Drove down to the gas station to fill up. Opened the cap and got a huge rush of vapors. Tremendous amount of pressure building up in there.

Went home and did some diagnostics. Found out that the CCP solenoid isn't being energized by the ECM. Did some more diagnostics and found the following:

1.) I temporarily set the min vehicle speed for purge to 0. Also set min TPS for purge to 0 in order to force CCP on at idle. Cannister purge harness from ECM gets 13V when disconnected from the solenoid. Voltage drops to essenitally zero when I connect to the solenoid.

2.) Bench 12V power supply will energize the solenoid and activate the CCP just fine.

3.) Tried two other ECM's and got the same result.

4.) I noticed some descrepancies on the 7730 schematics. Some have green-yellow purge wire hooked up to A3. Others have it to F7. Since I added mine (Painless harness didn't come with it), I tried it on both terminals. Same results on both. would someone mind please checking their car to see what the green-yellow-stripe wire is connected to?

Somehow the ECM is not supplying enough current to keep the solenoid open? I did voltage drop and continuity tests across the harness with ECM disconnected and it seems to check out fine.

Any suggestions?

 

Ok, I checked a friend's 91 TPI Camaro and his is hooked up to F7. Hopefully I can get one more person to confirm this on another 7730 Camaro.

 

curious to what code? My gas cap has done the same thing for along time.

 

a) How much current is being drawn from 12V supply to energize CCP solenoid? Also, measure CCP solenoid coil resistance (harness disconnected) - my guess CCP coil resistance should be on the par with a typical BOSCH style relay (i.e 70 to 120 Ohms). Please post measured values.

b) Disconnect harness from CCP and connect a low power 12V light (something that draws <5 mA from 12 volt supply) or LED with a dropping resistor (make sure to get polarity right) - go for a short test drive and observe if ECM is controlling CCP circuit. If ECM turns on and off test light during your test drive then the problem is with CCP solenoid needing higher current sink circuit (which is rather odd). An intermediate relay can be set up as buffer between CCP solenoid and ECM, but I do not believe it is the problem!

//RF

 

What type of vent is there for the tank? If it is a sealed system, then at the least you should add a 3rd gen "honk" valve. This is the pressure/vacuum relief valve that sits above the rear axle. It is plumbed to the tank to control min/max tank pressure.

Also note that as little as 3 psi can easily build up in the tank and is normal. With a 1/4 tank of fuel that is a lot of compressed 'air.' On a stock 3rd gen in the summer it is enough to nearly blow the cap over your shoulder when opening it for a fill up.

RBob.

 

Ok, I'm getting 34 ohms when I measure the resistance on the solenoid by itself. So with a 13V voltage drop across the solenoid, then by Ohm's law, V=IR, it should be pulling .38 amps. I don't have a functioning ammeter, unfortunately.

I'll have to do part b) at a later date.

BTW, forgot to mention... I have a different solenoid that I tried too (from my previous incorrect CCP cannister). It did the same thing- 13V disconnected, 0V connected. That solenoid measures out at 25 ohms. Therefore, theoretical current draw would be .52 amps.

According to the schematic, the solenoid gets a constant 12V supply from a common switched ignition source (which it shares with other circuits). ECM appears to have a switched ground (the green-yellow stripe wire) that it uses to energize the solenoid. There must be some current limiter on this ground wire in the ECM.

Maybe I need a new solenoid? Anyone know where to get one? I hear they're hard to find these days.

 

It's the factory 1971 evap lines and tank. I'm planning to mimic the TPI setup soon with that valve and all. However, if the solenoid isn't being energized, then none of that will buy me anything.

 

Actually the only thing I can say about this right now is that the ECM is producing enough current to drive the multi-meter to 13V (multimeters probably draw a very miniscule current to measure voltage).

And since I forced the CCP on at idle in the calibration in the chip, I can pretty much run the test at idle without having to drive.

EDIT: Scratch that. I just hooked up my scanner and the CCP is not being commanded on at idle.

 

Just took it out for a drive. ECM is commanding the CCP on when I'm at speed.

What I did was to disconnect the CCP hose from the fuel tank vent line. I then hooked up a vacuum gauge with a long hose to the tank port on the CCP cannister and then put the vacuum gauge in the car. Also capped off the air intake port on the cannister so that I'd see manifold vauum on the gauge.

I drove the car and observed the gauge. It moved very little. Then when I got home, I pulled the cap off the air intake port and got a small sucking sound. That seems to be telling me that the solenoid is energizing??

 

The idea is to determine (positively) if ECM has control over CCP line. DVM can be used as an indicator instead of light or LED along with a scanner to see if ECM is actuating (in code sense) CCP line. Until that is done we're are still kinda guessing here. And yes a typical DVM input impedance is in megaOhms when set to measure voltage.
It is dark (and late right now), but I am going to measure my CCP coil resistance tomorrow morning, meanwhile I just measured another BOSCH style relay and it came in at 75 Ohms. At 13.8 Volts this relay will draw 184mA vs. 405mA for your first (34 Ohm) CCP solenoid (5.6W). IMHO that solenoid is a current hog!

//RF

 

The canister purge action does not pull vapors from the tank. The tank pressure pushes vapors into the canister. The canister itself is vented to the atmosphere. So a vacuum gauge on the line to the tank won't show vacuum during an active purge cycle.

The vapor pressure control valve prevents the tank from pushing vapors into the canister until the tank pressure exceeds 5 KPa (0.72 psi).

The tank vent (honk valve) is there as a safety for excess pressure, and to allow air ingress as the fuel is consumed.

With tank pressure it will vent at 5.5 - 7.6 KPa (0.8 - 1.1 psi).

Under vacuum it will allow air ingress between 0.88" - 1.9" Hg.

There are two types of canisters in use. A solid bottom and an open (filtered) bottom. The solid bottom types have a vent on top marked AIR. This is to be open to the atmosphere.

Note that while in a purge cycle the ECM checks the BLM to be sure that it doesn't drop too low. If it does the ECM will back off the purge volume. The purge is controlled via PWM of the solenoid.

The pressure information listed above is from the '92 FSM.

RBob.


{edit: just to add, the CCP DC% is in the ALDL data stream. It is worthwhile to check it).

 

Ok, I'm feeling very much like a dope. I drove it around some more last night with the scanner and the CCP purge was off. Duty cycle was zero. Came back, poked around in the calibration a little more only to discover that the CCP was turned off in the calibration switches (in Tunercat). So I switched it back on and went for another quick drive around the block and now the CCP is being commanded on (I can see it in the data stream).

Like you said, the duty cycle varies with driving condition. It was a quick drive aruond the block since it was late, so I'll have to test it out on a somewhat warm day again to see if I still have the pressure build up in the tank.

In any event, I was curious as to how the CCP got switched off so I checked my factory ANHT calibration (which was my baseline) and it was off there too. Then I checked my AUJP and AXCN and it was on. Somehow the ANHT had it switched off to start with. Very wierd.

This CCP thing has me pulling my hair out. For the longest time I had the wrong cannister and a seemingly unsolveable stalling issue, but no real tank overpressure issue. Then I get the correct cannister, solve the stalling problem, and then introduce the over-pressure issue!

 

Measured my CCP solenoid = 32 Ohms. This one was sourced from '90 B-Body.

//RF

 

So then mine sounds pretty typical.

 

i can't find the specs, but 30~40 ohms is fine.

the power side should not go to 0 volts when you plug in the solenoid.
this is a problem some where. check one of the other components thats powered by the same circuit and see if its power goes to 0 with the CCP solenoid plugged in. if it
doesn't, i would trace the power wire from the CCP solenoid back into the harness to the splice connection.

 

Actually, it turns out what was happening was that the ECM was not even commanding the CCP on when I was taking the measurement at idle. I tried forcing it on at idle within the calibration but it didn't work. The scanner confirmed that the CCP remained off. There must have been just enough of a connection to ground within the ECM on the green/yellow-stripe wire to see 13V across the multi-meter, but since the ECM had the CCP off, the connection wasn't enough to energize the solenoid. That explains why the voltage dropped to zero when I plugged it in.

Next time I drive it, per RF Masters' suggestion, what I'll do is hook some wires up and monitor the voltage when the scanner says that the CCP has been commanded on. I know the solenoid works because I tested it with a bench power supply. So if the CCP is commanded on and I still see 13-14V across the solenoid, then I know that the system is working properly at the cannister.

Note that in a previous post above, I did discover that the CCP was actually turned off in the calibration switches table in my factory ANHT calibration that I used as a start point. Which means I've been driving around all this time with an inoperative cannister purge. That alone may explain the entire problem. Any rate, I switched it back on and confirmed that the CCP does get commanded on with a short drive up the street and back. It's just a matter of confirming the solenoid operation while driving too.

I also know that my vacuum hook ups are correct since I have a factory 1991 TPI Z28 Camaro to visually reference first-hand. At which point now, if the fuel tank excessive overpressure condition still exists, then I know I need to work the fuel tank end of the car.

Once again, I really appreciate everyone's time reading my posts and offering suggestions.

 

Ok, I think figured it out.

Went for a long drive today. Stopped and opened the fuel cap and got some pressure release. Came back and removed the inline purge valve and drove it some more. Not so much as slight hiss when removing the cap.

I got that purge valve from a junk yard. Looks like that was the culprit. I'll probably get a new one now. Ebay's selling them for ~$15.

 

The purge valve should be between the canister and the engine, not between tank and canister. As RBob said, it shouldn't have any effect on tank pressure if installed in the right place.

 

I think that he is referring to the purge vapor pressure control valve. It goes between the tank purge line and the canister. It will open once tank pressure exceeds 0.72 psi (5 KPa). Which is one reason why our tanks hold pressure.

RBob.

 

Yeah, I was thinking the same thing, and hope he has it right, but it would explain things a bit if it was wrong.

 

Yup, that's how I have it. I checked my friend's '91 TPI again to verify.

 

Ok, I got another valve from a friend and installed it. Seems to work now. Went for a drive, came back and opened the cap. Just a slight amount of pressure. Seems to agree with Rbob's assessment of how much pressure is designed into the system. But it's nowhere near the blow-the-cap-off pressure I was getting before.

Today wasn't that warm out, so it'll be interesting to see how it behaves when we start into warmer weather.

Thanks again guys.

 

is this the white valve on drivers side over top of the rear axle?