I need some help to pass emissions. In Indiana they use a large funnel that collects all the air from the rear of the car for testing. In 2 tests I have had high HC readings. CO and NOX where fine. A check with the portable HC sniffer found HC's inside the gas filler door. (they check the gas cap before the emissions test)

When I got home I opened the gas filler door and could smell gas and The tank is never under pressure when I open it. I have never smelled gas at any other time and have never seen gas on the ground under the car.
The engine runs like a top, no miss fires, blow by, SES light, etc. it was tuned up about 600 miles ago and I rebuilt the fuel pressure Reg. after the first test. HC went up by .10 after that, Mileage increased from 13 to 18 MPG combined driving.

The car is a stock 1992 RS with a 305 TBI auto trans. 126,000 miles.

I need to check my evaporative emissions system but I'm not sure of the best way to go about looking for leaks in the system. A vacuum pump would tell me if there is a leak but not where. Any one have an idea of what I could try?

my latest readings are:
HC 1.50 Standard 0.80
CO 5.50 Standard 15.00
NOx 1.75 Standard 2.50

No access to a scanner right now so I have no idea what the BLM's or the 02 volts are but I don't think that's where the problem lies.

 

You may have a leak in the evap system (roten hose). In Cali Smog Tech have special adapters to pressurize gas tank and see if holds pressure. These adapters are expensive and hard to come by.

Alternatively locate a charcoal canister (also known as FUEL VAPOR CANISTER) in the front, by the radiator. One of the lines is connected to TB, while the other is connected to the tank. Disconnect tank line and apply vacuum to the tank line. With engine idling system apply vacuum with hand held pump - evap line should hold vacuum otherwise you have a leak somewhere in the evap line or gas tank. It is not uncommon for old gas caps to leak (they test gas caps here in Cali).

//RF

 

Thanks for the info. They test caps here to but I did notice a little light rust where the O-ring seals on the filler neck. I guess it could be leaking there. I'll see if I can't clean it up a little, It can't hurt.

 

No luck finding a scanner so I just ordered an ALDL cable for the laptop. I need to make sure everything else is good to go in addition to looking for that darn vapor leak.

 

I picked up a hand vacuum pump after work and started checking things out. The vent line to the tank will hold a vacuum. It seems the tank will hold a vacuum too (never try and draw a vacuum on a fuel tank that is only 1/4 full with a hand held pump ) Looking at the tank sending unit from the rear of the car there are 4 lines. Last 2 on the right Appear to be fuel feed and return. 2nd from the left is the vent line to the purge canister. Any one know what the first line on the left is? It will only hold about 5 kpa of vacuum. Like maybe I'm opening a check valve @ 5 kpa.

Other things I did, Pull tank sending unit clean and lube O-ring seal and Carefully put it back together. Put some ATF on the fuel cap gasket to help make sure it had a good seal. Tank sending unit was pulled from above as last owner cut a hole in the floor to change the fuel pump. This is why I have no idea what the fourth hose is. I also noticed that my idle increased when hot from 650 to about 750 after I put every thing back together. A quick check didn't reveal any vacuum leaks but I was running out of daylight. I'll check again tomorrow.

 

There was a recall years ago for pinhole leaks around the fuel tank/filler neck area.

 

I live in NW Indiana and have emissions testing also. The gas cap test is separate to test the seal and vent. It has nothing to do with the sniffer test. It is a standard tailpipe sniffer test to check emissions from your engine. They do not test the evap system on these older cars.

Obviously it is best to determine why your HC are high and fix it. You are probably looking at a rich fuel condition or a spark issue. But (knowing Indiana rules) if you are in a spot where you just need to pass the test so you can use your car, then I suggest you put a couple of containers of ISO-HEET (not the standard HEET) in half a tank of fuel, drive the car for 15-20 minutes and go get it tested. The HC reading will be much lower.

good luck

 

Alvanwie Thanks for the info. I took mine to Portage to get it tested and they put a large funnel device behind the car. It's purpose as I understand it was to sample all the air from behind the car not just the exhaust. You are correct they do not test the evap system as such but the funnel sniffs for HCs from any source on the car. I'm just trying to cover all my bases here as I do need to get it to pass and soon. This car will be my cruiser at some point but for right now it has to be my daily driver.

Don't think this kid knew what he was talking about but he told me that.
Valpo Only uses the exhaust probe. Since I live between the to test centers I think I'll go there and see If I can get it to pass. Don't know why they would be different though.

Pukka Thanks for the info, might just save me from chasing my tail at some point.

PS, Alvanwie you car looks great. I hope to have mine looking as good some day.

 

I can't imagine how long it took to pump down a fuel tank. I did a vacuum brake booster once, and THAT took a while.

There is a vent valve on the tank. That is the forth hose. Plastic mushroom looking device secured above the rear axle on the drivers side. It will vent excess pressure and allow air into the tank. The 5 KPa sounds about right for allowing air in. It vents at about 1.5 psi of pressure.

One problem I was having with the gas cap is that it would not always seat onto the neck. It would get tight and click before seating all the way down for a seal. To fix this I used a round file (rat tail) and deepened the grooved area of the cap threads. Then used a flat file and knocked down the tops of the threads.

Just a little, not much is required to be removed. Until I did this I would always get fumes from the fill door area. In the Summer it was so bad I wouldn't put the car in the garage after a fill up. Not until the car cooled off some.

RBob.

 

Thanks, RBob. I suspected the fourth hose was a vent but was not sure. I'm leaning toward a bad vent valve. I tested it with vacuum but had no way to put pressure to it. I didn't get much of a vacuum on the tank, just enough to tell it was holding a slight vacuum. I never would have thought of an intermittent problem with the gas cap! I'll be sure to give the cap and filler neck a better look.
Thanks again.

 

a hand vacuum pump took awhile with a nearly empty tank. common leak is around the filler neck where it meets the tank..i epoxied the hell out of it in the car didnt want to pull the tank..worked like a champ

 

Thanks undercover89tbi.
I did notice that I have some movement between the filler neck and the tank. I didn't see any fuel stains on the outside of the tank though. since it's a common problem and I'm going to be under the car I might as well epoxy mine as well. Better safe than sorry.

 

yeah i forgot to mention that my tank never had any pressure when i took off the cap and always was a gas smell back by my car but never any signs of leaking. now it has a lot of presssure when i take off the cap. that should deffinately help you unless you have somewhere else bad too..

 

undercover89tbi, I'm glad to know that I can repair that with epoxy. I was thinking I would have to pull the tank to either braze or silver solder it. Since I cant get it in the garage right now and the drive way Is kinda soggy that thought didn't make me very happy I didn't get much pressure on the tank just enough to here a long hiss when I took my finger off the vent line. I'm going to try and do some more testing today while the weather is nice.
Thanks again for your input.

 

I think I have found the problem. It would appear that when the fuel pump was changed the first time the shop that did the work put something in the filer neck and pried on it. This dented the sealing surface for the cap and probably destroyed the seal between the neck and the tank.
On the bright side, I have a good friend that owns a repair shop And I'm long past due to visit him.
OH, and my ALDL cable came in and I can't wait to start data logging and learning to tune my own chips!!!

Thanks to everyone that took the time to answer and to get me to look at things that I might not have considered. I hope I can return the favor some day.

 

hi, im looking to buy an 86 Trans Am with a 305 TPI. I went to go look at it today & it looks & runs great ONLY problem is that it didnt pass smog due to the emissions test in CA. the guy said it was because of an evaporative leak from the gas tank...he also stated that his mechanic took a look & said it was the gasket on the fuel sending unit. ive never changed that before so if i do buy the Trans Am what problems am I looking to get into? Any help is really appreciated.
THANK YOU

 

You'll have to drop gas tank (make sure that it is empty) to replace $3 dollar gasket. While you are it may as well replace fuel pump (hoses and sending unit if OE). Before hand check all evap system hoses connecting to CCP canister. After 20+ years in service rubber hoses tend to crack.

//RF

 

Did they say it was the evaporative emissions system? Or is that just a guess? If you get to retest for free you're pretty close I would start easy and just check your timing and look for vacuum leaks with a spray can of carb cleaner.