No problems since (~4,000 Miles)

 

[Jglew82]

On my 350 I had to put in heat boots on #s 5 - 8, my headers melted 2 of the boots within 2 hours of running. No problems since (~4,000 Miles)

100%, I had the melting issue with my headers too. An inexpensive set of sleeves saved the day.