cat heat sheild missing and running poorly

[D]

My cat heat sheild fell off on my 91 5speed. Will the missing heat sheild damage the cat or anything elese? My truck is currently missing and i've changed wires, plugs, tps, cleaned mas, could a bad cat cause it to miss from idle to 3000 only under partial throttle, how bout the o2 sensor, also the check engine light never turns on. Has a relatively new regulator and still gets 15mpg, idles and starts well just runs poorly under 1/2 to 3/4 throttle acceleration or going up hills, requires more down shifting to stay out of the miss range.

 

[mdrut]

Could be anything, vacuum is where I would start, then check out the emissions system, then fuel and electrical..The heat shield has nothing to do with performance, it's just there for safety purposes..

 

[D]

checked vacum leaks, guage reads normally, also the Egr is by passesd I took the botom of the air box off and pluged the vacume line.

 

[mdrut]

hmmm, if you are sure it is not vacuum, move on to the electrical system..check your plug gap, check for a lose wire, jumping spark, loose connection or corroded connection, do a pressure check on the fuel rail, also check the compression of each cylinder..It could be the O2, but not likely if you are not getting a "light".. Have you run the codes yet?

 

[D]

No I havent run any codes yet. Can you get codes with out the check engine light on? The plugs are all gapped to .054? or whatever the specs say, it does seem like a large gap though. I had to use the spark plug gapper and a feeler guage to measure that gap. Also the plugs all look fine (no oil on the #5) The truck has 167000 miles and not very much engine work done except for what I have mentioned. I think that the 02 could be origional. I couldn't find any recipets for it.