catalytic converter check

[#2875]

i've been helping my buddy in town with a low mileage stock (and i mean stock) ty; it's having some pesky low end stumble issues and i've checked everything checkable but the exhaust and the trans. i did a search and read about a way to check for a clogged converter by checking vacuum at idle and then at 2500 rpm. can somebody please elaborate on this technique (because i am a tard) and is this an accurate way to check the cat? would the fact that it runs well under boost and in the upper r's eliminate the cat being the prob? other suggestions?

 

[gmcturbo]

plugs, wires, cap, rotor. ive had luck with ngk ur6s. had a simillar prob a while ago

 

[#2875]

thanks for the help but i checked all of that. he likes the ac plugs which are new and he has new wires which we ohmed. i dropped in a known good distributor (my old one) and also my msd billet along with a my blaster coil. there was no change. we started it in his garage with no lights and observed no arcing whatsoever.

just in case somebody is about to chime in and say FUEL his a/f mixture is sweet all the way through the dyno pull and we also had a puel pressure gauge on his rail that was rock solid while it was falling on it's face.

the only thing that is weird is his factory boost gauge is bouncing all over the place when it "doesn't want to go". i know stock is inaccurate but i refuse to pull my gauge out and hook it up to his system and i don't have a spare but the laptop confirms the apparent jumps in boost before it comes out of it and goes back to it's steady increase.

 

[TT350]

I’m not sure about this but if the factory boost gage is run by the map sensor it may be bad! You may also check the tps sensor. Ether one can cause a stumble.

 

[#2875]

tps is fine, that's an EASY check. map sensor is cool too... at least it reads the same as the mechanical gauge... again, a no-brainer. i have checked all that shit.

 

[TT350]

Well sounds like you got it figured out! Pay a shop $50.00-$75.00 and have them put it on a Snap-On scanner they can see all the sensors in real time. If the tech has half a brain he can find the problem in no time.

 

[ItsMyTy]

you can check the cat the way you mentioned

however if it's only a low end problem, i'd bet your problem is elsewhere because cat problems show up worse at higher load/rpm conditions.

 

[#2875]

that's kind of what i was thinking too plus the rpm check was all good i'm just not sure if i'm doing it right.

what has me stumped is everything checks out fine. aside from the stumble the truck runs fine, idles fine, pulls really well in the mid to high range. i have swapped all his sensors with mine and still nothing comes up. a vacuum check had me wanting to steal his tbody.

tt350, i'm not sure what the hell you are talking about. i don't have it figured out, that's why i'm asking for help. the scan tool is mine, it's turbolink. the dyno belongs to a friend of mine and we haven't charged him a cent. besides there is no shop or tech within 150 miles of here that has an awd dyno and a snap on scanner.