Hello All,

I need some help with my trusty 1997 Buick LeSabre. It has around 257,000 miles and decided to start stumbling above about a quarter throttle. It will go along just fine below ~quarter throttle. As I ask more of it the problem gets worse, up to about 3/4 throttle where it will cut out completely unless I back off the throttle.

Air cleaner is new.

Fuel pump is new.

Fuel filter is new.

Same behavior with MAF connected and disconnected.

Same behavior with EGR connected and disconnected.

Same behavior with FPR connected to vacuum and disconnected.

TPS follows appropriately on a scanner with pedal motion.

Spark plugs are about 57% worn out.

Catalytic converter is getting tired but apparently still okay. It gave me an [catalyst efficiency below threshold] code off and on a couple of years ago but hasn't shown any codes for at least a couple of years.

Fuel trims hover near zero all the time.

The only code it shows right now is P0300.

It sounds and acts like it is running out of fuel, like I'm adding air and that'* it.

No other signs leading up to this. Only thing I can think of is that it has had a 1-cylinder misfire every once in a while when really cold, and that would clear up in about 30 seconds . . . but then it stopped doing that about a year ago. It did this for maybe a year before it cured itself. I suspected the ICM at the time, but then it cleared up completely so I thought no more of it. I put the fuel pump and filter in because these have a lot of miles on them and the sender was bad anyways. Problem was there before and after. The problem actually got a bit better when I put a piece of cardboard in front of the air filter that blocked off about 50% of the air flow. Same problem happened at probably 40% instead of 25%.

I'm thinking maybe the 257,000 mile ICM and/or coils are going bad finally?

This one has me a bit stumped. Any thoughts on this would be appreciated!

Thanks!

Is it breathing ok?

Like intake and exhaust? Yeah, no obstructions.

"TPS follows appropriately on a scanner with pedal motion."
That won't show a glitch.

"Spark plugs are about 57% worn out."
That'* amazing.

I'd do some testing or record it with a good scan tool to see if you can get some direction.
I'm not convinced it'* breathing right. I think the cat may be partly clogged.

Do this put a vacuum gauge on it and watch as you go from idle to heavy load (power brake).. I power brake full throttle and allow it to go into converter stall and hold it there for at least a five seconds.The vacuum should shoot up and stay high. Hold the throttle down. Does the vacuum drop at all? If so we have a plugged exhaust.

I've seen failing TPSes be glitchy doing this. Smooth pedal motion shows jagged motion on the scanner and/or bad spots. Just wanted to show that it seems happy in my case.

Well, I changed them (for the second time) at around 200,000 miles, and they're 100,000-mile plugs with 57,000 miles on them so . . . 57% .

Gotcha. This has been haunting my thoughts but didn't want to admit it. I'll pull the forward oxygen sensor and see if that improves it.

. . . and I'll do this as well.

Will update with results!

Followup: It was a bad catalytic converter. It would rattle a little if I pounded it really hard (heel of hand, not hammer etc.).

Replaced it, then replaced:

- Plugs with ACDelco 41-979 (old ones had over 50,000 miles on them, but looked to be burning okay)
- Wires with ACDelco 9746U (old ones were the second set, installed around 2008, seemed okay, replaced for good measure)
- Coils with Delphi GN10123 (old ones were original)
- ICM with Standard Motor Products LX364T (old one was original)
- Oxygen sensors with Delphi ES20001 and Delphi ES20002
- Exhaust hanger donuts with Walker 35352 (one was missing, and the other was pretty old)

Runs great now. Have already put a few hundred miles on it. I figure it is good until 357,000 miles now, right?

Funny story: I ordered the plugs from Amazon. They arrived in a rip-open style bag with three plugs out of their boxes and one plug in an NGK box. The one in the NGK box was used and all chewed up on the outside like someone had taken pliers to it . . . and all six had different gaps. I figure that once they escaped their boxes, their gaps were adjusted by throwing the package during shipment. All of them got returned and I bought a good set locally.