[Birddog1970]

Unfortunately many issues with the fresh rebuild on my 70 396 (burning oil, etc). Took on a decent drive on the highway to hopefully "seat the rings" and ran into another issue. After driving for about 15 min varying speeds from 50 - 70 not getting on it hard, I stopped at the light to turn around and when I tried to go in gear it sputtered and stalled. Gas pedal felt like a sponge. Got it started and pulled over for a bit. No leaks, etc. Still felt sluggish but I got it back on the highway and it drove fine home.

Here are a few things I noticed.

1) before this run my oil pressure was 50 psi at ide, now its about 30 at idle and 50 at speed
2) I heard some hissing after I pulled in the garage, pulled off my "vented" gas tank cap and whoosh it almost blew out of my hand

So what do you think may have caused the new drop in oil pressure (maybe just low on oil since I am burning oil?

Should my "vented" cap do that? Thought the purpose was to not build up pressure? If it can't vent what symptoms would that cause.

Thanks for your help, this 1 1/2 year resto is really disappointing with all these motor issues. FYI -- I think the oil burning is oil passing my valve seals or guides, still trouble shooting it.

Dave

 

[webfoot]

I had the same thing happen as you, got a new cap and apparently it was a non-venting type, blew right out of my hand and I got a face full of fumes. I put the old vented-style one back on and it worked fine. (with extra ventilation due to a cracked rubber seal.) Although I must say, during the tank of fuel I ran with that cap on it didn't run any different.

30 psi at idle sounds pretty good to me, perhaps the oil never fully warmed up before?

 

[Yellomalibu]

My C10 Blazer does that all the time. The vented gas caps are vented the other direction. One way valve, so as the fuel level drops it doesn't pull a vacuum which would fight the fuel pump.
I don't know if there's supposed to be a vent elsewhere in the gas tank... but if there is, it's not working. In the mean time, I suggest "grounding yourself" by touching the car before you unscrew the cap... and don't try to smoke a cig while you are gassing up.

Haven't followed your threads, but I will say it's common for oil pressure to be lower when up to temp compared to when the motor is cold.

 

[Birddog1970]

Here is an update. I went out to the car after it sat for an hour and there is a small puddle of fuel coming from somewhere on top of the tank towards the axle. A quick check it doesn't appear that the lines are off/cut .. is it possible all that pressure (full tank of gas btw) had it come up where the sending unit mates?

I've driven the car about 80 miles in town before todays run so it has been up to op tem many times with the oil pressure at 50 psi idle ... trying to trouble shoot all these issues is difficult so seeing if one could be related to the other or I have a list of separate issues.

My gas cap says "vented" on it (70 SS with a non-vented tank), although it is one of those high quality repro's. What issues could back pressure of fuel vapors cause?

Also, now that I think about it I thought I smelled gas on my dipstick (not strong but there). Hard to tell since the car is blowing blue smoke ... I heard improper venting of the gas tank could cause this....wonder if that blue smoke that burns my eyes and nose after awhile is gas getting into the fuel

Thanks for the responses.

 

[NickeyChevelle]

It sounds to me like you have a Non-vented gas tank that you're using a Vented Gas Cap on? I don't think the "vented gas cap" is really vented: I think a "vented gas cap" is designed and ment to be used only on a "vented gas tank." Gas caps have not been vented since the introduction of evaporative emission controls/SMOG control on Chevrolet cars in the 1970 model year. All modern vehicles use a non-vented cap. The gas cap is an integral part of the design of the fuel and emission control system, and calibrated for the application. Internal fuel tank pressures can be adversly affected by the use of an improper cap.

I think you need to look at your fuel system as a SYSTEM: Do you have the factory polution control system or the Evaporative Emissions Control (EEC) system stuff hooked up and working? Did you have the polution controls/EEC hooked up prior to your rebuild? Did everything work okay then? The '70's Non-vented tank is designed to work with the polution control system (the EEC system sucks those gas tank fumes out of the tank), if you don't have the EEC system hooked up then maybe you shouldn't have a Non-vented tank.

My '69 has a "vented gas tank" and it uses a "vented gas cap." Because there is no EEC system on my car there is a vent on the front of the gas tank. That vent works in conjunction with the vented gas cap as a system.

Some guys have been drilling holes in their gas caps to actually vent the gas tank... I'm not sold on that idea, but it is less expensive that installing a vented gas tank and vented gas cap.

I think your first step would be to buy a Non-vented gas cap and try it out. http://www.napaonline.com/Search/Detail.aspx?A=BK_7031034_0299411576&An=599001+101970+50012+2012020

 

[SS_Dave]

The 69 vented tank has a line that goes to a check valve above the rear axle.
The cap is air tight.
The 70, I believe is a vented cap.
No need for another tank vent.
If your cap blew off in your hand, it is either not the right cap or the vent is not working.

Check your oil level to make sure the fuel pressure did not force gas past the needle valve and into the crankcase.
This would thin the oil enough to lower the oil pressure.
If the oil smells like gas, change it and get a vented cap.

 

[Dean]

The 69 vented tank has a line that goes to a check valve above the rear axle.
The cap is air tight.
The 70, I believe is a vented cap.
No need for another tank vent.
If your cap blew off in your hand, it is either not the right cap or the vent is not working.

Check your oil level to make sure the fuel pressure did not force gas past the needle valve and into the crankcase.
This would thin the oil enough to lower the oil pressure.
If the oil smells like gas, change it and get a vented cap.

For the record, there is no check valve on 69 gas tank vents.

This has foam rubber inside with a restrictor plug on top

Sensitive content, not recommended for those under 18

Show Content

 

[Birddog1970]

My tank is a non-vented tank so I need the vented gas cap. I dont have nor seen any sinces of an EEC system on my car. I'm going to see if I can find another vented cap or possibly drill a small hole in the cap as others have done.

 

[OLDED]

I had the same issues of stalling as Birddog1970 had where it stalled at idle after a 30 minute Atlanta traffic drive and wouldn't start back up. I pushed it to the side of traffic and let it set for 30 minutes or so and it started and ran for a couple of minutes again and died. I took the cap off, started it up and drove home in peace. I did have to test the traction on the rear tires to see if the full amount of fuel was reaching the carb though - and guess what, it got a full load.

Long story - shortened, I drilled a couple of 1/8" holes in the top of the cap to let air pass -500 miles ago and all is good. No pressure or vacuum build up.

 

[SixActual]

I bought my '70 SS NEW and it came with a Vented Gas Cap and a non-vented fuel tank.

I noticed that when the air temp was high, the engine was hot, and I removed the cap to fill the tank up with fuel, it would also "WHOOSH" out fuel vapor, nothing else. I also noticed a short upward movement of the filler neck on the tank the moment I removed the Cap. In addition, the fuel gauge indicated I had more fuel than I thought before I removed the Cap. I guess the pressure had an effect on the Fuel Level Gauge as well.

It should be noted that I never experienced any problems with the engine starving of fuel or oil pressure concerns, and that I still have the Original Cap.


I'm unsure if these annoyances had anything had to do with the new LS-6 engine introduced that year or not. I don't recall if GM had Recalls back then for that issue, if in fact, it was a serious problem requiring a Recall.

I've lived with that since I've owned the car and never gave it any serious thought, other than to keep my grape clear of the filler neck when I remove the cap.

 

[Ed.Camino]

Sorry to here of your problems with the build, might help to think of them as seperate issues and resolve them 1 at a time.
Had a similar fuel tank issues years ago (I hate saying that) with a 71 El Camino, (dont remember if it was EEC or not). I had pulled the tank to have it cleaned or something and after reinstalling the tank I would have the same symptoms as described in your post. After a couple or 3 days I dropped the tank again and discovered I had pinched 1 or maybe 2 hoses between the top of the tank and the ..... whatever them things are called that the top of the tank comes up against.
Replaced the hose(s?) and made sure they were not pinched on tank reinstall and all the stalling, rough idle, and whooshing went away.
What was happening was as gas was pumped from the tank, no air was getting in to replace the space that the gas had occupied, causing a vacuum of sorts that the pump could not overcome, starving the carb. The blowback when removing the cap was caused by the inrush of air into the tank which would blow vapors out after the vacuum was eliminated.
It's interesting to note that on the 2 71's I have now, both have EEC and both had pinched hoses as desribed above. They were not running when I got them so dont know what the affect was.
El Camino has an EEC pressure differential valve (for lack of a better term right now) above the tank, where in the Chevelle I believe it is behind the rear seat. Just a plastic rectangle box looking think with 4 or 5 hoses tied into the tank and EEC canister.
If your tank has hose fittings in each front upper corner you have an EEC tank.
You need a cap to match the type fuel tank in the vehicle, and if EEC it needs to be working also.
I bet you got a pinched hose.

 

[dogsridewith]

I'm not saying there isn't, but are you sure there is a valve("pressure differntial valve") in that collector can behind the seat of my stock 71?

 

[RickH]

I'd say to focus on what Ed.Camino said:
What was happening was as gas was pumped from the tank, no air was getting in to replace the space that the gas had occupied, causing a vacuum of sorts that the pump could not overcome, starving the carb. The blowback when removing the cap was caused by the inrush of air into the tank which would blow vapors out after the vacuum was eliminated.

I had the same problem on my '71: whoosh when removing gas cap, and starving for gas when under a heavy load. It took a long time for me realize the two problems were related. I had removed that canister behind the back seat and plugged the extra lines coming off the corners of the gas tank. When I replaced the canister, and hooked up the lines, both problems were solved.

 

[Ed.Camino]

My bad, it's not called a "pressure differential valve" , it is called a seperator.

None of the GM manuals I have show a 71 without an EEC system which consists of the gas tank, seperator, canister and required hoses.

Quote from Fisher Body 1971 service manual
"On all coupe and sedan style bodies, gas tank fuel emissions seperator extends upward through piercing in rear compartment pan and is attached by screws to both rear compartment pan and to forward side of rear seat back panel (cargo barrier). The compartment pan attaching area requires a gasket to assure a positive seal against intrusion of fumes, dust, or water into body interior.
For information regarding removal, installation or general service procedures of fuel emissions seperator, refer to the applicable car division chassis service manual."

El camino and wagon have the seperator in a different location.
The pic was scanned from the assembly manual
The "can" or "charcoal canister" is located under the hood
Sorry, I have no info on 1970

 

[bobbyd38]

I'm replacing my Fuel Tank in my 72 Chevelle that I'm doing a Frame off restoration into a RestoMod. I have No Emission control parts and don't want them. Ground Up has a "Chevelle Fuel Tank Without EEC" Code: XTR-70A is this what I need to order?

The original tank I took out of the car has about 4 ports some went into the canister behind the seat and others went to the front of the Car for EEC.

What about a Fuel Cap? Anyone know what I should buy?

I simply want to buy and install the correct fuel system/parts in the car and not have any problems down the road, like the ones experienced in this thread.

Thanks,
Bob