I've been trying to figure out why the truck has been running so irradically and someone suggested that it may be vapor lock. It runs fine when I initially start it but as soon as it heats up I notice that there's no fuel coming through the filter and it barely runs. The fuel pump is brand new so I ruled that out. I'm looking into an electric unit with a low pressure switch to see if that will help. Any other suggestions?
Vapor lock is pretty common. As you track down the heat sources, keep in mind that the fuel that flows very slowly thru the fuel lines will pick up heat at every place the fuel line is not exposed to free flowing air and is exposed to warm engine parts, or is exposed to HOT air from heat-radiating parts. That includes not only engine area, but also farther back where an exhaust pipe may run close by, or a hot-air escape path sometimes between frame rails. That means the old recommended method of wrapping a spring or other coiled wire around the fuel line will help only in the cooler-air areas to help get rid of heat. Do NOT use that method near the hot areas or you will pick up more heat. The old trick of clothes pins, or wrapping the fuel line, or slipping a length of slit-open rubber fuel line over the metal line will work near hotspots or hot radiator air where you pick up more heat, but do not use that insulating method near cool-air areas, since you want the use the cool air to remove heat from the line. An electric fuel pump near the rear is a good way to reduce boiling because there is less suction line (suction invites boiling- lower boiling point), and more fuel line under pressure (raises boiling point slightly), but the best thing to do is to keep the fuel recirculating instead of just sitting in the fuel line waiting to slowly move forward to the carb, all the while soaking up more heat. Here is a copy of a post I made in a different thread about percolating carbs. It has info on what worked very well for me. *************************************************** What I did for my Stude pickup when it kept boiling the carb was to turn off the electric pump as I approached a parking lot or driveway before I came to a stop, so the carb wouldn't be full. After I got tired of that, or tired of hard starting when I forgot to turn the pump off early, was to buy a fuel filter I used to see on AMC's and other smog-carbed cars of the early 80's. Many of them, if not most makes of cars, used what looked like an ordinary fuel filter with an extra outlet. That extra outlet is for a fuel return line that runs back to the tank. The reason for most vapor-lock (also helps with most carb boiling probs), is that the fuel inches it's way along inside the fuel line, picking up more and more heat as it slowly crawls along. By the time it reaches the carb it is either boiling or very close to it. Adding the filter with the built in tiny orifice and return line will keep the fuel moving along, the hot fuel will return to the tank for cooling, and cooler fuel will replace it in the lines near the carb. That usually keeps slow moving fuel from just sitting there collecting heat. I used to see those filters hanging on the racks with the "normal" filters at Wal Mart Ace Napa and everywhere. If they are no longer as common as they used to be, just go to the parts counter, tell them you drive a "1977 PACER" and need a fuel filter. I don't know if ALL the filters will have the tiny orifice in the return outlet to keep you from starving the carb, but the AMC ones certainly did. If yours doesn't have a restrictor orifice built in, you may dump too much fuel pressure back to the tank, and have to restrict your return line in order to feed the carb, but every one of the AMC fuel filters I have used were just right. Here are some pics of what they usually looked like, so you can know when you find one. Just make sure the return line isn't too free-flowing or you could starve the carb. All the ones I have used have had the restrictor built into the small extra-outlet. I just can't assure you that ALL of them use one. The ones I have seen on factory AMC cars have all been mounted not too far from the carb. If you mount them too far back in the fuel system, it will defeat the purpose of keeping the fuel cool at the carb. <!-- / message --><!-- attachments --> P.S. This style of filter and recirc flow, will only work when mounted after the pump, on the pressure (output) side of it. The closer you mount the recirculating filter to the front near the carb, the less hot fuel you will have in the system. IF YOU DON'T WANT TO USE AN ELECTRIC PUMP, AND WANT TO KEEP THE FUEL SYSTEM AS STOCK AS POSSIBLE, THIS RECIRCULATING FILTER CAN BE USED VERY SUCCESSFULLY WITH THE STOCK MECHANICAL PUMP IF YOU MOUNT THE FILTER NEAR THE CARB, AND RUN ONE SIMPLE RETURN LINE TO THE TANK. THAT IS HOW AMC DID IT. IT WILL STILL KEEP THE FUEL COOL BY VIRTUE OF A SLOW CONSTANT FLOW.
That filter is a great idea. A quickie test for ya would be to run rubber lines, oh and loose the metal/glass filter. Plastics and rubber won't heat soak so much.
Great stuff!! I've been dealing with some vapor lock issues this summer... I'll be using information for sure...
I'm using rubber lines up to the mechanical pump. A steel line runs up the side of the block then transitions into the rubber line, filter, fuel pressure guage then carb. The steel line may be too close to the block huggers.The tank is vented. I installed a rollover vent valve a few months ago, but I try driving it w/o the cap on to see if that helps. Got rid of the glass filter after hearing a few horror stories of them breaking and torching the vehicle. I like the idea of the pacer filter, if all else fails I may try that in conjunction with an electrical pump if I can still find that type of filter.
CLOGGED FUEL LINE and/or Pickup in the tank. Not to be Captain Obvious here, but is it getting enough gas when it is COLD? Are you sure you just don't have crud obstructing your gas inlet in the tank? (I've had several cars with this problem over the years, and the only way to know for sure is to pull your tank and take a look in there.) When you first start the car there is enough gas in the line to keep everything seeming okay, but as the engine runs and especially if you drive it needs more gas that the pump just can't get from the tank. The result is low flow or no flow at the filter or carb.
Robert, if you have a guage where you could hook it at or very near the carburator do that and measure the P S I when the engine is warm . It should read at 3-7 P S I . If not , thers not sufficent pressure to run the truck . add the electric at the tank and then run the mechanical along with it , together , they wil act in tandem . scrubba
I see you are running lots of rubber line on the suction side? I would use as much steel line as I could, then use rubber only for the connections and flex areas. There is a hazard in running lots of rubber line, going around bends, heating it up, then pulling a suction thru it. Not just the safety concerns, but also a softened hose flattening around bends, and long straight runs when a suction is applied. It may be harder to pull fuel through it than you think. I would use as much steel line as I could, then add rubber only where needed. The fuel should flow easier. Then I would add the "bypass-type fuel filter" and a return line. In my experience it has had a 100% success rate.
Gasoline can boil around 180 degrees or less depending on the grade. If the engine is running near that temperature it can be a real headache. If the fuel pump is already at that temperature the fuel can flash into vapor right there and there is no pumping action. Even if you go with an electric fuel pump the fuel can boil in the carb if the engine is too hot. If you go with an electric fuel pump with a regulator you can bypass the stock pump and eliminate that hot point. Add a plastic insulator plate under the carb to help reduce the chance of boiling in the carb. Add insulation to the fuel line to reduce heat transfer from the hot engine compartment to the metal fuel line. If you're running a 180 degree thermostat, go down to a 160. If possible, add a hose to the carb to gain cold air to the carb. This will aso help in the HP dept. Anything you can do to reduce fuel temp will help. Bill
I do have a guage and that reminds me that I forgot to mention that I'm getting some strange readings. Sometimes the needle bounces around wildly and then at times it holds steady at about 5-6 psi. Any thoughts?
I have a 53' Mercury and had the same proublems. I finaly gave up and put a new Carter fuel pump on the car (from MAC's). I mounted it on the frame of the car just in front of the rear wheel.The nice think about the new Carters is that they are set for between 3.5 and 4.5 lbs, so you do not need a fuel regulator. I did put a new mech fuel pump on the car too. When I begin to have proublems, I simply hit the switch for the pump and once the car picks back up, I turn off the switch and continue on with the mech pump. I did a bypass hose around the pump with a check valve from Graingers (they have a store in Denver). The check valve is a perfect fit and cheep.Yes it is not the cleanest instalation (too many hose clamps) but it works great....and qualty of fuel is not an issue. Good luck..... <!-- / message --><!-- attachments -->
I replaced the mechanical pump with an electrical one and most of the rubber lines with steel ones. Started and ran fine, Pressure guage held rock steady @ 5 psi. headed out for a test drive and it did the same thing...ran like @($%@&!on & on top of that the brakes started locking up agsin!. The only other alternative is to check the carb. I'll see if one of my buds will let me borrow one just to test this theory. I'm getting pretty tired of this!!!!!!!
Would like more information about the horror stories for the glass filters. Are you talking about the Carter glass filters with ceramic insert?
As for vapor lock, I'm very opinionated. I say there's basically no such thing. I'm sure I may stir up some comments on this but, just stay with me. My first car was a '40 Ford and the second was a '49. I live in Texas, so obviously it's mucho hot. Here's my point...these cars did not give problems back then, so IF everything is operating "like new", the cars will run like they were designed to do. Sounds like you have a fuel problem...low pressure, low volume, tank vent problem, bad filter, sucking air, etc.
I was told by my old timer mechanic friends that the fuel quality back then was much better than it is now... They told me that vapor lock happens on the old cars because the fuel is crappier and boils off too easily due to the use of fuel injection.... Any truth to this? I had some vapor lock issues after I adjusted the carb too lean, it went away when I set it back
imnezrider : You are missing the point son , fuel has changed radicaly inthe the last few years. The vapor pressure [boiling point] has gotten much lower in the last few years. modern injection systems with pumps in the tank are not affected because the entire fuel system is under pressure which prevents the fuel from gasing [boiling]. Az used to have MBTE in the fuel in the summer,2 years ago they changed to alcohol year around. First 95* day here my 65 Ranchero ,200-6, promply became un driveable due to boiling fuel . Fix= a 3 line return style filter at the carb. Been 115+ around here for the last month been fine now. This year my 27 T Roadster started this shit ,been fine the last 2 summers. Fix= move fuel outlet to bottom of tank for gravity feed, rather than lifting it up from a top suction.
ah yes, old school chemistry class.... the higher the pressure, the higher the boil point.... that's why we wear pressure suits in space...your blood boils in a vacuum
Have you checked the fuel pressure at the exact moment when you have the troubles? Does it stay high or does it go away to almost nothing? Have you checked to be sure of what it is actually doing? Clogged filter. Clogged pickup screen. Fuel cap not vented, creating a vacuum in tank after a few minutes of running. Rubber hose collapsing. Accidental cut inside rubber line creating a rubber flap that restricts flow (more common than you think)(often comes from pushing a rubber hose over a metal line or fitting). Kink in line somewhere. Smashed section in line somewhere. Cracked hose or line or cracked solder joint letting in air on the suction side. Loose clamp letting air in. Some heat source somewhere is still heating the fuel to boiling. Restricted passage at carb inlet letting in only enough fuel to fill the carb bowl at idle, but running the bowl dry during higher demand while driving. Fuel filter too tiny or possibly defective. Bad gasket on glass bowl type filter, letting air in. Crack at fitting on plastic type filter letting air in. Someone used silicone sealer at fuel gauge sender unit, and now it's in the fuel? yes, I have seen that cause all sorts of strange problems when an invisible glob gets loose and moves around undetected. Possibly not even a fuel problem. I have seen a weak ignition give those same symptoms when the spark is so weak it gets snuffed out when you put a light load on the engine and give it more throttle. In one case it was a bad condensor, in another an internally damaged coil wire with super high resistance, in another, a failing coil arcing internally, in another the points rubbing block was worn down so the points wouldn't open as far as they were supposed to. Yes, gas does boil. Yes, vapor lock is real, and it has existed for decades. No, those old old old vapor lock remedies from yesteryear did not come about because one person had a bad dream long long ago, it has existed for many many years, and is not from imagination or a "faulty owner". It is caused simply by heat getting to the fuel. Either keep the heat from getting to the fuel, or recurculate the fuel to keep cooler fuel coming, or run a high pressure that you don't want to use with a carb. Recirculating the fuel by adding the right filter-with-a-bypass, and a return line to the tank is the simplest, easiest, and most trouble free way to prevent boiling. Other methods work, but I think this is the simplest and most reliable way.
just another place to look, the gas line on a friend's merc was bundled together with the tranny cooling lines for a few feet. that contact was all it took to heat the fuel.
Remove your fuel line before your pump and put some regulated air pressure through the line with your cap off and listen to the tank for the air coming out. Also check your rubber line from your tank to frame for a crack or leak. I had a gto that ran great, never gave me any problems, but sucked gas like there was no tommrow. It ended up that the hose was leaking only when driving, changed it and milieage went up. Your's could be sucking air or collapsing in on it's self, or swelled shut inside if it's old or cheap fuel hose.
It's best when it happens in a fully loaded station wagon in the middle of the dessert. My old 327 had a summer of 'lock' the year I moved to L.A. Looks like you're in good hands here on the H.A.M.B.
Talked to some of the local racers and was told stories of them breaking causing fires. I've broken a few myself but thankfully, never had a fire.
I'm outta ideas. I swapped the carb and it's still running lousy. The tank is vented and clean. New fuel lines, filters, fuel pump, distributer & coil. Using fresh fuel.
Biggest question of all-- Everybody wants to know- At the very moment when it is happening, when you shut off the engine, quickly unhook the fuel line by the carb, and you run the pump for a few seconds, have you been getting a good spray of fuel or air-and-liquid spitting or just air bubbles with a small bit of fuel? What does it look like when you do that? Then when you reconnect, does it drive OK for a while afterwards, or keep on with the trouble? What has been happening at the exact moment? Knowing that info is the only way to troubleshoot further. If you get a good gusher of fuel, hook it back up, and still immediately runs lousey, then we are barking up the wrong tree, and need to look elsewhere. "Input. I need input"
a few quick questions.... 1. What engine is it....I reread and did not see this.... you have an AD truck...is this a 235? Or 216? 2. Next question.....you do a tune up on it recently? Like new points and what not?
Gotta ask. Have you tried a different brand of gasoline? If your using a major brand, try an off brand and vice-versa. Several years ago, I had a one ton Ford van that would vapor lock like crazy with some brands of fuel, never with others. I know that all gasoline is supposed to be the same, but .... Bill
I'm running a 327 with a Edelbrock Preformer intake & carb (1406 600cfm) Plugs & points are new. No problems with fuel flow. It idles fine, but after it warms up and I take it for a spin it starts running like crap. Runs decent at idle speed.
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