I was driving home Sat. evening, going up a long fairly steep hill when all of a sudden I lost alot of power. I had it smashed to the floor to keep it at 55 mph. The valves started to knock badly and the check engine light come on. After I got to the top of the hill it seemed to regain it's normal power, however the valves still sound horrible when in a high gear.

My check engine light showed that it was a misfire in cylinders 1 and 8. Also showed a random misfire. About 2 weeks ago it threw out an 02 sensor code but I just reset it didn't see if it was showing lean or misfiring.

About 2 months ago I had the cat replaced and the shop said that the honeycomb had busted up and went into the muffler and was clogging the muffler.

I get home Sat. evening thinking that my exhuast is clogging off and I am going to get a new muffler on Tues. However, I went to Autozone today to order a new Flowmastetr, the guy says that if it were the muffler clogging the valves would not be knocking.

My plenum is also leaking and I just haven't had time to replace it. I told the Autozone guy that well if it's not the exhaust maybe it's the plenum leaking that is causing the cylinders to misfire. He says no that he thinks that it is the fuel pump or the filter is clogged. Which if this is the cause, isn't the filter inside the tank with the pump?

What do you guys think?
It's a 97 5.2l auto.

 

Yes the filter is inside the fuel tank, But if your valves are making noise, You may have lost a push rod or rocker.

 

The valves are only knocking when it is under power in a high gear say going up hill on interstate at 75mph and 1800 rpm. They are not knocking at idle or higher rpm's.

 

valve issue is likely the plenum gasket I would not think its the fuel filter/pump I would replace the muffler any way(if it is stock) Have you replaced your timing chain yet? and then if it still is acting up a new o2 sensor

 

I haven't replaced the timing chain. It could have been done before I owned it but don't know. The o2 sensors were replaced when I put the new cat on it. I think I am going to do the plenum then muffler after that. That is, unless anybody else has any suggestions. I'll check the timing belt tomm.
thanks

 

Could it be spark knocking? Before you go spending big bucks throw some octane boost and maybe some drygas in the tank.

 

You catalytic converter is clogged. The telltale symptoms are lack of power uphill and the random misfires, especially with cylinders #6 and #8 (as they are closest to the cat and will get the most backpressure).

Take your truck to a muffler shop and have them run a pressure test.

 

Could be your cat or a bend in the exhaust pipe. Been wheeling lately? If your exhaust is restricted, it will spark knock/rattle bad and cause a misfire. But go with the easy stuff first. Run some octane boost in the gas. Could just be bad gas.