catalytic converter?

[Scottyjr]

Hello all. Working on a 2000 Grand Prix 3100. My son's car and he is complaining about very bad mileage. About a year ago I did a LIM job on it and, off and,on after that, it had a misfire. Finally found the problem but I'm suspicious that I might have screwed up the cat. So yesterday I got underneath it and drilled a hole in the pre cat pipe and attached a fitting and pressure guage. At idle it read below 1psi. Reving it brought it to just under 3 and during a steep hill climb under a heavy foot it read a wee bit above 9psi. I put it back in the garage and reved it again and noticed something peculiar. Anytime I got the RPM above 3700 the engine started to oscillate between 4000 and momentarily down to about 2000 and quickly back up to 4000. The drops in RPM only lasted about a second and were very consistent and regular. Now I wonder if perhaps the cat is also the caused of the fluctuations in RPM. Any Thoughts? - Scotty

[mickaz]

if you see a quick oscilation in RPM, it might be the gauge. for a bad fuel mileage you might have leaking injectors, a bad 02 or a egr(?)

[walterdude]

When you say the RPMs drop when you hit 4 grand are you in neuteral or park?? As far as I know there is a park/neuteral rev limiter at 4k that will do just as you say. My 93 Z34 and 99 Lumina (3100) both do that....
As far as a misfire and exhaust pressure,, don't know... Perhaps someone else does??
Good luck,,,
Tom..

[robertisaar]

misfiring CAN kill a cat pretty quickly, though plugging it is not always the case...

[Scottyjr]

When you say the RPMs drop when you hit 4 grand are you in neuteral or park?? As far as I know there is a park/neuteral rev limiter at 4k that will do just as you say. My 93 Z34 and 99 Lumina (3100) both do that....
As far as a misfire and exhaust pressure,, don't know... Perhaps someone else does??
Good luck,,,
Tom..

Ah! That's a handy bit of information. It makes sense that nothing like that symptom occurred while tromping down on the gas while driving. Thanks Mate! - Scotty

[Scottyjr]

I still want to try to determine if the cat is bad. I read several sets of instructions using a vacuum guage. I will try that and maybe also drill a hole in the exhaust line after the cat and see what the pressure reading is there. - Scotty

[robertisaar]

pressure/vacuum readings won't tell you if the catalysts are worn out...only an e-test can do that. some junkyards have a machine that "measures" the life of it as well, but i don't know how they do it.

[RedZMonte]

a properly functioning cat should increase aprox 150 degrees from the front to the back... was told this by a emmissions shop. you have a lazer temp gauge?

S

[robertisaar]

that is true, due to either the endothermic or exothermic reaction that takes place(i can't remember which).

[Scottyjr]

Well now I'm thinking that the cat may be good. Using a vacuum guage at idle the reading was 18. Reving to 2500 brought the reading briefly to near zero but quickly recovered to 18 while holding the engine speed at 2500. My understanding from other reading is that that is correct behavior. I also monitored the O2 sensors on a fully warmed engine. Below are the screenshots. Relative guages are at the right of the graph in both images. Seems I got the positions of the guages of the O2 sensors swapped in the images so be aware of that.



This shows the engine at idle. The downstream O2 sensor (green) shows dropping off to a low value at the end of the graph. That is the point where the engine speed was increased to about 2250RPM. The faint magenta vertical line at about the 80 mark on the time line base is when the guages got their information



This graph begins with the engine at idle and then increased to 2250 RPM. The faint magenta vertical line at about the 60 mark on the time line base is when the guages got their information



As suggested, I also took temp readings at the front and rear of the cat. Both readings were 363 degrees +/- 5 degrees.

Again, from other readign, it looks like the cat is working very well. Anybody have an analysis of this information? Thanks - Scotty