Vapor lock

[Clone TIE Pilot]

I am still having an issue with a rough and uneven idle despite everything checking out. No vacuum leaks, fuel mixture dwell at the correct 30 degree range, correct ignition timing, etc. I have come to suspect it may be a vapor lock issue as the symptoms become worse as the engine heat soaks and the idle smooths out if I apply some propane to the air intake. It seems to be starving for fuel. I also have a off idle hesitation that becomes worse the harder I accelerate from a dead stop. With the high fuel prices I suspect my local gasoline may have higher ethanol content as both my Ranger and CVPI are not running quite as well recently either.

From I have read LG4 and L69s had vapor lock issues even back in the 80's when they were new and modern ethanol blended fuels boils even easier than the gasoline of lore. Currently running the stock return style L69 mechanical fuel pump running up to a Holley dead head regulator and to the E4ME Qjet which uses the thick style mounting gasket. Back in the day the dealership fix was to install a in tank electric booster pump to force feed fuel to the original mechanical pump and an electric cooler fan for the Qjet. The old emission legal ZZ4 350 swap kit for F bodies included a new in tank electric fuel pump and return regulator to completely replace the 305 mechanical fuel pump.

Probably the best fix is to convert to a in tank electric fuel pump but I rather not go that extreme if I can help it. Currently thinking of installing a phenolic mechanical fuel pump spacer, fuel line insulation, and maybe one of those external fuel filters with a vapor port. Since I still have the stock EVAP system and charcoal can, I assume I just run the vapor line from the fuel filter and tee it into the tank line into the charcoal can?

 

[ck80]

I am still having an issue with a rough and uneven idle despite everything checking out. No vacuum leaks, fuel mixture dwell at the correct 30 degree range, correct ignition timing, etc. I have come to suspect it may be a vapor lock issue as the symptoms become worse as the engine heat soaks and the idle smooths out if I apply some propane to the air intake. It seems to be starving for fuel. I also have a off idle hesitation that becomes worse the harder I accelerate from a dead stop. With the high fuel prices I suspect my local gasoline may have higher ethanol content as both my Ranger and CVPI are not running quite as well recently either.

From I have read LG4 and L69s had vapor lock issues even back in the 80's when they were new and modern ethanol blended fuels boils even easier than the gasoline of lore. Currently running the stock return style L69 mechanical fuel pump running up to a Holley dead head regulator and to the E4ME Qjet which uses the thick style mounting gasket. Back in the day the dealership fix was to install a in tank electric booster pump to force feed fuel to the original mechanical pump and an electric cooler fan for the Qjet. The old emission legal ZZ4 350 swap kit for F bodies included a new in tank electric fuel pump and return regulator to completely replace the 305 mechanical fuel pump.

Probably the best fix is to convert to a in tank electric fuel pump but I rather not go that extreme if I can help it. Currently thinking of installing a phenolic mechanical fuel pump spacer, fuel line insulation, and maybe one of those external fuel filters with a vapor port. Since I still have the stock EVAP system and charcoal can, I assume I just run the vapor line from the fuel filter and tee it into the tank line into the charcoal can?

Couple thoughts there...

1) blame Brandon and e15.

2) I'd wonder if venting that much extra vapor routed into the charcoal canister would result in saturation of the granules?

It's one thing to vent vapor from limited evaporation. It's a whole second issue to force feed boiling off fuel down It's throat. I'd wonder how long that could hold up as a fix since it wasn't designed for that frequency/volume?

When you think of how the system was arranged, when you pumped liquid fuel out of the tank, the source of air to backfill the lost volume was fresh air pulled in via the return line, coming through the charcoal canister. Only when parked and pressured rose did anything reverse direction into the canister.

Although some limited fuel vapors were routed back to the canister while running, its nowhere near the pressure or HC load youre probably contemplating? Just spitballing here.

You'd also possibly create some form of backpressure turbulence by teeing into the existing line like that. Thing about it, when you dump the new boil off vapors in whichever feed is strongest will push past the junction with the losing side left to build reverse pressure until it can overpower. Try it with water flows off a hose and faucet in the sink or two water hoses. You'll see current deflection. Picture in a closed system, no stream can be pushed aside to alter flow so you get turbulence and some pressure load, and who knows if it'd build enough to blow a line free or just do nothing.

Regardless, in the scenario I have to imagine neither would vent as freely as intended which could lead to running complications... but could be an interesting experiment.

What about as an alternative you had a ball check type valve to keep the line closed when not running with pressure buildup, and space permitting put a very small spacer ring on top of the air horn under the air cleaner and vent the fuel vapor past check valve into the incoming aircharge. It'd be ever so slightly rich in those moments, but, otherwise wouldn't impact running and I'm guessing ecm would adjust much like it does to give more fuel in cold conditions than hot already based on o2 readings.

Again... just spit balling. Most all of that may be meaningless and worth what you paid for it

 

[Clone TIE Pilot]

I am not familar with vapor venting fuel filters. For older pre EVAP cars it seems people just run a return line back to the tank. But for cars equip with EVAP I only find very vague advise of just connecting the fuel filter vapor port to the charcoal can. Perhaps it would be better to just tee into the pump return line with a one way checkvalve? I am concerned if I tee it into the line between charcoal can and control valve it would create a rich issue.

At least my old 46 farm tractor seems unaffected.

 

[Ugly1]

There is insulating the fuel line from the pump to the carb, and they also make a specific set of insulating spacers that goes under the carb for that problem. Cheap way I’ve seen/read is insulating the line with aluminum foil wrapped around it to reflect the heat. But double checking that as a old school fix is worth a look.

 

[Clone TIE Pilot]

I will try insulating the fuel line and phenolic (heat insulated) fuel pump spacer first. The L69 fuel pump has a built in large fuel reservoir but with it being bolted to the engine block it can conduct heat from it. If that doesn't work I may elect to go to an in tank electric pump setup like the old ZZ3 350 swap kits for 3rd gen LG4/L69 Camaros. Rockauto has Monte Carlo 4.3 TBI fuel senders with electric pump for $47, but still requires a whole mess of additional parts to make it work in a carbed car.

Did find a gas station that sells ethanol free gas buts its far from my house, its more expensive, and its reported they don't always have it in stock.

 

[78Delta88]

I will try insulating the fuel line and phenolic (heat insulated) fuel pump spacer first. The L69 fuel pump has a built in large fuel reservoir but with it being bolted to the engine block it can conduct heat from it. If that doesn't work I may elect to go to an in tank electric pump setup like the old ZZ3 350 swap kits for 3rd gen LG4/L69 Camaros. Rockauto has Monte Carlo 4.3 TBI fuel senders with electric pump for $47, but still requires a whole mess of additional parts to make it work in a carbed car.

Did find a gas station that sells ethanol free gas buts its far from my house, its more expensive, and its reported they don't always have it in stock.

I can post part numbers later tomorrow, but having built several G and A Body street/show in and around Tucson, 100*F + on some days, delt with this before. The stock FP has the small vent return, if you're does , get rid of it. Get a basic mechanical FP, oreilly/auto zone etc... The basic FP just has the in and out ports. Cap the small line going back to the tank. Depending on how your evap system is setup you might need vented gas cap..., maybe not... I went though the very issue you describe and all new FPs, and found that instead of sending vapor back to the tank it was pumping and sending raw fuel back, and yes pumping air bubbles up to the carb. Quality control is just not what it used to be. You describe symptoms of insufficient fuel reaching the carb. So figuring your carb is good, etc..., etc..., just do the simple first. Holley ... Edelbrock... Excellent carbs but they don't run on bubbles. It sound like you have the fuel regulator with the return. If you do you can tap the return into the line you capped from changing the FP. Also if/when you drop the tank, check all your rubber hoses. The 86 era fuel hose rubber will not handle today's gas and its additives. Seen to many restored 80's vintage burnt to the ground. Use the fuel injection line designed for today's gas. This is also true for the old vacuum lines.

 

[69hurstolds]

Seen to many restored 80's vintage

Where you seeing these unicorns? I've yet to see anyone other than 88hurstolds actually restore a vintage 80s G-body to its former glory. Pretty much everything else has been modified. Of course, if they're all burning up, it would make sense nobody else is seeing them.

 

[airboatgreg]

I doubt it is a vapor lock. It is running lean because it smooths out when you add propane. Take a can of carb cleaner with a straw and spray around the base gasket, crimp off vacuum hoses. Don't forget the booster. Tighten intake bolts from middle out, With the choke open and engine hot, look down the primary side of the carb and see if it is dripping from one of the venturies. If it is you have a plugged carb. With car at idle and in park manually open and close the secondary throttle plates as they will stick and create a lean issue. Have you adjuster the idle mixture screws? If you don't find a vacuum leak have engine warm and rev motor up, close the choke plate manually and let off throttle, open choke back as it nears stall. Many times this will unplug a stuck idle circuit.

 

[Clone TIE Pilot]

I can post part numbers later tomorrow, but having built several G and A Body street/show in and around Tucson, 100*F + on some days, delt with this before. The stock FP has the small vent return, if you're does , get rid of it. Get a basic mechanical FP, oreilly/auto zone etc... The basic FP just has the in and out ports. Cap the small line going back to the tank. Depending on how your evap system is setup you might need vented gas cap..., maybe not... I went though the very issue you describe and all new FPs, and found that instead of sending vapor back to the tank it was pumping and sending raw fuel back, and yes pumping air bubbles up to the carb. Quality control is just not what it used to be. You describe symptoms of insufficient fuel reaching the carb. So figuring your carb is good, etc..., etc..., just do the simple first. Holley ... Edelbrock... Excellent carbs but they don't run on bubbles. It sound like you have the fuel regulator with the return. If you do you can tap the return into the line you capped from changing the FP. Also if/when you drop the tank, check all your rubber hoses. The 86 era fuel hose rubber will not handle today's gas and its additives. Seen to many restored 80's vintage burnt to the ground. Use the fuel injection line designed for today's gas. This is also true for the old vacuum lines.

I read in the factory service manual that is the idea, the 3 port pumps circulate fuel from and back to the tank to keep it cool and only bleed fuel to the carb when it needs it, similar to a return style EFI setup. But with today's poor CQ, new pumps may have incorrect specs. Its very common for mechanical fuel pumps to put out too much PSI and unseat float needles which is I installed a dead head regulator. Its been suggested to me to switch to a 2 port pump and a use a return style regulator.

Its not vacuum leaks. I tested everything with a unlit torch and found no leaks. Also tested vacuum items such as the brake booster with a Mityvac, they all hold vacuum.

 

[86LK]

as 78Delta88 said, check your rubber fuel lines. the internal coating on them starts to delaminate and clog the lines causing it to run like sht. we had this happen on a gas golf cart at work we used to move stuff daily. about once a year it would start acting up, we'd have to pull some lines and clean them out then reinstall. and those things are just dumb stupid gas engines. check/replace your rubber lines and fittings