Hi.
I've been having hesitation problems for a while now and I finally gave up and took it to the mechanic for diagnosis. THis is what they gave me!

1. Vacum at ilde was at 17psi (I belive) right on with the specs.
2. When put in gear idle drops to 3 and when powerbraking idle is at 0-1.
He told me that he suspects the catalytic converter.
Fuel was within specs
The timing was right on and the whole igntion system worked flawlessly.

Do you think a clogged, or partially clogged cat can do this???

 

sounds abit fishy..like they are trying to sell you a cat...easy way to check...disconnect the cat from the Y-pipe and take it for a quick spin...problem goes away, the cat is your prob., but I doubt it

 

Partially clogged cats always manifest themselves as a lack of power above 1/2 throttle. Ie: pulling a steep hill or passing.
Mostly clogged cats are easy to spot, they're the guy that can't get above 45 mph. I suspect a lean condition. It would help to know if it is TBI, TPI, or 4bbl.

 

They were not trying to sell me the cat. They don't do exhaust work in their shop.
They are the only ones that are honest and that have done work on my parent's car.

O vac. while powerbreaking. And 17 at idle what does that say???

Its a TBI!

Could the cat be partially clogged allowing me to still drive but with less power??

 

I wasn't saying they out to rip you off. I just disaggree with their call. It could be a partially clogged cat if it had been that way since you had it. If so, you are going to be really suprised how much power it makes without a stopped up cat.
I bet your cat is welded on huh?
A quick check without breaking out the sawzall is to do a stall test first (Block 4 wheels, park & foot brake on max, record RPM at WOT stall) then back the nuts off of the flange that holds the exhaust pipe to each exhaust manifold to the end of the stud. This will give about 1/4" gap between the manifold and pipe.
Load test (powerbrake) it this way for no longer than 60 seconds as exhaust will burn wires coming off of the starter.
Compare the RPM readings with the first test.
If the cat is stopped up you will see a BIG difference.
...I still think it's a lean condition...

 

The problem is that the bolts on the collectors are hard to get at and they are different than the 3 bolt collector. These just have 2 bolts and they are really hard to brake apaprt.

What do you mean by lean condition.

Not enough fuel???

 

when i was in tech school i ran into this problom, my auto teacher told me a good way to tell if it is the cat, first hook ok a vac gauge and get a base line at idle then bring the rpm up to around 3000 (in N gear) and hold it there and have a friend watch the vac gauge, if it is a cloged cat or any ext block the vac will start to drop after a min or 2, it worked for my on a camry,give it a shoot and let us know

 

They already tested it like that. The vacum dropped to 1psi when powerbraking.

 

Sounds about normal.

What I can't believe is that the idle vacuum is pretty much normal, when you drop it into gear, all hell breaks loose. 3 in/hg of vacuum?????? That thing should be popping MAP codes, blowing black smoke, and drowning in fuel.

Now take a vacuum measurement at about 3,000 rpm in Park/Neutral. You should see MORE vacuum than at idle. If it slowly starts dropping, suspect fuel delivery or a clogged exhaust.

 

They checked the fuel preassure. It was within spec idling in gear and in park. I think it was aroun 9psi in park and I think it got up to 13-14psi when they were powerbreaking it.

The car almost floods itself when I am powerbraking.

 

The other thing that makes me think its the cat is all the parts I threw at it and nothing changed.
New, Ignition (everything including distributor, tested perfectly).
New sensors, IAC, TPS, 02, CTS, EGR tested fine.
New fuel pump, new strainer, new fuel filter.
New TBI with new injectors and new FPR.
Semi new alt. new battery. etc...
Heads reworked and have been fine untill this happened.
The cat is the only thing I can think of.
And yes its welded!

 

Vacuum (stated in inches of mercury) will always drop when power braking. Vacuum is created by restricting the air allowed into the engine - the cylinders "suck" harder than the mostly closed throttle allow the air to enter them. When you open the throttle but restrict the RPMs (by loading the engine against the torque converter), you're eliminating the restriction of air flow into the engine - therefore, vacuum drops.

You might get better answers on the TBI forum.

 

Have you done a compression or leakdown test?

 

Wait a minute, this is a 350 with the 305 TBI on it, correct?

I don't see any PROM tuning on your list. TBI is a speed density system, very sensitive to any changes in flow, such as comes with exhaust change even, let alone displacement changes. You can't expect it to run right unless the PROM is tuned for your configuration. That's why I recommended the TBI forum - they talk about it all the time.

 

ok just unbolt the cat and run it and see if it inproves if it dose, you know what needs to be done, if it dosent "keep truck'in"