engine vacuum. im still thinking cat. low vacuum at higher rpms would indicate excess exhaust backpressure, could also be several other things that would cause a bad reason, and i dont know your mechanical skill set, however, to interperate a vacuum test correctly, you need to know a good amount about the engine. make sure that you use a good gauge, liquid dampend, itll look like you have sticking valves on a cheap gauge and wont be able to tell too much.

 

I am relatively good mechanically, and will try anything. I will take a vacuum reading and report back. Can the test be taken at high rpms with no load? (sitting still).

Is there any particular reading I should be looking for?

Thanks,


Mike

 

Almost forgot, My idle sometimes drops way down when I come to a stop or just after I give it gas and let off.

 

do it with no load. at idle, you should have about 17-19inHg, should be very steady. as you raise rpms, do this slowly needle should drop a little, but go back to normal if you hold the throttle at one place, it should also be steady(it will drop when rpms go up, and raise when they go down, but when theyre stable...idle, 2000, 3500, they should all look the same) from about 3500 when you let off throttle it should jump up to about 22 or so, and should drop back to normal by the time the engine idles out(pay attention to this) a clogged cat would show up as it fluctuating too much at high rpm, however, that same reading could mean other things too, good diags off vacuum gauges are not really something you learn over night, it takes doing a lot of them to understand the minor differences between a burnt valve and a bad plug.

 

Dave,
If it were a cat, wouldn't it have thrown a code by now?

 

1138 could be the code its throwing. a lot of 02 codes, including cat proformance, are disabled if there are other o2 codes in the system. the ecu is seeing that the sensor never goes lean, so it doesnt expect a good value from the downstream sensor. in the end, it could be a lot of different things, and without my hands on it, this is a guess. it needs to actually be diag'd, but a vacuum reading is the second place id go in the shop, after seeing what my o2 voltages are, but for someone without t4, that is kind of hard to do