79′ F150 460cu. Nobody Can Solve The Problem. Please Help
Posted in Crank, Oil and Piston | Asked on May 15, 2010
Below is the question about 79′ F150 460cu. Nobody Can Solve The Problem. Please Help, 79′ F150 460cu. Issue has everyone stumped, even a couple pros.
Vehicle will start and run absolutely perfect when completely cool. About the time the thermostat opens it falls on its face. Hardly keep running, exhaust sounds like it has a cold. Replaced plugs, wires, distributor, cap, rotor button, coil, control module, fuel filters, air filter. Tested fuel pressure, piston compression, timing, oil level fine, oil pressure good. Turn it off, let cool, runs beautiful, until hot. Never runs above 210-215, usually lower. Has no EGR valve, O2 sensor or catalytic converter.
The choke is manual and never needed unless below 32F. I did swap the carb out with another I knew to be good, problem was exactly the same.
If it is an intake manifold leak, which I’ve been considering, where in the intake would it be leaking to cause said problem. And more importantly how to find it, the spray around with carb-cleaner trick didn’t yield results.
Also, the thermostat isn’t even a year old, the radiator is maybe 3 years old, the truck is seldom driven because it is a gas hog off-road beast. I have two temperature gauges, block temp and thermostat housing temp. The stay within 10 degrees. Engine block temp drops slightly, momentarily, when thermostat opens. Temp stays well within normal range. Has high flow mechanical fan plus (2) two 14″ electric fans on thermostat switch. Over-heating seems ruled out.
Again, intake manifold vacuum leak hasn’t been ruled out entirely, but need a better understanding on where such a a leak would be to affect entire engine performance. When I think of manifold leaks I think of carb/manifold gasket leaking, or piston running lean from sucking in too much air on that port. I even isolated the brake power booster and transmission module to insure they were not the culprit. I have no lean running pistons and only one manifold vacuum port to check. When truck is running fine the vacuum sets steady at 19, when running bad it drops to around 15-16. I would assume a leak would show a more significant drop, rather than what I have dismissed as low idle vacuum.
Thanks everyone for the ideas so far, I am still poking at it. I decided to replace the factory 190F thermostat with a 160F to see if that makes a difference with internal temps. I didn’t used to run a thermostat in it, but put one in last fall for faster heat in the winter. Seems silly but it may be the problem. I am going to look into that resistor thing one of you mentioned, see what I can find. Again, thanks for all the answers so far.
OK, some of you obviously didn’t read the list of stuff I already checked or replaced – for instance anything distributor related. That was the first thing I went after, the magnetic pick-ups in those are notorious for failing in my experience.
I like the information about the resistor before the coil, I checked the voltage before and after issue occurs – no difference.
I find that now it seems to just idle low BUT smooth, still no acceleration at all. Vacuum reads around 15-16-ish, a little low for the norm of 19 I’m usually getting.
Running out of options, I’m in the process of replacing the intake manifold gaskets, I am optimistic this will solve it.
There are 7 Answers for "79′ F150 460cu. Nobody Can Solve The Problem. Please Help"
no coolant or clogged radiator?
i would just replace coolant and go for an after market radiator and replace all the lines. maybe your fuel might even be to much octane and heat for the engine. change oil and all that good stuff. good luck
Im going to take a stab a your problem. Everything I thought of you’ve changed but two items. FIRST, Im certain its in the ignition system. Just doest open until it gets warm. Fords had an electronic deal under the dist. It was under the plate I think. That’s one the other—–for years Ford used a resistance module to reduce voltage to the coil from 12v to 8.9 or so. I still have one on my 65 Sunbeam Tiger but later years they removed it and put it in line. Its not easy to find w/out a dealer schematic but its a resistor that makes the wire appear thicker where its located. When the car starts- truck here- that resistor is by passed but when they key returns to run it then has ignition power thru the resistor- IF it is breaking down when it warms up- well you see where Im coming from. Im a Ford person- PLEASE email me when U find the problem. achd73@yahoo.
Good luck. TonytheTiger
If it happens exactly when the thermostat opens, look for something related to the thermostat. When the T-stat opens, it re-routes coolant from recirculating in the block to circulating through the radiator. If the radiator or one of the hoses is blocked, coolant pressure will slow down your water pump and possibly cause the engine to run really badly. See if you can run water (garden hose) through the radiator and through the block. Or take out the thermostat entirely and see if the problem still happens. Oh yes, thought of something else–you have two electric fans on the t-stat. If they’re drawing more power than they should, that will load your alternator and possibly slow your idle by a couple hundred rpms. Just another thing to check, but these fans should be really easy to unplug.
If that’s not it, run the engine with the air cleaner housing off, watch the carb as it warms up to see if anything moves.
The problem sounds like it would be vac related, but I don’t think there’s anything on your truck that uses vac at a temperature. Likely your lower vac is caused by lower rpms when it’s running badly. Some cars have an intake warmer valve in the intake snorkel. If you’ve got one of those, isolate it to rule out.
Are you getting good flow through the exhaust pipe? Plugged muffler could do this.
why do you think it is an air-intake problem if it occurs so reliably when the temp goes up? everything you describe makes it sound like a thermal problem. Maybe your block temp gauges are not accurate or are just not sampling in the places that are overheating. Try running the heater when the thermostat opens in addition to having the fans go. Make sure no air bubbles in the system. The next step i think is to try a new radiator.
You probably have a vacuum advance on yours. Check it. If it’s an automatic transmission, check the modulator on it. It’s built the same way.
You may in fact NOT have a vacuum leak,but-
If you suspect one, try these tricks.
A. Get a piece of vacuum hose long enough to stick in your ear and then poke around all areas you suspect a leak to be. You’d be amazed where a hose will go. You’ll hear it readily, especially if you stick a cigarette butt in your other ear first. (I know, but it works…)
B. Attach a propane torch bottle to the vacuum hose-DO NOT LIGHT IT!!- then turn it on slightly, fishing the hose end around suspected areas of leaks. This WILL change your engine rpm slightly or more, if you’re listening for it. If you think you found a suspect- follow up with the ‘ear’ method again. Good luck!
the pickup coil in the distributor is bad or the intake manifold is leaking when warmed up it can leak inside the engine to the crankcase as well as externally block the pcv and spray some 2+2 in oil fill cap if rpm increases problem found