CPenn

Hi everyone, 1980. 4 spd manual. Spent the winter working on tranny, rebuilt carb, some electronics. Note that this car sat for 5 years in a garage before I got it. Got it running good on all four jacks going 70 mph! Snow melted here so I took it off the jacks and took it for a drive. Got about 1/3 mile and it sputtered and stalled out. Would not start. No fuel in the bowl. Pulled it back to the house and checked the float valve on carb. It was fine. Note that when I rebuilt carb I added an inline fuel filter under the hood. Anyway, after checking the float valve I then found that no fuel was being delivered to the carb so I figured the fuel pump died, although I could hear it hum. Strange thing is that the next morning the fuel pump worked fine. I figured the fuel pump was on/off and needed replaced. Got a new fuel pump and replaced the fuel filter located next to the fuel pump.

I figured that would fix it. Wrong. Took it for a ride. Got two miles. Same thing. Although this time, right before it completely stalled out, I heard a loud "POP!" sound from the tank. Pulled it back to the house and I figured that there was junk in the fuel line. I blew the lines out with compressed air. One thing I noticed was that when I blew air into the hose that went into the fuel tank, it pressurized the tank. In fact, it pressurized it so much that when I took the air hose off, fuel started flowing out of that fuel hose to relieve the pressure. Put it all back together and drove down the road. Same thing happened again. Pulled it back. The next morning, it pumped fuel to the carb just fine when I started it up.

Could this be a problem with the fuel cut valve? I think that there is a charcoal cannister that sits on top of the tank for venting, could that be clogged? The strange thing is that I can start it and idle it for hours or bring up to high rpm standing still and it is fine. I even had it on jacks over the winter and had wheels turning all 4 gears with no problems. It only does this stalling fuel delivery problem when the car is actually moving.

I appreciate any insight on this. Thanks!

yeti

iTrader: (1)

I'm probably wrong but make sure the tank can vent. When I bought my 79 the PO had the carb deadheaded with the return and evap line looped. The car would drive fine but anytime I went to get fuel you'd hear it suck or blow out air (I imagine there was just enough of a leak to atmosphere that it didn't cause major issues. Your car sounds to be stock but the general description sounds like that's what happening and I assume by the time your driven a couple miles it's pulled so much vacuum that it's too hard to pull fuel out. Next time this happens try opening the fuel cap, priming the fuel, and trying again.

Maxwedge

iTrader: (1)

I was thinking similarly. Your tank isn't venting properly so once some vacuum builds in the tank, the pump can't pull fuel from it. I think the vapor canister on SA's is in the top of the air cleaner. I've only had FB's that carry it on the passenger side shock tower. There are 3 metal lines running from the tank to the firewall - fuel delivery, fuel return and vent line to the charcoal canister.

CPenn

Guys, thanks, this all makes sense. I identified each of the components that you mentioned. I'll work on it this week and let you know what happens.

In taking apart/inspecting some of this, do you see any problem with pushing compressed air into the metal line for the vent, specifically pushing air from the engine compartment back to the tank? I was thinking of doing that while checking on the charcoal cannister. AlthoughI blew out the fuel supply and return lines over the weekend, I did not blow out the vent line (I did not even know it existed).
Thanks

yeti

iTrader: (1)

Again, I don't really know and without looking at a diagram I would think blowing air could possibly be okay as long as the fuel cap is off and you are okay with blowing whatever is in the line into the tank which may have the possibility to clog something else later on. I would think low pressure should be fine though.

Please verify that there are no check valves or anything else that may get damaged in the route though.

CPenn

Good point. Looking at a previous thread (https://www.rx7club.com/1st-generati...k-help-941997/) it seems that the vent line in the engine compartment is connected to the "vapor valve" located on top of the tank. That could be a one-way valve so blowing in it from the engine toward the rear may do no good. The charcoal canister with the air horn seemed okay as I was able to blow air through it via my mouth. I also used my Haynes manual to test the air check valve located in the engine compartment and it was fine. I am going to remove the "cut and check" valve located next to the fuel tank and test it. If that tests fine, then I'll remove the tank and test the vapor valve located on top of it (and clean the tank while its off).

CPenn

After removing the hose that connects the top of the tank to the cut and check valve, and also removing the vent hose in the engine compartment where it connects, I blew air into the hose to the tank in order to see if the vent would let the pressure out. It did not allow much air to come out. Only a little came out of the vent line in the engine compartment. In fact, I could hear the air escaping a little around the fuel cap. I am guessing that the vent on top of the tank is partly plugged/stuck. I am going to remove the tank and install a new vent plus a new cut and check valve. I'll post back with results.

Maxwedge

iTrader: (1)

Dumb question, but you did visually inspect the vent line under the car? It's easy to crush if someone drove over something or jacked in the wrong area.

I just ask because I don't know of a "normal" reason the vent line would get clogged. Nothing but vapor and air pass through it, not fuel or sediment from a rusty tank.

CPenn

Good point. So I disconnected the vent where it comes out of the tank, and also where it connects in the engine compartment on the other side. That way I could test air flow through the metal line alone. It moved air just fine. Apparently the problem is at the tank. So then I took the tank off on Saturday. Removed the vapor vent; the valve in it appears to not be working. Should have a new one before tomorrow.

Cleaned the tank while I had it off. I used a gallon of "Evapo-Rust", which worked great! It is not a solvent so I could even bring it into the basement where it is warmer. It uses oxalate instead of solvents. Oxalate is a naturally occurring organic acid, it is the same stuff that is in "Bar Keepers Friend". So next time I have to clean something rusty, I'll probably just use Bar Keepers Friend due to the lesser cost.

CPenn

Finally got a chance to take it on the road after replacing the cut and check valve and the gas tank vapor valve. That fixed the problem. I cannot say which of those two was the problem. Possibly both. It certainly seemed like the vapor valve was letting no air out based on my tests with compressed air. So in summary, the fuel tank was not venting due to either a bad cut and check valve and/or fuel tank valve, which caused the fuel pump to draw a vacuum. When the pump could no longer pull the fuel from the tank due to excessive vacuum, it stalled out. This fixed it. Brought the speedometer up to 75 mph, which was the fastest I could go and still feel a tiny bit safe on a back country road with no lines painted. It sure felt fast being so close to the ground. I never drove a sports car before.

Take care.

boyee

iTrader: (3)

Hi @CPenn May I ask where you got your valves from? Seems like Mazda no longer sells the check and cut nor the vaporizer valves. Thanks!

CPenn

Ebay. Grand Tex auto. https://www.ebay.com/usr/gtex_auto?ul_noapp=true

boyee

iTrader: (3)

thanks! ebay FTW - found one here https://www.ebay.com/itm/18486771898...wAAOSwn6Vgt~VI

also i managed to get a new fuel cut valve from Jim Ellis Mazda so we’ll see if that shows up

boyee

iTrader: (3)

To contribute to this thread what worked for me was also replacing the fuel vaporizer vent valve. The fuel cut valve was not the root cause although it sounded possible.