Engine dying while cruising - vented fuel cap problem?

Took the car out today for it’s first longer trip (40+ miles one way), but about 10 miles in after cruising at 65-70, the engine starts cutting out and then dies. After pulling over, it it starts back up after cranking for a few seconds. Unfortunately, it cutout a few more times on the way back home. It seems to run fine normally as I replaced plugs, wires, rotor, cap, installed 12v Pertronix coil (powered from relay), and it already has a Pertronix igniter. Intermittent problems like this make it hard to troubleshoot…I assume it’s fuel or spark.

I was po’ed, so left it parked for a few hours and left it alone. Ironic as I was headed out to a local cruise up in Golden…took the 4Runner.

I decided to check the fuel cap tonight as I replaced it a few weeks ago…a Motorad MGC40 from WCCC. When I opened the cap, it appears the tank had a pretty good vacuum. From my understanding, cap should let air in (but not out), so there shouldn’t be a vacuum in the tank.

I’m curious if the engine cutting out intermittently is this simple…a bad cap? I assume I could perform a quick test by driving it with a loose cap that would let air in? I’m just a bit gun shy in taking it out now.

Yes. That probably is your problem.

Are you sure it had a vacuum? That is definitely a problem.

I’m fairly confident…could hear inrush of air when loosening the cap.

The air rush thing is a big indictor of the wrong cay. Just drill a small hole in the sealer part of the cap. Not clear thru it just to the underside of the metal cap.

Great advise on how to make the car catch on fire while driving.

I checked out the Motorad cap a bit closer. After cleaning it off, I found I could suck on the valve and it would let air in. If so, curious why it’s not letting air in when it’s on the car?

Will see if I can find another cap…none of the local parts stores have them in stock.

C6GY-9030-A is the correct cap they come up on ebay I have had one on my 390 car for 17 years and never an issue.

Just received a Stant cap…turns out its Motorad as well.

Will try and take it out later this week with the cap loose for now just to verify whether it is a venting issue causing the engine to die after a few miles.

I just lost my fuel cap today. Is it too dangerous to drive at all without it before the new one arrives??

I wouldn’t…seems to easy for gas to slosh out when taking off.

It’s Stant G40. Typically around $15 including shipping on eBay. I bought one today. There are perhaps 30 of them listed from $12 to $100.

Thanks Royce…that what I needed and just purchased as well.

Now I kinda wanna check mine for a cap problem. I have a random stall as well but only seems to happen while hot and in the worse possible place like an intersection or pulling into traffic. It’s one of the reasons I’ve kept it close to home as I can’t trust it yet. I’ve replaced the cap with a typical stant replacement but I did not have the original to compare too.

I had originally thought a possible vapor lock,heat soak issue with the ignition, or coil issue. Seems to happen more when over 100 ambient temperature. I have not dedicated much time to finding it yet but I’ll look it to the cap first.

Funny you mentioned these things…same possible issues I thought of as well, as mine only seemed to occur once the car has heated up. Unfortunately, I don’t have anything to compare it against as these are the first longer trips I’ve taken since purchase.

I only thought about the gas cap based on the vacuum I noticed in the tank after removing the cap.

If that doesn’t work, I think I’m stuck parts swapping since this is an intermittent issue, although I’m going to put a 1/4” phenolic style spacer under the carb first. After that, would try (unless someone has a better suggestion) to swap these one at a time in this order:

  • different coil and Pertronix unit…I already have spares
  • different 2 barrel…I still have the old Holley two barrel I can try (I replaced with new Holley 2 barrel 350 cfm last fall)
  • relay supplying 12v to coil and Pertronix…relay is triggered off old resistor wire…seems odd to have an intermittent issue with this
  • fuel pump…may not be needed as I could tell carb had fuel in the bowl via clear float window after last engine die event

Keeping my fingers crossed on cap issue. I’m a bit paranoid as testing may leave me stranded, but it has restarted after cranking for 3-5 seconds, where normally it starts up almost instantly.

Im in the same boat with short drives since getting my car. I got it in pieces and slowing assembling over the years, Ive driven it in varies stages to move around and just keep it moving. Now that its more roadable and has tags Ive been just kicking ideas around on what would cause the stall. Actually forgot about it again until last time I drove it and it just about stalled pulling onto a main road. I have not done any really diag on it trying to isolate it but the fuel cap concern is something I have not thought about much. Now that summer is in full swing here Im very hesitant to take it out as our temps are no joke and they will find a weak link if you have one. On the other hand its a good way to sort out any and all issues. The engine is one of the last things I have yet to put my hands on much on my car. I know I have a few areas Im concerned with and want to address before I feel comfortable taking it on the road for very long. Im tempted to take it out for a small fathers day car show(and I dont like showing at car shows :laughing: ) but worried Ill make it there and back!

All of my ignition is aftermarket including the carb. Installed by the PO and driven very little afterwards until parked but its early to Mid 90s MSD stuff so best not to rule it out just yet, Ive rewired to clean up the install and move the cdi box to a better location but the stall still comes up occasionally. I know I need to reseal the carb yet again for a surge at cruise speeds, Our fuel is killer on carbs! In all honestly I probably wont spend too much with the ignition and carb as I want to do EFI anyway so if I find an issue there that will be my excuse! :laughing:

relay supplying 12v to coil and Pertronix…relay is triggered off old resistor wire…seems odd to have an intermittent issue with this

Do you have a 12 volt coil? You could be over heating the 6 volt coil.

Yes, I have a new Pertronix 12v coil…believe it is the 1.5 ohm version.

I was thinking the same as I want to convert to EFI due to our Denver altitude, but have heard its better to work these issues out first as EFI will magnify any existing issues. I was waiting until after I installed the Blue Thunder 4 barrel intake from my existing 2 barrel, but may rethink this by going to the 2 barrel Sniper sooner just for drivability…believe xr7g428 made that suggestion earlier this year.

The car runs well outside of this current issue…just want to nail it down.

My self and others have had heat problems with Pertronix coils, they have sent me replacements without complaining. Gets hot and no spark leave hood open 10 minutes or so and good to go. I’ve gone back to ‘old yeller’.

Yes and No. Base engine concerns aside. It is a good idea to make sure you are as problem free as possible but depending on which way you go with the EFI conversion and where your concern is it may not matter. For example I plan to go with the distributor, cdi box and new coil along at the same time as EFI as to take advantage of the EFI adjusted timing. If my problem lies within the fuel system, carb, cdi box, distributor its not going to matter much with how I plan to do things. I have been putting the EFI towards the end of the list due to cost and necessity but I am approaching that point I want to start acquiring the parts for it but would be nice to make some drives while I wait