Caprice Evap System Concerns

[1987 GMC Jimmy]

I've noticed a few issues with the evap system that I worry are affecting economy/performance. I fueled today, and when I opened the cap it hissed like opening a soda, and I could very audibly hear the tank popping like it was expanding. Sometimes when I fuel the pump shuts off prematurely multiple times and sometimes it fuels perfectly. Sometimes it'll run a little off, and when it does this, I can usually smell fuel vapors near the tank and near the canister. No fuel leaks of any kind and no vacuum leaks. These issues have been gradually getting worse, and it wasn't doing it a few months ago. I don't understand why the evaporative emissions is so complicated on this vehicle. It's got the canister, the purge/control valve (Rochester Products 17085891), two separate check valves, and three lines that go to the carb from the control valve. If I could get rid of these little check valves, I would for the sake of simplification. I haven't tested the canister for blockage yet, but the filter is fairly new. The only other thing that I can think of is it needs a new purge valve, but I'd like to know if those extra little things can go. If anyone has any suggestions, please offer them up. I don't think the fuel cap is necessarily an issue at the moment because it'll shut off when fueling, but maybe there're multiple problems.

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[Cuba]

Well the fact you have hissing when pulling off the cap leads me to suspect the vent valve in the tank. I would start by disconnecting the vent/evap tube on the pickup assembly. With the tubes bent towards you, it should be the far right tube, maybe 5/16". 1st tube is fuel line to pump at 3/8" (far left), then the skinny one at 1/4" which I believe is the return, then the last one which should be the vent. Hoping that your setup allows for easy access to the pickup assembly in the tank. ...

For testing purposes, you should be able to run the vehicle for 5 or 10 mns, this is with the vapor hose unhooked. If you no longer have pressure buildup (hissing upon cap removal), there's a blockage in the evap hose system. If you still have hissing at the cap, the vent on the tank pickup is blocked up. In most cases, it's just an oversized checkball maybe 5/8" to 3/4" and I've seen them seize in its carrier. So when pressures rise in the tank, no vapors are allowed to release causing the tank to pop out and sometimes causes fuel to vacuum out the canister. In fact starts pouring out of the canister until the pressure is reduced in the tank... at least on electric fuel pump driven vehicles 11-13psi.

 

[Cuba]

With all the Evap junk in your setup, I'm trying to keep it as simple as possible. With the vent hose from the tank removed or unhooked, it should pinpoint where you need to focus on. Either on the tank area or Evap system. Hopefully it's more simple one and it's just a pickup assembly change.

 

[1987 GMC Jimmy]

I went ahead and tested some stuff out. No blockage between the canister and the tank. Air flows both ways. It's not the easiest, but you mentioned a check ball in that little cage in the sender, and I can't produce as much vacuum with my mouth as the car can. I also checked the canister for blockage, and it was free. I did find some stuff near the canister control valve that was weird. There's some little check valve that's not vacuum powered (assuming thermal), and I'm gonna get rid of it because I don't think it's doing anything.

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What I did find was a one way check valve between the control valve and the carburetor that was blocked or stuck in both directions. I'm assuming this obstruction was causing the tank venting issues. I suppose my options are either putting a brake booster check valve where this one was or having nothing there. It's a GM part (25518739), and I'm guessing they put it on there for a reason, but I'm sure it'll live without it.

 

[Cuba]

Nice! Good find. I've never seen that piece but I'm sure you can modify it like you said. The emissions junk is highly overrated.

 

[1987 GMC Jimmy]

Thanks! I ran it like that, and it seems to be fine. Idles a little better and stench of fuel vapors seems to have subsided. I'll get rid of a lot of it one day. I can appreciate what some of it does. Some of it not so much.

 

[CorvairGeek]

The mystery part is something that must have been added in the '86 model. Haven't seen that on the '84 / '85. I thought the evaporative system was already complicated enough! I had already replaced everything in the system on mine (except canister) because of fuel vapor / odor issues over the years.
My '84 has always required fuel to be added slowly, or it shuts off the pump and may spit back fuel. Very sensitive about the fuel nozzle position too. Mine has even had the fuel tank changed (ruptured inside a United Van Lines moving truck, fire, long story) with no change in behavior. I think GM may have known about the issue, as the '85 Sedan has a unique rear panel that is easier to repair / refinish around the fuel filler. Most people would never notice the minor change.
My '78 that had the no lead restrictor broken out did not have the problem.

 

[1987 GMC Jimmy]

Yeah, mine would shut the gas pump off multiple times but only sometimes, and it would overfill and dump fuel on the ground. We'll see how it does now. That check valve and that other thing are gone now. I've been having issues with accelerating from part-throttle in the 40-60 mph range when the car's warm, and I wanted to rule the evap system issues out before I started digging into the distributor and what not.

 

[CorvairGeek]

Yeah, mine would shut the gas pump off multiple times but only sometimes, and it would overfill and dump fuel on the ground. We'll see how it does now. That check valve and that other thing are gone now. I've been having issues with accelerating from part-throttle in the 40-60 mph range when the car's warm, and I wanted to rule the evap system issues out before I started digging into the distributor and what not.

That sounds a lot like excessive EGR, especially with your 2.56 axle ratio.
My folks had a special order (GM employee) '85 wagon that felt like it had a misfire when it shifted into overdrive with the EGR active. Others with wagons complained of the same thing.
I disconnected the EGR on my '84 when it had 2K miles on it. Unplugged the EGR solenoid too because of how noisy it got when it was old. Always had to use premium fuel at sea level, but it is fine living at 2500 feet. Mine is the last year under 9:1 compression without a knock sensor, so anything newer is more complex.

 

[1987 GMC Jimmy]

That sounds a lot like excessive EGR, especially with your 2.56 axle ratio.
My folks had a special order (GM employee) '85 wagon that felt like it had a misfire when it shifted into overdrive with the EGR active. Others with wagons complained of the same thing.
I disconnected the EGR on my '84 when it had 2K miles on it. Unplugged the EGR solenoid too because of how noisy it got when it was old. Always had to use premium fuel at sea level, but it is fine living at 2500 feet. Mine is the last year under 9:1 compression without a knock sensor, so anything newer is more complex.

What would be the best approach to repairing it if that's the case? I replaced the EGR valve a year ago, and it's ran fine until a couple months ago. Some days it'll act up, and some days it's perfect.

 

[CorvairGeek]

Probably worth removing the valve and making sure the ports are clean and don't have carbon jamming them up.
You can always disconnect the vacuum at the valve (plug the vacuum line) and check driveablity without it too. Just beware of severe detonation, especially when the torque converter locks in 3rd, and after shifting into OD.
I don't recall if the '86 has the EGR solenoid towards the back, passengers side of the intake manifold. In the event it fails or is electrically unplugged (I can hear mine clicking as it should as soon as the car is in gear, just got loud with age so I unplugged it- EGR is disconnected), it will allow unregulated vacuum through to the valve (much like a pre-computer engine) and cause driveability issues too from excessive EGR.

 

[1987 GMC Jimmy]

Probably worth removing the valve and making sure the ports are clean and don't have carbon jamming them up.
You can always disconnect the vacuum at the valve (plug the vacuum line) and check driveablity without it too. Just beware of severe detonation, especially when the torque converter locks in 3rd, and after shifting into OD.
I don't recall if the '86 has the EGR solenoid towards the back, passengers side of the intake manifold. In the event it fails or is electrically unplugged (I can hear mine clicking as it should as soon as the car is in gear, just got loud with age so I unplugged it- EGR is disconnected), it will allow unregulated vacuum through to the valve (much like a pre-computer engine) and cause driveability issues too from excessive EGR.

I went ahead and tested the valve with my Mityvac. It would hold a little vacuum, but it lost it all within a few seconds. I figured that since its likely a positive back pressure valve, maybe the car needs to be running to properly test it. I tried to pump it up with the car running, and it didn't hold vacuum for a second. I plugged the line and ran it with the valve out of the loop and it ran perfect. Pretty bad for a 13-14 month old EGR valve. It's still under warranty, though. That made perfect sense to look at, but I totally let it go because it was a new part. Hopefully a new one remedies the issue.

 

[1987 GMC Jimmy]

I installed a new valve, and it seemed better at first, but then it started running worse. It would idle down too far in drive and reverse and stall when I pressed the gas so I'm back to disconnecting the line from the valve and plugging it. The only thing I could guess is that maybe the EGR solenoid is having issues, but that's the only hypothesis besides the aforementioned one. Unfortunately, it appears to be a discontinued item, and I don't think a TBI part would work, but I'm not sure. O'Reilly has a BWD part number on their website, and it says I can order it so I'll see if they can still get one.