Rough idle, P0300, Help!

[wiscomick]

Alright clan, need some wisdom. Truck is a 97 C2500 5.7 Automatic. Last night I was on the highway and the truck started misfiring like a mother out of nowhere. Rough idle, slow acceleration, blinking Service Engine Soon light.

Got it home, scanned it, came up with P0300. Replaced coil, plugs, and wires this morning, no difference. Pulled a compression test, everything looks fine.

Truck has recently been throwing a code for O2 Bank 2 Downstream no signal, but I have my doubts about that being this issue. I think it's just a dead post-cat O2. There's no heat difference post-cat on either pipe leading into the muffer so I'm ruling out a clogged cat.

In the last 6 months I have replaced intake gaskets, PCV, cap and rotor, air filter, fuel tank, sending unit, fuel pump, upstream O2 sensors, the works. I'm out of ideas. Scan info, compression test, and plug visuals are posted below, seems to be misfiring on 3 and 5 pretty hard. Starting to suspect maybe the injection spider is leaking? I have no idea.

Of course, I'm in the middle of 4 days off that I took to tackle work around the house and immediately my truck goes down when I need it most. Any help would be great.

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[RawbDidIt]

Bank 1 looks rich compared to bank 2. Confirmed by 2 rich looking spark plugs on right side of picture. I'd look at fuel system and sensors, with it being on just one side, it may well be injectors.

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[RawbDidIt]

Could be running rich to compensate for a vacuum leak in those 2 cylinders. Don't suppose you have fuel trim numbers?

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[wiscomick]

Spark plugs are sitting as they sit in the engine, with the front right one in the pic being cylinder number one, if that helps.

 

[wiscomick]

Unfortunately I don't have fuel trim numbers.

 

[Schurkey]

I think your scan tool is lyin' to you.

1. P0300 is "random misfire" but your misfire logs show it's specific to 3 and 5. I'd expect P0303 and P0305 if the scan tool was telling the truth. Usually, this is due to a scan tool reporting generic codes instead of the "true" specific codes.

2. Three oxygen sensor reports, including Bank 1, Sensor 3--but no Bank 1 sensor 2? No Bank 2, sensor 2? I'd have expected four O2 sensors, but maybe that's a big-block thing. At least, my '97 K2500 7.4L has four O2 sensors monitoring ahead and behind twin catalytic converters.

Also, is your chart showing 100 psi cranking compression in the upper left cylinder? That can't be good.

If I'm seeing your plugs in proper order--lower right is #1--then #6 is every bit as bad-looking as #5, but isn't misfiring. #3 looks the same as #4, but only #3 is misfiring. #3 looks completely different from #5, but they both ARE misfiring. Interesting.

I would verify spark to #3 and #5 with a spark-tester calibrated for HEI. If you've got enough spark to RELIABLY fire a genuine spark tester, you've got enough spark to run the engine. At that point, I'd be pricing a new spider assembly.





By the way--when the MIL is flashing, it's telling you to shut the engine off because you're in danger of damaging the catalyst. Once you get the engine running properly, you may want to verify that the catalyst cleaning-up the left bank is still functional. It may have overheated and melted internally.

 

[RawbDidIt]

I think your scan tool is lyin' to you.

1. P0300 is "random misfire" but your misfire logs show it's specific to 3 and 5. I'd expect P0303 and P0305 if the scan tool was telling the truth. Usually, this is due to a scan tool reporting generic codes instead of the "true" specific codes.

2. Three oxygen sensor reports, including Bank 1, Sensor 3--but no Bank 1 sensor 2? No Bank 2, sensor 2? I'd have expected four O2 sensors, but maybe that's a big-block thing. At least, my '97 K2500 7.4L has four O2 sensors monitoring ahead and behind twin catalytic converters.

Also, is your chart showing 100 psi cranking compression in the upper left cylinder? That can't be good.

If I'm seeing your plugs in proper order--lower right is #1--then #6 is every bit as bad-looking as #5, but isn't misfiring. #3 looks the same as #4, but only #3 is misfiring. #3 looks completely different from #5, but they both ARE misfiring. Interesting.

I would verify spark to #3 and #5 with a spark-tester calibrated for HEI. If you've got enough spark to RELIABLY fire a genuine spark tester, you've got enough spark to run the engine. At that point, I'd be pricing a new spider assembly.





By the way--when the MIL is flashing, it's telling you to shut the engine off because you're in danger of damaging the catalyst. Once you get the engine running properly, you may want to verify that the catalyst cleaning-up the left bank is still functional. It may have overheated and melted internally.

I was wondering the same thing regarding the o2 sensors, dismissed it as sensor "3" actually being sensor 2, and the missing sensor being the one that wasn't registering. Didn't realize until after I posted that the plugs were actually in some order, just saw 2 plugs that looked carbon fouled and jumped the gun.

How long are your cylinders holding compression? Could be a bad valve (crossing fingers that's not the issue) or the spider as previously mentioned. The fact that it's only in 2 cylinders eliminates a lot of sensors and the basic stuff. Pretty much leaves fuel injection (could be seals or injector), valves, spark plugs, or distributor. Cylinder leak down test would be my next step. Check for compression leaks, if you find one, pour a tablespoon of oil into the cylinder and retest. If the leak stops, it's the piston rings, if it doesn't it's either a valve or a leak from your spark plug.

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[Schurkey]

Check for compression leaks, if you find one, pour a tablespoon of oil into the cylinder and retest. If the leak stops, it's the piston rings, if it doesn't it's either a valve or a leak from your spark plug.

I've never seen that actually work. Given the slanted cylinders of a V-8, I've never understood how the oil knows how to travel "uphill" to seal the cylinder on the intake manifold side.

Given that his compression test seems fine except for #8 which isn't misfiring, I'd say the rings and valves are "good enough" even if #8 does seem to be in some distress.

 

[RawbDidIt]

I've never seen that actually work. Given the slanted cylinders of a V-8, I've never understood how the oil knows how to travel "uphill" to seal the cylinder on the intake manifold side.

Given that his compression test seems fine except for #8 which isn't misfiring, I'd say the rings and valves are "good enough" even if #8 does seem to be in some distress.

Yeah, probably right, probably isn't the issue, but couldn't hurt to check. As for the oil sealing the piston rings, I must admit, I've never done it on a V configured engine, worked like a charm on an old 4 cylinder I was working on. You may be onto something there, but you can still check where the air is coming out and tell where the leak is. Just because an engine produces compression in spec doesn't necessarily mean there isn't a significant leak, if this was true, a leak down test would never be necessary, you'd only ever do a compression test.

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[Schurkey]

Just because an engine produces compression in spec doesn't necessarily mean there isn't a significant leak, if this was true, a leak down test would never be necessary, you'd only ever do a compression test.

If an engine passes a compression test, there is USUALLY no need for a leakdown test if it has no other symptoms of failure. The leakdown test determines WHERE the compression leak is--intake valve, exhaust valve, rings, or compression loss into the cooling system.

OTOH, there's no harm in doing a leakdown test; and with low compression in #8 he should do at least that cylinder, and one of the "good" ones as a comparison.