Head scratcher

[L31MaxExpress]

Think I figured out what killed the cats on my 97. Felt a little funny and here is why. I wanted to confirm the plugged cats were the weird misfire feeling, before I put the new ones in place. I have not put the new cats on yet, just 3" tubing. 10 minutes of cruising around. Seems to misfire at light throttle, 1,500-2,500 rpm ballpark. Lay into it harder and does not show any misfiring. I want to blame the cam overlap, but I would certainly expect to see near even misfiring on all cylinders if it were overlap. If it were ignition I would expect worse misfire under actual load ditto for valve spring issues. You can really feel when both 3 & 4 misfire in the same cycle.

After it cools off I am going to jack it up, put it on stands, pull the front wheels off and do some compression readings on the cylinders I can reach through the wheel wells. Since cylinder 1 is good, I will compare 1 to 2, 3 and 4, pulling 1 plug at a time. I do not understand how half the engine runs perfectly, 1/4 of it slightly acts up and 1/4 of it is totally unhappy except under heavy load. At this point literally thinking lifters. If the compression and spark plugs do not show any anomolies literally will have to assume it is lifters and proceed with the cam, lifter and intake swap. When I do this swap, I will move the injectors around too. Put the ones in the "bad" cylinders into the good cylinders in the event it is an injector flow issue at low pulsewidths.

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[0xDEADBEEF]

3 & 4 share a rod journal. That's all I got.

 

[Moofus02]

It's harder to fire across the plug gap at light throttle or idle than heavy throttle. Less stuff for the spark to skip across. I've got a 5.7 with a weak cylinder on each bank. Ears cap and rotors. I can kill one in 3 hours if I let it idle. If it doesn't get idle time they last quite a while

 

[L31MaxExpress]

It's harder to fire across the plug gap at light throttle or idle than heavy throttle. Less stuff for the spark to skip across. I've got a 5.7 with a weak cylinder on each bank. Ears cap and rotors. I can kill one in 3 hours if I let it idle. If it doesn't get idle time they last quite a while

I have done testing with the old ignition scopes. When you snap the throttle the secondary ignition voltage drastically climbs in response to the elevated cylinder pressure.

 

[Supercharged111]

I'll add this: I'd replaced a cap and rotor last year on the 1500 due to cross firing and I went with brass contacts for corrosion resistance. I must have a ventilation issue because I found a horrific amount of corrosion in there just a few days ago. Truck became a hard starting pig, would idle well, popped and farted with light throttle, ran well with cruising throttle, and broke up again with a high load. The lean cruise also pissed it off which makes sense, but the rest of the symptoms do not jive IMO with high resistance on the ignition circuit.

 

[Moofus02]

I have done testing with the old ignition scopes. When you snap the throttle the secondary ignition voltage drastically climbs in response to the elevated cylinder pressure.

I want one of those scopes.
Not arguing just my observation that wide plug gaps and lean mixture low compression etc will kill cap and rotors idling. I've got a miss at light throttle that goes away when I get in it but i really think the low compression is a couple bad valves. Easier to put a cap and rotor on it every 3 to 4 months than change the engine. Been like this for 160k miles

 

[L31MaxExpress]

I'll add this: I'd replaced a cap and rotor last year on the 1500 due to cross firing and I went with brass contacts for corrosion resistance. I must have a ventilation issue because I found a horrific amount of corrosion in there just a few days ago. Truck became a hard starting pig, would idle well, popped and farted with light throttle, ran well with cruising throttle, and broke up again with a high load. The lean cruise also pissed it off which makes sense, but the rest of the symptoms do not jive IMO with high resistance on the ignition circuit.

My distributor cap and rotor are Mercury marine brass terminal, replaced them about a year ago when the ~5 year old MSD gave up, maybe have 2,500 miles on them. Plugs and wires at that point too. My distributor base is vented like an optispark. Never really have had a corrosion Issue with it, just burned through caps and rotors. Mine starts easy, idles good, and runs flawless under a fairly heavy load. Mine misses at very light acceleration throttle, light throttle cruise, and deceleration when it is not in DFCO. Lean cruise disabled because I was working on getting the narrow bands dialed back in. I actually had to target a richer 02 voltage to get the wideband back to stoich.

It has cooled off enough both in the shop and the engine from running it earlier that I am about to tear into it. Already have it on jack stands and front wheels are off. One thing I am going to look carefully at is the crush seal on those plugs especially. 3 & 4 are the hardest plugs to get to without jacking up the thing, removing the front wheels and going through the inner fenders I changed the last set of plugs laying on my back with an offset 5/8" wrench, maybe I did not get them crushed enough and they are pulling air into the cylinder when the manifold vacuum increases off idle at 2,000 rpm. At idle it has lower vacuum, WOT very little vacuum but at 2,500 rpm it pulls over 22 in/hg and I noticed it misfires worse at 1,500-2,500 rpm with no load and the misfire progressively goes away in severity as the throttle is opened. Deceleration it will pull 25+ in/hg at 2,500 rpm and I can feel it miss pretty severely until it goes into DFCO on a throttle lift at speed. When I go up some slight grades it goes away, coasting down the other side, it misfires, shudders and carries on. Had a similar issue years ago with a worn distributor gear, but the CMP reading stays spot on around 2* now with nearly no variation where it was all over the place with the worn gear.

I am going to do both a cranking compression test and an idling compression test on the front 4 cylinders here momentarily. Will also confirm plug gaps are still the 0.044" I had them set at.

 

[L31MaxExpress]

I want one of those scopes.
Not arguing just my observation that wide plug gaps and lean mixture low compression etc will kill cap and rotors idling. I've got a miss at light throttle that goes away when I get in it but i really think the low compression is a couple bad valves. Easier to put a cap and rotor on it every 3 to 4 months than change the engine. Been like this for 160k miles

I do not see the exact model, but the one I have experience using was similar. Also had a 5 gas analyzer and a distributor curve machine in the same area of the shop. It is amazing to me what we used 20 years ago to diagonse stuff when the SES light was not there with a code pertaining to a specific cylinder. I have also tested a buch of stuff with an O'scope. Ford MAP sensors that were frequency based, TPS, knock sensor, etc. Ever seen a wet flow bench for a carb? Shop had one of those too. The machine could literally calculate the airflow and fuel flow with the test fluid and give you a very close approximation of the air/fuel ratio the carburetor would deliver before it ever went back on the engine.

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[L31MaxExpress]

Still cranking the same ~200 psi it has since the Rhoads lifters came out of it. May have been 210 last time I checked it but that was with all the plugs out. So no piston, ring or valve sealing issues. Pistons and valves look beautiful from what I can see with my inspection camera.

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[Moofus02]

I do not see the exact model, but the one I have experience using was similar. Also had a 5 gas analyzer and a distributor curve machine in the same area of the shop. It is amazing to me what we used 20 years ago to diagonse stuff when the SES light was not there with a code pertaining to a specific cylinder. I have also tested a buch of stuff with an O'scope. Ford MAP sensors that were frequency based, TPS, knock sensor, etc. Ever seen a wet flow bench for a carb? Shop had one of those too. The machine could literally calculate the airflow and fuel flow with the test fluid and give you a very close approximation of the air/fuel ratio the carburetor would deliver before it ever went back on the engine.

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I remember those machines from when I was young. Never seen a wet flow for a carb but it sounds cool. Not many old guys around anymore that know how too use that stuff lol. I'm still amazed at how many guys I know that will build an ls and use a aftermarket computer instead of using a gm computer and tunning it. Your knowledge is impressive and it surprises me how many guys on some of the forums argue with you just to do it. Thanks for all the knowledge you share.
Never used a distributer machine but have done it the hard way in a motor with timing tape and a light lol