I just came back in. Fuel pressure was as follows:

Engine running /vac hose disconnect - 43lb
engine running / vac hose connected approx 39lb

Fp within 10 minutes of shutdown still around 40 lbs
Fp after about 45 minutes of shutdown around 18lb.

I'm not sure if that amount of pressure drop is typical.

 

The fuel pressure bleed off sounds a little quick but I dont think that would cause any problems because its not bleeding off that fast... Your pressure seems high enough also... Hmmmmm... Is your air filter new/clean?

 

we're all just totally guessing here...please give your oxygen and CO2 readings...they are important to determine rich mix or lean miss/incomplete combustion etc, etc.

basically copy (all) the info from your test record.

 

Get this book . keep it next to your FSM. If you want to understand how your vette works, this is a must read. Covers cause's of smog falures.

Corvette Mike

 

I have had that book for years along with some others and I have referenced all of them. I have what I believe to be a pretty good understanding of how the system works. When I looked at the data from my scanner all of the parameters were in their normal ranges.

Air filter has about 6000 normal driving miles on it.

My CO2 was 12.40, which I believe is measured as a percent.
There is no O2 reading.

HC - 557 / pass 220
CO 1.05 / pass 1.20
These were the only three readings listed.

 

How old is your big cat?

 

Cat is about 8 yrs old. Pre cats are gone due to LT headers. All emission hookups are present.

No unusual smell coming from the exhaust.

 

A really healthy catalyst will be in the high 13's to 15 percent CO2 with the proper fuel mixture.

O2 would be high if you had a cylinder misfire (unburned/ incomplete combustion passes air through with the gasoline hc's).

The system will run rich if the thermostat is stuck open and running cool (CO upstream of the cat should be more like .7 at idle- if you can measure it).

If your engine burned oil for a long time or had a misfire for a long time, it will weaken the cat converter. Does your year have an air pump..the diverter valve should be operational..that really helps get the cat temp up and give it fresh air to react with-once again, the o2 reading is helpful).

 

The engine generally doesn't burn oil. I rarely have to add oil between changes. We'll check the air diverter valve tomorrow.

 

I just got the car back from the shop.They needed to do some "tweaks" but I got my sticker.

 

Be more specific, this doesn't tell us anything about what fixed it. Tweaked what? What did you replace?

It's annoying looking through a bunch of unfinished threads with no real answers.

 

86Pacer - I hear your pain on unfinished threads. I put tweaks in " "s because I believe the only thing they actually told me they could see wrong was a dirty MAF which they cleaned. They hooked up their scanner and didn't see anything although the tech felt there was something "screwy" with the ECM because the MAF was reading a very small amount of airflow when the key was turned on. It has always been that way and I never really thought it was a problem given that there is air in the MAF even when the engine isn't running. If anyone knows for sure that this isn't true, then I would possibly consider replacing the MAF.

Unfortunately the economic realities of running a business come to bear here. I seem to have an intermittent miss (although I only hear it when in park at around 23-2500 rpms). Ideally the shop would hook up their scanner. oscilliscope, fuel pressure gauge and whatever else together and they could probably nail down the problem. Unfortunately the time it would take to do that at $75/hour makes it very costly to properly diagnoise the problem, if it did at all. I think they might have felt presenting someone hundreds of dollars of labor charges to fix what some would consider more of a bother than a necessity as anti-customer, especially knowing that I am unemploted right now.

There is no reason why this car can't run clean and pass emissions based on what was done to it. When I eventually get reemployed, I will most likely replace spark plug wires, cap& rotor and maybe even get those Bosch III type injectors. The other thing I will be willing to try is giving up on my expensive dyno tune (never satisfied anyway) and try and work with PCMforless on a new chip.

I apologize that this thread is ending up not offering a real solution and I appreciate all of the reply's to my thread. All I can say is at least I'm covered for two more years. I will report back to the forum when I am able to make the other changes.

thanks

Glenn

 

I'm only asking what lowered that 557 HC down to under the 220 max limit to let you pass, since it can be something else to check for someone else with this. Apperently just some MAF cleaner spray. What's your HC and CO now?

Here's my before and after on the non vette just for kicks.

Not bad for 286,000 miles.

 

Perhaps I shouldn't speculate, but sometimes when a shop has 2 or 3 hours of diagnostic into a car and maybe a couple of parts, they would rather 'pass' the car and collect a little labor rather than attempt to collect for labor on a failed car. Maybe the MAF was way off...sometimes you have to spend a lot of time to poke around on all the possibilities- you'll never know for sure unless you see the car run on a 4 or 5 gas with your own eyes.

 

I'd be real impressed if that was still the original catalyst. seems like you fixed an air leak/misfire on that one.

 

No one single thing fixed it. Fuel injectos alone dropped it down to low 200's. The cat and vac leaks lowered it the rest of the way to almost nothing. I did a pre-test between each repair to verify. It was a combination of bad fuel injectors, vac leaks, and a new more efficient OBD II compliant cat for OBD I car per new CA regulation. Old cat had 285,000 original miles. Which makes me that much more curious as to how an HC reading of 557 was lowered to compliant levels with no parts, yet the car still isn't running right apparently.

 

They found the "AIR" tube had fallen off........badumpbump

 

They of course didn't tell me but I wouldn't be surprised.

 

BLM's swinging lean when you let off the gas is often a header leak so it dumps a bunch of fuel it doesn't need. Sometimes you'll see the smoke and think it's a valve seal problem. Those behind you usually get a good whiff of your exhaust. Since BLM is retained in memory and used at startup until it goes closed loop, that can make it rich.

Removing the precats and keeping the stock cat usually delays lightoff. Let it idle long enough - sometimes it'll get there. Otherwise, it should be good to go with Closed Loop. Having to get it "hot" is folklore. Design and air pump takes care of heating things up.

FP is normal.

A functioning MAF reads/senses the air around it and the data stream should spit that out with the key on.

A MAF that isn't sensing the correct air flow will cause it to be rich or lean in most cells.

I'd guess it had a header leak and a cold CAT. Hard to say what the Dealer did, but you might want to check your header bolts for correct torque.

 

The new cat is just scrubbing the HC's if the motor isn't burning completely clean. It'll run a little hotter than it would otherwise. I wasn't sure how the state was handling older cars with the cat replacement laws. I'm not sure what an LT1 pair would cost, but since you can't buy second hand ones anymore, I may start looking for original GM ones out of state to have a set so I never have to get gouged at the muffler shop for $500 or whatever on a certified one.