Tough miss/backfire problem under load - 1992 Lesabre

Hello. I've got a head scratcher. I have an engine that misses and backfires badly upon heavy acceleration once the car is fully warmed up. It has done it with two different transmissions. It barely does it at all when first started in the morning. The difficulty is that I can't imagine what could cause it.

I have replaced with either new or known good parts (I have three 92 Lesabres) the spark plugs, spark plug wires, coil pack, ignition module, crank sensor, computer, injectors, fuel pump, fuel tank, fuel filter, fuel pressure regulator, MAF sensor, and throttle intake.

The engine was rebuilt by me a couple years ago. It was a poor man's rebuild. I put new rod and crank bearings in. Checked the specs on the crank (good.) New piston rings, new timing chain and gears (and cam magnet,) new valve guides, a thorough cleaning of everything including individual valves (ALL carbon was removed.) I honed the cylinders. I got a different camshaft from a junk yard because the original was pitted and worn on a few lobes. The replacement was under spec but did not look worn. (Spec was supposed to be .250 and the cam was .243 or something like that.) I hoped it would be close enough to not have to buy a new cam for over $200. The engine was lubed with engine assembly grease during the process and put into service within a week of building it. At first it ran great. At some point later it developed the miss/backfire. It may have only been a month or two before becoming a problem.

Clue 1: I had to pull the oil pan a month ago and found metal shavings in the pan. I pulled all the rod caps and found that cylinder 3 was abnormally worn and likely the cause of the shavings. I replaced that one cylinder's rod bearings. The engine runs the exact same now as it did before the change.

Clue 2: I had the exact same engine in a different 92 Lesabre a few months back and I thought the problem was likely a clogged catalytic converter (after I had replaced all of the above parts.) Now that it is in a different car I know that is not the problem.

I'm thinking possibly it is something internal in the engine such a valve or cam problem, but I'm not sure.

Any ideas?

PS I am now planning to pull the motor if I can't figure it out and disassemble. I have another perfect running engine with 139K sitting around. I am debating whether to install the 139K engine as is and get what I can out of it before rebuilding or rebuilding now while it is out of the car. I also got a 97 Lesabre tranny out of the pick and pull that I am going to install after swapping the wire harnesses and making the solenoids/temp sensor match my 92 setup. My current tranny is starting to shudder under load at times.

Have you performed a compression & leak-down test on all cylinders? Have you looked at the spark plug electrodes to compare firing on all six plugs while removed from the engine? What kind of ignition wires are you using & what is the resistance of each one? (Check resistance while flexing the wire & note if any change in reading)

Have you performed a compression & leak-down test on all cylinders? Have you looked at the spark plug electrodes to compare firing on all six plugs while removed from the engine? What kind of ignition wires are you using & what is the resistance of each one? (Check resistance while flexing the wire & note if any change in reading)

I just did a leak down test. Each cylinder was between 2-3% with a warm (not hot) engine. I can't do a compression test yet because I think I loaned out my compression tester to my wife's cousin (I can't find it so they may still have it.) The spark plugs are tanish brown on four plugs and whitish orange on two plugs (3 and 5.) There is no wetness on the electrodes. The wires are black wire with "8mm Double Silicone Ultimate Heat Protection" on them. I also have gray wires on a different 92 with "8mm Hi Energy Suppression Cable."

My coil packs resistance is 5.79K, 5.81K, and 5.81K.

My spark plug wire resistance is:

#1 4.7K-5.2K
#2 10.5K
#3 5.5K
#4 9.2K
#5 5.5-9.5K (goes up when wiggling) (Also they gray #5 is 7.5K)
#6 9.6K

I did notice my coolant is slowly disappearing out of my overflow tank.

Therefore my new theory is that my head gasket has failed between cylinders 3 and 5 and that coolant is seeping into the combustion chambers fouling up the combustion process. The head gaskets were brand new when I rebuilt the engine but perhaps the heads or block were slightly warped. I'll check it more carefully this time. Time to tear it down.

My current tranny is starting to shudder under load at times.
I was going to start a thread. What is it that causes the trans to shudder?

Chris.

I was going to start a thread. What is it that causes the trans to shudder?

Chris.

I do not know the internal cause. But it happens under load (heavy acceleration or going uphill.) To get it to quit I have to momentarily give it less gas and then accelerate again a little slower. On a bad hill I could see having real trouble.

I need your 97 Trans more then you do.

Chris.