Ok so I'm not a mechanic by any means but this problem has severely baffled me. I have a 74 mav with a 250 straight 6cyc. originally i had notitced my car began to run a little rough, nothing major i adjusting the timing because it was severely retarded and then adjusted the mixture and idle speed. It seemed to fix the problem. three months later i go to put the car into drive push on the gas and it immidiately dies. However, if I let the car coast in drive for approx 10 mins i can push on the gas and it begins to run, but its rough until i hit 30 mph, then it goes away. I noticed that I had a fuel leak aroung the fuel pump so i replaced it, which required me to change the oil, adjust timing and mixture. Then, I test drove it again and the problem still exists. I checked the vacuum pressure which happened to be really low near the vacuum advance so i replaced that, but now i have also noticed a large amount of air coming from the breather cap on the block. I replaced the gaskets under the carb and all of the vacuum lines. however now i have an even bigger vacuum leak. The hose that also acts as a breather on the block that connects under the carb causes a major vacuum leak and the car will only stay running if the hose is disconnected and the connecter under the carb is plugged. I have also replaced all of the spark plugs and wires. Im not sure if this is something from ignition or a major vacuum problem that i am missing. Please any advice will help
First, I would remove all the vacuum lines, except what is absolutely needed. The 74 has EGR and all kinds of crap that could affect the idle if not in perfect shape. Plug the egr, or even better, remove it. All the vacuum lines you really NEED are the one to the distributor, one to the firewall for heater flap control, and one to the tranny, if C4. There should also be a big one from the valve cover to the front of the carb with a PCV valve in it (I think the I6s have that). Then, new distributor cap and rotor, plug wires if they are too old. After that, start over with carb adjustments. Timing won't kill the engine, if it is a little off, just make it idle slower or faster, unless you have it way retarded. Then, if that doesn't work, I would look into rebuilding or swapping the carb. Also, make sure the fuel is new and clean, the tank is clean, and the filters are not clogged. This is all just your basic 30-year-old car tune-up. You really have a lot of areas to look at before you nail down where the problem might be. But, all of this can be done for around $50, so it shouldn't break the bank. Let us know what you find, we should be able to help you get it running smoothly, but we need details...
The PCV valve is required if you like your piston rings in good shape. EGR is not required for it to run but is required in almost all states that have inspections.(even in Texas unless you are the inspector) I'd check the EGR Valve to make sure its not stuck open. I'd take a good look at the plugs. Black soot? Rich. Extra white? Lean. Red? Crappy low octane fuel with bad additives. Yellow/green? You like the gas pedal too much and probably could use a colder plug or you got cooling problems. wet with gas? Ignition system or flooding. Wet with oil? get a motor. Plugs can tell you a lot about how your car runs and what's wrong.
Comick They check your EGR? My inspector says "if it runs, horn works, headlights, brake lights, and wipers, you PASS!" My first inspection had no hood, engine stalled if you let off the gas, and brakes were shot, still passed.
Ok so i removed the vacuum lines and connected the ones that were important. Here is some info on the engine 250 6cyc automatic 2dr plugs 2,3,5 were completey gummed up even after just doing a tune up a few months ago compression 1=105 2=105 3=95 4=95 5=70 6=80 WHen i bought the car the owner told me it only had 88,000 original miles but when i looked at the compression I'm starting to think otherwise. But that could also be another issue. The carb is only three years old. I replaced the PCV valve thinking that would fix the vacuum problem however, even with a new valve it still causes a major vacuum leak. I did notice something very interesting the vacuum lines that go to the tranny were cut which would explain why the car would shift through all the gears before i hit 30mph. The distributer is new along with all the wires and plugs. The car does not stall as much now but I'm still not sure about the PCV valve. I really appreciate all the advice.
you must have made a wrong turn somewhere. nice country up there. i've been here 16 years. moved here from washington state.
I would like to see the compression test run again. test the compression after the car is warmed up to normal operating temp. Pull all the plugs and then run the test with the throttle held wide open while cranking. You are looking for no more than 15 PSI difference between bores. If you have more deviation than that or pressure below 100 you need a new motor. If the transmission vacuum line had been cut it would shift at 4000 rpm and on the street you would never get it in 3rd gear. The vacuum hoses that you are talking about do not go to the transmission. A pcv valve will leak vacuum. It is a controlled leak that is used to pull the oil and fuel fumes out of the crankcase and burn them. Unless you put it in backwards the leak that is there is quite normal. Did the 74's have a curb idle soleniod? I know the the 72 and 73 did. If it is equipped with a curb idle solenoid then you adjust your idle speed in gear with it. It allows you to have an engine running smooth and strong and still shut off when you Turn off the power. To find vacuum leaks, spray carb cleaner around the carb,vacuum hoses, cylinder heads and intake while the engine is idling. Make sure you keep this stuff away from hoses and belts or that you rinse them off ASAP with water. If you find vacuum leaks fix them as you go. If the symptoms continue then check the EGR valve. Replacing the valve is essential, but you need make sure that it is the problem. When the engine is idling remove the vacuum line to the EGR valve while listening to the sound of the motor. If the idle gets worse the the valve is working. If the idle remains unchanged then the valve is bad.
One thing that no one has mentioned yet is the vacume booster if you have power brakes. Sometimes the diaphram in the booster will get a hole in it and you will end up with a vacume leak the you can not find. Sometimes you can hear the leak from inside the car sometimes you cant. So if you have power brakes, plug off the hose going to the booster and see if it makes any difference in the idle. Just another 2 pennies
You mention that you are "not a mechanic, by any means" but you are swift enough to run a compression check...
I took an auto class in high school. It's not fixing or replacing the parts that is the issue for me its just learning how to diagnose the problem.
My Mavericks are always in 3rd by time they hit 30. You definately need to have those vaccume lines hooked up to the vaccume modulator on the trans though. If the PCV valve is new and the hose going to it isn't leaking, then your problem is elsewhere. If all else fails, you can totally unhook every vaccume line and plug them all off. Start the car (don't try to drive it or anything) and see if it's still running bad. If not, hook all the vaccume lines back up one at a time until you figgure out which one is giving you the problem. If it dosen't run right with all of them disconnected and plugged off, I would start looking towards the fuel system.
Looking at the compression readings and having burnt up a couple of 250's I'd say its time for a rebuild. 302???!!! Yeah 68 and newer. I just read the book about a week ago. Keep going to the guy you are using.
i wouldnt say he needs a new motor... i would say he would need to have it rebuilt or freshened up with some new rings... but after 80k miles, it might be worth rebuilding the engine... or upgrade to a 302 since there are more parts and more power in a 302 over a 250
